Cell Cycle and M Phase Quiz

Cell Cycle

• Two major phases based on cellular activities readily visible with a light microscope:
  • M phase: Cell undergoes division (approx. 1 hour)
    • Mitosis: process of nuclear division, separating replicated DNA into two nuclei
    • Cytokinesis: process of dividing the cytoplasm of a cell in two. This happens after nuclear division in mitosis.
• Interphase: Cell grows and performs metabolic activities (duration depends on cell type)
  • G1: Most protein synthesis; basal metabolic activities
  • S: DNA replication, histone synthesis, and chromosome duplication
  • G2: Protein synthesis and pre-mitotic activities

Cell Cycle Variations

• Cell cycle length varies significantly:
  • 30 minutes in a cleaving frog embryo (lacking G1 and G2 phases)
  • Days to weeks in most cells
  • Several months in slowly growing tissues (e.g., mammalian liver)

Cell Categorization

• Cells are categorized based on their capacity/frequency to grow and divide:
  • Highly specialized cells: Do not undergo cell division (neurons, muscle, red blood cells, osteoclasts).
  • Differentiated cells: Divide only upon induction (liver cells, lymphocytes).
  • Unspecialized cells (stem cells): High mitotic activity (stem cells in various adult tissues, basal layer of epithelia).

Stem Cell Division

• Stem cells may undergo asymmetric cell division
  • One daughter cell remains uncommitted and similar to the parent cell
  • The other daughter cell differentiates.

Mitosis (Cell Division)

• Mitosis: Produces two daughter cells identical to the parental cell.
• Serves to produce new cells throughout the body.
• Billion divisions occur to produce complex multicellular organisms from a single cell (zygote).
• Continuously occurs in developed organisms.
• Renews tissues, regenerates damaged tissues, and maintains structural and functional homeostasis.

Mitosis Stages (detailed in subsequent sections)

• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

Cytokinesis (detailed in subsequent sections)

• Separation of newly formed cells.
• Involves processes initiated in late anaphase.

Interphase - G1 Phase

• Cell grows and increases its volume.
• Builds structures needed to support increased volume.
• Metabolic activation (protein and RNA synthesis).
• Duplication of centrosomes and cytoplasmic organelles.
• Controls the extracellular environment and cell size to support DNA replication.

Interphase - G0 Phase

• Some cells can stop cycling for an indefinite time (quiescent state).
• This is due to unfavorable environmental conditions.
• Arrests replication, metabolic rate reduces.
• Can be reversible or irreversible.
• May retain ability to divide upon changes in environmental conditions.
• Terminally differentiated cells: Progress towards specific function (e.g. neurons, muscle), permanently reside in G0.
• Senescent cells: Damaged DNA; enter G0 to prevent damage propagation.

Interphase - S Phase

• Each chromosome duplicates its chromatid, becoming 'di-chromatid'.
• Histone synthesis occurs.
• Free histones incorporated into new nucleosomes.
• Chromatin domains re-establish (maintaining epigenetic regulation scheme).
• DNA checked for damage.

Interphase - G2 Phase

• Cells grow rapidly, increasing in size.
• Active protein synthesis occurs.
• Duplicated chromosomes are checked for errors.
• Double-strand breaks are repaired.
• Prepares for mitosis.

Cell Cycle Regulation

• Cell controls a wide variety of information packages (integrity and activity of molecules, organelles, chemical signals, mechanical signals).
• Decisions about progress in the cell cycle are made based on these signals.
• Decisions occur at specific checkpoints.

Cell Cycle Checkpoints (detailed in subsequent sections)

• G1-S checkpoint : The cell verifies the availability of growth factors, nutrients, and enzymes before starting S phase.
• G2-M checkpoint : the cells checks that DNA synthesis is complete and correct. If DNA contains error or damages, or is incomplete, the gate is closed and mitosis cannot start.
• Mitotic checkpoint (spindle checkpoint) : at the end of metaphase, the cell checks that all kinetochores are correctly attached to sister chromatids on the metaphase plate

Cyclins and Cyclin-Dependent Kinases (CDKs)

• Regulation of passage between cell cycle stages.
• CYCLIN: regulatory subunit
• CYCLIN-dependent KINASE (CDK): transfers phosphate groups, activating specific protein substrates.

Cyclins and CDKS activity control

• Activity of kinases is regulated by cyclin levels, which fluctuate throughout the cell cycle.
• Multiple cyclins and CDKs expressed at different phases of the cell cycle.
• They regulate various checkpoints in the cell cycle.

p53 Role in Cell Cycle Regulation

• p53 is a transcription factor essential for controlling cell cycle processes.
• Important for responding to cellular stresses (e.g., DNA damage).
• Activated p53 regulates production of proteins involved in DNA repair, apoptosis, and cell-cycle arrest.
• p53 is activated when cellular stress signals are present, like DNA damage and nutrients and is deactivated when the cell's health is good.

Meiosis

• Reductional cell division, distributing replicated homologous chromosomes.
• Two sequential divisions ("Meiosis I" and "Meiosis II") without an intervening DNA replication phase.
  • Meiosis I: Separates homologous chromosomes.
  • Meiosis II: Separates sister chromatids.

Meiosis I Subphases

• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

Meiosis Recombination

• Crossing over occurs during prophase I, leading to new combinations of maternal and paternal alleles.

Meiosis I Stages (detailed in subsequent sections)

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

Consequences of Meiotic Nondisjunction

• Failure of homologous chromosomes or sister chromatids to separate properly during meiosis I or II.
• Can result in aneuploidy (abnormal number of chromosomes) in offspring.
• Some resulting conditions have distinct human karyotype abnormalities that can be seen using karyotyping.

Human Aneuploidies

• Trisomy 21 (Down's syndrome)
• Trisomy 18 (Edwards' syndrome)
• Trisomy 13 (Patau's syndrome)
• 47, XXY (Klinefelter's syndrome)
• 45, X (Turner's syndrome)

Meiosis in Vertebrates

• Meiosis occurs in gonads to produce gametes.
• Male: Spermatogonia undergo meiosis to form spermatozoa.
• Female: Oogonia undergo meiosis to form oocytes.

Test your understanding of the cell cycle, focusing on the M phase and its key processes. This quiz covers various phases, including G1, G2, cytokinesis, and more. Perfect for biology students wanting to solidify their knowledge of cell division.