Chapter 3.3: Cell Cycle and Cell Division

1. Cell Cycle

• Definition: The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells.
• Phases: The cell cycle is divided into two basic phases:
  • Interphase: The phase between two successive M phases. It is the resting phase during which the cell is preparing for division by undergoing both cell growth and DNA replication.
    • G₁ Phase (Gap 1): The interval between mitosis and initiation of DNA replication. The cell is metabolically active and continuously grows.
    • S Phase (Synthesis): The period during which DNA synthesis or replication takes place. The amount of DNA per cell doubles.
    • G₂ Phase (Gap 2): The period during which proteins are synthesized in preparation for mitosis while cell growth continues.
  • M Phase (Mitosis Phase): The phase where the actual cell division or mitosis occurs. It starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis).

2. Mitosis (Equational Division)

• Definition: A type of cell division in which a parent cell divides into two genetically identical daughter cells. The chromosome number in the daughter cells is the same as that in the parent cell.
• Stages:
  • Prophase: Condensation of chromatin material into compact mitotic chromosomes. Initiation of the assembly of the mitotic spindle. The nuclear envelope disintegrates.
  • Metaphase: The chromosomes, now consisting of two sister chromatids, align at the metaphase plate (equator). The spindle fibres attach to the kinetochores of the chromosomes.
  • Anaphase: The centromeres split and the sister chromatids separate, moving to opposite poles of the cell.
  • Telophase: The chromosomes decondense and lose their individuality. The nuclear envelope reassembles around the chromosome clusters at each pole.
• Cytokinesis: The division of the cytoplasm, which usually follows karyokinesis. In animal cells, it occurs by the formation of a cleavage furrow. In plant cells, it occurs by the formation of a cell plate.
• Significance: Growth, repair, and asexual reproduction.

3. Meiosis (Reductional Division)

• Definition: A type of cell division that reduces the chromosome number by half, producing four haploid daughter cells. It occurs in diploid organisms to form gametes.
• Stages: Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.

Meiosis I

• Prophase I: The longest phase of meiosis. It is subdivided into five phases:
  • Leptotene: Chromosomes become gradually visible under the light microscope.
  • Zygotene: Pairing of homologous chromosomes (synapsis). The paired chromosomes are called bivalents.
  • Pachytene: Crossing over occurs between non-sister chromatids of the homologous chromosomes. This is the exchange of genetic material, which leads to recombination.
  • Diplotene: The synaptonemal complex dissolves, and the homologous chromosomes of the bivalents separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata.
  • Diakinesis: The chiasmata terminalize. The nuclear envelope breaks down.
• Metaphase I: The bivalents align on the equatorial plate.
• Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
• Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows, and this is called as a dyad of cells.

Meiosis II

• It is similar to mitosis.

• Prophase II: The nuclear envelope disappears.

• Metaphase II: The chromosomes align at the equator.

• Anaphase II: The centromeres split, and the sister chromatids move to opposite poles.

• Telophase II: The nuclear envelope reforms, and cytokinesis follows, resulting in four haploid daughter cells (tetrad).

• Significance:

  • Formation of gametes for sexual reproduction.
  • Maintains the chromosome number in a species.
  • Introduces genetic variation.

4. Differences between Mitosis and Meiosis

1. C-value and the Eukaryotic Genome

• Definition of C-value: The C-value is the constant amount of DNA, measured in picograms (pg), found in a haploid nucleus (e.g., a gamete) of a given eukaryotic species. It is a fundamental measure of genome size. For instance, the human C-value is approximately 3.2 pg, which corresponds to about 3 billion base pairs.

• The C-value Paradox: This long-standing puzzle in genetics refers to the lack of correlation between the C-value of a species and its organismal complexity. For example, the lily plant (Lilium longiflorum) has a C-value of about 36 pg, over 10 times that of humans, while the pufferfish (Takifugu rubripes), a complex vertebrate, has a C-value of only 0.4 pg. This paradox is resolved by understanding that a large portion of eukaryotic genomes consists of non-coding DNA. This includes:

  • Introns: Non-coding sequences within genes that are spliced out during mRNA processing.
  • Repetitive DNA: Sequences that are repeated thousands or even millions of times. This includes satellite DNA, minisatellites, and microsatellites.
  • Transposable Elements: "Jumping genes" that can move around the genome. Therefore, the complexity of an organism is more related to the number and regulation of its genes rather than the sheer amount of DNA.

