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
diploid
2n
2n = 4
single stranded
homologous
chromosomes
double stranded
homologous chromosomes
How do we make sperm & eggs?
• Must reduce 46 chromosomes  23
23
46
meiosis
23
egg
23
46
23
sperm
gametes
46
zygote
fertilization
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!
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
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://cochrane.rockyview.ab.ca/Members/carolynjmcleod/biology-30/chapter-16celldivision/BIO%2030%20%20SPERMATOGENESIS%20AND%20OOGENESIS%20ANIMATI
ONS.doc/view
Spermatogenesis
Oogenesis
= 4 haploid sperm
=1 egg and 3 polar bodies
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter28/animation__spermatog
enesis__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
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://www.youtube.com/watch?v=kVMb4Js9
9tA
http://www.youtube.com/watch?v=iCL6d0Ow
Kt8
Meiosis Square Dance
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter12/animations.html#