Cell Division: Mitosis and Meiosis

Introduction to Cell Types and Genetics Terminology

• Types of Eukaryotic Body Cells:
  • Somatic Cells: These are non-reproductive cells that undergo mitosis for growth and maintenance.
  • Sex Cells (Gametes/Germline): These are cells involved in sexual reproduction and are produced through meiosis.
• Key Transitions:
  • Gametes: Reproductive cells (sperm or egg) containing half the genetic material.
  • Zygote: A fertilized egg formed by the fusion of an egg and a sperm ($n=23$ egg + $n=23$ sperm $\rightarrow$ $2n=46$ zygote).

Cell Cycle Checkpoints and Control

• Definition: Checkpoints determine whether the cell cycle proceeds or is halted. They ensure the cell is ready to continue by checking for errors in replication, growth, or division.
• Major Checkpoints:
  • The $G_1$ Checkpoint (at the $G_1/S$ transition):
    • This is the primary decision point for a cell.
    • It determines if the cell should choose to divide or enter $G_0$ (a state of specialization with no more divisions).
    • Once a cell passes this point and enters the $S$ phase, it is irreversibly committed to division.
    • Checklist: Cell size, nutrients, growth factors, and DNA damage.
  • The $G_2$ Checkpoint (at the $G_2/M$ transition):
    • Ensures cell division proceeds smoothly to produce healthy daughter cells with complete, undamaged DNA.
    • If errors or damage are detected, the cell pauses for repairs.
    • Apoptosis: If damage is irreparable, the cell undergoes programmed cell death.
    • Checklist: DNA damage and DNA replication completeness.
  • The $M$ Checkpoint (Spindle Checkpoint):
    • Occurs during the transition from metaphase to anaphase.
    • The cell examines if all sister chromatids are correctly attached to the spindle microtubules.
    • Because separating sister chromatids in anaphase is an irreversible step, the cycle halts until all chromosomes are firmly attached to at least two spindle fibers from opposite poles.
    • Checklist: Chromosome attachment to spindle at the metaphase plate.
• Cancer Cells: Occur when good cells "go bad" by ignoring normal cell cycle signals and undergoing rapid proliferation.

DNA Packing and Structural States

• Chromatin: The extended, uncondensed form of DNA molecules present when the cell is not dividing. DNA replication and transcription can only occur in this state.
• Chromosomes: Pre-division condensed forms of DNA. Each chromosome consists of a single molecule of DNA. Condensation makes it easier to sort and organize replicated DNA into daughter cells.

Mitosis: Division of Somatic Cells

• Process Overview: A single eukaryotic somatic cell divides into two identical daughter cells.
• Chromosome Count: Daughter cells retain the same number of chromosomes as the parent cell ($2n \rightarrow 2n$).
• The Four Sub-phases:
  1. Prophase:
     • Chromosomes condense.
     • Spindle fibers form (specialized microtubules radiating from centrioles).
     • Chromosomes are captured by the spindle fibers.
  2. Metaphase:
     • Chromosomes align along the equator (metaphase plate) of the cell.
     • One chromatid faces each pole.
  3. Anaphase:
     • Sister chromatids separate.
     • Spindle fibers attached to kinetochores shorten, pulling chromatids toward the poles.
     • Free spindle fibers lengthen, pushing the poles of the cell apart.
  4. Telophase:
     • Spindle fibers disintegrate.
     • Nuclear envelopes form around both groups of chromosomes.
     • Chromosomes revert to their extended (chromatin) state.
     • Cytokinesis occurs, enclosing each nucleus into a separate cell.

Cytokinesis: Animal vs. Plant Cells

• Animal Cells: Undergo cytokinesis via a cleavage furrow. A ring of microtubules contracts and pinches the cell in half.
• Plant Cells: Undergo cytokinesis by forming a cell plate between the two daughter nuclei.

Meiosis: Sex Cell (Gamete) Formation

• Process Overview: A single germ cell divides into four unique haploid daughter cells. This process reduces the chromosome number by half ($2n \rightarrow n$).
• Location: Occurs only in the gonads (testes or ovaries).
  • Spermatogenesis: Male gamete production.
  • Oogenesis: Female gamete production.
• Stages: Meiosis consists of two distinct nuclear divisions: Meiosis I and Meiosis II.

Meiosis I: Reductional Division

• Interphase I: Similar to mitosis; chromosomes replicate during $S$ phase. Each duplicated chromosome consists of two identical sister chromatids attached at centromeres. Centrioles replicate as well.
• Prophase I:
  • The longest and most complex phase (occupying approximately $90\%$ of the time).
  • Chromosomes condense.
  • Synapsis: Homologous chromosomes come together to form a tetrad (two chromosomes or four chromatids).
  • Crossing Over: Non-sister chromatids exchange segments of DNA at specific locations called chiasmata (singular: chiasma). This produces variation so chromatids are a mix of maternal and paternal DNA.
• Metaphase I:
  • Shortest phase.
  • Tetrads align on the metaphase plate.
  • Independent Assortment: The orientation of homologous pairs to the poles is random. This is another source of variation.
  • Formula for combinations: $2^n$. For example, if $2n = 4$, then $n = 2$, resulting in $2^2 = 4$ possible combinations.
• Anaphase I:
  • Homologous chromosomes separate and move toward opposite poles.
  • Note: Sister chromatids remain attached at their centromeres.
• Telophase I:
  • Each pole has a haploid set of chromosomes.
  • Cytokinesis occurs, forming two haploid daughter cells.

Homologous Chromosomes and Karyotypes

• Homologous Chromosomes: Pairs (one maternal, one paternal) that are similar in size and shape. They carry genes for the same inherited traits at the same locus (position).
• Human Chromosome Set: Humans have $23$ pairs of homologous chromosomes ($46$ total).
  • Autosomes: Sets $1$ through $22$. These code for most offspring traits.
  • Sex Chromosomes: The $23^{rd}$ set. They determine the sex of the offspring.
    • $XX$: Female.
    • $XY$: Male. The $Y$ chromosome "decides" the sex.
• Karyotype: A method of organizing a cell's chromosomes by number, size, and type.
• Chromosomal Abnormalities:
  • Trisomy 21 (Downs Syndrome): Characterized by an extra $21^{st}$ chromosome.

Meiosis II: Equational Division

• Note: There is no Interphase II (or it is very short, with no further DNA replication).
• Stages:
  • Prophase II: Similar to mitosis prophase.
  • Metaphase II: Chromosomes align at the equator (metaphase plate).
  • Anaphase II: Sister chromatids separate and move toward opposite poles.
  • Telophase II: Nuclei form, cytokinesis occurs, and four unique haploid daughter cells (gametes) are produced.