| Gametes (zi-goat) | - Sperm cells in men, egg cells in women
- Join together at fertilisation to form zygote, which divides into a new organism |
| Diploid number | - Normal body cells have a diploid no. (2n) of chromosomes
- Meaning each cell has 2 of each chromosome, 1 from mum 1 from dad |
| Haploid | - Gametes have haploid (n) no. of chromosomes
- There's only 1 copy of each chromosome |
| Fertilisation | - Any sperm can fertilise any egg, random
- Produces zygotes with different combinations of chromosome to both parents
- Mixing of genetic material in sexual reproduction increases genetic diversity within species |
| During fertilisation | - Haploid sperm fuses with haploid egg
- Making cell with normal diploid no. of chromosomes
- 1/2 these chromosomes from sperm, 1/2 from egg |
| Meiosis | - Type of cell division
- Takes place in reproductive organs
- Cells divide by meiosis are diploid at start
- Haploid at end, chromosome no. halves
- Without it, you'd get double no. of chromosomes, not good |
| Meiosis process (1) | - Before meiosis starts, DNA unravels + replicates
- So there's 2 copies of each chromosomes, chromatids |
| Meiosis process (2) | - DNA condenses, forming double-armed chromosomes
- Each made from 2 sister chromatids
- Sister chromatids joined in middle by centromere |
| Meiosis process (3) | - Meiosis I (first division)
- Chromosomes arrange themselves into homologous pairs |
| Meiosis process (4) | - These homologous pairs are then separated, halving the chromosome no. |
| Meiosis process (5) | - Meiosis II (second division)
- Pairs of sister chromatids that make up each chromosome are separated (the centromere is divided) |
| Meiosis process (6) | - Produces 4 haploid cells (gametes) that are genetically different |
| + Meiosis process (1) | - Interphase, chromosomes replicate, but not yet visible
- Prophase I, chromosomes condense become visible
- Late prophase I, HC arrange themselves in pairs |
| + Meiosis process (2) | - Metaphase I, HP line up on the equator of spindle
- Anaphase I, HC separate
- Telophase I, two daughter nuclei formed
- Cytokinesis I, cytoplasm divides |
| + Meiosis process (3) | - Prophase II, chromosomes become visible again
- Metaphase II, individual chromosomes line up on new spindle |
| + Meiosis process (4) | - Anaphase II, chromosomes separating
- Telophase II, 4 haploid nuclei formed
- Cytokinesis II, cytoplasm divides to form 4 haploid (gamete) cells |
| Main events during meiosis that lead to genetic variation | - Crossing over chromatids
- Independent segregation of chromosomes |
| Crossing over chromatids | - Crossing over of chromatids in meiosis I means that the 4 daughter cells have chromatids with different alleles
- Increasing genetic variation |
| Process of crossing over (1) | - The chromatids of each pair become twisted around 1 another |
| Process of crossing over (2) | - During this twisting process tensions are created
- Proportions of the chromatids break off |
| Process of crossing over (3) | - These broken portions might then re-join with the chromatids of its homologous pair, recombination |
| Process of crossing over (4) | - Usually it is same portions of HCs that are exchanged |
| Process of crossing over (5) | - Produces new genetic combinations of maternal + paternal alleles |
| Independent segregation of chromosomes | - Each pair of homologous chromosomes in cells made up of 1 chromosome from mum + 1 from dad
- When homologous pairs are separated in meiosis I, which chromosome ends up in which daughter cells is random
- So 4 daughter cells have completely different combinations of those maternal + paternal chromosomes
- Leads to genetic variation |
| Chromosome combinations in meiosis calculations | 2^n where n = no. of pairs of homologous chromosomes
(2^n)² where n = no. of pairs of homologous chromosomes |
| Differences between mitosis + meiosis (MITOSIS) | - Produces cells with same no. of chromosomes as parent cell
- Daughter cells are genetically identical to each other + parent cell
- Produces 2 daughter cells |
| Differences between mitosis + meiosis (MEIOSIS) | - Produces cells with 1/2 the no. of chromosomes as parent cell
- Haploid cells are genetically different from each other + parent cell
- Produces 4 haploid cells |
| + Independent assortment of homologous chromosomes | - HCs are randomly distributed to daughter cells, and different chromosomes segregate independently of each other
- So gametes have unique combinations of chromosomes
- Met 1 |
| + Crossing over of chromatids | - Pro 1
- Starts before the chromosomes condense
- HCs held together to form a bivalent
- Crossing point of sister chromatids is called a chiasma
- Leads to exchange of alleles |
| + Explain how meiosis leads to genetic variation of offspring? | - Independent assortment of HC
- Met 1
- Independent assortment of sister chromatids
- Met2
- Crossing over of HC (Pro 1)
- Forming recombinants
- By exchanging alleles |
