Sexual reproduction and cell specialisation

gametes and mieosis

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Sexual reproduction overview

  • most common way of reproduction
  • Sexual reproduction is the prouction of a new individual resulting from the goining of two specialised cells called gametes. the individual which results is not genetically identical to the parents, but contains a genetic information from both.

Disadvantages

  • more risky because it relies on two gametes meeting, it is not easy to find a mate
  • more expesive fro the bodies resources because it involves special sex organs

Advantages

  • increases genetic variety, in a changing environment it is more likely that one or more of the offspring will have a combination of dna to help it survive going on to reproduce
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What are gametes

  • in the nucleus the cell contains 46 pairs of  chromosomes called a diploid cell (2n) - number of chromosomes are characteristic of the species.
  • however if 2 diploid cells combined to make a new organism it would be overloaded with genetic infomation.
  • to avoid this haploid (n) cells are created called gametes formed with only one set of chromosomes half 26
  • sexual reproduction occurs when two haploid cells come into contact and fuse to form a new diploid cell called a zygote - process is fertilizaion
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The formation of gametes

  • formed in the sex organs sometimes called Gonads
  • in flowering plants the sex organs are the anthers in the male which produce the gameta pollen and Ovaries in the female produce the gamete ovules.
  • in animals the male gonads are the testes which produce the gamete sperm or spermatozoa. and the female gametes are the overies which produce the gamete ova or eggs.
  • male gametes are: many, mini, motile
  • female gametes are:  few, fat, fixed
  • in bacteria genetic infomation is exchanged throught a strand of cytoplasm called a sex pilius
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Meiosis in the formation of gametes

  • to bring about the reduction int he 46 chromosomes a process of meiosis is used to divide the diploid cell into a haploid cell containing 23 chromosomes (gamete)
  • it happens only in the sex organs
  • in animals the gametes are formed directly from meiosis in plants meiosis forms special cells called microspores in males and megaspores in females which go on to produce gametes.
  • meiosis basis of variation that allows species to evolve
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meiosis the process

like a double circuit of mitosis division:

  • Prophase 1 : each pair of homologus chromosome appears in the condensed form with two chromatids
  • Metaphase 1 : the spindles form and the pairs of chromosomes line up in the middle down the metaphase plate crossing over occurs.
  • Anaphase 1 : one chromosome ( a pair of chromatids) from each homologus pair move to each end of the cell as a result the chromosome number in each cell is half that of the original.
  • Telophase 1 : the nuclear membrane reforms and the cells begin to divide
  • Metaphase 2 : new spindles are formed and the chromosomes (still made up of pairs of chromatids) line up along the metaphase plate (middle) of the cell
  • Anaphase 2 : the centrometeres now divide and the chromatids move to opposite ends of the cell
  • Telophase 2 : nuclear envelope reforms , the chromosomes return to their interphase state and cytokinesis occurs, giving four daughter cells each with half the chromosome number of original diploid cells.
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(http://www.revisionworld.co.uk/files/meiosisphase1%20copy.jpg)

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crossing over in meiosis

  • crossing over happens in Metaphase 1
  • the homologus pair of chromosomes which line up next to each other cross there single chromatids over breaking and recombining with the chromatid of the chromosome next to it
  • this process introduces further variation to the gametes

(http://media-3.web.britannica.com/eb-media/75/94975-036-651CE673.gif)

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the importance of meiosis in introducing genetic v

two main ways variation is introduced :

independant or random assortment:

  • chromosomes that came from the individual's parents are distributed into the gametes so into their offspring at random
  • crossing over

Crossing over or recombination

  • when the maternal chromatids are 'cut and stiched' together. The point where the chromatids break are called chiasmata. These are important in two ways to exchange genetic material. The second is through errors in the process leads to further mutation.
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