In order for an organization to sustain itself and grow, it must have an effective leader and the same holds true for the highly organized cell. In this chapter, we complete our tour of a typical body cell with a look at the cell’s “chief supervisor,” the nucleus. After describing its form and chemical makeup, we will explain how the nucleus controls the cell’s day-to-day activities through the action of its genetic material, DNA. Finally, as a logical connection to an upcoming chapter, the study of groups of cells called tissues, we will conclude this chapter by explaining how cells reproduce.
FUNCTION OF THE NUCLEUS
The most recognizable organelle in most cells is the nucleus, and its enormous size compared to other organelles suggests that it plays a major role in the normal functioning of the cell. The nucleus is the largest organelle in our typical cell, and is so-named because it resembles a round seed (nucleus, nut). It serves as a storehouse and library for genetic material, which, in the form of DNA molecules, provides the chemical “blueprints” for constructing the cell’s proteins. Because the nucleus directs protein synthesis, and because proteins (notably enzymes) control most of the cell’s metabolic activities, the nucleus is nicknamed the cell’s “control center.”
The number of nuclei within a cell relates directly to the amount of protein the cell must synthesize to maintain its structure and metabolic activity. Most cells in the body are uninucleate (oo-ni-NŪ-klē-āt; uni, single) having only one nucleus, but a few types of cells are multinucleate, having more than one nucleus. Examples of multinucleate cells that synthesize large amounts of protein include skeletal muscle cells and certain cells in bones and liver. Red blood cells are unique in that they develop as uninucleate cells inside bones but become anucleate (ā-NŪ-klē-āt) when they eject their nucleus prior to entering the blood (the prefix a- means “without”). This seemingly bizarre act enables red blood cells to carry more oxygen, but it also leaves the cells without any genetic material. Consequently, red blood cells can’t replace worn-out proteins and live only three or four months.
STRUCTURE OF THE NUCLEUS
The most noticeable features of the nucleus are the nuclear envelope, nucleoplasm, and nucleolus (Figure 5-1). The nuclear envelope encloses the nucleus and consists of an outer and inner membrane. The outer membrane is continuous with the rough endoplasmic reticulum, and like the rough ER, has numerous ribosomes on its surface. Large, circular openings, called nuclear pores, exist where the inner and outer membranes fuse and allow the nucleus to exchange materials with the cytosol. Nuclear pores are large enough to allow ions and small molecules (including nucleotides) to move freely into and out of the nucleus. However, macromolecules such as proteins and RNA can move into or out of the nucleus only with the help of proteins that line the nuclear pores. The exact mechanism by which this transport occurs is unknown, but it is a process that requires the cell to expend energy.
Figure 5-1. Parts of the nucleus
The inner membrane of the nuclear envelope surrounds the nucleoplasm (NŪ-klē-ō-plazm), the fluid inside the nucleus. Nucleoplasm has a consistency similar to cytosol and its major organic components include DNA, RNA, ribosomal proteins, enzymes, and nucleotides. A nucleolus (nū-KLĒ-ō-lus) is a dark-staining region inside the nucleoplasm where RNA molecules and proteins come together to form large or small ribosomal subunits. The ribosomal subunits eventually exit the nucleus through the nuclear pores and come together in the cytosol to form functional ribosomes. Although it appears to have a well-defined border, a nucleolus does not have a membrane. What is more, the size of a nucleolus is ever changing, growing when new ribosomal subunits become part of it and shrinking when subunits leave it to enter the cytoplasm. Cells that synthesize large amounts of protein need many ribosomes and, therefore, usually have more than one nucleolus inside the nucleus.
ORGANIZATION OF THE GENETIC MATERIAL
Genetic material consists of DNA and protein bound together in a way that allows it to exist either as fine, entangled threads during cell growth or as highly condensed, well-defined packages during cell division. The genetic material exists in the nucleus of a non-dividing cell as fine, intertwining threads called chromosomes (KRŌ-mō-somz; chromo, color; somes, bodies). The structural material that comprises a chromosome is chromatin (KRŌ-ma-tin), so-named because it appears dark when stained with certain dyes. A closer look at chromatin reveals that it is a complex union between DNA and a group of globular proteins called histones (HIS-tōnz). Histone proteins look like small marbles and bunch together in groups of eight. A short segment of a double helix DNA molecule wraps around these clusters of histones, much like a thread wraps around a spool. The entire DNA-histone complex resembles a beaded necklace, in which each “bead” is a nucleosome (NŪ-klē-ō-sōm). Figure 5-2 shows the structure of a nuclear chromosome.
Figure 5-2. Structure of a chromosome
Chromosome Number
The number of chromosomes in the nucleus of a human gamete (sex cell—egg or a sperm) is 23, referred to as the human’s haploid number (n). Each of these chromosomes is different; that is, each one contains genetic information for different inheritable traits. One of the 23 different chromosomes is called the sex chromosome, because it plays a role in determining the sex of the person. The other 22 chromosomes in the gamete are called autosomes.
Within the nucleus of a human somatic cell, the 23 different chromosomes exist in pairs; therefore, the number of chromosomes in the nucleus of a typical body cell is 46, referred to as the human’s diploid number (2n; DIP-loyd). The two chromosomes within a given pair are said to be homologous (hō-MOL-o-gus), because they contain genetic information for the same inheritable traits (homo, same; logo, words). One member of each homologous pair of chromosomes comes from the mother and the other member of the pair comes from the father. Chromosomes derived from the mother are maternal chromosomes (muh-TER-nal; mater, mother), while those derived from the father are paternal chromosomes (puh-TER-nal; pater, father).
To make sure you understand the numbers and terms just described, let’s recap what you just read. The haploid number (n) refers to the number of different kinds of chromosomes in an organism. For humans, n=23. The diploid number is the number of chromosomes present when there are two copies (one maternal and one paternal) for each kind of chromosome. For humans, 2n=46. Human gametes are haploid (n), containing only 23 chromosomes (22 autosomes and 1 sex chromosome). Human somatic cells are diploid (2n), containing 46 chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes).
