Meiosis

1
Meiosis
AP Biology Unit 3
2
Meiosis

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

3
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

4
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.

5
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

6
No DNA is copied between Meiosis I and II
7
Metaphase I homologous pairs line up in the
middle of the cell
Anaphase I homologous pairs pulled apart
8
Metaphase II Chromosomes line up single file
9
(No Transcript)
10
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

11
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

12
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

13
Sexual vs. Asexual Reproduction
Asexual Reproduction Sexual Reproduction
Who does it? Bacteria, Yeast Hydra, Fungi Animals, Plants
Methods (a few examples) Budding, Binary Fission Pollination, Fertilization
Advantages Very quick, produces clones More genetic diversity
Disadvantages Less genetic diversity only through mutations Uses more energy, offspring may not be as fit
14
Example of Asexual Reproduction-- Budding
15
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

16
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

17
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

18
Recombination Frequency

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

19
Linkage Maps

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

20
Sample Problem

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

Genes Recombination Frequency
A B 40
A C 10
A D 15
B D 25
B C 30
C D 5
21
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

22
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

23
Nondisjunction

• Meiotic spindle works incorrectly.
• Chromosomes fail to separate properly.
• May occur during meiosis I or II
• Meiosis I homologous chromosomes dont separate
• Meiosis II sister chromatids dont separate

24
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)

25
Trisomic vs. Monosomic

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

26
Meiosis Problems

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

27
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

28
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