Cell cycle

• 1. Introduction

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  The cell cycle is the sequence of events that leads a single cell to divide into two daughter cells. Every organism alive today, from the simplest bacterium to a human being, depends on this cycle to grow, repair, and reproduce. In multicellular animals and plants, the cycle is how a fertilized egg becomes an entire organism. In the intestinal lining, certain cells complete the whole sequence in as little as 9 to 10 hours. In resting mouse skin, stem cells may take more than 200 hours to do the same thing. What controls this variation? What keeps the cycle from running wild, spawning tumors? And what happens when the machinery that guards the process goes wrong? Three scientists won the 2001 Nobel Prize in Physiology or Medicine for identifying the molecular switches at the heart of those answers.

• 2. Four Phases, One Goal

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  Eukaryotic cells, those with a nucleus, divide through four distinct phases: G1, S, G2, and M. The first three together form interphase, which typically occupies at least 91% of the total cycle time. During G1, the cell restocks its proteins and multiplies organelles such as mitochondria and ribosomes. Then S phase begins when DNA synthesis starts. By the time S phase ends, every chromosome has been copied; each now consists of two sister chromatids joined together. RNA transcription and general protein synthesis drop to very low levels during S phase, with one notable exception: histone production, which the cell concentrates almost entirely in this window. G2 follows, a period of rapid protein synthesis and growth that readies the cell for division. Microtubules begin reorganizing into a spindle during G2, a structure that will later pull the copied chromosomes apart. M phase itself, mitosis plus cytokinesis, accounts for roughly only 10% of the entire cycle.

• 3. Checkpoint by Checkpoint

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  Three main checkpoints stand guard at critical transitions. At the G1/S checkpoint, the cell verifies it has enough raw materials, including nucleotide bases and DNA synthase, to fully replicate its genome. A malnourished or damaged cell stalls here. The G2/M checkpoint confirms the cell has built enough cytoplasm and phospholipids to sustain two daughter cells, and also evaluates whether the timing is right for division. In a developing embryo, for example, many cells must divide in synchrony; the G2/M checkpoint is how that coordination is enforced. The metaphase checkpoint is the narrowest gate: it fires only after the cell has already committed to mitosis, verifying that the spindle has formed and every chromosome is aligned at the spindle equator before anaphase begins. Normal human cells are estimated to convert about 1% of single-strand DNA damage into roughly 50 double-strand breaks per cell per cell cycle. These breaks are usually repaired accurately, but errors are considered a significant contributor to the rate of cancer in humans.

• 4. Cyclins, CDKs, and the Nobel Prize

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  Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse shared the 2001 Nobel Prize in Physiology or Medicine for their work identifying the two families of molecules that drive the cycle forward: cyclins and cyclin-dependent kinases, or CDKs. CDKs are present in cells at steady concentrations throughout the cycle. Cyclins, by contrast, appear and disappear at specific moments in response to molecular signals, which is where their name originates. Only when a cyclin binds its CDK partner does the pair become active. The activated complex then performs phosphorylation, adding a phosphate group to target proteins to switch them on or off in a coordinated sequence. In yeast, a single CDK, Cdc28 in Saccharomyces cerevisiae and Cdc2 in Schizosaccharomyces pombe, runs the entire cycle. Animals evolved whole families of CDKs. Cdk1 governs entry into mitosis; Cdk2, Cdk4, and Cdk6 regulate the transition into S phase. Experiments with mice showed that cells lacking Cdk2, Cdk4, and Cdk6 simultaneously can still progress through the basic cycle, but Cdk1 knockouts are lethal, suggesting an ancestral CDK1-type kinase is the ultimate driver.