DNA EXPRESSION
Each DNA molecule in the nucleus consists of functional segments called genes (jēnz) that provide a chemical code specifying the number, type, and arrangement (sequence) of amino acids in proteins. The word gene implies that information within these segments of DNA can be used to generate proteins. Recall that two or more amino acids linked together comprise a peptide; therefore, since proteins consist of many amino acids, biochemists refer to them as polypeptides. Biochemists have determined that genes contain chemical instructions necessary to synthesize polypeptides, and this concept became the one gene-one polypeptide theory. All the genes on all the different types of chromosomes in your cells make up your genome (je-NŌM). An example of how genes might align on a chromosome is shown in Figure 5-3. The small segments that have no labels represent “promoters” and other regions not encoded directly onto RNA molecules. The names of actual genes would be written in italics using lowercase letters; e.g., genes that code for the enzymes sucrase and lactase might be written suc and lac, respectively. The Human Genome Project is attempting to identify every gene on every chromosome in the human species. Identifying gene location would allow researchers and clinicians to know which genes are “linked” (located on the same chromosome) and might allow defective genes to be replaced.
Figure 5-3. Genes on a chromosome
The Genetic Code
What exactly is the genetic information inside a gene that helps direct the sequencing of amino acids in a protein? First, recall the DNA of living organisms is a double helix molecule, comprised of two polymers of nucleotides held together by hydrogen bonds. Each nucleotide consists of a sugar (deoxyribose), a phosphate ion, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), or guanine (G). Think of the letters A, T, C, and G as the alphabet for the cell’s genetic language (code), which consists of 3-letter “words” called triplets. With four letters available to arrange in groups of three, there are 43 or 64 triplets possible. Sixty triplets specify amino acids that can become part of a protein; three triplets function as “stop” signals for protein synthesis; and the remaining triplet functions as a “start” signal for protein synthesis. Since the body contains only 20 different kinds of amino acids, more than one DNA triplet can specify the same amino acid. For example, the triplets CAT, CAG, CAC, and CAA all specify the amino acid valine.
The genes in a non-dividing cell remain inside the nucleus and do not guide protein synthesis directly, but instead employ RNA molecules to carry genetic information to ribosomes in the cytoplasm. Biochemists refer to the phenomenon whereby genetic information passes from a DNA molecule to an RNA molecule and ultimately to protein as the central dogma of molecular biology (dogma, opinion). We can summarize the central dogma as follows:
However, DNA does not become RNA, and RNA does not become protein. Instead, genetic information in DNA guides the synthesis of RNA, and then the information in RNA guides the synthesis of a protein. The central dogma involves two major processes: transcription and translation.
TRANSCRIPTION
Transcription, or RNA synthesis, represents the first step in the central dogma whereby genetic information passes from one type of nucleic acid to another type (trans, across; script, writing). The normal pathway is to transfer information from DNA into a molecule of RNA. To grasp the significance of this process, consider the following analogy. Let’s say that you want to build a certain type of house (which we will say is comparable to a protein). You can find a blueprint (gene) for this house in a special book (chromosome) located in the library (the cell’s nucleus). The book cannot leave the library, however, but you can copy (transcribe) the blueprint and take the copied version (RNA) to the construction site (ribosome). Although the blueprint copy is not the same kind of paper (nucleic acid) as the original blueprint, it contains the appropriate information for building the specific house (synthesizing the specific protein). Figure 5-4 provides an overview of transcription.
Figure 5-4. Overview of transcription
How does a cell determine which genes to transcribe? Before transcription can begin, gene activation must occur, which means a special chemical must “turn on” the gene, making it ready for transcription. Identifying gene-activating chemicals, called transcription factors, and learning how they work is a major goal of modern biochemistry research. Suffice it to say, negative feedback regulates the concentrations of transcription factors within the nucleus. For example, a deficiency of a particular cellular protein causes increased production of transcription factors that activate the gene responsible for generating that protein. Just before transcription begins, transcription factors uncoil the chromatin and expose a portion of the DNA molecule called a promoter (prō-MŌ-ter), located next to the gene of interest. The gene is now ready to be transcribed.
Transcription, like other synthesis reactions in a cell, requires an enzyme. In this case, the enzyme is RNA polymerase (puh-LIM-er-ās), so-named because it constructs a polymer called RNA. After binding to the promoter located on the DNA, the RNA polymerase breaks the hydrogen bonds between the parallel strands of the DNA’s double helix, effectively “unzipping” the DNA molecule in that region. Separating the double helix is necessary to expose the DNA triplets that specify the amino acids in a protein. This initial phase in transcription is comparable to opening a book of blueprints to the page showing the blueprint that you want to copy. Furthermore, let’s say that this blueprint exists on only one page in the open book. Similarly, RNA polymerase binds to only one strand of the DNA double helix; this strand is the DNA template and it contains the base sequence used to direct the synthesis of an RNA molecule.
In the second phase of transcription, RNA polymerase moves along the DNA template toward the template’s 5’ end. While doing this, the RNA polymerase pulls in RNA nucleotides from the nucleoplasm and aligns their bases (A, U, C, or G)alongside complementary bases on the DNAtemplate. (Recall from Chapter 3, RNA substitutes uracil for thymine; thus, uracil is complementaryto adenine.) Hydrogen bonds form between thecomplementary bases, temporarily holding them together. When the RNA polymerase positions two RNA nucleotides side by side, it forms a covalent bond between them. In this way, RNA polymerase constructs an RNA molecule having a base sequence that is complementary to the base sequence on the DNA template. Since the polymerase moves along the template in a 3’ to 5’ direction, it constructs the RNA molecule in a 5’ to 3’ direction. In other words, it always adds new RNA nucleotides to the 3’ end. The portion of the RNA trailing behind the RNA polymerase detaches from the template, allowing the separated DNA strands in that region to come back together, much like re-zipping a zipper, to form a double helix (see Figure 5-5).
