1. Meiosis 3.3 & 10.1

2. Essential Idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
3.3
Meiosis
Understandings:
One diploid nucleus divides by meiosis to produce four haploid nuclei
The halving of the chromosome number allows a sexual life cycle with fusion of gametes
DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids
The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation
Orientation of pairs of homologous chromosomes prior to separation is random
Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number
Crossing over and random orientation promotes genetic variation
Fusion of gametes from different parents promotes genetic variation
Applications:
Non-disjunction can cause Down syndrome and other chromosome abnormalities
Studies showing age of parents influences changes of non-disjunction
Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks
Skill:
Draw diagrams to show the stages of meiosis resulting in the formation of four haploid cells

3. Chromosomes replicate in interphase before meiosis.
Essential idea: Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells
10.1
Meiosis
Understandings:
Chromosomes replicate in interphase before meiosis.
Crossing over is the exchange of DNA material between non-sister homologous chromatids.
Crossing over produces new combinations of alleles on the chromosomes of haploid cells.
Chiasmata formation between non-sister chromatids in a bivalent can result in an exchange of alleles.
Homologous chromosomes separate in meiosis I.
Sister chromatids separate in meiosis II.
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
Skills:
Drawing diagrams to show chiasmata formed by crossing over,

4. Production of gametes (egg and sperm cells)
A. Need to have half as many chromosomes as somatic cells so that when they fuse chromosome number is maintained
B. Called a reduction division

5. Remember, DNA has been duplicated during S phase before meiosis begins
A. Same number of chromosomes, but now sister chromatids

6. Increasing genetic diversity
A. Crossing over – exchange of genetic material between homologous chromosomes
1. Chromatids overlap, forming chiasmata, and swap segments

7. 2. Allows DNA from a person’s maternal chromosomes to mix with DNA from paternal chromosomes a. Chromosomes that end up in gametes are a mosaic (increases variety)

8. Random Orientation
1. The way maternal and
paternal chromosomes
line up on metaphase
plate is entirely by chance
2. Makes chances of
creating the same gamete
twice very slim

10. Sexual Reproduction
1. Chances of making the same gamete twice (NOT including crossing over) can be calculated by 2n
a. Ex. Humans 223 = 1 in 8,388,608
2. Now compound that by adding a second parent
3. Compound it again with crossing over

11. Phases of Meiosis
Prophase I
1. Chromosomes condense
2. Homologues pair up forming bivalents and crossing over occurs
3. Spindle fibers form

12. Metaphase I
Nuclear membrane disintegrates
Homologous pairs line up together on metaphase plate in random orientation

13. Anaphase I
1. Spindle fibers separate homologous pairs and pull them to opposite poles of cell

14. Telophase I
Spindle fibers disintegrate
Usually chromosome de-condense
Often cytokinesis occurs

15. Prophase II
1. Chromosomes condense again
2. New spindle fibers form

16. Metaphase II
1. Chromosomes line up single file on the metaphase plate (random orientation)
2. Spindle fibers attach to sister chromatids

17. Anaphase II
1. Sister chromatids pulled to opposite poles
2. Cleavage furrow starts to form in animal cells

18. Telophase II
1. Chromosomes de-condense
2. Nuclear envelopes form in each of the four haploid nuclei
3. Cytokinesis results in 4 cells

19. Make a chart as a table:
Brainstorm similarities and differences between mitosis and meiosis.

20. Non-disjunction
A. Sometimes homologous pairs or sister chromatids are not properly separated

21. Can result in monosomy (only one copy of a chromosome) or trisomy (3 copies of a chromosome)
1. Down’s Syndrome – 3 copies of chromosome 21

22. Most common in sex chromosomes
a. Turner Syndrome (XO)
b. Klinefelter Syndrome (XXY)
c. XYY Syndrome
d. Triple X Syndrome
Other examples
a. Trisomy 13 – Patau Syndrome
b. Trisomy 18 – Edward Syndrome

23. Non-disjunction and maternal age

25. Obtaining cells for karyotyping
Amniocentesis – use needle to remove amniotic fluid from around the fetus
Chorionic villi – tissue from placental tissue in uterine wall
Amnio miscarriage risk about 1%, villi about 2%