Meiosis

Homologous chromosomes – identical chromosomes…same length, centromere, same staining pattern…exception…males sex chromosome

1-22 autosomes 23 sex chromsomes

Homologous chromosomes• Paired chromosomes– both chromosomes of a pair carry “matching” genes

• control same inherited characters• homologous = same information

diploid2n2n = 4

single stranded homologouschromosomes

double strandedhomologous chromosomes

How do we make sperm & eggs?• Must reduce 46 chromosomes 23

23

2346

egg

sperm

46

meiosis 46

fertilization

23

23

gametes

zygote

Gametes – single set of 22 chromosomes 1 sex chromosome…(n)=23

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!

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

Meiosis I – Division of homologous pairs

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

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

Spermatogenesis Oogenesis

= 4 haploid sperm =1 egg and 3 polar bodies

http://cochrane.rockyview.ab.ca/Members/carolynjmcleod/biology-30/chapter-16-cell-division/BIO%2030%20%20SPERMATOGENESIS%20AND%20OOGENESIS%20ANIMATIONS.doc/view

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

1. Independent Assortment of homologous chromosome in meiosis I 2. Crossing over during prophase I 3. 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://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

http://www.youtube.com/watch?v=iCL6d0OwKt8Meiosis Square Dance

http://www.youtube.com/watch?v=kVMb4Js99tA