Meiosis

SNAB Edexcel for 2016 AS exams 

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  • Created by: KatyN
  • Created on: 01-06-16 10:35

Gametes - Mammalian

Gametes are adapted for their role in sexual reproduction.

  • Ovum is a large cell incapable of movement. It is wafted by cilliated cells along an oviduct form the ovary to the uterus. It has a protein and lipid rich cytoplasm as food reserves for the developing embryo and has a surrounding layer of jelly-like substance called the zona pellucida and follicle cells surrounding that.
  • The sperm is tiny in comparison to the Ovum (5um compared to 100um of ovum) and is motile due to its flagellum powered by the ATP provided by mitochondria. The sperm also contains a large lysosome at its head called an acrosome, full of digestive enzymes to help digest through the follicle cells and zona pellucida in the Ovum. 

                            (http://www.old-ib.bioninja.com.au/_Media/egg-and-sperm_med.jpeg)

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The fundamental difference

The main difference between gamete cells and somatic cells is the fact that gametes are haploid, whereas somatic cells are diploid. This means that gametes hold half of the number of chromosomes than a normal cell. Having half the amount of chromosomes means that reproduction (aka the fusion of two cells ) creates a cell with the correct amount of chromosomes. 

For example, Humans have 46 chromosomes per cell. If reproduction occured with two diploid cells then the offspring would have 92 chromosomes, and then their offspring would have 182 chromosomes-  Haploid cells keep the number of chromosomes consistent

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Meiosis produces gametes

Meiosis is similar to mitosis, except it includes 2 divisions instead of just 1. The first division is the same as mitosis, with DNA replication, chromsomes condensing and attaching to spindles at the cell equator before being pulled apart by shortening spindle fibres. 

In the second division, the pairs of sister chromosomes are seperated, halving the chromosome number. As a result four new daughter cells that are genetically different from each other are produced. 

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Meiosis produces gametes

Meiosis is similar to mitosis, except it includes 2 divisions instead of just 1. The first division is the same as mitosis, with DNA replication, chromsomes condensing and attaching to spindles at the cell equator before being pulled apart by shortening spindle fibres. 

In the second division, the pairs of sister chromosomes are seperated, halving the chromosome number. As a result four new daughter cells that are genetically different from each other are produced. 

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Meiosis produces gametes- IMG

(http://barakleech.weebly.com/uploads/1/7/0/8/17080900/333508092.gif)

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Independent Assortment

The four daughter cells produced in mieosis all have completely different genetic combinations. The independent assortment of the chromosome pairs in the parent cells as they line up  during metaphase is a source of genetic variation and produces different combinations of chromosomes from the parents to go in each cell. In an organism with a large amount of chromosomes )like a human's 23 pairs) the number of possible combinations is so large it is unlikeley that two daughter cells would have the same genetic combination. 

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Crossing Over

During the first division of meiosis, homologous pairs of chromosomes meet and pair up with other homologous pairs. Two of the chromatids in each pair then cross over and twist around each other. These twisted parts break off of their original chromatid and rejoin the other chromatid, recombining the genetic material.

The chromatids still contain the same genes but now have a different combination of alleles which means that each of the four new cells formed in meiotic division contain chromatids with different alleles


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