For life to continue and flourish, cells must be able to divide. Tissue growth and cellular damage repair are also necessary to maintain an organism throughout its life. All these rely on the dynamic processes of DNA replication and the cell cycle. The mechanisms highlighted in this section are tightly regulated and represent only part of the life cycle of a cell.
DNA replication is the process by which new DNA is copied from an original DNA template. It is one phase of the highly coordinated cell cycle, and it requires a variety of enzymes with special functions. The creation of a complementary DNA strand from a template strand is described as semi-conservative replication. The result of semi-conservative replication is two separate double-stranded DNA molecules, each of which is composed of an original “parent” template strand and a newly synthesized “daughter” DNA strand.
DNA replication progresses in three steps referred to as initiation, elongation, and termination. During initiation, enzymes are recruited to specific sites along the DNA sequence (Figure 3.12). For example, an initiator enzyme, called helicase, “unwinds” DNA by breaking the hydrogen bonds between the two parent strands. The unraveling of the helix into two separated strands exposes the strands and creates a fork, which is the active site of DNA replication.
Elongation is the assembly of new DNA daughter strands from the exposed original parent strands. The two parent strands can further be classified as leading strand or lagging strand and are distinguished by the direction of replication. Enzymes called DNA polymerases read parent template strands in a specific direction. Complementary nucleotides are added, and the newly formed daughter strands will grow. On the leading parent strand, a DNA polymerase will create one continuous strand. The lagging parent strand is created in several disconnected sections and other enzymes fill in the missing nucleotide gaps between these sections.
Finally, termination refers to the end of DNA replication activity. It is signaled by a stop sequence in the DNA that is recognized by machinery at the replication fork. The end result of DNA replication is that the number of chromosomes are doubled so that the cell can divide into two.
DNA replication should result in the creation of two identical DNA nucleotide sequences. However, although DNA polymerases are quite precise during DNA replication, copying mistakes are estimated to occur every 107 DNA nucleotides. Variation from the original DNA sequence is known as a mutation. The different types of mutations will be discussed in greater detail in Chapter 4. Briefly, mutations can result in single nucleotide changes, as well as the insertion or deletion of nucleotides and repeated sequences. Depending on where they occur in the genome, mutations can be deleterious (harmful). For example, mutations may occur in regions that control cell cycle regulation, which can result in cancer (see Special Topic: The Cell Cycle and Immortality of Cancer Cells). Many other types of mutations, however, are not harmful to an organism.
Regardless of their effect, the cell attempts to reduce the frequency of mutations that occur during DNA replication. To accomplish this, there are polymerases with proofreading capacities that can identify and correct mismatched nucleotides. These safeguards reduce the frequency of DNA mutations so that they only occur every 109 nucleotides.
Mitotic Cell Division
There are two types of cells in the body: germ cells (sperm and egg) and somatic cells. The body and its various tissues comprises somatic cells. Organisms that contain two sets of chromosomes in their somatic cells are called diploid organisms. Humans have 46 chromosomes and they are diploid because they inherit one set of chromosomes (n = 23) from each parent. As a result, they have 23 matching pairs of chromosomes, which are known as homologous chromosomes. As seen in Figure 3.13, homologous chromosome pairs vary in size and are generally numbered from largest (chromosome 1) to smallest (chromosome 22) with the exception of the 23rd pair, which is made up of the sex chromosomes (X and Y). Typically, the female sex is XX and the male sex is XY. Individuals inherit an X chromosome from their chromosomal mother and an X or Y from their chromosomal father.
To grow and repair tissues, somatic cells must divide. As discussed previously, for cell division to occur, a cell must first replicate its genetic material. During DNA replication, each chromosome produces double the amount of genetic information. The duplicated arms of chromosomes are known as sister chromatids, and they are attached at the centromeric region. To elaborate, the number of chromosomes stays the same (n = 46); however, the amount of genetic material is doubled in the cell as the result of replication.
Mitosis is the process of somatic cell division that gives rise to two diploid daughter cells. Figure 3.14 includes a brief overview of mitosis. Once DNA and other organelles in the cell have finished replication, mitotic spindle fibers physically align each chromosome at the center of the cell. Next, the spindle fibers divide the sister chromatids and move each one to opposite sides of the cell. At this phase, there are 46 chromosomes on each side of a human cell. The cell can now divide into two fully separated daughter cells.
Meiotic Cell Division
Gametogenesis is the production of gametes (sperm and egg cells); it involves two rounds of cell division called meiosis. Similar to mitosis, the parent cell in meiosis is diploid. However, meiosis has a few key differences, including the number of daughter cells produced (four cells, which require two rounds of cell division to produce) and the number of chromosomes each daughter cell has (see Figure 3.14).
