What are the overall goals of Cell Division with Meiosis? Where and when is this cell division happening?
Meiosis is all about being genetically different. The reason our cells go through meiosis is to form four daughter cells from two cell divisions, meiosis I and meiosis II. The first cell division is when homologous chromosomes separate to form two haploid cells. Second, the sister chromatids separate to form a total of four daughter cells. In this process cells have a higher genetic variation from independent assortment and crossing over. Independent assortment takes place in metaphase of meiosis I. When the homologues chromosomes line up in metaphase different chromosome pairs may line up on a different side than it’s other like chromosomes. This process can add up to more than 8 million variations from the original genetic code. Crossing over also helps with genetic variation in the prophase stage of meiosis I. When chromosome pairs are lined up ends of chromatids often pull apart but a chiasma site on the homologous chromosomes keep the two pairs of homologous chromosomes together. Then a part of a homologous chromosome sticks with the opposite set of homologous chromosomes and vice versa. Both of these processes take place to form a great genetic variation to make species more apt to evolve in a changing world.
How do the chromosomes found in cells following Meiosis compare with those in G1 cells just starting the cell cycle?
Chromosomes at the end of meiosis are half the size of those just starting G1. A chromosome at the beginning of the cell cycle are called diploids, composed of 46 chromosomes total. At the end meiosis cells are called haploids that are composed of only 23 chromosomes.
During meiosis, why do the homologous chromosomes separate first, and then the sister chromatids? What does each separation achieve, in terms of reaching the ultimate goal of Meiosis. Give a complete explanation for this, including the difference between homologous chromosomes and sister chromatids.
The biggest reason that homologous chromosomes separate before sister chromatids is so crossing over can still take place. Homologous chromosomes are a pair of chromosomes of the same length, and the same pattern that codes for certain characteristics in the body. Sister chromatids are copies of a chromosome attached at the centromere. Two sister chromatids make up one chromosome. Crossing over happens when the homologous chromosomes move next to each other in meiosis I and exchange strands of DNA when connected at the chiasma site. Thus, homologous chromosomes separate first to better fulfill the ultimate goal of meiosis, producing genetic variation. Without this order of operations crossing over would never take place, and without crossing over meiosis would lose a great deal of variation.
- Meiosis I - Homologous chromosome pairs (2 of each chromosome #1-23) line up next to each other and separate. During this time:
a. Independent Assortment - Each homologous pair randomly decides which chromosome will line up on the left or the right. When each . homologous pair separates and moves to an opposite end of the cell, it creates greater than 8 million possible combinations of alleles for . chromosomes 1-23.
b. Crossing Over - homologous chromosomes swap alleles.
2. Meiosis II - Sister Chromatids separate from one another
3. Cytokinesis to separate cells
4. 4 genetically unique cells become sperm or egg – each with only ONE copy of each chromosome (so 23 total).
5. Fertilization of egg with sperm creates an embryo that will undergo cell cycle with Mitosis to make the 10 trillion cells of a human.