3.2 & 3.3

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  • Created by: Aniqa
  • Created on: 05-01-13 23:06

MEIOSIS

Nuclear division for the production of gametes.

Allows GENETIC VARIATION: differences that exist between genetic material in individuals.

1. CROSSING OVER:

- Homologous pairs come together

- 2 chromatids twist around eachother 

- Parts break off and recombine onto other chromatids.

2. INDEPENDANT ASSORTMENT:

- Gametes produced

- 4 daughter cells produced with different combinations of chromosomes.

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Meiosis diagrams

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GAMETOGENESIS

Gametes which make sexual production possible, restore the full complement of chromosomes and allow genetic variation.

  • SPERM: Many, mini and mobile.

Produced in testis

Suspended in semen

Penetrate the zona pellucida layer around the ovum

  • OVA: Few, fat, fixed.

Contain food

Supplies nutrients

Higher proportion of material

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FERTILISATION: Humans

1. ACROSOME REACTION: Digestive enzymes are released, chanel through zona pellucida to penetrate to cell surface membrane.


2. MEMBRANE FUSE: Fuse to allow sperm nucleus to enter cytoplasm.


3. CORTICAL REACTION: Vesicles move and fuse with membrane, releasing contents through exocytosis causing changes on surface of eggs.


4. MEIOSIS RESTART: PMAT, MAT


5. FERTILISATION: Restoration of full complement of chromosomes. 

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FERTILISATION: Plants

  • Pollen tube begins to grow out of cells through stigma into the style
  • Grows towards ovary by elongation
  • Generative nucleus travels down
  • Divides by mitosis and forms 2 male nuclei
  • Passed to ovule for double fertilisation

- One forms diploid zygote

- One fuses with nuclei of two polar bodies; forms Endosperm nucleus.

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CELL DIFFERENTIATION

TOTIPOTENCY: Ability to produce all cell types including specialised and extraembryonic cells.

PLURIPOTENCY: Ability of a stem cell to produce specialised cell but not extraembryonic.

Undifferentiated become differentiated when they develop structure, due to differential gene expression.

STEM CELLS are UNDIFFERENTIATED: give rise to other cell types.

  • EMBRYONIC STEM CELLS: found in embryos

Easy to extract and grow. Rejection and infections are possible.

  • ADULT STEM CELLS: found in bone marrow

Rejection avoided, difficult to extract and infections possible. More difficult to produce cell types.

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TISSUE CULTURE: show totipotency

- Correct stimulus given to specialised cells

- Some genes activated or deactivated

- mRNA made from active genes

- Translated into proteins which modify the cell and control processes

- Cell becomes specialised

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