1. Meiosis
Overview video

2. Purpose of Meiosis Also called a reduction division
Daughter cells end up with half the number of chromosomes as the parent cell
Goes from diploid (2N) to haploid (1N)
Daughter cells contain one of each chromosome and one sex chromosome
Produces gametes (sperm and eggs)

3. Sperm surround an egg prior to fertilization
Oogenesis in Rana ovary
Spermatogenesis
Developing sperm

4. Homologous Chromosomes
Two chromosomes (1 from each parent) that look the same
Same size
Same banding
Carry the same genes
Alleles of the genes may differ
Both carry gene for eye color but one chromosome carries the blue allele and one carries the brown allele
Pair up and split during meiosis

5. Formation of the chiasmata
Occurs when two chromatids overlap in prophase 1
Bivalents form
Two homologous chromosomes
One from mom and one from dad
Can occur between sister chromatids
Creates recombinants…new combinations of genes
Can occur between non-sister chromatids
Forms a cross shaped structure called a chiasmata
Segments of the non-sister chromatids break off and reattach on the other chromatid
Bozeman video….watch it at home

9. The process Meiosis I Meiosis II reduction of chromosomes
Separation of homologous chromosomes into separate cells
Random orientation of chromosomes: no two gametes are identical
Cells go from 2N to 1N
Crossing over occurs during prophase 1
Meiosis II
Separation of sister chromatids into individual chromosomes

10. Chromosomes during Meiosis I
Interphase
Cell growth and DNA replication
Prophase I
Chromosomes condense
Nucleolus becomes invisible
Spindle forms
Synapsis
Homologous chromosomes pair up side by side
Now called bivalent and cross-over points are called chiasmata
Nuclear membrane disappears
Metaphase I
Bivalents move to the equator of the cells
Anaphase I
Homologous pairs split up, one chromosome of each pair goes to each pole
Telophase I
Chromosomes arrive at the poles
Spindle disappears

11. 10.1.1: Meiosis II Prophase II Metaphase II Anaphase II Telophase II
New spindle is formed at right angles to the previous spindle
Metaphase II
Chromosomes move to the equator
Anaphase II
Chromosomes separate
Chromatids move to opposite poles
Telophase II
Chromosomes arrive at the poles
Spindle disappears
Nuclear membrane reappears
Nucleolus becomes visible
Chromosomes become chromatin

12. Mendel and Independent Assortment
Each allele moves independently of all other alleles
Exceptions to this rule exist
Think hair color and eye color
Alleles are often located very close to one another on the same chromosome

14. Non-disjunction Chromosomes can fail to separate during meiosis.
Trisomy 21
Trisomy 18
Trisomy 13
Photos: Cytogenetics Dept. Waikato Hospital
Chromosomes can fail to separate during meiosis.
This is called non-disjunction.
It results in abnormal numbers of chromosomes in the gametes.
Non-disjunction can occur:
if chromosomes fail to separate at anaphase I.
if sister chromatids fail to separate during anaphase II
Fertilization of an abnormal gamete with a normal gamete (or vice versa) results in an abnormal chromosome number.
This is known as an aneuploidy, e.g. trisomies occur where there are three instead of the normal pair of a chromosome (right).

15. Karyotyping
The process of finding the chromosomal characteristics of a cell
Chromosomes are stained to show banding

16. Pre-natal testing
Karyotyping is generally done during pre-natal testing
Chronic villus
Done at 11 to 12 weeks gestation
Cells are taken from the chronic villus (cells that became the placenta)
Risk of miscarriage is 1%
Advantage is diagnosis in early pregnancy
Amniocentesis
Done in the 16th week of pregnancy
Takes amniotic fluid that surrounds the fetus
Cells are cultured and pictures of the dividing cells are taken
Creates a karyotype
Risk of miscarriage is 0.5%.
Recommended to all women over the age of 35 due to increased risk of non-disjunction

19. Genetic Variation
The number of different types of gametes that can be created due to crossing over during prophase I is infinite
Each time the crossing over occurs the gamete produced will be different than if straight meiosis were to occur
Crossing over can take place on any chromosome
Every gamete is different
Fertilization takes a unique gamete from dad and a unique gamete from mom to create a new individual

20. Hank Time