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Meiosis
Understanding the Concept of Meiosis,
The Meiosis Cycle and this entire concept can explain genetic variation across generations, otherwise known as Hereditary variation. This may explain why you have similar traits to your mother or father. Traits are passed down from parents to offspring through what we call genes or DNA. We inherit our parent's chromosomes which are bundles of DNA.
As we wrote about earlier last week, the Cell Cycle briefly covers Mitosis, which differs from Meiosis. However, the phases are similar to what we learned "PMAT", many. For starters, the key difference is that Mitosis can replicate and repair cells that are asexual (meaning they do not reproduce). Mitosis also produces identical cells, which we know as "clones". These are exact copies of the parent cell, and can only be genetically varied through mutations. In Meiosis this involves two parent cells, one from the male/female, and create unique offspring. This will be more noticeable as I cover the steps and key factors that differentiate Meiosis from typical cell division.
Homologous Chromosomes... What are they?
These are a pair of chromosomes that have similar traits but NOT identical to each other. These traits that may similar can range from size, length, centromere position, and carry the same genetic information. One homologous chromosome being inherited from dad, and the other from mom.
Key Term(s):
Karyotypes, are a display of chromosome pairs that are ordered by size and length.
Somatic Cells (body cells) are diploid or 2n two complete sets of each chromosome, humans have: 2n=46
Gametes Cells (sex cells) are haploid or n which is one set of each chromosome, humans have sperm/egg: n=23
Types of Chromosomes:
Autosomes: chromosomes that do NOT determine gender(sex), humans have 22 pairs
Sex chromosomes: X and Y, eggs: X (humans have 22+X), while sperm: X or Y (humans: 22+X OR 22+Y)
Meiosis
Meiosis is a process that creates haploid gamete cells in sexually reproducing diploid organisms, resulting in daughter cells with half the number of chromosomes as the parent cell.
Involves two rounds of cell division, Meiosis I and Meiosis II.
Key events:
Prophase I: synapsis and crossing over --> leads to genetic variation
Metaphase I: tetrads (homologous pairs) line up at the metaphase plate
Anaphase I: homologous pairs seperate
Meiosis I
Meiosis I is entered following Interphase where the cell goes through G1, S, and G2. (DNA copied during S phase)
Prophase I: Synapsis of homologous chromosomes pair up and physically connect to each other forming what we call a "tetrad". This is also done through the process of crossing over/recombination which takes place at chiasmata and DNA is exchanged between homologous pairs. Every chromatid produced has unique combination of DNA.
Metaphase I: Independent orientation tetrads line up at the metaphase plate
Anaphase I: Pairs of homologous chromosomes separate, while sister chromatids are still attached
Telophase I & Cytokinesis: Nuclei and cytoplasm divide, resulting in 2 cells now making a haploid (2) set of chromosomes in each cell
Meiosis II
Meiosis II is following subsequently after Meiosis I
Prophase II: No crossing over, and the spindle forms
Metaphase II: Chromosomes line up at the metaphase plate due to the crossing over that occurred in prophase I creating genetically unique chromatids
Anaphase II: Sister chromatids are able to move towards opposite poles of the cell
Telophase II and Cytokinesis: Now there are 4 haploid cells, with nuclei reappearing, and each daughter cell being genetically unique
Genetic Variation... How?
1. Crossing Over, produces recombinant chromosomes exchanging genetic material
2. Independent Assortment of Chromosomes, leads to chromosomes being randomly orientated along the metaphase plate during Metaphase I, can orient either with the maternal or paternal chromosomes closer to a given pole
3. Random fertilization, implies that any sperm can fertilize any egg
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