Figure 5-5. Direction of transcription
Transcription stops when the RNA polymerase comes to a region of the gene called the terminator. Reaching the terminator causes RNA polymerase to detach from the DNA template and release the newly formed RNA molecule, called an RNA transcript. Let’s now look at the three types of RNA generated by transcription.
Types of RNA
Transcription of various genes is responsible for producing all RNA molecules used by a cell for protein synthesis, but not all RNAs have the same function in protein synthesis. RNA comes in three different functional forms: ribosomal RNA, messenger RNA, and transfer RNA.
Ribosomal RNA. Ribosomal RNA (rRNA) helps structurally reinforce a ribosome and may play a role in linking amino acids together during protein synthesis. Soon after their synthesis, rRNA molecules bind to special proteins inside the nucleus to form large and small ribosomal subunits. When needed for protein synthesis, these ribosomal subunits pass through nuclear pores and come together in the cytoplasm to form functional ribosomes. Biochemists have identified five different chromosomes in the nucleus of a human cell that generate rRNA molecules during transcription.
Messenger RNA. Carrying a copy of the DNA’s genetic message for protein synthesis from the nucleus to a ribosome is the function of messenger (mRNA) molecules. Each “word” in the mRNA’s message consists of three nucleotide bases called codons (KŌ-donz), so-named because they represent the codes for specific amino acids. Codons are complementary to triplets in DNA. For example, transcribing the DNA triplet CAT results in the codon GUA. Since there are 64 possible DNA triplets, there are 64 possible codons. While a single codon can specify only one of the 20 different kinds of amino acids in the body, more than one codon can specify the same amino acid. Redundancy in the genetic coding system means that changing one of the bases in a triplet or codon may not necessarily change the amino acid specified originally. Table 5-1 lists all possible codons and the amino acids they specify. (Note the three “stop” codons do not code for amino acids.) You do not need to memorize this table, but you do need to know the “start” codon (AUG) and its corresponding amino acid (methionine; meth-I.-ō-nēn).
Before an mRNA molecule carries the DNA’s genetic message to a ribosome, it undergoes a process called mRNA processing. Before processing occurs, the mRNA, known as pre-mRNA, contains nonfunctional segments called introns (EN-tronz) that do not properly code for segments of a protein. Special enzymes in the nucleus remove the introns and splice together the remaining functional segments called exons (KS-onz). Introns are so-named because they remain inside the nucleus after mRNA processing. Exons are so-named because they bond together and exit the nucleus. The functional mRNA that ends up at a ribosome may be considerably shorter than the original pre-mRNA (see Figure 5-6).
Figure 5-6. Processing mRNA
Table 5-1. The Genetic Code – Codons on mRNA and corresponding amino acids for which they code.
| Adenine as first base | Cytosine as first base | Guanine as first base | Uracil as first base |
|---|---|---|---|
| AAA Lysine | CAA Glutamine | GAA Glutamine | UAA (Stop) |
| AAC Asparagine | CAC Histidine | GAC Asparagine | UAC Tyrosine |
| AAG Lysine | CAG Glycine | GAG Glutamine | UAG (Stop) |
| AAU Asparagine | CAU Histidine | GAU Asparagine | UAU Tyrosine |
| ACA Threonine | CCA Proline | GCA Alanine | UCA Serine |
| ACC Threonine | CCC Proline | GCC Alanine | UCC Serine |
| ACG Threonine | CCG Proline | GCG Alanine | UCG Serine |
| ACU Threonine | CCU Proline | GCU Alanine | UCU Serine |
| AGA Arginine | CGA Arginine | GGA Glycine | UGA (Stop) |
| AGC Serine | CGC Arginine | GGC Glycine | UGC Cystine |
| AGG Arginine | CGG Arginine | GGG Glycine | UGG Tryptophan |
| AGU Serine | CGU Arginine | GGU Glycine | UGU Cystine |
| AUA Isoleucine | CUA Leucine | GUA Valine | UUA Leucine |
| AUC Isoleucine | CUC Leucine | GUC Valine | UUC Phenylalanine |
| AUG Methionine (Start) | CUG Leucine | GUG Valine | UUG Leucine |
| AUU Isoleucine | CUU Leucine | GUU Valine | UUU Phenylalanine |
DNA Transcription in Human Cells
In addition to cleaning up a pre-mRNA molecule by removing introns, mRNA processing offers another advantage. By splicing exons together in different sequences, a single gene can generate more than one functional mRNA molecule. This explains how the human genome’s approximately 30,000 functional genes can give rise to more than 100,000 different proteins in the body.
Transfer RNA. Bringing amino acids to a ribosome so they can come together to form a protein is the function of transfer RNA (tRNA). This relatively small, clover-shaped molecule consists of less than 100 nucleotides. Hydrogen bonds between complementary bases in certain regions of the molecule are responsible for a tRNA’s unique shape. One end of the tRNA displays three nitrogenous bases, called an anticodon, which can form hydrogen bonds with complementary codons on mRNA. For example, a tRNA with the anticodon CAU will bind only to the mRNA codon GUA. At the other end of the tRNA, an enzyme in the cytoplasm attaches a specific amino acid. The tRNA then carries its amino acid passenger to the ribosome to become part of a protein. In this respect, tRNA is like a delivery truck that carries building materials to a construction site. Let’s now look at how tRNA and mRNA interact at a ribosome.
TRANSCRIPTION
Translation is the process by which a cell uses the sequence of codons on an mRNA molecule to determine the sequence of particular amino acids in a protein. To grasp the relevance of the word translation, think of mRNA as a translator that converts words (triplets) of one language (DNA) into appropriate words (amino acids) of another language (protein). In our blueprint analogy, the RNA polymerase is like the carpenter who “translates” the information on a photocopied house blueprint (mRNA) in order to build an actual house (protein).
