Meiosis

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Meiosis Biology 130 Lecture 8
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The basic sexual cycle
Zygote
2n
2n
2n
Mitosis
2n
2n
2n
2n
2n
2n
2n
Syngamy
Gametes
n
n
Meiosis
1n
1n
1n
1n
1n
1n
1n
Mitosis
Spores
1n
1n
1n
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Meiosis
In meiosis, homologous chromosomes are separated
Mitosis
1N
Syngamy or fertilization
Mitosis
Meiosis
2N
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Chromosomes
In diploid phases of life cycles, there are pairs
of chromosomes. The members of each pair are
called homologous chromosomes.
Long arm
Two chromosomes
One chromatid per chromosome
Centromere
Short arm
Second pair of homologous chromosomes, one
chromatid each
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Fig. 9.4
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Chromosomes
During S phase of the cell cycle, all chromosomes
are replicated
Two chromatids per chromosome
Two chromosomes, four chromatids
Second pair of homologous chromosomes, two
chromatids each
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Chromosomes in meiosis
In preparation for meiosis, homologous
chromosomes come together. Because each
chromosome had two chromatids, the result is four
chromatids together in a tetrad and also called
a bivalent.
Two chromatids per chromosome
Two chromosomes, four chromatids
Second pair of homologous chromosomes, two
chromatids each
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Crossing over
An important event occurs at this stage. Parts of
the chromatids can interchange.
Two chromatids per chromosome
Two chromosomes, four chromatids
Second pair of homologous chromosomes, two
chromatids each
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Crossing over

• Occurs so frequently that the chromosomes can no
  longer be considered as coming from the paternal
  or maternal parent but each is a new, unique,
  mix.
• Segements will usually stay together, but you
  have about a 50/50 chance of the two ends of a
  chromosome coming from the same parent.
• There can be multiple crossovers, so even if the
  two ends come from the same parent, sections in
  the middle may come from the other parent.

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Meiosis Prophase I Metaphase I
In prophase I homo- logous chromosomes pair up
(unlike in mitosis). Once paired, crossing over
can occur.
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Anaphase I Metaphase II
In anaphase I homologs are pulled apart. BUT, the
centromeres do not divide. Notice that now
there are just two chromosomes (four chromatids).
Metaphase II is like metaphase of mitosis,
chromatids are separated.
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Metaphase II Anaphase II Anaphase from mitosis
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Anaphase II Late Telophase II
The result of meiosis is four haploid cells with
unique genetic makeup.
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Mitosis and meiosis

• Mitosis
• Common
• One cell to two
• DNA content to 1/2
• Exact reproduction
• Homologs do not pair
• Diploid or haploid cells

• Meiosis
• Rare, only in germ cells
• One cell to four
• DNA content to 1/4
• Allows mixing of genes
• No exact copies
• Homologs pair
• Only diploid cells

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Linkage maps

• Typically, there are 500 to 2000 genes per
  chromosome
• For the genes that show linkage (i.e., are on the
  same chromosome) they can be arranged by how
  often a cross-over event separates them
• Consider 3 linked genes most of the time they
  are together (ABC)
• 10 of the time Ab 10 of the time Bc, 1 AbC

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Linkage maps

• 10 of the time Ab 10 of the time Bc, never
  AbC
• Cross-over occurs 10 of the time between A and B
  and 10 of the time between B and C

A
C
B
Cross over 10 of the time
Cross over 10 of the time
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Linkage maps
The frequency of crossing over allows the order
of the genes to be determined.
Cross over 20 of the time
A
C
B
Cross over 10 of the time
Cross over 10 of the time
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Linkage maps
The distance is measured in centiMorgans (cM) and
is how often (divided by 100) a cross over event
occurs between two genes. A is 10 cM from B but
20 cM from C
Cross over 20 of the time
A
C
B
Cross over 10 of the time
Cross over 10 of the time
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A few more terms

• Gene Basic unit of inheritance, encodes one
  protein
• Locus Location on a chromosome
• Allele Version of a gene

In some cases, there may be more than one gene
for a specific protein. These genes will be found
at different loci on the chromosome. If there
were two loci with a gene for a particular
protein, an individual could have four alleles
for that protein.
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Duplication of parts of chromosomes
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