Meiosis

Why do we need meiosis? Meiosis is necessary to halve the number of chromosomes going into the sex cells Why halve the chromosomes in gametes? At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

Meiosis makes for a lot of genetic diversity Meiosis makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.

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:

Meiosis I Interphase Each of the chromosomes replicate

Prophase I Extremely IMPORTANT!!! It is during this phase that crossing over can occur. Crossing Over is the exchange of segments during synapsis.

Metaphase I The chromosomes line up at the equator(Middle) attached by their centromeres to spindle fibers from centrioles. Still in homologous pairs

Anaphase I The spindle pulls the chromosomes apart Chromosomes appear as pairs (unlike mitosis)

Telophase I The cytoplasm divides, forming two new daughter cells. cytokinesis

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 ! DNA does not replicate!

Prophase II Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

Metaphase II The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

Anaphase II The centromeres of sister chromatids finally separate The sister chromatids of each pair move toward opposite poles Now individual chromosomes

Telophase II and Cytokinesis Nuclei form at opposite poles of the cell and cytokinesis occurs After completion of cytokinesis there are four daughter cells All are haploid (n)

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes. In humans, n = 23 and 223 = 6,000,0000. That’s a lot of diversity by this mechanism alone.

Another 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.

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

Mitosis vs. Meiosis

Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

Boy or Girl? The Y Chromosome “Decides”

Meiosis – division error Chromosome pair

Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?