Translation occurs in the cytoplasm and involves the collective efforts of mRNA, tRNA, and small and large ribosomal subunits. Before translation begins, mRNA molecules and the ribosomal subunits exit the nucleus through nuclear pores in the nuclear membrane. In the cytoplasm, the ribosomal subunits come together around the mRNA to form a functional ribosome. The mRNA then shifts through the ribosome one codon at a time. Each time the mRNA shifts, a tRNA molecule brings in the next amino acid specified by the codon occupying a precise location within the ribosome. Within the ribosome-mRNA complex, a covalent bond forms between two adjacent amino acids so that the ribosome builds a protein one amino acid at a time. Translation involves five major steps (see Figure 5-7 and the description of each step that follows).
- Binding mRNA with small ribosomal subunit: After entering the cytoplasm, mRNA binds to an mRNA-binding site located on the small ribosomal subunit. The region of the mRNA that binds first with the small ribosomal subunit displays the “start” codon having the base sequence AUG. Then a tRNA with the complimentary anticodon UAC binds with the start codon. This “first” tRNA, called the initiator tRNA because it initiates the translation of the genetic code, always carries the amino acid methionine.
- Formation of ribosome and entry of next tRNA: After binding with the mRNA, the small ribosomal subunit locks together with a large ribosomal subunit to form a functional ribosome. The initiator tRNA and its methionine amino acid now fit into a region called the P site located on the large ribosomal subunit. The P, or peptidyl (PEP-ti-dil) site is so-named because it will hold the growing polypeptide (protein) when another amino acid arrives at the ribosome. The next tRNA that can enter the ribosome has an anticodon that is complementary to the next codon on the mRNA. After entering the ribosome, the tRNA binds to the complementary codon and occupies a site in the large ribosomal subunit called the A site. The “A” stands for aminoacyl (a-mē-nō-AS-il), implying that this site receives the incoming amino acids, which the ribosome will add to the growing polypeptide, held in the P site.
- Formation of a peptide bond: With tRNA molecules occupying the P and A sites in the ribosome, an enzyme in the large ribosomal subunit forms a peptide (covalent) bond between the two adjacent amino acids resting atop their respective tRNAs. At this point, the two amino acids together comprise a dipeptide, which is held by the tRNA in the A site.
Figure 5-7. Translation
- Translocation: After a peptide bond forms, the P site releases its tRNA, which no longer holds an amino acid. At the same time, the ribosome shifts the mRNA so that the third codon enters the ribosome. During this shifting process, called translocation, the tRNA located in the A site moves to the P site, leaving the A site empty. The next tRNA, which has an anticodon that is complementary to the third codon, enters the ribosome and binds to the A site. A peptide bond then forms between the second and third amino acids so that the dipeptide becomes a tripeptide, containing three amino acids. Trans location and peptide bond formation occur again and again resulting in an oligopeptide, and finally a polypeptide.
- The stop codon ends translation. Translation ends when translocation pulls a “stop” codon (UGA, UAG, or UAA) into the A site of the ribosome. At that moment, the tRNA molecule located in the P site releases the polypeptide and detaches from the P site. The large and small ribosomal subunits then disconnect from one another and release the mRNA. The released polypeptide twists and coils to develop different levels of complexity and may perform a specific duty as one of the TRICCS proteins.
Note:
A ribosome can combine 3-4 amino acids per second; therefore, it takes about 1 minute to synthesize a protein consisting of 200 amino acids, but several hours to make titin, the body’s largest protein made of over 30,000 amino acids.
Each ribosomal subunit and RNA molecule mentioned above may participate in translation over one thousand times before finally wearing out. After detaching from mRNA, the ribosomal subunits are free to bind again to the same mRNA or they can bind to a different mRNA. Likewise, after detaching from a ribosome’s P site, a tRNA is free to pick up another amino acid (specific for that tRNA) and bring it to either the same ribosome or a different ribosome. Furthermore, a single mRNA molecule can undergo translation by more than one ribosome at a time. When two or more ribosomes translate the same mRNA at the same time, the ribosomes collectively make up a polyribosome. A polyribosome enables a cell to generate large amounts of a specific protein in a relatively short amount of time with relatively few mRNA molecules.
Like all other compounds within a cell, the chemical components of translation have a limited life span. While some mRNA molecules may endure repeated translation for several hours or even a day or two before degrading, other mRNAs degrade and break apart after only a few minutes. The mechanisms responsible for this degradation are somewhat complicated, so let’s just think of mRNA as being like a cotton thread that eventually wears out and breaks because we have been pulling it repeatedly through a bead (ribosome) or a series of beads (polyribosome). Of course, even the bead would wear out eventually. When needed, a cell can call upon its nucleus to synthesize new RNA molecules through transcription to replace the worn-out strands. Translation of specific mRNAs generates new ribosomal proteins, which enter the nucleus to become part of new ribosomal subunits.
Site of Protein Synthesis
What determines whether a cell synthesizes a particular protein in the cytosol (on a free ribosome) or on the rough ER? Interestingly, the synthesis of all proteins begins at free ribosomes, and then the ribosome either remains free in the cytosol to complete translation or attaches itself to the rough ER (becoming a bound ribosome) where it completes translation. If completion of translation is to occur at the rough R, the first few amino acids to become part of the growing polypeptide function as a leader sequence. While the ribosome continues with translation, the leader sequence of the growing polypeptide attaches to a signal recognition particle (SRP) in the cytosol. The SRP then enables the ribosome to attach to a receptor site on the rough ER. As translation continues, the growing polypeptide chain enters the ER through a tiny pore at the receptor site. Proteins made in this way are modified inside the R and Golgi complex. After incorporation into a vesicle that pinches off the Golgi complex, the new protein may be: (1) inside a lysosome; (2) inserted into the cell membrane, or (3) secreted into the extracellular fluid.
