Ib Biology Hl Notes
Genes and Genetics
1. 2.
each chromosome exists as two genetically identical chromatids attached to their centromere. Each chromosome appears as two chromatids attached to a centromere. In the first meiotic division chromosomes align in homologous pairs. Points of contact form between members of the same homologous pair. The points of contact or crossing over between members of a homologous pair are the chiasmata.
3. The homologous pairs move to the equator of the cell. Equal lengths of the chromatids of the same homologous pair have broken off and crossed over. The chromosomes align at random and independantly. Only one possible arrangement is shown. 4. Homologous pairs align at random at the equator of the cell. This shows the early separation of the chromosomes of each of the homologous pairs. 5. Homologous pairs are separated. This shows the cell at an early stage of meiotic division. The cell membrane is starting to pinch inwards. 6. The cell divides to form two cells each with a haploid set of chromosomes. Cell division is complete and two cells containing a haploid set of chromosomes are formed. 7. In the second meiotic division the chromosomes in each cell align independently and randomly at the equator of the cell. 8. The chromatids in each cell are separated. The chromatids are pulled towards the opposite poles of the cell. 9. The cells divide. The cell membrane is starting to pinch inwards. 10. Cell division is complete, resulting in the formation of four cells each with the haploid number. 11.
Meiosis - Functions
Halving the chromosome number - meiosis consists of two nuclear divisions (meiosis I and meiosis II) but the chromosomes replicate once. Producing four daughter cells. Each are haploid Producing Genetic Variety - through prophase I and through random assortment during metaphase I. In addition, random fertilisation also produces variety since any gamete has an equal chance on combining.
Crossing Over - during prophase
1. 2.
each chromosome exists as two genetically identical chromatids attached to their centromere. Each chromosome appears as two chromatids attached to a centromere. In the first meiotic division chromosomes align in homologous pairs. Points of contact form between members of the same homologous pair. The points of contact or crossing over between members of a homologous pair are the chiasmata.
3. The homologous pairs move to the equator of the cell. Equal lengths of the chromatids of the same homologous pair have broken off and crossed over. The chromosomes align at random and independantly. Only one possible arrangement is shown. 4. Homologous pairs align at random at the equator of the cell. This shows the early separation of the chromosomes of each of the homologous pairs. 5. Homologous pairs are separated. This shows the cell at an early stage of meiotic division. The cell membrane is starting to pinch inwards. 6. The cell divides to form two cells each with a haploid set of chromosomes. Cell division is complete and two cells containing a haploid set of chromosomes are formed. 7. In the second meiotic division the chromosomes in each cell align independently and randomly at the equator of the cell. 8. The chromatids in each cell are separated. The chromatids are pulled towards the opposite poles of the cell. 9. The cells divide. The cell membrane is starting to pinch inwards. 10. Cell division is complete, resulting in the formation of four cells each with the haploid number. 11.
Meiosis - Functions
Halving the chromosome number - meiosis consists of two nuclear divisions (meiosis I and meiosis II) but the chromosomes replicate once. Producing four daughter cells. Each are haploid Producing Genetic Variety - through prophase I and through random assortment during metaphase I. In addition, random fertilisation also produces variety since any gamete has an equal chance on combining.
Crossing Over - during prophase