During the first round of division (known as meiosis I), each chromosome (n = 46) replicates its DNA so that sister chromatids are formed. Next, with the help of spindle fibers, homologous chromosomes align near the center of the cell and sister chromatids physically swap genetic material. In other words, the sister chromatids of matching chromosomes cross over with each other at matching DNA nucleotide positions. The occurrence of homologous chromosomes crossing over, swapping DNA, and then rejoining segments is called genetic recombination. The “genetic shuffling” that occurs in gametes increases organismal genetic diversity by creating new combinations of genes on chromosomes that are different from the parent cell. Genetic mutations can also arise during recombination. For example, there may be an unequal swapping of genetic material that occurs between the two sister chromatids, which can result in deletions or duplications of DNA nucleotides. Once genetic recombination is complete, homologous chromosomes are separated and two daughter cells are formed.
The daughter cells after the first round of meiosis are haploid, meaning they only have one set of chromosomes (n = 23). During the second round of cell division (known as meiosis II), sister chromatids are separated and two additional haploid daughter cells are formed. Therefore, the four resulting daughter cells have one set of chromosomes (n = 23), and they also have a genetic composition that is not identical to the parent cells nor to each other.
Although both sperm and egg gamete production undergo meiosis, they differ in the final number of viable daughter cells. In the case of spermatogenesis, four mature sperm cells are produced. Although four egg cells are also produced in oogenesis, only one of these egg cells will result in an ovum (mature egg). During fertilization, an egg cell and sperm cell fuse, which creates a diploid cell that develops into an embryo. The ovum also provides the cellular organelles necessary for embryonic cell division. This includes mitochondria, which is why humans, and most other multicellular eukaryotes, have the same mtDNA sequence as their mothers.
Chromosomal Disorders: Aneuploidies
During mitosis or meiosis, entire deletions or duplications of chromosomes can occur due to error. For example, homologous chromosomes may fail to separate properly, so one daughter cell may end up with an extra chromosome while the other daughter cell has one less. Cells with an unexpected (or abnormal) number of chromosomes are known as aneuploid. Adult or embryonic cells can be tested for chromosome number (karyotyping). Aneuploid cells are typically detrimental to a dividing cell or developing embryo, which can lead to a loss of pregnancy. However, the occurrence of individuals being born with three copies of the 21st chromosome is relatively common; this genetic condition is known as Down Syndrome. Moreover, individuals can also be born with aneuploid sex chromosome conditions such as XXY, XXX, and XO (referring to only one X chromosome).
Special Topic: The Cell Cycle and Immortality of Cancer Cells
DNA replication is part of a series of preparatory phases that a cell undergoes prior to cell division, collectively known as interphase (Figure 3.15). During interphase, the cell not only doubles its chromosomes through DNA replication, but it also increases its metabolic capacity to provide energy for growth and division. Transition into each phase of the cell cycle is tightly controlled by proteins that serve as checkpoints. If a cell fails to pass a checkpoint, then DNA replication and/or cell division will not continue. Some of the reasons why a cell may fail at a checkpoint is DNA damage, lack of nutrients to continue the process, or insufficient size. In turn, a cell may undergo apoptosis, which is a mechanism for cell death.
Unchecked cellular growth is a distinguishing hallmark of cancer. In other words, as cancer cells grow and proliferate, they acquire the capacity to avoid death and replicate indefinitely. This uncontrolled and continuous cell division is also known as “immortality.” As previously mentioned, most cells lose the ability to divide due to shortening of telomeres on the ends of chromosomes over time. One way in which cancer cells retain replicative immortality is that the length of their telomeres is continuously protected. Chemotherapy, often used to treat cancer, targets the cell cycle (especially cell division) to halt the propagation of genetically abnormal cells. Another therapeutic approach that continues to be investigated is targeting telomere activity to stop the division of cancer cells.
Researchers have exploited the immortality of cancer cells for molecular research. The oldest immortal cell line is HeLa cells (Figure 3.16), which were harvested from Henrietta Lacks, an African American woman diagnosed with cervical cancer in 1955. At that time, extracted cells frequently died during experiments, but surprisingly HeLa cells continued to replicate. Propagation of Lacks’s cell line has significantly contributed to medical research, including contributing to ongoing cancer research and helping to test the polio vaccine in the 1950s. However, Lacks had not given her consent for her tumor biopsy to be used in cell culture research. Moreover, her family was unaware of the extraction and remarkable application of her cells for two decades. The history of HeLa cell origin was first revealed in 1976. The controversy voiced by the Lacks family was included in an extensive account of HeLa cells published in Rebecca Skloot’s 2010 book, The Immortal Life of Henrietta Lacks. A film based on the book was also released in 2017 (Wolfe 2017).