If completion of translation is to occur in the cytosol, the polypeptide may or may not have a leader sequence. (If a leader sequence is present, it has a different amino acid sequence than the one found on proteins completed at the rough ER.) If there is no leader sequence, the polypeptide will be completed at the free ribosome and remain in the cytosol. If a leader sequence is present, it will specify that the new polypeptide will enter the nucleus, a mitochondrion, or a peroxisome. In summary, the destination of proteins completed at free ribosomes depends on the presence or absence of a leader sequence. Moreover, the type of organelle into which some of these proteins enter depends on the specific type of leader sequence present on the polypeptide.
CELLULAR DIFFERENTIATION
Since the trillions of cells in a human body arise from a single cell (fertilized egg) and have virtually the same genes, how does the body come to possess more than 200 different kinds of cells? The answer relates primarily to how different cells utilize their genetic material. While all nucleated cells in the body have virtually the same genes, some kinds of cells transcribe certain genes that other types of cells do not transcribe. Consequently, some cells make certain types of proteins that other cells do not make. In other words, although cells may have the same kind of library (nucleus) with the same kinds of books (chromosomes), not every cell utilizes the same pages (genes) from those books. Cellular differentiation is the process by which similar cells become different from one another through the expression of different genes.
Cellular differentiation occurs primarily due to a cell’s interaction with its extracellular environment. Let’s consider how this happens. When a cell divides, the resulting daughter cells are genetically identical to the dividing (parent) cell. After repeated cell divisions, the cells cluster in masses so that some cells exist completely surrounded by other cells. Due in part to the accumulation of various cellular products in the extracellular fluid, cells in the middle of the cluster live surrounded by a slightly different chemical environment than cells along the edge of the cluster. Consequently, various chemicals may either activate or deactivate certain genes in some cells, while the same genes in other cells remain unaffected. In turn, cells that produce different types of proteins become structurally and functionally different from one another.
THE CELL CYCLE
A human body can grow, replace damaged and worn-out cells, and experience cellular differentiation only because many of its cells can reproduce themselves. Cell division, or cellular reproduction, results when a cell splits to form two new cells called daughter cells. The series of events that occur in the life of a typical cell from the moment it forms until it divides comprise the cell cycle. The word “cycle” implies a return to a beginning point; in the cell cycle, daughter cells grow and divide producing more daughter cells, which in turn, grow and divide, and so on. A few types of cells, including neurons and skeletal muscle cells, do not divide; therefore, they do not go through a cell cycle.
In addition to body growth, cell replacement, and cellular differentiation, cell division is important because it maintains a high surface area to volume (SA/V) ratio for cells. Understanding this fact will help you understand why cells are so small. First, the plasma membrane represents the cell’s surface area through which exchange of nutrients and wastes can occur. Second, as a cell grows larger, its volume and surface area both increase, but the surface area increases at a slower rate. Consequently, the SA/V for a large cell is lower than that of a smaller cell. This means that the larger cell has a smaller proportion of its mass (volume) exposed to the extracellular fluid than does the smaller cell. The larger cell may not have enough surface area through which to absorb nutrients or expel wastes in order to survive. By dividing in two, a cell can produce two daughter cells, each with a higher SA/V ratio than the dividing cell.
In Figure 5-8, you can see the effects of different sizes and shapes on there are different you can see the effect of increasing size on the SA/V ratio. For part (a), a small cell is 1 unit long on each side, so its SA is 1 x 1 x 6 sides = 6 square units (written 6 units2), while its volume is 1 x 1 x 1 = 1 cubit unit (written 1 unit3). Therefore, its SA/V ratio is 6, meaning it has 6 units2 available for each unit3 of volume (or mass). For part (b), the SA is 24 units2 and the V is 8 units3, so the SA/V = 3, that is, it has 3 units2 of SA for each unit3 of volume. We can take the same 8 units3 of volume from part (b) and arrange the blocks in a row and not change the volume. However, the SA of this arrangement (shape) is now 34 instead of 24.
Figure 5-8. Effect of size and shape on surface area to volume ratio. (a) 1 cube (b) 8 cubes as a box (c) 8 cubes as a chain
As you can see, changing the arrangement of the blocks, while not affecting the volume can affect the SA/V ratio. Think about your hand having a specific volume. Its surface can vary, however, depending on whether you hold you hand out with an open palm or make a fist. While both have the same volume, the open-palm hand has a higher SA/V. Now let’s move on to see how cells can increase their SA/V through cell division.
The cell cycle involves three major phases: interphase, when the cell is growing and preparing for cell division; mitosis, when the cell’s genetic material divides to form two nuclei; and cytokinesis, when the cell’s cytoplasm divides to form two daughter cells. The length of time that a cell spends in each stage varies, but interphase is always the longest. Rapidly dividing epithelial cells in the intestine may spend 10-12 hours in interphase, but only an hour in mitosis and cytokinesis. At the other extreme, liver cells may spend a year or longer in interphase before they enter mitosis. We will now describe the major events that occur in a typical cell with a life cycle lasting about 24 hours. Figure 5-9 illustrates this typical cell cycle.
INTERPHASE
A typical cell spends most of its time in the cell cycle carrying out normal metabolic activities and preparing to divide. Because this part of the cell cycle includes cellular activities that occur between cell divisions, cell biologists refer to it as interphase (inter-, between). In our typical cell, interphase lasts 15-20 hours and includes three sub-phases: G1, S, and G2. The S stands for “synthesis,” and denotes the time when the cell is synthesizing DNA molecules prior to mitosis. The G’s stand for “gaps,” which refer to the times before and after DNA replication but does not include mitosis. To say that a cell is in G1 phase suggests that the cell will eventually enter the S and G2 phase. For this reason, cells that will not divide are in a G0 phase (G zero).
