MEIOSIS

Meiosis The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n) Meiosis is sexual reproduction. Two divisions (meiosis I and meiosis II).

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

Meiosis Sex cells divide to produce gametes (sperm or egg). Gametes have half the # of chromosomes. Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis Meiosis is similar to mitosis with some chromosomal differences.

Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) meiosis II human sex cell diploid (2n) n=23 meiosis I

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

Interphase I Similar to mitosis interphase. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. Centriole pairs also replicate.

Interphase I Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. 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 two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size. Homologous pairs (tetrads) 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. 22 pairs of autosomes b. 01 pair of sex chromosomes

Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type.

Homologous Chromosomes eye color locus hair color Paternal Maternal

Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues.

Autosomes (The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets 1 - 22

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

In Humans the “Sex Chromosomes” are the 23rd set Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male

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

Crossing Over Crossing over (variation) may occur between nonsister chromatids at the chiasmata. Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) are the sites of crossing over.

Crossing Over - variation Tetrad nonsister chromatids chiasmata: site of crossing over variation

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.

Sex Chromosomes XX chromosome - female XY chromosome - male

Prophase I centrioles spindle fiber aster fibers

Metaphase I Shortest phase Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

Metaphase I metaphase plate OR metaphase plate

Question: In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

Answer 223 = ~8 million combinations Formula: 2n Human chromosomes: 2n = 46 n = 23 223 = ~8 million combinations

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

Anaphase I

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

Telophase I

Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis

Prophase II same as prophase in mitosis

Metaphase II same as metaphase in mitosis metaphase plate

Anaphase II same as anaphase in mitosis sister chromatids separate

Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced. gametes = sperm or egg

Telophase II

Meiosis n=2 n=2 2n=4 sperm haploid (n) meiosis II meiosis I sex cell diploid (2n)

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

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

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)

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

Question: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes

Sources www.ursulinehs.org/powerpoint/meiosis.ppt www.biology4teachers.com/Cell%20Division/MEIOSIS.ppt http://www.iteachbio.com/Life%20Science/LifeFunctionsandTheCell/Meiosis.ppt