Cell Types, Unit 4- Cell Cycle, and Cell Division: Mitosis and Meiosis

Cell Types Based on Chromosome Content

Gametes

Gametes are specialized reproductive cells that carry half the number of chromosomes found in somatic cells.

• Definition: Gametes are haploid (n), containing 23 different chromosomes.

• Examples: Ova (eggs) and sperm.

Somatic Cells

Somatic cells include all body cells except gametes and their precursors. They are produced by mitosis and are diploid (2n).

• Chromosome Number: 46 chromosomes (23 pairs) in humans.

• Homologous Chromosomes: Each pair consists of one chromosome from each parent, similar in length, centromere position, and gene loci.

• Genes and Alleles: Genes are units of heredity; alleles are different versions of a gene.

• Autosomes vs. Sex Chromosomes:

  • 22 pairs are autosomes (non-sex chromosomes).

  • 1 pair are sex chromosomes (XX = female, XY = male).

Cell Cycle – For Growth and Repair of Tissues

The cell cycle describes the life of a cell from its formation to its own division. Diploid somatic cells (2n) divide to produce genetically identical diploid cells.

• Phases:

  1. Interphase: Cell growth and DNA replication.

  2. Mitotic (M) Phase: Cell division (mitosis and cytokinesis).

Interphase

• G1 Phase: Growth, metabolism, and preparation for DNA replication.

• S Phase: DNA replication; chromosomes duplicate but remain as chromatin.

• G2 Phase: Further growth and preparation for mitosis; centrosome replication completes.

Mitotic Phase

• Mitosis: Division of nuclear material (chromosomes) in four stages:

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

• Cytokinesis: Division of the cytoplasm.

Somatic Cell Formation: Mitosis

Prophase

• Chromosomes condense and become visible.

• Nuclear envelope breaks up.

• Centrosomes move to opposite poles.

• Spindle microtubules grow and attach to kinetochores.

Metaphase

• Chromosomes align at the metaphase plate (cell equator).

Anaphase

• Sister chromatids separate at the centromere and move to opposite poles.

• Results in 92 individual chromosomes (in humans) temporarily.

Telophase

• Chromosomes decondense back to chromatin.

• Nuclear envelope reforms.

• Cytokinesis divides the cytoplasm, resulting in two identical diploid cells.

Gamete Formation: Meiosis (Reproductive Nuclear Division)

Meiosis is a two-division process that reduces chromosome number by half, producing haploid gametes.

• Purpose: To produce gametes (sperm and ova) with 23 chromosomes each (haploid, n).

• Phases: One round of DNA replication, followed by two cell divisions (Meiosis I and Meiosis II).

Meiosis I

• Prophase I: Homologous chromosomes pair up and exchange genetic material (crossing over).

• Metaphase I: Tetrads (paired homologous chromosomes) align at the equator.

• Anaphase I: Homologous chromosomes separate to opposite poles; sister chromatids remain together.

• Telophase I: Two haploid cells form, each with 23 chromosomes (each chromosome still has two chromatids).

Meiosis II

• Similar to mitosis; sister chromatids separate.

• Results in four haploid gametes, each with 23 chromosomes.

Comparison Table: Mitosis vs. Meiosis I vs. Meiosis II

	Mitosis	Meiosis I	Meiosis II
Prophase	46 chromosomes condense	Tetrads form	23 chromosomes condense
Metaphase	46 chromosomes line up at equator	23 tetrads line up at equator	23 chromosomes line up at equator
Anaphase	Kinetochores split & 46 chromosomes migrate to each pole	Tetrads split & 23 replicated chromosomes migrate to each pole	Kinetochores split, sister chromatids separate, 23 chromosomes migrate
After cytokinesis	2 identical diploid cells (46 chromosomes each)	2 haploid cells (23 chromosomes with sister chromatids)	4 haploid gametes (23 chromosomes each)

Why Meiosis?

• Ensures that offspring have the same chromosome number as parents.

• Combines 23 chromosomes from each parent to form a diploid zygote (46 chromosomes).

Key Equations:

• Diploid number: $2n$

• Haploid number: $n$

• Fertilization: $n + n = 2n$

Additional info: The notes provide a concise overview of chromosome content, cell cycle phases, and the processes of mitosis and meiosis, including a comparative table for clarity.