Meiosis

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Meiosis
AP Biology Unit 3
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Meiosis

• Process that occurs to form haploid cells from
  diploid cells
• Forms gametes sperm and egg

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Homologous Chromosomes

• A pair of chromosomes containing the same sets of
  genes
• One chromosome from each parent
• Dont necessarily contain identical genetic
  material
• Ex. You get one chromosome 4 from mom and one
  chromosome 4 from dad

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Homologous Chromosomes vs. Sister Chromatids

• Homologous chromosomes contain the same
  information, but arent identical to one another
• Sister chromatids are identical copies of the
  same chromosome.

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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 cell

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No DNA is copied between Meiosis I and II
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Metaphase I homologous pairs line up in the
middle of the cell
Anaphase I homologous pairs pulled apart
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Metaphase II Chromosomes line up single file
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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 non-sister chromatids in the
  pair exchange genetic material

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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

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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

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Sexual vs. Asexual Reproduction
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Example of Asexual Reproduction-- Budding
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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

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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 theyll be
  separated by crossing over
• If 2 genes are far from one another on a
  chromosome there is a greater chance theyll be
  separated by crossing over

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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

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Recombination Frequency

• If 2 genes have a high recombination frequency,
  are they close together or far apart?
• Far apart greater chance of being separated

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Linkage Maps

• A map (diagram of relative positions of genes)
  can be made by considering recombination
  frequencies

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Sample Problem

• What is the order of genes given the following
  recombination frequencies?

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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, lets say gene A
• A is closer to C (10) compared to D (15)
• A ----- (10)------ C --- (5) --- D

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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

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Nondisjunction

• Meiotic spindle works incorrectly.
• Chromosomes fail to separate properly.
• May occur during meiosis I or II

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Polyploidy vs. Aneuploidy

• Polyploidy having too many chromosomes ( more
  than 2 sets of a chromosome)
• Aneuploidy having an abnormal number of
  chromosomes (either too few or too many)

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Trisomic vs. Monosomic

• Trisomic having 3 copies of a chromosome
• Monosomic only having 1 copy of a chromosome

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Meiosis Problems

• Extra or missing parts of chromosomes can also
  occur due to errors in cross over

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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

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Karyotypes

• Karyotypes are used to determine if there are
  chromosomal abnormalities
• Only tells you if there are the right number of
  chromosomes
• Doesnt tell you if specific genes are mutated