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
Don’t 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 aren’t 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

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
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 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

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. A ----- (10)------ C --- (5) --- D
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

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 don’t separate
Meiosis II: sister chromatids don’t 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