Daily Newsletter March 22, 2012
Suggested Reading: Meiosis and Fertilization
& Recombination Between Homologous DNA Sequences.
Today's Topic: Meiosis
On Tuesday, the topic discussed was mitosis. We saw that mitosis was standard nuclear division. When coupled with cytokinesis, we saw that the goal was taking one parental cell and creating two identical daughter cells.
With meiosis, we will again experience nuclear division, but with a different goal than mitosis. The goal of meiosis is to produce gametic (reproductive) cells. As such, we will 'always' couple meiosis with cytokinesis.
Goal of Meiosis: From one diploid parental nuclei, generate four genetically unique haploid nuclei.
Goal of Meiosis-Cytokinesis: From one diploid parental cell, generate four genetically unique haploid cells.
The production of gametic cells requires two nuclear division events: a reduction division, and an equatorial division.
Important Note: All forms of nuclear division have the same basic four stages. Prophase is the set-up, Metaphase is when chromosomes align, Anaphase is when chromosome/chromatid seperate, and Telophase is when the nucleus returns to "norm".
The reduction division occurs in Meiosis I, and as the name implies, we are "reducing" something. Specifically, we are reducing chromosomal number. REMEMBER: a diploid organism (2N) carries two copies of each chromosome. During a reduction division, the copies are separated and moved to opposite sides (poles) of the cell.
Homologous Chromosomes: This term refers to the two copies of each chromosome. For example: Chromosome 1 in humans is the largest of the chromosomes. Individuals have two copies of chromosome 1: one from their mother and one from their father. These two individual examples of chromosome 1 are considered homologous. Homologous chromosomes carry the same genes, but each homolog (one copy) will have a unique set of alleles (one allele for each gene).
The important part of Meiosis I is what occurs in Prophase I.
During Prophase I, homologous chromosomes are brought together. The homologs possess DNA sequence similarities, and are able to bind to each other. This results in a cross-over event (recombination). During cross-over, alleles are swapped between the two chromosomes, with the end result being genetically unique allele patterns on each chromosome.
This event is one of the most critical ways that eukaryotic organisms ensure the diversity of their populations. Why?
Daily Challenge: We have discussed the main stages of nuclear division, and you have readings from your textbook and supplemental reading on meiosis. In your own words, describe the process of meiosis, complete with a discussion of synapsis, chisamata, and cross-over. Why is the recombination event so critical to population diversity?
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