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).

Meiosis Sex cells divide to produce gametes (sperm or egg). Gametes have half the # of chromosomes. Occurs only in gonads (testes or ovaries).

Three sexual life cycles differing in the timing of meiosis and fertilization

Moss with Sporophytes Gametophytes

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

Spermatogenesis diploid (2n) Spermato- cyte meiosis II meiosis I sperm haploid(n) meiosis II 2n=46 Spermato- cyte diploid (2n) n=23 meiosis I

Oogenesis diploid (2n) polar bodies Oocyte meiosis II meiosis I ovum haploid(n) meiosis II 2n=46 Oocyte diploid (2n) n=23 meiosis I polar bodies

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

Interphase Nucleus and nucleolus visible. chromatin cell membrane 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.

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. a. 22 pairs of autosomes b. 01 pair of sex chromosomes

Homologous Chromosomes eye color locus hair color Paternal Maternal

Prophase I centrioles spindle fiber aster fibers

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Crossing Over Crossing over (variation) may occur between nonsister chromatids. Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

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

Metaphase I equator OR equator

Metaphase I Shortest phase Tetrads align on the equator INDEPENDENT ASSORTMENT OCCURS: genes for different traits sort independently of one another into gametes

Anaphase I

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

Telophase I

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

Meiosis II No interphase (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis but the cells are different from each other

Prophase II same as prophase in mitosis

Metaphase II same as metaphase in mitosis equator

Anaphase II same as anaphase in mitosis sister chromatids separate

Telophase II

Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced; 3 polar bodies and an egg in females, 4 sperm in males gametes = sperm or egg

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

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

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

The Results of Alternative Arrangements of Two Homologous Chromosome Pairs on the Metaphase Plate in Meiosis I

A Comparison of Mitosis and Meiosis

Meiosis Animations: http://www.mhhe.com/biosci/bio_animations/08_MH_Meiosis_Web/ http://www.wiley.com/college/test/0471787159/biology_basics/animations/meiosis.html Comparison of Mitosis and Meiosis: http://highered.mheducation.com/sites/9834092339/student_view0/chapter11/comparison_of_meiosis_and_mitosis.html http://highered.mheducation.com/sites/0072495855/student_view0/chapter3/animation__how_meiosis_works.html http://highered.mheducation.com/sites/9834092339/student_view0/chapter11/meiosis_with_crossing_over.html http://www.biologyalive.com/life/classes/apbiology/documents/Unit%208/15_Lectures_PPT/media/15_06CrossingOver_A.swf http://highered.mheducation.com/sites/9834092339/student_view0/chapter11/meiosis_i.html http://highered.mheducation.com/sites/0072495855/student_view0/chapter28/animation__stages_of_meiosis.html http://www.wiley.com/college/trefil/0470118547/animations/ch21/meiosis/meiosis.html http://www.sumanasinc.com/webcontent/animations/content/meiosis.html http://highered.mheducation.com/sites/0072495855/student_view0/chapter3/animation__random_orientation_of_chromosomes_during_meiosis.html Independent Assortment: http://www.sumanasinc.com/webcontent/animations/content/independentassortmentnm.html

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)

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

Answer: 10 chromosomes

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

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

1) Three major differences between meiosis and the mitosis are:

2) Compare the number of chromatids present in human cells before and after DNA replication occurs in interphase. 3) Compare and contrast oogenesis with spermatogenesis.

Determine the number of chromo- somes in a cell whose diploid (2n) number is 10 after: a. meiosis I b. meiosis II c. mitosis

What is the primary difference between sexual life cycles in animals when compared to fungi and plants?

Failure of chromosomes to separate properly during meiosis may result in a/an ____________ Disorder, such as Down Syndrome. Explain the evolutionary reason that Sickle Cell Anemia is more common among African Americans.

If you understand the process of meiosis then you know that: It occurs only to make sex cells/gametes. 2) Four different cells are produced. 3) Crossing over (exchange among DNA of tetrads) occurs in Prophase I to make the cells genetically different. 4) The amount of DNA is reduced by half in Meiosis I.