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Meiosis
AP Biology
Unit 3
Meiosis
• Process that occurs to form haploid cells
from diploid cells
• Forms gametes– sperm and egg
Homologous Chromosomes
• A pair of chromosomes
containing the same sets of
genes
– One chromosome from
each parent
– Don’t necessarily contain
identical genetic material
– Ex. You get one
chromosome #4 from mom
and one chromosome #4
from dad
Homologous Chromosomes vs.
Sister Chromatids
• Homologous
chromosomes contain the
same information, but
aren’t identical to one
another
• Sister chromatids are
identical copies of the
same chromosome.
Meiosis Overview
• Chromosomes are replicated to form sister
chromatids before meiosis
• 2 stages
– Meiosis I  homologous pairs are separated
– Meiosis II  sister chromatids are separated
• Results in 4 haploid daughter cells
– In humans, this means there are 23
chromosomes in each haploid cell
No DNA is copied
between Meiosis I
and II
Metaphase I: homologous pairs
line up in the middle of the cell
Anaphase I: homologous
pairs pulled apart
Metaphase II:
Chromosomes line
up single file
Unique to Meiosis
• Synapsis and Crossing
Over in Prophase I
– Synapsis = when
homologous chromosomes
line up next to one another
 form a tetrad
– Crossing over = the nonsister chromatids in the pair
exchange genetic material
Crossing Over and Genetic Diversity
• Crossing Over leads to
more genetic diversity in
a species
• Why?
• By exchanging some
genetic material, the
haploid cells formed
through meiosis are no
longer identical– more
variety
Unique to Meiosis
• Homologous Chromosomes line up next to
each other (form tetrads)
• Homologous chromosomes, NOT sister
chromatids, separate from each other during
Anaphase I
Sexual vs. Asexual Reproduction
Asexual
Reproduction
Sexual
Reproduction
Bacteria, Yeast
Hydra, Fungi
Budding, Binary
Methods (a
few examples) Fission
Animals, Plants
Very quick, produces
clones
Disadvantages Less genetic
diversity– only
through mutations
More genetic
diversity
Uses more energy,
offspring may not
be as fit
Who does it?
Advantages
Pollination,
Fertilization
Example of Asexual
Reproduction-- Budding
Recombination
• Another name for Crossing Over
• Occurs in Prophase I when homologous
chromosomes pair up
• Results in genetic variation
• In humans, 1-3 crossover events per
chromosome
Recombination Frequency
• Likelihood of crossing over between 2
genes is directly related to the distance
between them
– If 2 genes are close to one another on a
chromosome there is less of a chance they’ll be
separated by crossing over
– If 2 genes are far from one another on a
chromosome there is a greater chance they’ll be
separated by crossing over
Recombination Frequency
• When 2 genes are separated by crossing
over we say they have recombined.
• High recombination frequency = likely to
be separated by crossing over
• Low recombination frequency = not likely
to be separated by crossing over
Recombination Frequency
• If 2 genes have a high recombination
frequency, are they close together or far
apart?
– Far apart– greater % chance of being separated
Linkage Maps
• A map (diagram of relative positions of
genes) can be made by considering
recombination frequencies
Sample Problem
• What is the
order of genes
given the
following
recombination
frequencies?
Genes
A&B
Recombination
Frequency
40%
A&C
10%
A&D
15%
B&D
25%
B&C
30%
C&D
5%
Answer
• Step 1: Start by determining which genes
are closest together
– C & D are closest  5% recombination freq.
C--- (5) --- D
• Now, determine the position of one more
gene relative to these, let’s say gene A
– A is closer to C (10%) compared to D (15%)
A ----- (10)------ C --- (5) --- D
Answer (continued)
• Now, put in the final gene
– B is farthest from A, then C, then D according
to the numbers
A ----- (10) ------- C --- (5) --- D ---- (25) ---- B
Nondisjunction
• Meiotic spindle works
incorrectly.
• Chromosomes fail to
separate properly.
• May occur during
meiosis I or II
– Meiosis I: homologous
chromosomes don’t
separate
– Meiosis II: sister
chromatids don’t separate
Polyploidy vs. Aneuploidy
• Polyploidy = having too many
chromosomes ( more than 2 copies/sets of a
chromosome)
• Aneuploidy = having an abnormal number
of chromosomes (either too few or too
many)
Trisomic vs. Monosomic
• Trisomic = having 3 copies of a
chromosome
• Monosomic = only having 1 copy of a
chromosome
Meiosis Problems
• Extra or missing
parts of
chromosomes
can also occur
due to errors in
crossing over
Why is this a problem?
• Why might an increase of DNA result in
physical problems?
– Proteins overproduced, not enough regulators
to regulate all genes, etc.
• Why might a decrease of DNA result in
physical problems?
– Missing genes or portions of genes, regulatory
proteins not made, etc
Karyotypes
• Karyotypes are used to determine if there
are chromosomal abnormalities
– Only tells you if there are the right number of
chromosomes
– Does NOT tell you if specific genes are
mutated