MEIOSIS & Fertilization Objective: E1 - Recognize the significance of meiosis to sexual reproduction, understand how gametes are formed, & recognize the process of fertilization.

Homologous Chromosomes Homologous refers to chromosomes that each have a corresponding chromosome from the opposite-sex parent. Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes

Number of Chromosomes Diploid cells contain both sets of chromosomes Haploid cells contain a single set of chromosomes

Organisms that reproduce Sexually are made up of two different types of cells. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova (eggs) are gametes.

Diploid (2n)  Haploid (n) Meiosis is describe as reduction division because the number of chromosomes per cell are cut in half through the separation of homologous chromosomes. Diploid (2n)  Haploid (n)

Meiosis (called Meiosis I and Meiosis II) Has 2 cell divisions with only one duplication of chromosomes. So, 1 diploid cell becomes 4 haploid cells.

In prophase I of meiosis, each chromosome pairs with its corresponding homologous chromosome to form a Tetrad (shown below) eye color locus hair color Paternal Maternal

Crossing Over -Process in which homologous chromosomes exchange portions of their chromatids during Prophase I of meiosis - Creates variation (diversity) in the offspring’s traits Tetrad nonsister chromatids chiasmata: site of crossing over variation

Gametogenesis & Fertilization A. Cytokinesis of meiosis differs in males and females 1. Male – Spermatogenesis a. Cytokinesis is even b. All 4 gametes survive

Gametogenesis & Fertilization 2. Female – Oogenesis a. Cytokinesis is uneven b. Most of the cytoplasm goes to 1 cell = egg c. The other 3 (very small) = polar bodies

3. Fertilization Egg + sperm = fertilized egg (zygote) n + n = 2n - Fertilization restores the diploid number

10 chromosomes (haploid) Question: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: 10 chromosomes (haploid)

Sperm + Ovum (egg) Zygote Fertilization Sperm + Ovum (egg) Zygote fertilization ?

Cell Division Review: A centrioles spindle fiber B

Mitosis or Meiosis?

Etymology Haploid: haplo- (Gk. HAPLOS, single) + - oid (shape or form) Diploid: Diplo- (Gk. DIPLOOS, double) + -oid (shape or form)

Item State Pearland ISD F 6 3 G 76 75 H 14 18 J 4

Item State Pearland ISD F 22 24 G 20 H 51 49 J 7

Item State Pearland ISD F 21 14 G 17 H 5 4 J 57 67