MEIOSIS Meiosis The form of cell division by

MEIOSIS

Meiosis • The form of cell division by which gametes, with half the regular number of chromosomes, are produced. diploid (2 n) (complete set of chromosomes) haploid (n) (half the regular number of chromosomes) • Meiosis is needed for sexual reproduction • There are two divisions (meiosis I and meiosis II).

Meiosis • Reproductive cells divide to produce gametes (sperm or egg). • The gametes have half the regular # of chromosomes. • In humans gametes have 23 chromosomes. Meiosis in males: spermatogenesis Meiosis in females: oogenesis • Meiosis is similar to mitosis with some chromosomal differences.

Meiosis has two key outcomes: • • 1. Genetic Reduction: Meiosis is a form of cell division that produces daughter cells with half the number of chromosomes of the parent cell (23) 2. Genetic Recombination (Variation): The products of meiosis have different combinations of alleles. Genetic recombination gives rise to offspring that are genetically different from one another and their parents. This greatly increases genetic variation in a population

Meiosis

Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are similar in shape and size, and which carry genes controlling the same inherited traits. • Each locus (position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. a. b. 22 pairs of autosomes 1 pair of sex chromosomes

Interphase • Similar to mitotic interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. • Centriole pairs also replicate (in animal cells).

Meiosis I (four phases) • reduces the chromosome number in half in daughter cells. Called the reduction division. In humans 46 to 23 • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

Prophase I • Longest and most complex phase (90%). • Chromosomes condense. • Synapsis occurs: homologous chromosomes come together to form a tetrad. • Tetrad is a pair of homologous chromosomes or four chromatids (sister and non sister chromatids) that align in Prophase 1

Prophase 1

Crossing Over • Crossing over may occur between non sister chromatids at the chiasmata during Prophase 1. • During crossing over segments of nonsister chromatids overlap, break, and reattach to the other • Crossing over increases variation in the gametes produced • Chiasmata (chiasma) are the sites of crossing over.

Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • INDEPENDENT ASSORTMENT OCCURS: Orientation of homologous pair to poles is random. Turn to page 175 of your textbook

Metaphase 1 Homolougous chromosome pairs line up along the middle of the cell.

Independent Assortment

Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres.

Anaphase I Homologous pairs separate and migrate to the poles of the cell. Sister chromatids are still attached by a centromere.

Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed.

Meiosis II • There is no interphase II There is no DNA replication • Remember: Meiosis II is similar to mitosis

Prophase II

Metaphase 11

Anaphase 11

Telophase 11

Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs. • Four haploid daughter cells (gametes) are produced. gametes = sperm or egg

Genetic Variation • Important to population as the raw material for natural selection (evolution). • Question: What are three sexual sources of genetic variation?

Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!

Fertilization • The fusion of a sperm and egg to form a zygote • A zygote is a fertilized egg n=23 egg sperm n=23 2 n=46 zygote

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)

Meiosis Video • http: //highered. mcgrawhill. com/sites/0072495855/student_view 0/ch apter 28/animation__how_meiosis_works. htm l

• Meiosis Video : http: //www. sumanasinc. com/webcontent/ani mations/content/meiosis. html • Meiosis square dance • Amoeba sister meiosis