Meiosis

Meiosis – A Source of Distinction Why do you share some but not all characteristics of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis.

Meiosis https://www.youtube.com/watch?v=mKWxeMMFTEU https://www.youtube.com/watch?v=toWK0fIyFlY&list=PLwL0Myd7Dk1HwPKULqkJoNkfOnT3aTzq6&index=2

What controls each characteristic or “trait” in an organism? The instructions for each trait are located in genes, which are segments of DNA on a chromosome. Gene= a segment of DNA that controls the production of one particular protein (1 gene = instructions for 1 protein) How many genes are on each of your 43 chromosomes? HUNDREDS

Chromosomes and Chromosome Number Reminder: Human body cells have 46 chromosomes. When your cells need to repair or replace or grow, they duplicate cells with 46 chromosomes; this is MITOSIS. But to first create life, cells need to undergo MEIOSIS. Each parent contributes 23 chromosomes- resulting in 23 pairs that match up- homologous chromosomes.

Homologous chromosomes have the same length and the same centromere position, and they carry genes that control the same inherited traits.

Organisms produce gametes, which are sex cells that have ½ the number of chromosomes. This number will vary from species to species. These cells are called haploid cells (n). Fertilization is the process where one haploid gamete (n) combines with another haploid cell (n). The cell will now have 2n chromosomes and is called a diploid cell.

Why do we need meiosis? Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species.

Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

The Stages of Meiosis: aka: Reduction Division- reduces the chromosome number by half Gametes (sex cells) are produced

Meiosis I : Separates Homologous Chromosomes Interphase Carry out normal cell function (metabolism) Each of the chromosomes replicate The result is two genetically identical sister chromatids which remain attached at their centromeres

Prophase I This is a crucial phase for meiosis. During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. Extremely IMPORTANT!!! It is during this phase that crossing over can occur. Crossing Over is the exchange of segments during synapsis- pieces of each chromosome switch places.

Metaphase I Spindle fibers attach to centromeres Homologous pairs of chromosomes line up together at the equator

Anaphase I Homologous chromosomes separate and move to either side of the cell (NOT as sister chromatids- the ENTIRE chromosome!) The cell goes from diploid (2n) to haploid (n)

Telophase I This is the end of the first meiotic cell division. Homologous chromosome reach the cell’s opposite poles. Each pole contains only one of the original pair of chromosomes. The sister chromatids are still attached but might not be identical to each other any longer due to crossing over. Chromosomes relax in the 2 new nuclei. Cytokinesis occurs but the cell does NOT enter interphase.

Figure 13.7 The stages of meiotic cell division: Meiosis I

Meiosis II : Separates sister chromatids Proceeds similar to mitosis THERE IS NO INTERPHASE II !

Prophase II Spindle forms and chromosomes condense

Metaphase II Chromosomes line up at the center

Anaphase II Centromeres split and sister chromatids NOW move to opposite poles

Telophase II and Cytokinesis Nuclei form around the 4 chromosomes After completion of cytokinesis there are four daughter cells All are haploid (n)

Figure 13.7 The stages of meiotic cell division: Meiosis II

Mitosis vs. Meiosis

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis

What is the end result of meiosis? 4 genetically different haploid cells What type of cell division do cells use in asexual reproduction? Mitosis – end result is 2 identical haploid gametes

What is the importance of meiosis in terms of sexual reproduction? Meiosis provides genetic variation. The 4 resulting haploid cells are all genetically different because of crossing over and random assortment. When sperm and egg join to form a new organism, there is genetic variability.

One Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Compare mitosis and meiosis Mitosis – 2 genetically identical diploid body cells Meiosis – 4 genetically different haploid sex cells

Mitosis vs. Meiosis

On page 276- answer questions 2,4, and 5 in COMPLETE SENTENCES or label drawings where appropriate. *NO CREDIT for incomplete answers or unclear, unlabeled drawings.