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MEIOSIS NOTES
1. Diploidy
• Normal cells are diploid (2n): there are two of each chromosome
◦ This means that there are 2 copies of every gene.
• Diploidy is useful because 2 copies of every gene means that there a backup copy if one gets mutated.
◦ Mutations are very frequent in the cells of large organisms. We wouldn’t survive with just one copy of
each gene.
2. The Problem of Doubling
• However, new organisms arise from the merger of two parental cells. If these parental cells were diploid, the
offspring would have twice as much DNA as the parents.
◦ This would would mean that over just three subsequent generations, the offspring would have 368
chromosomes.
• We know that offspring have the same amount of DNA as their parents, so how is it that the parents only pass
on half the genetic material?
3. Haploidy
• The answer is that the parental cells that merge to give rise to the offspring are not diploid. They are haploid.
◦ Haploid (1n): Having only one member of each chromosome pair.
• Cells become haploid via meiosis.
• Meiosis: Process where a diploid cell with two pairs of each chromosome gives rise to haploid sex cells
(gametes) with only one of each chromosome.
4. Homologous Pairs
• Homologous Chromosomes (homologues): Pair of chromosomes with the same sequence of genes (one
chromosome in a homologous pair come from the mother while the other comes from the father).
◦ Just because homologous chromosomes have the same order of genes, that is not to say that the genes are
identical.
▪ e.g., if a specific spot on the homologous pairs has a gene for blood type, one homologue's gene may
code for the A blood type, the other homologue's gene may code for B blood type
5. Meiosis
• Before meiosis starts, the DNA must be copied. This happens during Interphase.
• Interphase: Consists of G1, S and G2 phases
◦ The chromosomes are replicated during the S phase
• Meiosis consists of two stages:
◦ Meiosis I:
▪ Crossing over
▪ Reduction of chromosome # (from 2n to 1n)
▪ Homologous chromosomes are separated from one another
◦ Meiosis II:
▪ Division of chromosomes
▪ Sister chromatids are separated from one another
6. Meiosis I
• Prophase I: There is more going on in meiotic Prophase than in mitotic Prophase
◦ Nuclear membrane & nucleoli disappear
◦ Chromatin condenses to form chromatids
◦ Centrioles migrate to opposite ends and spindle fibers form
◦ Homologous chromosomes pair up and cross over
▪ Crossing Over
• Crossing over is the exchange of genetic material between homologous chromosomes
• Homologous chromosomes pair up to form tetrads and exchange pieces of their DNA.
◦ Tetrad: Two homologous chromosomes paired up
• This process allows for a reshuffling of genetic material.
• Metaphase I: Tetrads line up along the middle of the cell.
• Anaphase I: The tetrads separated, but the centromeres remain in tact.
◦ This means that the sister chromatids remain attached to one another and it is the homologous pairs that
are separated.
◦ In this step, the chromosome number is reduced by half. The cell went from being diploid to being
haploid.
• Telophase I:
◦ Nuclear membrane reforms.
◦ Chromatids de-condense to form chromatin.
◦ Nucleolus reappears
◦ Spindle apparatus breaks down.
• Cytokinesis I:
◦ The cell divides to form two cells.
◦ Each of these cells contains one chromosome of each homologous pair.
7. Interkinesis (AKA Interphase II)
• Rest phase between Meiosis I and Meiosis II.
• May be long, brief or non-existent (the DNA is NOT replicated during this phase if it occurs).
8. Meiosis II
• Prophase II:
◦ Nuclear membrane & nucleoli disappear
◦ Chromatin condenses to form chromatids.
◦ Sister chromatids are attached to one another by a centromere
◦ Centrioles migrate to opposite ends & spindle fibers form
• Metaphase II: Chromosomes line up along middle of cell.
• Anaphase II:
◦ The centromere of each chromosome splits
◦ The sister chromatids are separated and pulled to opposite ends by the spindle fibers.
• Telophase II:
◦ Nuclear membrane reforms.
◦ Chromatids de-condense to form chromatin.
◦ Nucleolus reappears
◦ Spindle apparatus breaks down.
• Cytokinesis II: The cell divides
9. Meiosis in Human Males
• In males, each of the four cells produced by meiosis will become sperm cells.
◦ The process of making sperm is always occurring from puberty onward.
10. Meiosis in Human Females
• Meiosis starts when the female is a fetus, but is halted during prophase I.
• During puberty, when ovulation occurs, the egg that is to be released finishes meiosis I.
• This produces two different types of cells: an oocyte and a polar body. The polar body may or may not
undergo meiosis II; either way, it will eventually die.
• The oocyte will begin meiosis II, but stops after prophase II. It will not finish meiosis II until it is fertilized
by a sperm. If the oocyte is fertilized, the cell contents are unevenly divided to give rise to two different
cells: a large ovum (which will develop into a new human being) and a small polar body which will die.
Sister Chromatids
{
Homologous Chromosomes
Protein
causes
freckles
Centromere
Protein
does
not
cause
freckles
Protein
causes
freckles
Causes
Friedrich
Ataxia
Centromere
Protein
for A
type
Causes
Friedrich
Ataxia
Protein
for A
type
Centromere
Protein
for B
type
CROSSING
OVER
Step 1: The homologous pairs of
each chromosome find each
other through random motion
Step 2: The sequences on
each homologous pair are
lined up
Step 3: The DNA breaks on
both chromosomes and the
DNA is physically exchanged
Step 4: The chromosomes
now have different
combinations of genes.
Does not
cause
Friedrich
Ataxia
OVERVIEW OF MEIOSIS
XX
XX
Crossing
Over
XX
XX
Meiosis I
X
X
X
X
Meiosis II
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Meiosis in Males
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<
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Meiosis in Females