Genetic information, variation and relationships between organisms (AQA AS Biology) PART 2 of 6 TOPICS
Genetic diversity can arise as a result of mutation or meiosis:
Gene mutations involve a change in the base sequence of chromosomes. It can happen during DNA replication or during base deletion (a base is removed) or base substitution (a base is substituted for another). Both of these cause changes to the triplets which can cause it to code for a different amino acid. Sometimes base substitution does not change the amino acid that is coded by the triplet as the genetic code is degenerate.
Mutagenic agents include x-rays, gamma rays and alpha particles and includes an isotope of cobalt and caesium. UV radiation with a wavelength (λ) above 260nm causes complications in DNA replication.
Gametes (sperm and egg cells) are haploid which is the cells having one copy of each chromosome. In the end a zygote is made which is diploid meaning that half the chromosomes are from the mother and half the chromosomes are from the father. The zygote then undergoes meiosis where it starts at interphase and ends at telophase similar to mitosis. The two homologous pairs of chromosomes line themselves up on the equator of the cell. The cell align themselves up on each pole of the cell and create a spindle where each fibre attaches to each centromere. The spindle fibres contact pulling each homologous chromosome to each end of the cell. The cell then splits by cytokinesis. This process is called meiosis 1. Meiosis 2 follows through which is the same as mitosis. Four daughter cells are created at the end of the whole of meiosis which are haploid. NB: Bear in mind that meiosis 1 produces two cells which undergo meiosis 2 separately to produce another two daughter cells each totalling up to four.
Independent segregation is where the four daughter cells have different combinations of chromosomes from maternal and paternal sides. This leads to genetic variation. Crossing over of chromatids only occurs in meiosis 1 where the allele on each chromatid is swapped over on to the chromosome.
Mitosis creates two daughter cells which are genetically identical to each other and to the parent cell. Meiosis creates four daughter cells which are genetically different from one another and to the parent cell.
Random fertilization increases variation further as a random sperm can fertilise a random egg.
NB: Notice that in the picture for meiosis where the cell has homologues aligned independently, the big red chromosome has a blue allele and the big red chromosome has a red allele showing that crossing has taken place to make the alleles swap over.