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2. The Cell Cycle Engine: Cyclins and Cyclin-Dependent Kinases (CDKs)

The progression through the cell cycle is driven by a family of protein kinases known as Cyclin-Dependent Kinases (CDKs). These enzymes phosphorylate (add a phosphate group to) specific target proteins, thereby activating or inactivating them to orchestrate the events of the cycle. However, CDKs are inactive on their own. They require a regulatory subunit called a cyclin to be active.

• Cyclins: These are the regulatory proteins whose concentrations rise and fall in a cyclical pattern throughout the cell cycle. The synthesis and degradation of cyclins at specific times ensure that the CDKs are active only when needed.
• CDKs: These are the catalytic subunits of the complex. Their protein levels remain relatively stable throughout the cycle, but their activity is dependent on the presence of their partner cyclin.

Major Cyclin-CDK Complexes and Their Roles:

Regulation of CDK Activity: A Multi-layered Control

The activity of Cyclin-CDK complexes is not just controlled by cyclin availability. It is fine-tuned by several other mechanisms:

1. Phosphorylation: For full activation, a CDK must be phosphorylated by a CDK-Activating Kinase (CAK). Conversely, inhibitory kinases like Wee1 can add an inhibitory phosphate to the CDK, keeping it inactive even when bound to a cyclin. This inhibitory phosphate must be removed by a phosphatase like Cdc25 for the complex to become active.
2. CDK Inhibitor Proteins (CKIs): These proteins bind to Cyclin-CDK complexes and block their activity. They are crucial for checkpoint control. There are two main families:
   • INK4 family (p16, p15, p18, p19): Specifically inhibit the G₁-phase Cyclin D-CDK4/6 complexes.
   • CIP/KIP family (p21, p27, p57): Inhibit a broader range of Cyclin-CDK complexes, including Cyclin E-CDK2 and Cyclin A-CDK2.

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3. Cell Cycle Checkpoints: The Molecular Guardians

Checkpoints are surveillance pathways that enforce the correct order of cell cycle events. They function by sending inhibitory signals to the core cell cycle machinery in response to errors like DNA damage or incomplete processes.

The G₁/S Checkpoint (The Restriction Point)

This is the most critical checkpoint, acting as the primary decision point for cell division. It assesses for DNA damage, cell size, nutrient availability, and the presence of growth factors (mitogens).

Molecular Pathway:

1. Signal Integration: Growth factors bind to cell surface receptors, activating signaling cascades (like the Ras/MAPK pathway).
2. Cyclin D Synthesis: These signals lead to the transcription and synthesis of Cyclin D.
3. pRb Phosphorylation: Cyclin D binds to and activates CDK4/6. The active Cyclin D-CDK4/6 complex begins to phosphorylate the Retinoblastoma protein (pRb). This is called hypophosphorylation.
4. E2F Release: In its active, unphosphorylated state, pRb is bound to the E2F family of transcription factors, inhibiting them. When pRb is hypophosphorylated by Cyclin D-CDK4/6, it partially releases E2F.
5. Positive Feedback: E2F then promotes the transcription of its own gene and, crucially, the gene for Cyclin E. Cyclin E binds to CDK2, and the active Cyclin E-CDK2 complex then hyperphosphorylates pRb, causing it to completely release E2F.
6. Commitment to S Phase: The now fully active E2F drives the transcription of all the genes necessary for S phase, including DNA polymerases, histone proteins, and Cyclin A. The cell has now passed the Restriction Point and is committed to completing the cell cycle.

DNA Damage Response at G₁/S:

• If DNA damage (e.g., a double-strand break) is detected, the sensor kinase ATM is activated.
• ATM phosphorylates and activates the checkpoint kinase Chk2.
• Both ATM and Chk2 phosphorylate and stabilize the tumor suppressor protein p53, preventing its degradation.
• p53 is a transcription factor that binds to the promoter of the gene for p21, a CKI.
• p21 protein binds to and inactivates the G₁/S Cyclin-CDK complexes (both D-CDK4/6 and E-CDK2), arresting the cell in G₁ and providing time for DNA repair. If the damage is too severe, p53 will induce apoptosis.

