2. 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).
3. Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
4. Meiosis
• Sex cells divide to produce gametes (sperm
or egg).
• Gametes have half the # of chromosomes.
• Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
• Meiosis is similar to mitosis with some
chromosomal differences.
7. Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
• Centriole pairs also replicate.
8. Interphase I
• Nucleus and nucleolus visible.
nuclear
membrane
nucleolus
cell membrane
chromatin
9. 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
10. Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes
come together to form a tetrad.
• Tetrad is two chromosomes or four
chromatids (sister and nonsister chromatids).
12. 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.
• Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes
13. Karyotype
• A method of organizing the chromosomes of a
cell in relation to number, size, and type.
17. 21 trisomy – Downs
Syndrome
Can you see the
extra 21st
chromosome?
Is this person
male or female?
18. Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
XX chromosome - female XY chromosome - male
In Humans the
“Sex
Chromosomes”
are the 23rd set
19. Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome
20. Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
• Chiasmata (chiasma) are the sites of crossing
over.
21. Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
variation
Tetrad
22. Another Way Meiosis Makes Lots of
Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of
different gamete types produced by independent
25. Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
37. Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember: four haploid daughter cells
produced.
gametes = sperm or egg
40. Variation
• Important to population as the raw
material for natural selection.
• Question:
What are the three sexual sources of
genetic variation?
41. Answer:
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!
42. Question:
• A cell containing 20 chromosomes (diploid)
at the beginning of meiosis would, at its
completion, produce cells containing how
many chromosomes?
