Cell Cycle 101: How One Cell Becomes Two
Foreword
Have you ever questioned how a wound heals? How does an embryo become a baby? How does a cancerous tumor grow? At the heart of these questions lies cell replication and division. The remarkable ability of a cell to grow and divide into two identical daughter cells underpins the success of life. This is, however, only conceivable because eukaryotic cells (that is, with nuclei) go through the cell cycle, a set of well-defined and tightly synchronized events. This process is divided in two main stages: the interphase, whereby a cell grows; and the mitotic phase, by which cells replicate (mitosis) and divide (cytokinesis). During this process deoxyribonucleic acid (DNA), also recognized as the "molecule of life," is accurately replicated and faithfully distributed to newly generated daughter cells. DNA is the molecule that carries and conveys hereditary material or genetic information from parents to offspring, thus holding all the information needed to construct and sustain an organism. The cell cycle thus functions as the master regulator of tissue growth, development and proliferation. It also ensures that old cells are removed in a timely manner as we age and replaced by new cells that proliferate in a tightly controlled fashion. This keeps the total number of viable cells constant, which is essential for life as we know it.
The Cell Cycle 101 series attempts to elucidate the most important aspects of the cell cycle: from its history and discovery to the underlying mechanisms that control cell division. This series aims to explore the intricacies of cell cycle control in development and the consequences of its failure in disease.
Therefore, this 101 series is divided into the following chapters:
2. Cell Cycle 101: How One Cell Becomes Two
Cell Cycle 101: How One Cell Becomes Two
Cells are exactly what their DNA instructs them to be. The observable uniformity, continuity, and robustness of some living forms reveal how reliably their genomes are conserved and how correctly they are duplicated. This is only conceivable because, before dividing into two daughter cells, a cell must go through a sequence of controlled and closely regulated events known as the cell cycle: a cell must first grow, duplicate its DNA, and then physically divide in order to generate identical cells. While prokaryotes reproduce by cell division (binary fission), multicellular organisms (eukaryotes) employ cell division for growth as well as cell and tissue maintenance and repair of cells and tissues. Compared to prokaryotes, eukaryotic cell cycles are far more complex. They typically contain an initial phase known as the interphase that prepares the cell for mitosis, the most important phase of a cell's life cycle. Each chromosome is duplicated in interphase, with one copy being later transferred to each of the two daughter cells during mitosis. Cytokinesis, which follows mitosis, allows for the physical division of the cytoplasm of a parental cell into two daughter cells. As a result, the content of the original cell is evenly distributed between the two daughter cells, which are therefore genetically identical.
Prokaryotic Cell Cycle
Cell division in prokaryotes, such as the bacteria E. coli, is referred to as binary fission. This process of biological reproduction generates two daughter cells that are genetically identical to the parental one, similar to mitosis in eukaryotes (Wong, 2021). Although conceptually similar to mitosis, binary fission serves a very different purpose, at least in unicellular organisms. While mitosis allows organisms to either grow larger or replace old, damaged cells with new ones; binary fission functions extend beyond cell replacement. In fact, binary fission is the mechanism by which single-cell prokaryotes reproduce into new individuals (Prokaryotic Cell Division, 2015). Compared to a eukaryote's cell cycle, binary fission in prokaryotes is a considerably simpler process. This is because, in contrast to eukaryotes, which have a significant number of linear (rod-shaped) chromosomes surrounded by a membrane-bound nucleus, prokaryotes only possess a single circular chromosome (Kaljevi et al., 2021). In prokaryotes, the chromosome is located in a region known as the nucleoid, which is instead linked to the cell membrane at a single point. Moreover, while DNA wraps around histone proteins in eukaryotic cells, most prokaryotes, however, do not require histones to store DNA, as they have a "naked" DNA that is far more accessible to replicating enzymes (Kumiznov, 2013). This type of asexual reproduction is therefore much faster, allowing for a tremendous population expansion in a very short period of time (Gibson, 2018).
