| Meiosis | 1. Meiosis produces haploid gametes
2. Gametes fuse during fertilisation to form a zygote. The diploid number is restored.
3. Ensures the diploid number remains the same after every generation. |
| What is meiosis | cell division to produces
4 daughter cells
gametes are haploid and genetically different |
| Diploid | pairs of chromosomes that match |
| Haploid | Chromosomes that do not match- half the number of chromosomes |
| Importances of meiosis | Produces sex cells which are HAPLOID
During fertilisation (egg and sperm fuse) the diploid number is restored
This ensures that the diploid number remains the same after every generation (DP = SAME)
Meiosis produces genetic variation in offspring which increases the chance of species survival if the environment changes adversely ( SURVIVAL) |
| What does random fertilisation of haploid gametes DO | further increases genetic variation within that species |
| Meiosis brief summary | Diploid cells (2n) divide and divide again into haploid cells (n) |
| Meiosis | 1. Meiosis produces haploid gametes
2. Gametes fuse during fertilisation to form a zygote. The diploid number is restored.
3. Ensures the diploid number remains the same after every generation. |
| Interphase | In interphase the following occurs:
1) DNA replication occurs
2) Organelles are synthesised
3) Increased protein synthesis occurs
4) Increased rate of respiration (to provide energy for processes 1-3) |
| Prophase | In prophase the following occurs:
1) Chromosomes condense so they become shorter and thicker.
2) Nuclear envelope breaks down.
IN MEIOSIS
BIVALENTS FORM CHROMOSOMES CAN GET TANGLED CAUSING CROSS OVER. |
| Metaphase | In metaphase the following occurs:
1) Spindle fibres form.
2) Centromeres attach to spindle fibres.
3) Chromosomes move along the spindle fibres to the middle of the cell. |
| Anaphase | In anaphase the following occurs:
1) Centromere splits.
2) Sister chromatids separate.
3) Sister chromatids pulled along spindle fibres to opposite poles of the cell. |
| Telephase | In telophase the following occurs:
1) Nuclei reform.
2) DNA becomes chromatin (loose).
3) Spindle fibres begins to break down.
4) Two genetically identical cells are formed. *CYTOKINESIS – the end of mitosis (where the cell splits to form two separate cells). |
| What causes genetic variation in meiosis | Cross over
independent assortment of homologous chromosomes |
| What is cross over | Bivalents form.
· Homologous chromosomes twist round each other.
· The point of cross over is called a chiasma.
· A section of DNA with a block of alleles breaks off, swaps position and reattaches (cross over).
· This creates new combinations of alleles |
| Independent assortment of chromosomes | Homologous chromosomes line up in pairs across the cell
· Maternal and paternal chromosomes line up randomly within each pair
· So that different combinations of chromosomes are produced in the gamete cells
· This creates different combinations of alleles. |
| What is chromosome non disjunction | Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis. CAUSES MUTATIONS |
| how does non disjunction occur | 1. Failure of a homologous pair of chromosomes to separate during the first division of meiosis
2. Failure of a pair of sister chromatids to separate during the second division of meiosis |
| Interphase known as the growth stage why | G1 – growth phase 1 (Increased protein and organelle
synthesis)
· S phase – DNA replication occurs.
· G2 – growth phase 2 |
| Mitosis brief summary | cell division to produces 2 genrtcally identical cells
1 division in one cycle
2 new cells produced
starting cell is diploid
new cell is diploid
all cells divide except sex cells |
| Meiosis brief summary | cell divisions to produce 4 genetically different sex cells
2 divisions in one cycle
4 new cells produved
starting cell diploid
new cell haploid = half number
only sex cells divide |
| Diploid = | pairs of chromosomes homologous |
| Haploid = | pairs of chromosomes but different |
