Meiosis
- Created by: Jacqui2
- Created on: 09-03-17 14:53
Chromosomes
- Single thread like structure
-Two identical chromatids
- Before cell divison, chromosomes are not visible and are dispersed as chromatin
- Just before cell divison, each chromosome condenses and becones visible
- Appear as two threads - two identical chromatids
Cells have 46 chromosomes - 23 homologous pair
- Diploid (2n)
- Haploid (n) - sex cells
Karyotype
- Cutting out chromosomes and putting them in pairs
- i.e Homologous pairs (identical genes - eyes)
Mitosis
- Doubles the number of cells without changing the genetic information
Important for:
- Producing an exact copy/clone/genetically identical
- Growth
- Takes place all over the body in animals
- Confined to meristems is plants
- Repair of tissues:
- Your skin and the cells lining your gut are constantly dying and being replaced
- Asexual reproduction results in offspring that are identical to the parent (yeast)
Mitosis can result in the production of large numbers of offspring in a realtively short period of time
Interphase (Mitosis&Meiosis)
Frist Growth (G1)
- The cell is acively growing
- Protein synthesis and the formation of new organelles take places
Synthesis (S)
- DNA and centrioles replicates and chromatids are formed
- Daughter DNA/chromatids are gentically identical
Second Growth (G2)
- Nuclear division
- ATP store built up and the spindle proteins are synthesised
- Chromosomes are thread llike in appaerance
- Mitochondria divide and the cell continues to grow
Prophase I (Mitosis&Meiosis)
- Chromosomes shorten and thicken
- Appearing as paired chromatids
- Homologous chromosomes pair up with ech other to form bivalents
- Chiasmata are seen where non-sister chromatids have broken and exchanged material with each other. This is know as crossing over and the resulting chromatids are known as recombiants. Chiasmata can be seen becase homologous cromosomes are chromatids start to repel each other.
- Nuclear envelope distergrates and the nucleolus disappears
- Pairs of centrioles move to opposite poles in animal cells
- The spindle form
Metaphase I (Mitosis&Meiosis)
Homologous chromosome pairs align themseves on the equator of the spindle
They are attached to the spindle via the centromeres
Ananphase I (Mitosis&Meiosis)
Homologous chromosome pairs are pulled apart to opposite poles at the centromeres
Shortening of the spindle fibres is due to the disassembly of the microtubules
Telophase I (Mitosis&Meiosis)
- Chromosomes (composed of two sister chromatids) reach respective poles and two new nnuclei form
- Chromosomes return briefly to interphase appearance- uncoiling to form chromatin
Prophase II (Meiosis)
Each daughter nucleus undergoes a second meiotic division at 90 degrees to the first- the spindle apparatus is formed at 90 degrees to the first one
Metaphase II (Meiosis)
Each chromosome (consiting of a pair of chromatids) aligns itself on the equator
Anaphase II (Meiosis)
Chromatids are pulled apart and move to opposite poles of the cell, they now form the chromosomes of the new daughter cellls
Telophase II (Meiosis)
Spindle disappears, nucleoli and nuclear membranes are reformed
Cytokinesis (Mitosis&Meiosis)
- Cytoplasm divides and a clevage furrow is formed
- Resulting of shortening of fibre of the cytoplasm
- Vesicles forming along the equatorial plate fuse
Cancer Check
- During G1 and G2, time is allocated for the cell to monitor the internal and external environment. (A final checkpoint is also present at the end of Metaphase)
- This only allows healthy cells to proceed
- If during any of these stages a fault is detected, the cell enter G0 and the cell cyclle is stopped
- Uncontrolled cell division results in the production of tumours - cancer
- Other genes are responsible for checking DNA damage and force the cell to terminate via autolysis
Cancer Treatments
- Vincristine - mitotic poison
- It binds to a protein called tubulin (constructs microtubules of spindle), therefore preventing anaphase
- Can cause side effects as a few healthy cell types are also affected
- It binds to a protein called tubulin (constructs microtubules of spindle), therefore preventing anaphase
- 5'Fluorouracil - inhibitor of an enzyme necessary for DNA replication
- Distrupts the production of nucleotides carying the nitrogenous base - Thymine
- Reffered as an antimetabolic as it is an S phase inhibitor
Genetic Variation
1. Random Segregation of Chromosomes
- During metaphase I pairs algin at the equator in two ways. Random order / chance
- During fertilisation any male gamete can join with any female gamete
- Thousands of millions of new gamete combinations are possible
2. Crossing Over
- Curing prophase I, chromatids twist around eachother which creates tension
- Corrseponding fragments of chromatid may get swapped
- Chismata - Non-sister chromatids breaks and rejoin
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