Like any other living cell, a prokaryotic cell that has committed to division must first replicate its DNA. Binary fission commences with DNA unravelling, which allows access for replication enzymes to start replicating DNA. The prokaryote chromosome comprises two specialized regions, the origin of replication and the terminus of replication, which are diametrically opposite to each other on the chromosome and correspond to the specific sites where DNA replication starts and finishes (Mackiewicz et al., 2001). Thus, DNA replication starts at the origin of replication, which is the first region to be duplicated. The circular chromosome opens at this site and the two strands of DNA are copied, with replication occurring in opposite directions on the two strands (Bock, 1996). As replication proceeds, the two origins migrate to opposite ends of the cell, carrying the rest of the chromosome with them and the cell lengthens, which facilitates the separation of newly produced chromosomes. Replication continues until the entire chromosome has been replicated and the replication enzymes meet on the opposite side. The division of the cytoplasm (a gel-like liquid that fills the whole cell and is bordered by the plasma membrane) can finally begin as the new chromosomes have left the cell's center and migrated to opposite ends of the cell. As the membrane contracts inward, a septum forms from the edge of the cell to its center. The septum eventually separates down the middle, releasing the two cells as two separate bacteria (Kuzminov, 2014; Vicente et al., 2006).
Eukaryotic Cell Cycle
Eukaryotic cell cycles are far more sophisticated than prokaryotic ones. Although DNA replication and cell division occur in both circumstances, the processes differ substantially. A key difference is how these organisms replicate their DNA. Because the average eukaryotic cell has 25 times more DNA than a prokaryotic cell, binary fission does not involve DNA condensation in chromosomes as observed in the eukaryotic cell cycle (BioRad, n.d.). Furthermore, DNA replication occurs in the nucleus of eukaryotic cells as opposed to the cytoplasm of prokaryotes. Another major difference lies in the stages at which DNA replication occurs. While prokaryotes continually replicate DNA throughout their relatively short cell cycle, eukaryotic cells only duplicate DNA at a specific stage of the cell cycle, known as the DNA synthesis phase or S phase (Alberts et al., 2002). Finally, although DNA replication in prokaryotes begins at a single replication origin, eukaryotic chromosomes are copied from many origins of replication, which initiate replication almost simultaneously during S phase (Barberis et al., 2010).
The typical cell cycle in eukaryotes consists of two main phases: the mitotic (M) phase, consisting of mitosis (nucleus division) and subsequent cytokinesis (cell division into two); and the interphase, the interlude between two M phases, during which the cell grows and copies its DNA and that consists of G1, S, and G2 phases. G1 and G2 are the gap phases, which come before or after the S phase, so that the cell prepares for DNA synthesis or mitosis, respectively (Manson et al., 2011). Mitosis is further divided into five subphases including prophase, prometaphase, metaphase, anaphase and telophase. DNA replication occurs during the S phase of the interphase, which lasts 8 to 10 hours and accounts for almost half of the cell cycle period in mammalian cells. After S phase, chromosome separation and cell division occur in M phase, which takes substantially less time, usually less than an hour (Alberts et al., 2002a). It is important to point out that strict regulatory mechanisms operate during the cell cycle (which will be further discussed in this series) to guarantee the timely and orderly sequence of events, in such a way that progress through each phase of the cell cycle is dependent on the successful completion of the preceding one (Wang, 2021).
Interphase: Time To Grow and Synthesize
Once a cell has committed to division, interphase begins and cells enter the first growth phase (G1 phase). While the cells' biosynthetic activities are markedly slowed down in the preceding M phase, they resume at a rapid pace in the G1 phase (Campbell et al., 2020; Jorgensen & Tyers, 2004). During this phase, all necessary proteins are produced and all cellular components increase in size (Polymenis & Aramayo, 2015). This includes organelles such as mitochondria, which contain their own DNA and are the cells' energy generators which convert oxygen and nutrients into adenosine triphosphate (ATP), the cellular energy currency; or ribosomes, the cellular machinery responsible for protein synthesis (Talking Glossary of Genomic and Genetic Terms, 2009). After growth is complete, cells enter S phase where DNA replication occurs.
