Cell Division and Reproduction

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Cell Division and Reproduction


  • The genetic material responsible for hereditary and variation is found in the nucleus
  • The nucleus contains long- thread like structures called chromosomes which mainly consist of DNA and histones. 
  • Individual chromosomes are only visible when a cell is divided, when it isn't only a mass can be seen known as chromatin.
  • Human body cells have 46 chromosomes in each nucleus and such a cell is described as diploid
  • The egg and sperm cells only have one set of chromosomes which is the haploid number 23.
  • The diploid number is the number of chromosomes in a zygote.
  • In body cells the chromosomes can be arranged in pairs, the two members of each pair are identical so are described as homologous pairs. 
  • Every chromosome has a centromere
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  • Produces 2 identical daughter cells.
  • The cell division of growth
  • The bulk of the cell cycle is spent in interphase. The organelles replicate and the cell doubles in size. 
  • Prophase- 

1) Longest stage of division

2) The chromosomes become visible because the chromatin fibres shorten and thicken

3) Each chromosome consists of 2 chromatids

4) The centrioles move to opposite poles

5) The nucleolus disappears and the nuclear membrane breaks down

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Mitosis 2

  • Metaphase - 1) The chromosomes assemble at the equator of the spindle

2) They become attached to the spindle at their centromeres

  • Anaphase - 1) The centromeres divide in two and the spindle fibres pull the daughter centromeres apart. The separated chromatids are pulled along behind.
  • Telophase - 1) The chromosomes reach the poles of the spindle

2) A nuclear membrane reforms around each of the two groups of chromosomes and the nucleoli disappear.

3) A membrane is formed

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Meiosis 1

  • During sexual reproduction, in the formation of the gametes, a reduction division called meiosis takes place
  • The daughter cells have a haploid number of chromosomes.
  • Meiosis is 2 divisions
  • To find meiosis it is necessary to look inside anthers and ovules
  • Meiosis is preceded an interphase stage during which the genetic material replicates.
  • Prophase 1

1) The chromosomes shorten and thicken so become visible

2) Homologous chromosomes pair together, a four stranded structure is thus formed

3) Non sister chromatids become joined called chiasmata

4) The chromatids may break and recombine crosswise which leads to genetic variation

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Meiosis 2

  • Metaphase 1

1) The pairs of chromosomes line up on the equator of the cell

2) The centromeres attach to the spindle

  • Anaphase 1

1) The spindle fibres shorten and pull the homologous chromosomes apart. 

2) One of each pair of homologous chromosomes is pulled to one end of the cell

  • Telophase 1

1) The chromosomes group together at opposite poles

2) Nuclear membranes form around each group

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Meiosis 3

  • Meiosis 2

1) This is similar to mitosis and during this phase the chromosomes line up on the equator and the chromatids separate

2) There are now 4 identical daughter cells each with a haploid nucleus, these are called gametes.

  • Meiosis is important as it leads to genetic variation. It does this by the production of haploid cells for fertilisation, the random assortment of chromosomes in metaphase and anaphase 1 and the crossing over of chromatids in prophase 1. 
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Mammalian Gametes


  • The human sperm consists of 3 main regions
  • The head contains a little cytoplasm filled with the nucleus. It is haploid
  • At the front of the sperm head is a thin cap called the acrosome which contains lysosomes and digestive enzymes
  • The mid-piece contains closely packed mitochondria which provides ATP for locomotion. The flagellum of the tail has contractile fibrils which propels the sperm forwards.


  • The egg cells has a much bigger volume then a sperm, egg cell has a haploid nucleus
  • It has a dense cytoplasm with mitochondria, rough ER, cortical granules and yolk droplets. 
  • The cortical granules contain enzymes that alter the structure of the outer egg when fertilised
  • The yolk droplets provide food for the embryo. 
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Mammalian Fertilisation

  • On it's journey to the egg the sperm undergoes capacitation. This makes the plasma membrane more permeable and makes the acrosome develop
  • The ovum is surrounded by follicle cells which form the corona radiata and a jelly layer called the zona pellucida.
  • On contact the acrosome reaction takes place. Enzymes are released from the acrosome which digest the zona pellucida.
  • The sperm fuses with the ovum membrane
  • The sperm nucleus shoots into the ovum
  • There is a change in the electrical potential which weakly blocks further fusion.
  • Enzymes released from lysosomes in the ovum thicken the jelly layer, preventing entry of more sperm
  • Nuclei fuse
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Plant Fertilisation

  • The male gametes are found inside the pollon grains which are made in the anthers
  • The nucleus of the pollon grain now divides into two by mitosis to form a pollon tube nucleus and a generative nucleus. 
  • Pollen grains are carried from one flower to another during pollination and are deposited on the stigma of the flower
  • Inside the female carpel of the flower is an ovary. Here one of more ovules develop. Inside each ovule is an embryonic sac with a female gamete nucleus.
  • When a pollon grain has landed on the stigma a  pollon tube is made by the pollon tube nucleus.
  • The male gametes then pass down this tube
  • The generative nucleus in the pollon grain divides to form two male nuclei.
  • The pollon tube enters an ovule
  • The pollon tube nucleus degenerated and the tip of the tube bursts open.
  • One nucleus fuses with the female gamete nucleus forming a zygote.
  • The other nucleus fuses with 2 polar nuclei forming an endosperm which is a food source.
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