Summary 6.1.1 Cellular Control Revision Notes (OCR A)
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A Level Biology A for OCR Student Book
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6.1.1 CELLULAR CONTROL
a. types of gene mutations and their possible effects on protein
production and function
Genetic mutation – a change to the genetic material.
Mutations are more likely to occur during DNA replication when the bases are
exposed.
They are made more likely by mutagens.
o Physical mutagens (e.g. X-rays) can break one or both DNA strands.
o Chemical mutagens (e.g. tar, free radicals) chemically alter bases in
DNA, changing the base sequence.
o Biological agents (e.g. viruses, base analogs, alkylating agents).
Mutations associated with mitosis are somatic mutations and are not passed
to offspring.
o These may be associated with the development of cancerous tumours.
Mutations associated with meiosis and gamete formation may be inherited by
offspring.
Gene mutations may affect protein production and function.
Types of gene mutation:
There are three main types of DNA mutation: substitution, insertion and
deletion.
o Substitution – one or more base pairs are substituted for another.
o Insertion – one or more nucleotides are inserted from a length of DNA.
o Deletion (indel) – one or more nucleotides are deleted from a length of
DNA.
Insertion and deletion mutations may cause a frameshift.
Point mutations:
There are three types of point mutation: silent, missense and nonsense.
Silent mutation – a point mutation involving a change to the base triplet,
where that triplet still codes for the same amino acid.
o This is possible due to the degenerate nature of the genetic code.
o All proteinogenic amino acids, except methionine, have more than one
base triplet code.
o The primary structure of the protein is unaffected.
Missense mutation – a change to the base triplet sequence that leads to a
change in the amino acid sequence in a protein.
o This changes the protein’s primary structure, so can alter the protein’s
overall shape and ability to carry out its normal function.
o A conservative mutation occurs when the new amino acid has similar
properties to the original. It is likely to cause less serious effects than a
non-conservative mutation.
o E.g. a missense mutation in the gene for haemoglobin results in
deoxygenated haemoglobin crystallising in erythrocytes, leading to
sickle cell anaemia.
Nonsense mutations – a mutation that alters a base triplet so that it becomes
a stop triplet.
, o This results in a truncated protein that will not function normally.
o This abnormal protein will likely be degraded within the cell.
o E.g. Duchenne muscular dystrophy is caused by a nonsense mutation.
Insertions and deletions:
If nucleotide base pairs, not in multiples of three, are inserted in the gene or
deleted from the gene, all the subsequent base triplets are altered.
o This is a frameshift. It can be caused by both insertions and deletions.
o This is due to the non-overlapping nature of the genetic code.
When the mRNA from such a mutated gene is translated, the amino acid
sequence after the frameshift is severely disrupted.
Insertions or deletions of a triplet of base pairs result in the addition or loss of
an amino acid.
o This is not a frameshift.
Chromosome mutations (just be aware of this):
These affect the whole chromosome or a number of chromosomes.
They can be caused by mutagens and normally occur during meiosis.
o Deletion – a section of chromosome breaks off and is lost within the
cell.
o Duplication – sections get duplicated on a chromosome.
o Translocation – a section of one chromosome breaks off and joins
another non-homologous chromosome.
o Inversion – a section of a chromosome breaks off, is reversed, and then
joins back onto the chromosome.
o Non-disjunction – the chromosomes do not separately correctly during
meiosis, so that gametes have more or fewer than the haploid number
of chromosomes.
The possible effects of gene mutations:
Mutations can have beneficial, neutral or harmful effects.
The large majority of gene mutations are neutral; with neither beneficial nor
harmful effects.
o The degenerate nature of the triplet code means that genetic
mutations may have no effect on the sequence of amino acids that are
coded for.
o E.g. the mutation that causes detached ear lobes.
Some are beneficial.
o E.g. a mutation that increases melanin production will be useful in
areas of high light intensity, where the melanin helps to protect against
sunburn and skin cancer.
Some are harmful.
o Genetic diseases e.g. Huntington’s disease, sickle cell anaemia.
b. the regulatory mechanisms that control gene expression at the
transcriptional level, post-transcriptional level and post-translational
level
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