Download - Meiosis
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