G1 phase
The G1 phase (1st gap phase) lasts 8-10 hours and is the period between when the cell forms and the time it begins replicating DNA. During G1 the chromatin inside the nucleus is uncoiled, allowing RNA polymerases to transcribe genes. The newly formed RNAs then leave the nucleus to participate in protein synthesis (translation) at ribosomes. Throughout G1 phase, the cell grows and reproduces its organelles, ensuring that after cell division the daughter cells will have all the necessary cellular components they need to survive. Near the end of G1, the cell replicates its centrosomes for use in mitosis.
S phase
The S phase (or DNA synthesis phase), which usually lasts 6-8 hours, is the period when the cell replicates each of the DNA molecules within the nucleus. During this phase, additional histone proteins generated at ribosomes enter the nucleus and bind with the newly synthesized DNA molecules to form new strands of chromatin. The nucleus of a human somatic cell entering the S phase contains 46 strands of chromatin; after S phase, the nucleus contains 92 strands of chromatin. Replicating the DNA molecules ensures that the future daughter cells will each have 46 strands of chromatin containing the same genetic information as that in the parent cell. This is crucial, since the daughter cells will need to synthesize different types of proteins to build cellular components and perform normal metabolic activities.
G2 phase
The G2 phase is the time after DNA replication when the cell makes final preparations for mitosis. Protein synthesis continues and the cell is still growing. The centrioles complete their replication and the nuclear membrane is still intact. The genetic material is still in the form of chromatin, but not yet visible with a light microscope. (Recall that each strand of chromatin consists of a double helix DNA molecule wrapped around clusters of histone proteins to form a chain of nucleosomes.) However, due to DNA replication in S phase, the 46 strands of chromatin from G1 phase have become 92 strands of chromatin. The G2 phase lasts 4-6 hours.
MITOSIS
During mitosis (mī-TŌ-sis) the replicated genetic material condenses and separates so that a copy of each original DNA molecule moves to opposite ends of the cell, yielding two identical nuclei. This process ensures that each daughter cell, formed later during cytokinesis, will have the same genetic material as its parent cell. Mitosis lasts about 2 hours in a typical cell and involves prophase, metaphase, anaphase, and telophase.
Prophase
Mitosis begins with prophase, when replicated strands of chromatin condense, centrosomes migrate, the mitotic spindle develops, and the nuclear membrane dismantles. During early prophase, the strands of chromatin undergo condensation, during which the nucleosomes coil repeatedly to form tighter and larger loops that eventually become visible under the light microscope. By the end of prophase, each strand of chromatin is fully condensed and called a chromatid. The two identical chromatids (called sister chromatids) resulting from the replication of a single strand of chromatin remain attached to one another at a region called the centromere (SEN-tro-mēr). Two sister chromatids held together by a centromere make up a replicated chromosome.
Figure 5-9. Parts of the cell cycle
Mitosis Stages in Human Cells
Why is it important to condense the DNA during prophase? Condensing the chromatin in prophase makes it easier for a cell to organize and separate its genetic material prior to cytokinesis. However, when tightly packed into chromosomes, the DNA’s genetic information is inaccessible to the cell to use for protein synthesis. In contrast, when a cell is not preparing to divide, the DNA molecule remains uncoiled and only loosely associated with the histone proteins, making the genetic information available for use in protein synthesis. Condensing 92 strands of chromatin into 46 replicated chromosomes is like coiling 92 long cotton threads around 46 spools to make them easier to manage until you need them later for sewing.
While chromatin condenses to form chromosomes, the other events that occur during prophase prepare the way for separating the sister chromatids from one another. First, the two centrosomes (replicated from a single centrosome during G1 phase) begin assembling microtubules. Some of these microtubules, called polar fibers, connect the two centrosomes to each other. As the polar fibers lengthen, they push the two centrosomes to opposite ends (poles) of the cell. Other microtubules, called kinetochore fibers, extend from both centrosomes and bind to each chromosome. The polar and kinetochore fibers together constitute the mitotic spindle. The two sister chromatids of each chromosome face different poles; thus, each sister chromatid normally binds only to kinetochore fibers originating from the centrosome they face. The kinetochore (ki-NE-tō-kor) is a protein-rich site on each chromatid that anchors a kinetochore fiber, and it is located in the centromere region of the chromosome. Near the end of prophase, the nuclear membrane dismantles, leaving the chromosomes suspended in the cytoplasm. This event will allow the sister chromatids to sepa-rate in a later phase of mitosis.
Metaphase
During metaphase, the replicated chromosomes align themselves along the equator or middle of the mitotic spindle. While aligned in this manner, the chromosomes occupy a region called the metaphase plate. During metaphase, the chromosomes jerk back and forth along the metaphase plate. This movement suggests that the kinetochores are attempting to pull the chromatids toward opposite poles along spindle fibers.
Anaphase
During anaphase, the sister chromatids pull apart and move toward opposite poles. To separate sister chromatids from one another, special enzymes must break the connection at the centromere. After this happens, motor molecules located in the kinetochores pull the sister chromatids, now called sister chromosomes, along the kinetochore fibers toward their respective poles. As the chromosomes move, the kinetochore fibers become shorter by losing tubulin subunits in the region of the kinetochore. A chromosome moving toward a centromere appears V-shaped, with its kinetochore at the “point” leading the way. At the same time, other motor molecules interacting with the polar fibers cause the centrosomes to move farther apart, which also helps pull the sister chromosomes apart. This action also lengthens the cell as it prepares to pinch in two.
Telophase
Telophase (TĒ-lō-fāz; telo, end), the last phase of mitosis, begins when all sister chromosomes stop moving having reached their respective poles around the centrosomes. The cell now has two identical sets of 46 chromosomes at opposite ends of the cell. During telophase, the mitotic spindle disassembles and vesicles in the form nuclear envelopes; the cell now has two cytoplasm collect around the masses of chromosomes and fuse to nuclei. Shortly thereafter, proteins enter the nuclei through newly constructed nuclear pores and bind with rRNA molecules to form nucleoli. In addition, the chromosomes uncoil to become fine strands of chromatin that RNA polymerases can transcribe for protein synthesis. The only thing left to do to complete the cell cycle is to divide the cytoplasm to form two cells.