DNA replication is not an easy task. The human genome contains around 333 billion DNA base pairs, each of which must be precisely duplicated when one of the trillion cells divides (National Human Genome Research, 2010). The DNA molecule has the shape of a double helix (Watson & Crick, 1953). During S-phase, an enzyme called helicase unwinds the double strand of DNA (similar to unzipping a zipper). Because DNA replication is semiconservative, each strand of the double helix functions as a template for the synthesis of a new, complementary strand (Meselson & Stahl, 1958). An enzyme called DNA polymerase then attaches nucleotides, the building blocks of DNA, to each of the individual template DNA strands using the complementary rule of base pairing: adenine attaches to thymine and cytosine attaches to guanine. This mechanism converts a parental molecule into two daughter molecules, with each newly created DNA molecule being identical to the original one, but possessing a new and an old strand.
S phase is also characterized by a significant decrease in gene expression and protein synthesis, except for those involved in histone formation. Besides regulating gene expression, histones also provide structural support to chromosomes (Günesdogan et al., 2014). The long DNA molecules that make up each chromosome must fit into the nucleus of the cell. To do this, the DNA wraps around complexes of histone proteins, resulting in highly condensed chromosomes (Annunziato, 2008). The number of histones must therefore double to keep up with DNA replication and ensure proper compaction and organization of genomic DNA (Maya et al., 2011). A human cell must manufacture 400 million histones in S phase to supply the histones needed to stabilize newly duplicated DNA (Armstrong & Spencer, 2021); Otherwise, the DNA may be exposed to deregulated levels of gene expression, which can lead to genome instability, that is the occurrence of mutations at high frequencies, which can be deleterious as they prevent faithful DNA replication (Maya-Mendoza et al., 2010; Wilky et al., 2019).
After conclusion of the S phase, cells move on to the second gap phase known as the G2 phase, where the primary function is to give the cells time to prepare for mitosis. Significant cell growth as well as substantial protein and lipid synthesis, the latter of which are essential for membrane biogenesis of the two daughter cells, are the main hallmarks of this phase (Wang, 2021). Spindle proteins are also produced during this phase, which ensure chromosomal alignment during mitosis. It is noteworthy that the two gap phases do not just serve as simple time delays to allow cell growth. They also provide the cell time to assess the internal and external environments to ensure conditions are appropriate before the cell enters the great upheavals of S phase and mitosis. The G1 phase is particularly important in this context, the duration of which depends heavily on environmental conditions and extracellular signals. Stress conditions including nutrient deprivation (Newcomb et al., 2003), osmotic stress (Bellí et al., 2001), and oxidative stress-induced DNA damage all lead to G1 arrest. Once a proper cellular response is mounted to overcome the stress, the G1 phase restarts allowing the cell cycle to proceed (Chang et al., 2017). Indeed, cell cycle progression is closely monitored by surveillance mechanisms that oversee the order, integrity, and fidelity of key cell cycle events - a role filled by signalling pathways known as cell cycle checkpoints, which will be further addressed in the next chapters of this series.
Mitosis: When One Becomes Two
The accurate segregation and division of a cell's already replicated DNA is the greatest obstacle a cell must overcome during division. This is due to the fact that each cell's whole genome is far larger than its diameter. The entire strand of DNA has to fit into the nucleus of a cell, i.e. it has to be packaged very neatly. This is accomplished by wrapping the DNA around structural histone proteins, which function like a scaffold, giving DNA a more compact structure (Mariño-Ramírez, 2015). Chromosome condensation, the landmark event at the beginning of prophase (the first phase of mitosis), is therefore critical for ensuring that DNA fits into the cell nucleus and that genetic information is accurately conveyed to daughter cells. The chromosomes in the nucleus are so closely packed during this phase that they can be seen under a microscope (Liang et al., 2015). Each chromosome now possesses two identical sister chromatids as a result of chromosomal duplication during the S phase.
