Cell Cycle (Meiosis)

Cell Reproduction

• Overview: meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in four genetically diverse gametes, essential for sexual reproduction.

Chromosomes

Structure and Function

• Chromosomes are rod-shaped structures made of DNA and protein.

• DNA wraps tightly around proteins called histones, which maintain chromosome shape and compact the DNA.

• A chromatid is one half of a chromosome that forms when DNA copies itself prior to cell division.

Chromosome Structure

Key Features

• Chromosomes consist of two chromatids joined at a centromere, which holds them together until cell division separates them.

• During interphase, chromosomes uncoil into chromatin for cellular activity and gene expression.

Chromosome Numbers

Organization in Animals

• Animal chromosomes are classified into sexes and autosomes.

• Sex chromosomes determine the organism's sex, typically two types: X and Y.

• Females have XX and males have XY.

• Humans possess 44 autosomes, categorized in pairs 1-22.

Chromosome Number

Homologous Pairs

• Every sexually reproducing organism inherits two homologous chromosomes for each autosome (one from each parent).

• Typical humans have 46 total chromosomes (44 autosomes + 2 sex chromosomes).

• A karyotype displays the chromosomes of a normally diving human cell.

Diploid vs Haploid

Definitions

• Somatic Cells: Body cells possessing two homologous chromosomes - termed diploid (2n).

• These include skin, blood, and muscle cells.

• Haploid Cells: Cells with one copy of chromosomes (gametes - egg and sperm).

Meiosis and Gamete Formation

Process Overview

• Meiosis reduces chromosome number by half involved in sexual reproduction.

• Begins with a germ cell (2n) and transitions to haploid gametes (n).

Meiosis I vs Mitosis

Key Differences

• Both undergo interphase (G1, S, G2), yet result in genetic variation through synapsis and crossing-over during prophase I.

Meiosis I Phases

1. Prophase I: Homologous chromosomes pair up (synapsis), forming tetrads, leading to genetic recombination.

2. Metaphase I: Tetrads line up at midline, spindle fibers attach.

3. Anaphase I: Homologous chromosomes move to opposite poles (independent assortment).

4. Telophase I: Cytokinesis forms two haploid cells with homologous pairs.

Meiosis II Overview

• Meiosis II occurs without interphase.

1. Prophase II: Chromosomes move to the midline.

2. Metaphase II: Chromatids align at the midline.

3. Anaphase II: Sister chromatids separate.

4. Telophase II: Nuclear membranes form around four new cells, each with half the original chromosome number.

Spermatogenesis

• Spermatogenesis is the sperm cell production process.

Oogenesis

• Oogenesis refers to mature egg cell formation, where only one viable egg is produced during meiosis, and the rest, polar bodies, degenerate.

Sexual Reproduction

• The fusion of sperm and egg produces genetically diverse offspring, essential for evolutionary adaptation.