Cell division and reproduction assessment statements

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  • Created by: brad
  • Created on: 13-05-13 17:22

Interphase and Mitosis

Interphase 

This can be split in to three parts: G1, S where DNA replication takes place and G2. 

Mitosis 

This can be set in to four parts: 

Prophse: 

  • Chromatin coils and condenses to form indivdual chromatds. 
  • The nucles envolope, pores and nucleolus disappear
  • The chromatids join up to form chromosomes  
  • The centrioles start to move to the polls of the cell and the spindle fibres start to form 

Metaphase:

  • The chromosomes line up next to each other 
  • The spindle fibres completely attach to the individual chromatids 
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Interphase and Mitosis cont.

Anaphase:

  • The spindle fibres start to retract 
  • This breaks the centromere and individual chromatids are pulled to the polls of the cell 

Telophase: 

  • The spindle fibres and centrioles dissapear 
  • The chromatids form chromosomes 
  • The nuclear membrane, envolope and nucleolus reform 
  • The cell statrs to split 

Cytokenesis: 

  • The cell splits in two creating two diploid cells 
  • The chromosomes decondense to form chromatin 
  • The two cells are indentical becuase the DNA from one cell was replicated 

Mitosis is done for growth, repair, asexual production 

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Meiosis

Homogolous chromosomes: Chromosomes that have the exact same gene loci are the same size and length. 

Meiosis starts off with the original diploid cell, the chromosomes are in the form of chromatin and everythin is in place. Now the cell goes through interphase (G1, S, G2) where DNA replication happens so there are 4 chromosomes. Now meiosis can start: 

Prophase 1: 

  • Here chromosomes coil up and condense so that they are visible. The hoomogolous chromosomes line up through random assortment. Where the chromosomes line up is completely random.  
  • The nuclear membrane, pores, envolope and nucleolus dissapear 
  • The centrioles form and start to create the spidle fibres.
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Meiosis cont.

Metaphase: 

  • Now the chromosomes are fully lined up the centrioles have reached the polls of the cell and have attahed to the chromosomes. 
  • Crossing over occurs here where the end of one chromatid swaps with the end of another so the genes of that bit are different The point where they switch is called the chismata. This creates genetic variation.

Anaphase: 

  • The spindle fibres retract pulling two chromosomes to either end of the cell. 

Telophase 

  • This is where the cell starts to split to form two diploid nuclei 
  • The nuclear membrane, pores, envolope and nucleolus reform for the moment 
  • The cell then completely divides and there may be a period of interphase
  • The spindle fibres and centrioles have dissapeared 
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Meiosis cont. 2

Prophase 2: 

  • Everything starts to dissapear again
  • The chromosomes line up next to each other 
  • The centrioles start to form spindle fibres again

Metaphse 2: 

  • The chromosomes are fully lined up 
  • The spindle fibres are fully attached to the individual chromatids 

Anaphase 2: 

  • The spindle fibres retract and start to pull the individual chromosomes to the polls of the cell 
  • The centromere has been broken
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Meiosis cont. 3

Telophase 2: 

  • The cell begins to split
  • Everything reforms and the spindle fibres dissapear 
  • The chromotids have reached the edge of the cell and form individual chromosomes 

Cytokenesis: 

  • The cell fully splits in two forming four haploid gametes 
  • The chromosomes decondense to form chromatin 

This process leads to infinte genetic variation and this is due to random assortment and crossing over. Random assortment means you could get any number of genes from your mum or dad and crossing over shuffles genes about. 

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Fertalisation in mammals Adaptations

Fertalisation here is the joining of a sperm cell and an egg cell.

Adaptations in a sperm cell: 

  • It contains a large amount of mitochondria. This provides in with a large amount of energy so that it can moves it flagellum to be able to swim up to the egg. 
  • It has a flagellum, this allows it to swim up to the egg. It is quite long so it can generate a lrage amount of force. 
  • It is streamlined. This allows it to get to the egg quickly and effciently so that it still has energy left. 
  • It has an acrosome. This contains enzymes are able to break down the jelly coat of the egg. 

Adaptations of the egg cell:

  • It cortical granules. These excytose chemicals that are used in slow block polyspermy. 
  • It has a jelly coat. stops polysperm and can form a fertalisation layer 
  • It is very large which increases its chances of being fertalised 
  • It release chemicals that attract sperm to it which increases the chances of fertalisation.  
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Fertalisation in mammals

The stages in fertalisation in mammals:

  • The sperm starts to swim up to the egg cell and the two meet. 
  • It causes the acrosome reaction to start. The acromsome releases hydrolytic enzymes which break down the head or the sperm and the jelly coat. Actin is released to elongate the head of the sperm so that it is touching the egg membrane. 
  • Fast block polyspermy in the egg happens where the edges of the cell turn to positive to stop any other sperm getting in. 
  • Slow block polyspermy starts where the cortical granules exocytose Ca+. This turns the jelly coat hard and in to a fertalisation layer which will stop any other sperm getting in.
  • Sperm tail dissapears 
  • The sperm nucleus moves in to the egg and the two nucleus fuse together and a zygote is formed. 
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Fertalisation in Plants

Pollination: The movement of pollen to the stigma of another plant. 

Fertalisation: The meeting of a male and a female gamete 

Seed dispersal: Getting seeds away from the parent plant

The stages of fertalisation in plants: 

  • The pollen grain lands on the stigma of another plant
  • Species specific receptors identify it as a foreign grain 
  • Light, water, air and gravity cause the pollen grain to open which lets out a generative nucleus 
  • This goes in to the stigma and a tube nucleus creates a pollen tube through the style to the micropyle. The tube nuclues contains enzymes that break down the style to create a path that is easy to get down. They also provide nutrients for the pollen tube to grow. 
  • In the pollent tube to generative nuclues divides by Mitosis to form two male gametes
  • Once at the micropyle the tube nuclues degenerates and the two male gametes enter the ovule.  
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Fertalisation in Plants cont.

  • Once the gemetes are in the ovule one of them fuses with the female egg to form a zygote 
  • The other fuses with 2 polar bodies to create a food store for the zygote called an endosperm 

Fertalsiation is extremely important in sexual reproduction becuase it means that there is increased genetic variation within a species. This leads to better adaptations within a species that will allow it to survive and to breed more. If it can brees more then the adaptations get passed on and over time these better adaptations become the norm. 

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