Chapter 11 Meiosis

Homologous Chromosomes

A pair of chromosomes (one from mom, one from dad) that code for the same traits but are not genetically identical.

Sister Chromatids

The two identical copies of a single chromosome, produced during DNA replication (S phase).

Gamete

A haploid sex cell (sperm or egg) used for sexual reproduction. In humans, they have 23 chromosomes.

Somatic Cell

Any cell of the body except for the gametes. These are diploid cells used for growth and repair.

Haploid (n)

A cell containing one set of chromosomes. Human gametes are haploid (n=23).

Diploid (2n)

A cell containing two sets of chromosomes, one from each parent. Human body cells are diploid (2n=46).

Process that forms haploid cells

Meiosis

When the diploid number is restored

At fertilization, when a haploid sperm fuses with a haploid egg to form a diploid zygote.

Two main stages of meiosis

Meiosis I and Meiosis II

Key event of Prophase I

Homologous chromosomes pair up to form tetrads and undergo crossing over.

Key event of Metaphase I

Tetrads line up at the middle of the cell. This is when independent assortment occurs.

Key event of Anaphase I

Homologous chromosomes are pulled apart and move to opposite poles.

Key event of Telophase I

Two new cells form, each now haploid (but each chromosome is still duplicated).

Key event of Meiosis II

The sister chromatids are separated (like mitosis of haploid cells).

Final result of Meiosis

Four genetically unique, haploid gametes.

Chromosome/Chromatid count at start of Meiosis I (human)

Chromosomes: 46 (duplicated). Chromatids: 92. The cell is Diploid.

Cell status after Meiosis I

Haploid (has one chromosome from each homologous pair, but chromosomes are still duplicated).

Phase that separates homologous chromosomes

Anaphase I

Phase that separates sister chromatids

Anaphase II

Crossing Over

During Prophase I, homologous chromosomes swap pieces of DNA, creating new gene combinations.

Independent Assortment

During Metaphase I, the random lining up of homologous pairs leads to many combinations of chromosomes in gametes.

Random Fertilization

Any one of millions of unique sperm can fertilize any unique egg, creating a novel zygote.

Purpose of Mitosis

Growth, repair, and asexual reproduction.

Purpose of Meiosis

Production of gametes for sexual reproduction.

Number of Divisions in Mitosis vs. Meiosis

Mitosis: One division. Meiosis: Two divisions (Meiosis I and II).

Daughter Cells of Mitosis vs. Meiosis

Mitosis: Two genetically identical, diploid cells. Meiosis: Four genetically unique, haploid cells.

Does crossing over occur in Mitosis?

No. Crossing over and homologous pairing are unique to Meiosis I.

Main advantage of sexual reproduction

Genetic diversity, which helps a species adapt to changing environments.

Potential disadvantage of sexual reproduction

Requires more time and energy to find a mate compared to fast, efficient asexual reproduction.