Genetic informtion, variation and relationships between organisms AQA AS Biology PART 2 of 6 TOPICS: Genetic diversity can arise as a result of mutation or during meiosis

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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 () 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

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