The document provides notes on meiosis and the origins of genetic variation. [1] Meiosis involves two divisions that produce gametes and involves processes like chromosome pairing, separation of homologous chromosomes, and cytokinesis. [2] Genetic variation arises from independent assortment of chromosomes, crossing over during prophase I, and random fertilization. [3] Nondisjunction can also occur, resulting in aneuploidy like trisomy which can cause conditions like Down syndrome.

1. Notes 11/06/09
**Exam December 7th, 1:30-4:30 pm. 4M47**
Meiosis = production of gametes [cells in ovaries and testes]
Phases of Meiosis 1
-Prophase 1: chromosomes coil up, nuclear membrane breaks down,
spindle apparatus begins to form, homologous chromosomes pair up. Sister
chromosomes cross over Spindle fibres attach to homologous chromosomes,
allows for pulling apart of chromosomes.
-Synapses – chromosome pairs.
-Metaphase 1: chromosomes can be arranged in any different
combination—independent assortment causes genetic variation.
-Anaphase 1: separation of homologous chromosomes
-Cytokinesis [separation of cytoplasm]
-Telophase 1: uncoil chromosomes, dissemble spindle apparatus, build up
nuclear membrane
Phases of Meiosis 2
-Begins with a haploid cell
-Split centromeres, drag daughter cells apart.
-Each cell contains 1 of each kind of homologous chromosomes, each
chromosome is single-stranded.
-Essentially the same as mitosis, but is different as it is the second stage
of mitosis.
Origins of Genetic Variation
-Sexual life cycles produce genetic variation among offspring
-Independent assortment of chromosomes
Pairs are lined up along metaphase plate in metaphase 1, get
rearranged for genetic variation.
-Possible genetic variation is much larger than the number of
people that have ever existed.
-Crossing over
Prophase 1 of meiosis.
DNA repair enzymes may mix up chromosome parts in repair,
so each cell has different genes.
Recombinant chromosomes => changes have been made by
shuffling the order/content of genes. Major source of genetic
variation
-Random fertilization
-No way to know which egg is going to attach to which sperm

2. and where fertilization is going to occur.
-Nondisjunction:
cells do not get dragged apart and are
dragged in the same direction [diagram page 146]. Daughter
cell has two copies of chromosome, other daughter cell is missing
chromosome. 2/4 gametes will have a copy of nondisjunction
chromosome, other pair does not have chromosome at all.
-Trisomy: extra copy of particular chromosome. Ex.
Trisomy 21 [Down Syndrome]. Happens in meiosis 1,
usually.
-Nondisjunction in meiosis 2 do not cause as many
problems as nondisjunction in meiosis 1.
-Other trisomys often do not develop [ex. 3, 8], and
spontaneous abortion occurs.
-Evolutionary adaptation depends on a population’s genetic variation.
-Chromosomal changes
-Deletion
-Chromosome is missing a piece of DNA
-Likely happening during crossing over.
-Duplication
-Segment being crossed over with another homologous
chromosome, both pieces of chromosomes added to
same sister chromatid. Other chromosome will have a
deletion.
-Inversion
-Chromosomes in a different order, may affect
functioning of genes if they are ordered incorrectly.
-Recriprocal Translocation
-Pieces of DNA switched between non-homologous pairs