CYTOKINESIS
Cytokinesis (sī-tō-kin-Ē-sis) is the last part of the cell cycle and involves division of the cytoplasm into two cells. Usually beginning in late anaphase, cytokinesis begins when a cleavage furrow or indentation forms around the cell’s midsection perpendicular to the mitotic spindle. The cleavage furrow develops when a contractile ring, consisting of actin microfilaments with associated myosin motor molecules, squeezes inward on the cytoplasm as if someone were tightening a belt around the cell. As the contraction ring tightens, the cleavage furrow deepens and eventually pinches the cell in two, forming two daughter cells. Because the contraction ring is perpendicular to the mitotic spindle, each daughter cell contains a nucleus. The daughter cells are genetically identical to each other and the parent cell, although they are smaller than the parent cell. After cytokinesis, each daughter cell is in G1 phase.
Note:
Remember that mitosis is nuclear division and cytokinesis is cytoplasmic division.
CLOSER LOOK AT DNA REPLICATION
To understand how daughter cells produced during cell division will be genetically identical to one another, we need take a closer look at how DNA replicates itself. To some extent, DNA replication resembles gene transcription in which an enzyme uses the DNA molecule as a template when forming an RNA molecule. However, unlike transcription, which utilizes only one strand of the DNA’s double helix as a template, DNA replication utilizes both strands.
DNA replication is a very precise process, producing two identical DNA molecules that are exact replicas of the original DNA molecule. However, after replication, the original (copied) DNA molecule no longer exists, but its two strands of DNA nucleotides form half of the new DNA molecules. Since this method of replication preserves half of the original DNA molecule in each new DNA molecule, biochemists refer to it as semiconservative replication (Figure 5-10).
Figure 5-10. Semiconservative replication of DNA
Before replication can begin, the parallel strands of the original DNA molecule must separate so that enzymes can “read” their nitrogenous bases while building new complementary DNA strands. An enzyme called helicase (HĒ-li-kās) “unzips” the DNA molecule by breaking hydrogen bonds between complementary base pairs holding the double helix together. The Y-shaped region where a helicase is unzipping the DNA is a replication fork. More than one helicase can work on a DNA molecule at one time, resulting in numerous replication forks. This allows other enzymes responsible for replication to attach to different regions of the same DNA molecule, thus speeding up the replication process.
After a helicase separates the DNA’s double helix into two strands (DNA templates), several different enzymes must bind to the templates before replication of the templates can occur. One of these enzymes, called a primase, first brings in RNA nucleotides that are complementary to the exposed DNA nucleotides on the template. Linking these RNA nucleotides together, the primase forms a short segment called the RNA primer. Once the RNA primer is in place, another enzyme, DNA polymerase III, moves in next to the primer. While moving along the DNA template, the DNA polymerase III pulls in free DNA nucleotides from the nucleoplasm and bonds them (via H-bonds) with complementary template. DNA polymerase III also forms covalent bonds between the new DNA nucleotides.
DNA polymerase can move along a DNA template only toward the template’s 5’ end; therefore, synthesis of a new DNA polymer occurs only in a 5’ to 3’ direction. As Figure 5-10 shows, one template is replicated continuously in the direction of the replication fork; the new DNA polymer formed on this template is called the leading strand. The other template is replicated in short segments, called Okazaki (ō-ka-SOK-ē) fragments, away from the replication fork, but still in a 5’ to 3’ direction. The new DNA polymer formed on this template is the lagging strand. Before Okazaki fragments can join together to elongate the lagging strand, a different polymerase, DNA polymerase I, must first remove the RNA primers. After this happens, yet another enzyme, DNA ligase (LI.-gās; liga, to join), must move in to join the Okazaki fragment to the lagging strand. Figure 5-11 shows the role of the major enzymes used in DNA replication.
Figure 5-11. Major enzymes used in DNA replication
DNA proofreading: To transfer genetic material accurately from parent cell to daughter cells, DNA replication must occur “letter for letter” without any mistakes. How can there be replication of a typical cell’s several million genes, some of which contain several thousand nucleotide bases, with virtually no copying errors? The answer lies with yet another type of enzyme, DNA polymerase II, which moves along the leading and lagging strands to check the accuracy of the pairing between the new nucleotides and the DNA template. This process is called DNA proofreading. If DNA polymerase II detects a pairing mistake, such as A-G or T-C, it clips out the wrong nucleotide and brings in another one to pair with the complementary base on the template. Proofreading reduces the chance that daughter cells will inherit genetic mistakes (mutations) that could impair their ability to synthesize certain proteins.
Note:
DNA proofreading fails to correct only about one error for every ten million base pairs replicated.
MUTATIONS
A mutation is a change in the base sequence of a gene. Since the order of the DNA triplets determines the order of amino acids in a protein, a change in the order of bases could have adverse effects on a protein’s composition. Mutations include substitutions, insertions, and deletions.
A substitution mutation involves changing (substituting) one nitrogenous base in a triplet. As a result, a single amino acid in the resulting polypeptide may change, but the other amino acids are unaffected. However, since more than one codon can code for the same amino acid, a substitution mutation may have no effect on the amino acid sequence at all. Consider the sentence below that contains only 3-letter words. Substituting an “A” for the first “E” in the following sentence changes the sentence slightly, but the meaning is still understood.
