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MEIOSIS
Modified with permission from
Robert Goodman, Biology Department,Citrus Community College
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 & meiosis II)
Meiosis
• Sex cells undergo meiosis to produce
gametes
– Gametes have half the # of chromosomes.
• Male: sperm
• Female: egg
– Meiosis only occurs in gonads
• Male Testes: spermatogenesis
• Female Ovaries: oogenesis
• Meiosis is similar to mitosis with some
chromosomal differences.
Spermatogenesis
Head
Mid piece
n=23
human
sex cell
Tail
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
Interphase I
• Similar to mitosis interphase
– Chromosomes replicate (S phase)
– Each duplicated chromosome consists of two
identical sister chromatids attached at their
centromeres
– Centriole pairs also replicate
Interphase I
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Meiosis I (four phases)
• Cell division reduces the
chromosome number by one-half.
• Four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I
Longest and most complex phase
(90%)
–Chromosomes condense
–Synapsis occurs
• Homologous chromosomes come
together to form a Tetrad
–Two homologous chromosomes which is
four chromatids (sister and nonsister
chromatids)
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
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.
– 22 pairs of autosomes
– 1 pair of sex chromosomes
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Sex Chromosomes
XX chromosome - female
XY chromosome - male
Crossing Over
• Crossing over (variation) may
occur between nonsister
chromatids at the certain areas.
–Segments of nonsister chromatids
break and reattach to the other
chromatid.
Crossing Over - Variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
VARIATION
Prophase I
spindle fiber
aster
fibers
centrioles
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
1. Example:
2. Then
3. Thus
2n = 4
n=2
22 = 4 combinations
Metaphase I
OR
metaphase plate
metaphase plate
Question:
In terms of Independent
Assortment
How many different
combinations of sperm could a
human male produce?
Answer
Formula: 2n
Human chromosomes: 2n = 46
n = 23
223
~8 million combinations
Anaphase I
Homologous chromosomes
separate and move towards the
poles
Sister chromatids remain
attached at their centromeres
Anaphase I
Telophase I
• Each pole now has haploid set
of chromosomes.
• Cytokinesis occurs and two
haploid daughter cells are
formed.
Telophase I
Meiosis II
• No interphase II
(or very short - no more DNA
replication)
• Remember: Meiosis II is similar to
mitosis
Prophase II
• Same as prophase in mitosis
Metaphase II
Same as metaphase in mitosis
metaphase plate
metaphase plate
Anaphase II
• Same as anaphase in mitosis
• Sister chromatids separate
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember: four haploid daughter
cells produced.
gametes = sperm or egg
Telophase II
Meiosis n=2
sex cell
sperm
n=2
n=2
2n=4
haploid (n)
n=2
diploid (2n)
n=2
n=2
meiosis I
meiosis II
Variation
• Important to population as the raw
material for Natural Selection.
• Question:
What are the three sexual sources of
genetic variation?
Answer:
1. Crossing over (prophase I)
2. Independent assortment (metaphase I)
3. Random fertilization
Remember: variation is good!
Question:
• A cell containing 20 chromosomes
(diploid) at the beginning of
meiosis would, at its completion,
produce cells containing how many
chromosomes?
Answer:
10 chromosomes (haploid)
Karyotype
• A method of organizing the chromosomes of a
cell in relation to number, size, and type.
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
Question:
A cell containing 40 chromatids at
the beginning of meiosis would, at
its completion, produce cells
containing how many
chromosomes?
Answer:
10 chromosomes