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.
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 *****
Penetrate the zona pellucida layer around the ovum
- OVA: Few, fat, fixed.
Higher proportion of material
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.
- 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.
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.
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