SC

4._Cellular_Division-1-1

Cell Physiology II

  • Overview of Mitosis and Meiosis

DNA Structure

  • Composed of two strands forming a double helix.

  • Strands made of nucleotides joined by covalent bonds along the helix.

Chromosome Structure (Figure 3.30)

  • Metaphase Chromosome: at midpoint of cell division.

  • Nucleosome: 10-nm diameter, consists of eight histone proteins wrapped by two winds of DNA double helix.

  • Chromatin: 2 nm diameter, forming a 'beads on a string' structure.

  • Tight Helical Fiber: 30-nm diameter structure.

  • Chromatid: 700-nm diameter; a single chromosome after DNA replication.

  • Looped Domain Structure: 300-nm diameter structure.

Cellular Division

  • DNA is organized into 46 chromosomes (23 pairs, diploid).

  • Each chromosome consists of identical copies, known as sister chromatids.

    • One chromatid from each parent (mom and dad).

  • Sister chromatids are attached at the centromere.

  • Chromosomes replicate during interphase, condensing during mitosis.

Types of Cellular Division

  • Somatic Cells: undergo Mitosis (2N to 2N).

  • Sex Cells: undergo Meiosis (2N to 1N).

Cell Cycle Overview

  • Most of a cell's life is in non-dividing state (Interphase).

  • The cell cycle includes: Interphase + Mitosis (which has stages of Prophase, Metaphase, Anaphase, and Telophase).

  • Cell cycle = IPMAT

Stages of Cell Cycle (Figure 17.7a)

  • Interphase: DNA is copied.

  • Mitosis: DNA is divided into two daughter cells.

  • Cytokinesis: Parent cell cleaved in half.

Mitosis Stages

  • Definition: Division of the cell nucleus.

  • Four stages: Prophase, Metaphase, Anaphase, Telophase.

  • In Prophase: replicated chromosomes condense, microtubules (spindle fibers) form and attach to chromosomes.

  • Metaphase: chromosomes line up on equator; microtubules organize them single file.

  • Anaphase: centromeres split; sister chromatids pulled to opposite poles.

  • Telophase: nuclear envelope reforms; chromosomes uncondense.

  • Cytokinesis: cytoplasm divided, producing daughter cells.

Control of Cell Division

  • "Stop" signals include:

    • Contact inhibition.

    • Growth-inhibiting factors from repressor genes.

    • Worn out telomeres (ends of chromosomes).

Summary of the Cell Cycle (IPMAT)

  • Includes all stages: Interphase, Prophase, Metaphase, Anaphase, Telophase.

  • Mitosis involves only PMAT stages.

Meiosis

  • Process for sex cells: involves two rounds of division (Meiosis I and II), leading to 23 single chromosomes (haploid).

  • Importance of haploid number when egg (23 chromosomes) and sperm (23 chromosomes) fuse to form a diploid zygote (46 chromosomes).

Meiosis Overview

  • Parent Cell undergoes DNA replication before Meiosis.

  • Results in 4 daughter cells after completing both rounds of division.

  • Meiosis I: separates homologous chromosomes.

  • Meiosis II: separates sister chromatids, similar to mitosis.

  • No DNA replication occurs between the two divisions.