Meiosis Notes Unit 6

Recall: Genes & Chromosomes DNA is divided into segments The segments are called genes which contain the instructions to make specific proteins Different genes are turned “on” in different types of cells

Alleles – alternative forms of a gene for each variation of a trait of an organism. Trait – any characteristic that is inherited.

Diploid Cells In body cells, chromosomes come in pairs (Mitosis) A cell with two of each kind of chromosome is called a diploid cell. It is called 2n or diploid number of chromosomes. Contains 2 chromosomes so there are 2 alleles for any given trait.

Haploid Cells Organisms that produce gametes or sex cells have cells that contain one of each kind of chromosome These cells are called a haploid cell (n). Contains 1 chromosome so there is only 1 allele for any given trait.

Homologous Chromosomes Homologous Chromosomes – The paired chromosomes. They are not always identical due to variations of alleles.

Why Meiosis? Meiosis – A different type of cell division where gametes have half the number of chromosomes as the parent. Therefore, when sex cells combine, they’ll create an offspring (baby) with the same number of chromosomes as the parents.

Meiosis: Reduction/Division Two divisions: Meiosis I & Meiosis II

Meiosis I & Crossing Over

Meiosis I – Begins with one diploid (2n) cell Meiosis I is very similar to Mitosis except for what happens to the chromosomes

Prophase I Like Mitosis: Nuclear membrane disappears Spindles form Chromatin condenses into chromatids Unlike Mitosis: Homologous chromosomes form “tetrads” Chromosomes cross over

What is “crossing-over”? Occurs when the tetrads form Crossing-over is an exchange of information between chromosomes

Metaphase I Homologous chromosomes line up at the metaphase plate. Microtubules (spindles) attach

Anaphase I Homologous chromosomes separate. Sister chromatids remain attached. Sister Chromatids

Telophase I and Cytokinesis: Two daughter cells are formed with each one containing only one chromosome of the homologous pair. The daughter cells are now haploid (have half of the original information).

Meiosis II: Gamete formation

Prophase II: DNA does not replicate (no Interphase before). Prophase II is just like mitotic prophase.

Metaphase II: All of the chromosomes line up at the metaphase plate.

Anaphase II: Centromeres divide Sister chromatids migrate separately to each pole.

Telophase II and Cytokinesis: Cell division is complete. Four haploid daughter cells are obtained.

Summary of Meiosis

Comparison Table of Mitosis and Meiosis Trait Mitosis Meiosis Chromosome Number compared to parent cell Same (Diploid) Half (Haploid) Genetic Similarities Same Different Type of Cell Body Cell Sex Cells (eggs & sperm) Number of Divisions I (one) I & II (two) Type of Reproduction Asexual Sexual (requires fertilization)