Meiosis

Homologous chromosomes – identical chromosomes …same length, centromere, same staining pattern …exception…males sex chromosome 1-22 autosomes sex chromsomes 23 of the 46 chromosomes are paternal 23 of the 46 are maternal

Homologous chromosomes Paired chromosomes both chromosomes of a pair carry “matching” genes control same inherited characters homologous = same information single stranded homologous chromosomes diploid 2n 2n = 4 double stranded homologous chromosomes

How do we make sperm & eggs? Must reduce 46 chromosomes  23 23 46 23 46 egg 23 meiosis 46 23 Gametes – single set of 22 chromosomes 1 sex chromosome…(n)=23 zygote fertilization sperm Gametes – single set of 22 chromosomes 1 sex chromosome…(n)=23 gametes

Meiosis: production of gametes Happens in gonads Conserves chromosome # diploid  haploid 2n  n humans: 46  23 meiosis reduces chromosome number makes gametes fertilization restores chromosome number haploid  diploid n  2n

Meiosis I – Division of homologous pairs Interphase I : -G1, S, G2 *replicates only once!

Meiosis I – Division of homologous pairs Prophase I : -chromosomes condense & form homologous chromosomes called tetrads or bivalents -crossing over occurs (exchange genes between homologous pairs -synapsis allows for maternal chromsomes to pair up w/ its paternal homolog -centrosomes move to poles, meiotic spindle forms -nuclear membrane disappears -90% of time At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope. • The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasmata (singular: chiasma). • The nucleolus disappears during prophase I. • In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell. • The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus. • Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.

Meiosis I – Division of homologous pairs Metaphase I : -homologous pairs align at the middle/equator -spindle fibers attach to kinetochore

Meiosis I – Division of homologous pairs Anaphase I : -homologous chromosomes move toward opposite poles *separate the pair

Meiosis I – Division of homologous pairs Telophase I & Cytokinesis : -two new nuclei form -homologous chromosomes form into chromatin -nuclear membrane reappears -divide cytoplasm

Meiosis I Animation

Meiosis II – Division of sister chromatids Interphase II : -G1, G2 * No replication!! Prophase II : -spindle apparatus forms -sister chromatids become visible -nuclear membrane disappears

Meiosis II – Division of sister chromatids Metaphase II : -sister chromatids at metaphase plate/midline -spindle fibers attach to kinetochores on each sister chromatid

Meiosis II – Division of sister chromatids Anaphase II : -sister chromatids move toward opp poles

Meiosis II – Division of sister chromatids Telophase II : -nuclear membrane reforms -chromatin forms -cytokinesis produces 4 haploid (n) cells

Meiosis II Animation

Meiosis I and II

Meiosis 1

Meiosis 2

Spermatogenesis Oogenesis http://cochrane.rockyview.ab.ca/Members/carolynjmcleod/biology-30/chapter-16-cell-division/BIO%2030%20%20SPERMATOGENESIS%20AND%20OOGENESIS%20ANIMATIONS.doc/view Spermatogenesis Oogenesis http://cochrane.rockyview.ab.ca/Members/carolynjmcleod/biology-30/chapter-16-cell-division/BIO%2030%20%20SPERMATOGENESIS%20AND%20OOGENESIS%20ANIMATIONS.doc/view = 4 haploid sperm =1 egg and 3 polar bodies http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__spermatogenesis__quiz_1_.html http://wps.aw.com/bc_martini_eap_4/40/10469/2680298.cw/content/index.html

One way meiosis generates genetic variability is through the different ways in which maternal and paternal chromosomes are combined in the daughter cells. The number of possible chromosome combinations in the haploid nuclei is potentially very large. In general, the number of possible chromosome combinations is 2n, where n is the number of chromosome pairs. For example, in fruit flies, which have 4 chromosome pairs, the number of possible combinations is 2n, or 16. For humans, with 23 chromosome pairs, there are over 8 million metaphase arrangements. 2 parents = zygote : 223 x 223 = over 70 trillion

3 Sources of Genetic Variability Independent Assortment of homologous chromosome in meiosis I Crossing over during prophase I Random fertilization of egg and sperm http://www.pbs.org/wgbh/nova/body/how-cells-divide.html

Differences across kingdoms Not all organisms use haploid & diploid stages in same way which one is dominant (2n or n) differs but still alternate between haploid & diploid must for sexual reproduction

http://www.youtube.com/watch?v=kVMb4Js99tA http://www.youtube.com/watch?v=iCL6d0OwKt8 Meiosis Square Dance http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#