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.