Significant changes occur also outside of the nucleus as the mitotic spindle develops. The spindle, which is made of microtubules, or strong fibers, ensures that chromosomes migrate to the cell's opposing poles, thus allowing for high-fidelity segregation. The next phase, prometaphase, starts with the breakdown of the physical barrier surrounding the nucleus, called the nuclear envelope; a process that allows for an equal distribution of the duplicated genetic material into two identical daughter nuclei (Scitable, 2015b). Chromosomes become fully condensed, shortening in size and becoming thicker, a process that facilitates segregation (Liang et al., 2015; Samejima et al., 2012). Metaphase begins when the duplicated chromosomes are aligned along the cell’s equator (the metaphase plate). The following phase, anaphase, is characterized by the detachment and migration of the separated chromatids to opposite sides of the cell by the pulling force of the spindle microtubules. Finally, telophase occurs when the chromosomes reach the poles and the cell nears the end of division. A nuclear membrane forms around each set of chromosomes, creating two separate nuclei within the same cell (Scitable, 2015a). Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis and ensures that each daughter cell possesses a nucleus. In a process triggered by a ring of protein filaments (the contractile ring), the cell membrane is pulled inwards and contracts, releasing two identical yet, separate cells, each with its own plasma membrane (Scitable, 2015d).
Meiosis: When One Becomes Four
In addition to mitosis, sex cells known as gametes (sperm and egg) can also divide through an alternative process of cell division known as meiosis. While the fundamental function of mitosis is cell regeneration, growth, and asexual reproduction, the primary purpose of meiosis is to create gametes for sexual reproduction. Although they serve very different purposes, most meiotic events are similar to those observed in mitosis (Bigler, 2021). Similar to mitosis, a cell undergoing meiosis must first go through interphase in which it grows and duplicates its DNA. Division by meiosis also involves progression through different stages (prophase, metaphase, anaphase and telophase). However, during meiosis cell division occurs twice in two separate rounds of division (meiosis I and II) (Scitable, 2015b).
Unlike mitosis, where a parent cell is divided into two identical daughter cells with the same chromosome number, meiosis creates four unique daughter cells with half the chromosome number of the parental cell. Since meiosis produces cells destined to become gametes (or reproductive cells), this reduction in chromosome number is critical. Otherwise, the fusion of two gametes during fertilization would result in offspring with double the number of chromosomes. During meiosis, DNA exchange occurs between paired chromosomes (DNA recombination), creating unique combinations of genetic material in each of the four daughter cells (Hunter, 2015). This creates four gametes that are neither genetically identical to each other nor to the original cell.
Conclusions
Every multicellular organism begins as a single-cell fertilized egg. This is because of meiosis, the process by which gametes are created. These are the only cells in our body that are haploid (carrying just one set of chromosomes; 1N), that is, having half the number of chromosomal sets as that of the original cell. This ensures that when fertilization occurs, all organisms have the right number of chromosomes. Meiosis is also the catalyst for the majority of genetic variation found in living organisms as a result of DNA recombination. When two gametes fuse together during fertilization, the variety is further amplified and new DNA combinations are created. As a result of the constant mixing of parental DNA during sexual reproduction, the diversity of life on Earth is incredibly rich. Meiosis, however, cannot support life as we know it. Only through several rounds of mitotic divisions, the fertilized egg can develop into an adult human made of ~50 trillion cells. Mitosis, in which one cell splits into two while retaining its genetic integrity (diploidy), supports the tremendous proliferation observed following conception. Furthermore, cell division not only allows for development but also replaces damaged or dead cells, thereby contributing to the success of life.
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