The G₂/M Checkpoint

This checkpoint ensures that DNA replication is fully complete and that there is no DNA damage before the cell commits to the dramatic events of mitosis.

Molecular Pathway of Mitotic Entry:

1. MPF Accumulation: Cyclin B is synthesized during S and G₂ and binds to CDK1, forming the complex known as MPF (Maturation-Promoting Factor).
2. Inhibitory Phosphorylation: Throughout G₂, MPF is held in an inactive state by the kinase Wee1, which adds an inhibitory phosphate to the CDK1 subunit.
3. Activation: At the end of G₂, the activating phosphatase Cdc25 is itself activated (via phosphorylation). Cdc25 removes the inhibitory phosphate from CDK1.
4. Mitotic Onset: The now active MPF triggers a cascade of phosphorylation events that drive the cell into mitosis. Its targets include:
   • Condensins: Proteins that drive chromosome condensation.
   • Nuclear Lamins: Their phosphorylation causes the breakdown of the nuclear envelope.
   • Microtubule-associated proteins: Promoting the formation of the mitotic spindle.

DNA Damage Response at G₂/M:

• If unreplicated or damaged DNA is detected, the sensor kinase ATR (for replication stress) or ATM (for breaks) is activated.
• They activate the checkpoint kinases Chk1 and Chk2.
• Chk1/Chk2 phosphorylate and inactivate the Cdc25 phosphatase.
• Since Cdc25 cannot remove the inhibitory phosphate from MPF, the cell remains arrested in G₂ until the DNA is repaired.

The Spindle Assembly Checkpoint (SAC)

This checkpoint operates during mitosis and prevents the separation of sister chromatids until every chromosome is correctly attached to the mitotic spindle. This is crucial for preventing aneuploidy.

Molecular Pathway:

1. The "Wait" Signal: A single kinetochore that is not properly attached to microtubules acts as a catalytic platform to generate a "wait" signal.
2. MCC Formation: Proteins including Mad2, BubR1, and Bub3 are recruited to the unattached kinetochore and assembled into the Mitotic Checkpoint Complex (MCC).
3. APC/C Inhibition: The MCC diffuses away and binds to the Anaphase-Promoting Complex/Cyclosome (APC/C), a large E3 ubiquitin ligase, keeping it inactive.
4. Attachment and Signal Extinction: When a microtubule attaches correctly to the kinetochore, the proteins that form the MCC are stripped away, and the "wait" signal is silenced.
5. APC/C Activation: Once all kinetochores are attached, the MCC no longer inhibits the APC/C. The APC/C, along with its co-activator Cdc20, becomes active.
6. Anaphase Trigger: The active APC/C targets two key proteins for ubiquitination and destruction by the proteasome:
   • Securin: This protein is an inhibitor of the enzyme separase. When securin is destroyed, separase becomes active.
   • Cyclin B: The destruction of Cyclin B inactivates MPF, which is necessary for the cell to exit mitosis and undergo cytokinesis.
7. Sister Chromatid Separation: The active separase enzyme cleaves the Scc1 subunit of the cohesin complex, the protein "glue" holding the sister chromatids together. This cleavage allows the sister chromatids to be pulled apart to opposite poles, marking the onset of anaphase.

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4. Mitosis and Meiosis: The Processes in Detail

(The detailed descriptions of the stages of Mitosis and Meiosis from the previous version remain here, now contextualized by the detailed regulatory information above.)

Mitosis (Equational Division)

• Prophase: Driven by MPF phosphorylation of condensins and lamins.
• Metaphase: Chromosomes align; the SAC is active and monitoring kinetochore attachments.
• Anaphase: Triggered by APC/C activation, securin degradation, and separase-mediated cleavage of cohesin.
• Telophase: Driven by the degradation of Cyclin B and the dephosphorylation of MPF substrates.

Meiosis (Reductional and Equational Division)

• Prophase I: The unique events of synapsis and crossing over occur.
• Anaphase I: Homologous chromosomes separate. Crucially, cohesin at the centromeres is protected by the protein Shugoshin (Sgo1), which prevents its cleavage by separase. This is why sister chromatids do not separate in Anaphase I.
• Meiosis II: Resembles a mitotic division. Shugoshin is degraded, allowing separase to cleave the centromeric cohesin at Anaphase II, finally separating the sister chromatids.

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5. Summary of Key Differences: Mitosis vs. Meiosis