2.4 Cell division and cell cycle

Cell division is the foundation of growth and reproduction in living organisms. The cell cycle, consisting of interphase and mitotic phase, orchestrates this complex process. Understanding its stages and regulation is crucial for grasping how cells multiply and maintain genetic stability.

Mitosis and meiosis are two types of cell division with distinct purposes. While mitosis produces identical daughter cells for growth and repair, meiosis creates diverse gametes for sexual reproduction. These processes highlight the intricate mechanisms cells employ to divide and pass on genetic information.

Cell cycle stages and regulation

Interphase: Preparation for cell division

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• The cell cycle consists of two main phases: interphase and mitotic phase (M phase)
• Interphase is further divided into G1, S, and G2 phases
• During G1 phase, the cell grows in size and prepares for DNA replication
  • The duration of G1 phase varies among different cell types (neurons, hepatocytes)
  • G1 phase can be influenced by external factors such as growth factors (epidermal growth factor) and cell-cell contact
• S phase is characterized by DNA replication, resulting in the duplication of the cell's genetic material
  • The initiation of DNA replication is tightly regulated to ensure that it occurs only once per cell cycle
• In G2 phase, the cell continues to grow and synthesizes proteins necessary for mitosis
  • The cell also checks for any DNA damage before proceeding to mitosis

Mitotic phase: Nuclear and cellular division

• The M phase consists of mitosis and cytokinesis
• Mitosis is the process of nuclear division
• Cytokinesis is the division of the cytoplasm, resulting in two genetically identical daughter cells

Cell cycle regulation by cyclin-dependent kinases and cyclins

• The cell cycle is regulated by cyclin-dependent kinases (CDKs) and their associated cyclins
• The levels of cyclins oscillate throughout the cell cycle, activating CDKs at specific points to drive the progression from one phase to the next
• CDK inhibitors, such as p21 and p27, can negatively regulate the activity of CDKs
  • CDK inhibitors halt the cell cycle progression in response to various signals (DNA damage, cellular stress)

Mitosis: Process and role in division

Stages of mitosis

• Mitosis is a process of nuclear division that results in the formation of two genetically identical daughter cells
• Mitosis is divided into four main stages: prophase, metaphase, anaphase, and telophase
• During prophase, the chromatin condenses into visible chromosomes, and the nuclear envelope breaks down
  • The centrosomes, which have duplicated during interphase, move to opposite poles of the cell and organize the mitotic spindle
• In metaphase, the chromosomes align at the equatorial plane of the cell
  • Chromosomes are attached to the mitotic spindle fibers at their centromeres
• During anaphase, the sister chromatids separate and are pulled towards opposite poles of the cell by the mitotic spindle fibers
• In telophase, the chromosomes decondense, and the nuclear envelope re-forms around the two sets of chromosomes

Cytokinesis: Division of the cytoplasm

• Cytokinesis is the final stage of cell division, which differs between animal and plant cells
• In animal cells, a contractile ring of actin and myosin filaments forms and constricts, pinching the cell into two
• In plant cells, a cell plate forms at the equatorial plane and grows centripetally to divide the cell

Importance of mitosis in growth, development, and tissue repair

• Mitosis is essential for growth, development, and tissue repair in multicellular organisms
• Mitosis ensures that the genetic material is equally distributed between the two daughter cells
• Mitotic cell division allows for the production of new cells to replace damaged or dead cells (skin, intestinal lining)

Cell cycle checkpoints for stability

Types of cell cycle checkpoints

• Cell cycle checkpoints are control mechanisms that ensure the proper progression of the cell cycle and maintain genomic stability
• The G1/S checkpoint, also known as the restriction point in mammalian cells, checks for the presence of growth factors, cell size, and any DNA damage before allowing the cell to enter S phase
• The intra-S phase checkpoint monitors the progress of DNA replication and can slow down or halt the process in response to DNA damage or replication stress
• The G2/M checkpoint assesses the completion of DNA replication and checks for any DNA damage before permitting the cell to enter mitosis
• The spindle assembly checkpoint (SAC) ensures proper attachment of chromosomes to the mitotic spindle during metaphase and prevents the onset of anaphase until all chromosomes are correctly aligned

Checkpoint proteins and cellular responses

• Checkpoint proteins, such as p53 and ATM/ATR, play crucial roles in detecting DNA damage and initiating appropriate cellular responses
• Cellular responses to checkpoint activation include cell cycle arrest, DNA repair, or apoptosis (programmed cell death)
• p53, a tumor suppressor protein, is activated in response to DNA damage and can induce cell cycle arrest or apoptosis to prevent the propagation of cells with genetic defects

Consequences of checkpoint defects

• Defects in cell cycle checkpoints can lead to genomic instability and the accumulation of mutations
• Checkpoint defects are associated with the development of various diseases, including cancer (breast cancer, colon cancer)
• Mutations in checkpoint genes, such as p53, are found in many types of cancer cells, allowing them to divide uncontrollably and evade apoptosis

Mitosis vs Meiosis: Key differences

Overview of mitosis and meiosis

• Mitosis is a process of cell division that results in the formation of two genetically identical daughter cells
• Meiosis is a specialized form of cell division that produces four genetically diverse haploid gametes or spores

Differences in purpose and occurrence

• Mitosis occurs in somatic cells for growth, development, and tissue repair
• Meiosis occurs in germ cells or reproductive organs to produce gametes for sexual reproduction (spermatocytes, oocytes)

Differences in the number of cell divisions and genetic material

• In mitosis, the genetic material is replicated once, followed by a single round of cell division
• In meiosis, the genetic material is replicated once, followed by two successive rounds of cell division (meiosis I and meiosis II)
• During mitosis, sister chromatids separate during anaphase
• In meiosis, homologous chromosomes separate in anaphase I, and sister chromatids separate in anaphase II
• Mitosis maintains the diploid chromosome number in daughter cells
• Meiosis halves the chromosome number, resulting in haploid gametes or spores

Genetic diversity in meiosis

• Crossing over and independent assortment of chromosomes during meiosis I contribute to genetic variation in the resulting gametes
• Mitosis produces genetically identical daughter cells
• The gametes or spores produced by meiosis are genetically diverse due to the recombination of genetic material during prophase I (crossing over)