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AQA Biology A level Diversity essay
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Essay on the causes and importance of diversity and variation in organisms.During meiosis, 2 processes occur which increase genetic diversity between daughter cells.
In prophase I, when homologous chromosomes pair up, crossing over can occur.
Chromatids wrap around each other and break off at the chiasmata. Alleles coding for the
different versions of the same characteristic are exchanged between the 2 chromosomes so
new allele combinations are formed. In metaphase I, homologous chromosomes line up at
the equator of the cell randomly - the orientation of each pair of homologous chromosomes
is independent of other pairs. In anaphase I, chromosomes are pulled apart to opposite
poles meaning that the assortment of alleles in each daughter cell is dependent on the
random position of chromosomes during metaphase I. Genetic variation between
generations is further caused by random fertilisation - any egg cells can be fertilised by any
sperm cell in mammals. Genetic variation is important in gametes as it prevents the chance
of inbreeding. Inbreeding increases the risk of recessive gene disorders as it causes more
recessive alleles to be homozygous since the gametes of parents are genetically similar.
Recessive gene disorders only occur when alleles are recessive homozygous.
Mutations are another cause of genetic diversity in organisms. Mutations are random
irreversible changes in DNA base sequences. Mutagenic agents promote mutations and
include different ionising radiations, viruses, and chemicals. Due to the genetic code being
degenerate, not all mutations (e.g. substitution mutations) are consequential, however some
mutations (e.g. addition, deletion) can cause frame shift which disrupts triplet codes for the
entire length of the base sequence following the mutation. Translocation mutations occur
when a piece of one chromosome breaks off and attaches to another chromosome or 2
chromosomes trade pieces. Mutations can be beneficial to a species long term as it can
allow species to evolve adapting to changes in their environment for survival. When
selection pressures change in a population, pre-existing genetic variation due to mutations
means that some members of the population are more adapted to survive to reproduce (e.g.
mutation in peppered moths that allowed them to camouflage better - hiding from predators).
This means that these select members survive to reproduce, increasing the allele frequency
of the beneficial allele in the next generation. If mutations did not cause existing genetic
variation, an entire species could be wiped out by a change in selection pressures.
Epigenetic changes occur when environmental factors cause reversible changes to gene
activity without altering the DNA base sequence. Increased DNA methylation, can lead to
decreased transcriptional activity in cells, which can lead to decreased expression of tumour
suppressor genes which regulate apoptosis and slows down cell division, leading to cancer.
This is because positively charged methyl groups attach to the cytosine-guanine binding
sites and are attracted to the negatively charged DNA. This causes the chromatin to be more
condensed so it is more difficult for transcription factors to bind to the promoter region to
initiate transcription. Increased acetylation can lead to increased transcriptional activity in
cells, leading to over-expression of proto-oncogenes which stimulate cell division into
oncogenes, leading to cancer. This is because negatively charged acetyl groups bind to
positively charged lysine on histones, removing a bond between the negatively charged DNA
and the histone. The DNA is less tightly wrapped around the histone allowing transcription
factors to access the promoter regions more easily. Epigenetics are important in scientific
research to develop ways to control gene expression in organisms.
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