THE-CAT-ATE-THE-RAT
becomes
THA-CAT-ATE-THE-RAT
Now let’s consider how a single substitution mutation can alter a single gene triplet sequence. First, let’s assume we have the following DNA sequence and its corresponding transcription and translation products:
DNA: CAT-CAT–CAT
mRNA: GUA-GUA-GUA
Polypeptide: Valine-Valine-Valine
DNA Mutation Effects in Human Cells
Point Mutations
Frameshift Mutations
Now, if a cytosine substitutes for the first adenine, notice how the amino acid sequence changes:
DNA: CAT-CAT–CAT
Mutation: CCT-CAT–CAT
mRNA: GGA-GUA–GUA
Polypeptide: glycine-valine-valine
This alteration of the protein’s primary structure could also alter its secondary and tertiary structure, thereby, preventing it from functioning normally. An example of a substitution mutation is sickle-cell anemia. Hemoglobin (Hb) is a quaternary protein consisting of four tertiary proteins. Each protein is 150 amino acids long. At a single location in two of these tertiary proteins, valine substitutes for glutamic acid, due to a single substitution in the gene coding for these proteins. This causes the hemoglobin to change the cell’s shape when [oxygen] is low, resulting in clogged blood vessels.
An insertion mutation causes the reading sequence to shift, which can significantly affect the codon sequence and, ultimately the amino acid sequence in the protein. Notice how the insertion of an “A” after the first H in the following sentence greatly affects the meaning of the sentence:
THE-CAT-ATE-THE-RAT
becomes
THA-ECA-TAT-ETH-ERA
A deletion mutation occurs when a nucleotide is removed from the gene. Deletions also cause a shift in the reading sequence of the gene. In our example, a deletion of the first E greatly affects the meaning of the sentence:
THE-CAT-ATE-THE-RAT
becomes
THC-ATA-TET-HER-AT
Insertions and deletions are frameshift mutations because they cause shifts in the reading sequence of the nucleotide bases. Substances that cause mutations are called mutagens. Mutagens that cause cancer are classified as carcinogenic (kar-sen-ō-JEN-ik).
CONTROL OF THE CELL CYCLE
A number of factors, both outside and inside a cell, are important in determining whether a cell will divide or not. Just as certain hormones and other chemicals released from surrounding cells can determine which genes a cell will transcribe, similar extracellular factors may either cause or prevent cell division. In addition, when normal dividing cells touch one another, they stop dividing. This external factor, called contact inhibition, is not a characteristic of cancer cells; therefore, cancer cells can divide uncontrollably producing large masses of abnormal cells (tumors) that can invade and destroy healthy tissues.
In recent years, cell biologists discovered that cells have a cell-cycle control system that operates through negative feedback and involves “molecular road blocks” at various stages or checkpoints in the cell’s life. In other words, a cell will not proceed past a certain stage in its life cycle unless the cell-cycle control system gives the “Go ahead” signal. For example, if nutrient deficiency or other factor prevents a cell from doubling its size, the cell will not enter S phase. Furthermore, a cell will not enter G2 and mitosis until the S phase is complete. The stop signal between G1 and S phase allowing the cell to “check” its size prevents cells that are too small from dividing. This control mechanism prevents future daughter cells from becoming smaller than previous daughter cells. The stop signal between S phase and mitosis allowing the cell to check the status of DNA replication ensures that daughter cells have the same genetic material as the parent cell.
A major intracellular stimulus that causes a cell to proceed into different stages of the cell cycle involves the interaction of two groups of proteins: cyclins and certain enzymes called cyclin-dependent kinases. Cyclins (SĪ-klinz) are so-named because their concentrations fluctuate (cycle) in a regular pattern during the cell cycle. Cyclin concentration increases steadily during interphase and drops off dramatically at the end of cell division. Cyclin-dependent kinases (Cdks) have a relatively constant concentration throughout the cell cycle. Binding a certain cyclin with a certain Cdk produces a protein complex called mitosis-promoting factor (MPF) that can stimulate the cell to proceed into mitosis. After mitosis and cytokinesis, the MPF breaks apart and enzymes break down the cyclin. Then throughout interphase, cyclin concentrations increase until the cell can produce enough MPF to trigger the next mitotic division. Regulation of the cell cycle is illustrated in Figure 5-12.
Figure 5-12. Regulation of the Cell Cycle
PREDETERMINED LIMITS ON CELL DIVISION
ven under the best extracellular conditions, most human cells have a genetic limit on how many times they can divide. Current research has identified the length of telomeres on chromosomes as one explanation for this limit on cell division. A telomere (TĒ-lō-mēr) is a repeating sequence of DNA nucleotides located at each end of a chromosome. In human telomeres, the repeating base sequence is TTAGGG, and it may repeat as many as 2000 times. Telomeres do not code for proteins, but serve three important functions: (1) enable DNA polymerases to replicate genes near the ends of a DNA molecule; (2) protect the ends of the DNA molecule against degradation by certain enzymes located in the nucleus; and (3) prevent the ends of chromosomes from sticking together.
A decrease in telomere length over time will ultimately prevent a cell from dividing and can lead to cell death. During each S phase, the end of a telomere may lose about 15 TTAGGG segments because DNA polymerase is unable to replicate them. (The reason for this failure to replicate is that DNA polymerase cannot position itself properly near the end of the telomere.) If a telomere early in life began with 2000 repeating segments, there would be none left after 133 cell divisions. When a telomere becomes too short, certain proteins in the nucleus prevent the replication of the DNA molecules; consequently, the cell can no longer divide. Furthermore, without telomeres to protect them, functional genes near the end of a chromosome are more susceptible to damage.
As you have seen, the world of the cell is small but complex. However, like the crafted gears of a fine-tuned clock, the intricate components of the cell carry out their activities with amazing precision. Since every tissue and organ relies upon cells to function, the details you remember from this chapter will serve as a solid foundation for all chapters ahead. In the next chapter, you will learn about the body’s tissues or groups of cells working together.
Flashcards
Master the processes of DNA expression and the cell cycle.
Glossary
Comprehensive terminology for the DNA Expression and the Cell Cycle chapter. Click on any term to see its definition.