• 5. The Rb Switch and Cancer

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  At the heart of the G1/S decision sits the retinoblastoma protein, Rb, which carries more than 14 potential phosphorylation sites. In a resting cell, unphosphorylated Rb acts as a tumor suppressor by blocking entry into the cell cycle. When a growth signal arrives, cyclin D binds CDK4/6 and the resulting complex begins phosphorylating Rb. Mono-phosphorylated Rb exists in 14 different isoforms, each with distinct binding affinities for the transcription factor E2F; these isoforms inhibit E2F target genes and hold the cell in G1. Progressive phosphorylation by cyclin D-CDK4/6 eventually drives Rb into a fully hyperphosphorylated state. In that state, Rb dissociates from E2F entirely, unleashing a cascade that includes the expression of cyclin E, cyclin A, DNA polymerase, and thymidine kinase. Cyclin E binds CDK2, and together they push the cell irreversibly from G1 into S phase. When cyclin D-CDK4/6 activity is deregulated, as it commonly is in cancer cells, this gate can be forced open without the appropriate signals. Three CDK4/6 inhibitors, palbociclib, ribociclib, and abemaciclib, received FDA approval to treat advanced hormone-receptor-positive, HER2-negative breast cancer. Palbociclib is taken orally, and the primary side effect is neutropenia, which is managed by adjusting the dose. Notably, this class of therapy only works in cancers where Rb is still expressed; tumors that have lost Rb are resistant from the outset.

• 6. Mitosis Up Close

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  Mitosis itself moves through five named stages: prophase, prometaphase, metaphase, anaphase, and telophase. One distinction stands out between animal and fungal cells. Animal cells undergo open mitosis, meaning the nuclear envelope breaks down entirely before the chromosomes separate. Fungi such as Aspergillus nidulans and Saccharomyces cerevisiae carry out closed mitosis, dividing the chromosomes inside an intact nucleus. After nuclear division, cytokinesis divides the cytoplasm and organelles. In animal cells, the membrane pinches inward along a groove that deepens until the two cells separate. In plant cells, a cell plate forms between the future daughter cells, and its position is set by a preprophase band of microtubules and actin filaments that appears before division even begins. There are cells where mitosis and cytokinesis are uncoupled; in a process called endoreplication, mitosis proceeds without cytokinesis, producing cells with multiple nuclei. This occurs prominently among fungi and slime molds, and even in certain stages of fruit fly embryonic development.

• 7. Prokaryotes and Archaea

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  Bacteria, which lack a nucleus, divide by binary fission and organize their cell cycle into three periods labeled B, C, and D. The B period runs from the end of one division to the start of DNA replication. The C period is when replication and chromosomal segregation happen. The D period bridges the completion of replication and the physical splitting of the cell. Bacterial regulation is more relaxed than in eukaryotes; in fast-growing species, a new round of DNA replication can begin before the previous cell division is even finished. Bacteria that use the protein FtsZ assemble it into a ring that marks the future division site. DNA replication is required for placing this ring at one end of the cell; if replication is blocked but FtsZ production continues, the ring forms at the center instead. Archaea occupy a middle ground. Their cycle closely resembles the eukaryotic G1-S-G2-M layout but lacks a fixed G0 resting phase and involves two separate division stages. Among the TACK archaea, Sulfolobus is typically arrested at G2 during stationary phase. The Euryarchaeota generally tolerate more variation, though Methanothermobacter thermautotrophicus stands out for cycling only through strictly diploid and tetraploid states.

• 8. Evolution of the Cycle

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  Before cells as we know them existed, self-replicating RNAs competed in a pre-cellular environment where growth meant the continuous production of more RNA. Parasitic RNAs that hijacked shared resources posed a fundamental threat. Fusing multiple RNAs into a genome imposed a much higher barrier: a parasite now had to incorporate itself into the entire genome to persist. Separating genomic RNA from functional RNA also allowed copy-number control to shift from competitive dynamics to regulated synthesis rates and RNA half-lives. Replacing RNA genomes with the more stable DNA molecule permitted larger genomes to evolve. That transition from self-catalysis to genome-directed enzyme synthesis had lasting consequences for the cell cycle, which must regulate functional protein production and genomic duplication through different mechanisms. Studies in Arabidopsis thaliana extended understanding of the G1/S transition beyond animals and yeast. Plants share conserved network features with opisthokonts, and many plant regulators have direct animal counterparts. Plants also carry a unique group of B-type CDKs whose roles span developmental functions and mitotic regulation. The Arabidopsis mutant cdka;1, lacking the plant's Cdk1 homolog CDKA;1, remains viable, which runs counter to the pattern seen in animals where Cdk1 loss is lethal. That single observation hints that the deepest rules of the cell cycle, though ancient and widespread, have continued to diverge across lineages in ways scientists are still unraveling.