DRILL #14Quote: You will never leave where you
are, until you decide where you'd rather be.
~Dexter Yager
Agenda: Meiosis Notes
Warm-up:1. Describe interphase.
2. After a bacterial cell has undergone binary fission, how many chromosome copies will each daughter cell contain?
Meiosis NotesCH 11
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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).
Meiosis• Sex cells divide to produce gametes (sperm
or egg).• Gametes have half the # of chromosomes.
• Occurs only in gonads (testes or ovaries).Male: spermatogenesisFemale: oogenesis
• Meiosis is similar to mitosis with some chromosomal differences.
Spermatogenesis
2n=46
humansex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
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.
Meiosis I (four phases)• Cell division that reduces the chromosome
number by one-half.
• four phases:a. prophase Ib. metaphase Ic. anaphase Id. telophase I
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).
Prophase I - Synapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
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. Having two copies of every gene can be an advantage. For example, if the one gene from Dad was damaged, Mom’s good gene could take over.
• Each locus (position of a gene) is in the same position on homologues.
• Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomesb. 01 pair of sex chromosomes
Homologous Chromosomes
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
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.
Crossing Over - variation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
Sex Chromosomes
XX chromosome - female XY chromosome - male
Prophase I
centriolesspindle fiber
asterfibers
Metaphase I• Shortest phase• Tetrads align on the metaphase plate.• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n
Example: 2n = 4then n = 2
thus 22 = 4 combinations
Metaphase I
metaphase plate
OR
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 = 8,388,608 million combinations
• Any one of these assortments can combine with any one of the 8,388,608 combinations of his partner!
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 platemetaphase 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
2n=4
sex cell
diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sperm
haploid (n)
meiosis II
In males, four sperm are produced and flagella soon develop.
In females, one large egg survives and three smaller eggs die.
Fertilization• The fusion of a sperm and egg to form a zygote.• A zygote is a fertilized egg
n=23egg
sperm n=23
2n=46zygote
Variation• Important to population as the raw
material for natural selection.
• Question:What are the three sexual sources ofgenetic 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.
Question:• A cell containing 40 chromatids at the
beginning of meiosis would, at its completion, produce cells containing how many chromosomes?
Answer:• 10 chromosomes
