November 4, 2011

Today we started notes for meiosis. You should have read Chapter 8 in the textbook already, which talks about mitosis and meiosis. Mitosis is still very important to know about, so GO BACK AND STUDY IT! Here are some of the important parts of today's notes:

(Note: know what the terms in purple mean)

• Mitosis has two divisions. The first division is the reduction of chromosomes and the second division is when the sister chromatids separate. The stages in the first division are:
  
  - Interphase: G1 (when the replication of chromosomes has not happened yet); S (synthesis - the DNA replicates); G2 (final preparation)
  - Prophase 1: There are 4 chromosomes in the parent cell, which means that there are 8 chromatids
  - Metaphase 1: Homologous chromosomes (chromosome pairs) line up (differently from mitosis, as shown in the picture - they line up with their pair).
  - Anaphase 1: The homologous chromosomes separate. The chromatids stay together because the centromeres are still intact
  - Telophase 1/Cytokinesis: The nuclear envelope comes back and 2 daughter cells are produced
  
  There is no second interphase GI or S between the two divisions
  
  The second division is similar to mitosis. The stages of the second division are:
  - Prophase 2: Each daughter cell has 2 chromosomes (which is why the first division is a "reduction") and 4 chromatids
  - Metaphase 2: The chromosomes line up (this time, the same as mitosis)
  - Anaphase 2: The chromatids separate
  - Telophase 2: In each of the 4 daughter cells, there are 2 chromosomes and no chromatids (this is because a chromatid is held together to a sister chromatid by a centromere). Each of the 4 daughter cells are genetically different
  


• Meiosis is the production of egg and sperm cells (which are gametes). It is sexual reproduction and produces a variation of offspring


• Each sperm or egg (sex cell) has half the number of chromosomes (23) as somatic - body - cells (46)


• Homologous chromosomes are 2 chromosomes that match in size, shape, and sequence of genes (remember that "homo" means "same")


• A tetrad is when 2 homologous chromosomes pair together. They are most likely to cross over during Prophase 1 (it is improbable, but not impossible for them to not cross over)


• Crossing over is essential for genetic variation (so you can look similar to your mom/dad but not identical)

• There are sex chromosomes (usually 2, non-matching pair) and autosomes (body cells, usually 44 in a human, paired)...2 + 44 = 46 chromosomes


• Diploid: the total number of chromosomes


• Haploid: half the number of chromosomes


• There is only 1 duplication of chromosomes but 2 divisions in meiosis


• At the end of Meiosis 1, there is haploid number, but there are still double chromosomes


• At the end of Meiosis 2, there are 4 daughter cells (genetically different), haploid, and single chromosomes


• The variety in species is because of:
  - Independent assortment - when homologous chromosomes pair at Metaphase 1, it is by chance. The effect is that the resulting gametes have a variety. Each one has 2 to the 23rd, or 8 million possible combinations!
  - Random fertilization - 1 egg/8 million fertilized by 1 sperm/8 million is about 4 trillion combinations!
  - Crossing over - the exchange of segments by homologous chromatids in genetic recombination, happens during prophase
  

Homework: Finish Spice Lab Report, UP 3-7, UP 9-11, cut out UP 21 (don't lose any - put them in an envelope, bring to class on Tuesday 11/8), UP 15-16B, and study mitosis and meiosis

Next Scribe: Vinise