II. Connections
1. Homologous chromosomes are nonidentical sex chromosomes that have the same length, shape, and assortment of genes that line up with each other during meiosis. Duplicated chromosomes are DNA strands that are replicated and stay attached at the centromere.
2. Kinetochore is a mass of protein and DNA in the centromere to which microtubules of the spindle attach. These microtubules which pull apart the connected chromatids are directed by microtubule organizing centers which dictate how microtubules will be organized and oriented in a given type of cell.
3. Somatic cells are also known as body cells which,, after mitosis, result in the creation of 2 diploid cells with two sets of chromosomes each. In reproductive sex cells, however, meiosis results in 4 haploid cells or gametes that contain only one set of chromosomes.
4. Nucleosomes are repeating subunits of chromatin occurring at intervals along a strand of DNA that are coiled around histones. Dehydration reactions are dangerous to these nucleosomes as they can disrupt, uncoil, and destroy the DNA structure.
III. Essentials
a. Prophase I
Chromosomes become visible and pair with their homologue. Microtubules begin to form, nuclear envelope starts to breakup
b. Metaphase I
Mics. Connect to both chromosomes and homologous pairs. Chromosomes align midway between the poles
c. Anaphase I
Mics. Shorten and move towards spindle pole, pulling chromosomes apart.
d. Telophase I
One of each type of chromosome has arrived at spindle poles, cytoplasm divides, forming two haploid cells
e. Prophase II
New bipolar spindle forms in each haploid cell, centrioles move to opp. Sides of cell, chromatids become tethered.
f. Metaphase II
Chromosomes line up at poles. Same process as Metaphase I.
g. Anaphase II
Sister chromatids break apart, mics. Move chromosomes toward spindle poles.
h. Telophase II
4 nuclei form as new nuclear envelope encloses each cluster of chromosomes. Daughter cells are haploid.
10.4 Outline
I. How meiosis introduces variation in Traits
A. Crossing over in Prophase
1. sister chromatids become stuck at certain point of chromosome
2. when Dna strands break, segments break off and fuse with opp chromatid. This is called gene swapping, creating vast variety in genes.
B. Metaphase I Alignments
1. random tethering of duplicated chromosomes to microtubules
a. more variation!
2. either chromosome partner can endu up at either spindle pole
3. This means that there are 8,388,608 possible combinations of maternal and paternal chromosomes in human sperm and egg cells.
Differences between Mitosis and Meiosis
Meiosis Mitosis
- Purpose: reproduction Purpose: growth and repair
- Occurs only in reproductive cells occurs in all somatic cells
- Results in 4 haploid gametes results in 2 Diploid cells
- Results in genetically variable cells results in genetically identical cells
- 2 rounds of division 1 round of division
- Results in cells w/ 23 chromosomes results in cells w/ 46 chromosomes
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