Meiosis Unit 11 continues…

MEIOSIS AND SEXUAL REPRODUCTION Human sex cells or gametes are called _______________ and ________________. Human sex cells contain ____ chromosomes. This is the ______________ number of chromosomes.

Why is it important that sex cells contain only HALF or the HAPLOID number of chromosomes?

Egg and sperm combine during a process called Fertilization produces a DIPLOID cell called the

# Chromosomes in cells during fertilization and growth of the zygote: gamete + gamete = zygote (egg) (sperm) Haploid haploid diploid diploid diploid

STAGES OF MEIOSIS: I. INTERPHASE I Similar to interphase in mitosis. What happens to DNA?

II. MEIOSIS I - PROPHASE I Similar to Prophase of Mitosis PLUS: 1. An event called ______________ takes place as homologous chromosomes come together to form Tetrads. Tetrad: 2. Crossing over occurs when the arms of homologous chromosomes exchange genetic material.

B. METAPHASE I 1. Tetrads line up randomly on the “equator” of the cell. Mendel’s THIRD LAW: RULE OF INDEPENDENT ASSORTMENT: Homologs move in a random fashion to either end of the cell. The two resulting cells end up with a random assortment of the mother and father’s chromosomes. 2. Spindle fibers attach to the tetrads at the centromeres.

C. ANAPHASE I 1. Spindle fibers contract causing homologous chromosomes in each tetrad to separate. 2. Homologs move to opposite ends of the cell.

D. TELOPHASE I: 1. Spindle fibers break down and nuclear membrane reforms. 2. Chromosomes unravel into chromatin. 3. Furrow in animal cell form (or cell plate in plant cells) as the cytoplasm begins to divide.

III. CYTOKINESIS/ INTERPHASE II Very short phase with little activity. A. Cell divides. B. NO DNA REPLICATION TAKES PLACE!!! C. Each cell contains the haploid number of double-arm chromosomes.

IV. MEIOSIS II

A. PROPHASE II: 1. Chromatin condenses to show a haploid number of double-arm chromosomes. 2. Nuclear membrane breaks down, spindle fibers form and centrioles in animal cells move to opposite ends of the cell.

B. METAPHASE II: 1. A haploid number of double-arm chromosomes line up on the equator of the cell. 2. Spindle fibers attach to centromeres joining sister chromatids.

C. ANAPHASE II 1. Spindle fibers contract causing centromeres to split. 2. Chromatids separate and a haploid number of single-arm chromosomes moves toward opposite ends or “poles” of the cell.

D. TELOPHASE II: 1. Spindle fibers disappear and nuclear membranes begin to form. 2. 3. 4. _______ cells are forming, each with a haploid number of single-arm chromosomes.

V. CYTOKINESIS A. Cytoplasm divides. B. Furrow or cell plate is completed. C. Four different cells have been formed, each with a __________ number of chromosomes.

The four cells have the same amount of DNA, the same number of chromosomes and the same types of genes represented in the chromosomes but they are NOT genetically identical cells! The sequence of nucleotides in the gene locations are very different..

GENETIC VARIATION IN THE PRODUCT OF MEIOSIS: Three ways that genetic variation (differences in the nucleotide sequence at a given gene location) can occur during the process of meiosis and sexual reproduction? a. b. If you continue this process to fertilization: c.

COMPARISON OF MITOSIS AND MEIOSIS: Kinds of cells produced Number of cells produced Chromosome number of parent cell

MITOSIS MEIOSIS Chromosome number of daughter cells Number of times the DNA is replicated Number of cell divisions Function Growth/repair Asexual reproduction Formation of gametes

Meiosis I--First division COMPARISON OF MITOSIS AND MEIOSIS: MEIOSIS INVOLVED ______ DIVISIONS OF NUCLEAR AND CYTOPLASMIC MATERIAL: Meiosis I--First division Meiosis II Second division

Why do you think Meiosis is called a reduction division process?

EGG AND SPERM PRODUCTION GAMETOGENESIS, or the production of gametes by the process of meiosis is of two types in humans: OOGENESIS: a. b. c. d.Occurs in human females during embryonic development and continues until menopause. e.Note that the cytoplasm is NOT equally divided between the eggs during this process. The result is one large, functional egg with a better chance of survival.

SPERMATOGENESIS a. b. c.