Monday, November 8, 2010


Yunsu Y.
Today we took notes on MEIOSIS.

Meiosis goes through reduction division, 2 part division. Reduction=reduce , Division= Separate sister chromatids. Meiosis occurs in the ovaries or testes. It starts with a diploid parent cell then goes into prophase 1, metaphase I, anapahse I, telophase I, prophase II, metaphase II, anaphase II, then telophase II. The cell goes through each step twice but it does not really go through interphase II. It "goes" through interphase II, but it is just a resting period, it does not go through the same steps like in interphase I because it does not need to synthesize more chromosomes. (No G1, S, or G2 just stays the same.) These phases are very similar to mitosis, but are slightly different and have different end products.

1. Metaphase I: the homologous pairs of chromosomes called tetrads, line up in the middle.
2. Telophase II: instead of having 2 daughters cells, like in mitosis, you end up with 4. They also are not genetically identical.

Through meiosis, egg and sperm (gametes) cells are produced, not like mitosis where many different kinds of cells can be produced. Remember because this is sexual reproduction there are variations among offsprings. This variations amoung offspring occurs because of independent assortment and random fertilization.

1. Independent Assortment: when the chromosomes line up, there is no specific way it is all by chance.
2. Random Fertilization: a random pairing of 1 egg to 1 sperm, many different combinations.

Crossing over: where genetic material gets exchanged, segments of homologous chromatids.
-Crossing over happens because the genetic material gets tangled and they cross, so when they divide the daughter cells are not all the same. This would occur in prophase I.
There are somatic cells and sex cells---
1. Somatic cells: out body cells which are diploid -46 chromosomes for humans.
2. Sex cells: Sperm and egg which are haploid -23 chromosomes for humans.
-The end products of meiosis are sex cells that are haploid cells (n) and the parent cells in the beginning are diploid cells (2n). The diploid is the total number of chromosomes, 2 sets. Haploid is half the number of chromosomes.
★It is important to remember that end products of meiosis are haploid. They have to be haploid so that when an egg meets a sperm it can create a full human equaling 46 chromosomes. (23+23=46)
-Each human has sex chromosomes, usually 2. Females have XX and males have XY, so since 2 are sex chromosomes there are 44 leftover chromosomes. These are called autosomes. (not sex)

These are karyotypes of female and males humans.
-On the left is the female and on the bottom is the male. You can tell by the difference of the last 2 sex chromosomes.
-You can see that the first somatic chromosome is the largest and the last somatic chromosome is the smallest.

We also watched a video on meiosis:
1. There are 4 egg cells and 4 sperms cells produced. All 4 of the sperm cells are viable but only 1 of the 4 eggs will be used and mature, the other 3 eggs are useless.
2. Centrioles move before the second division, making a right angle to the first location of the centriole at the first division.

Homework: UP. pgs 15-16B, Study Meiosis, Read Up pgs 13-14, Cut out pg 21 for tomorrows lab

Next scribe: Carey E

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