Mutations

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Mutations
A mutation is a change in the amount of, or arrangement of, the genetic material of a cell.
DNA is a double helix molecule and its structure is designed to make it very stable and prevent
mistakes in its genetic code.
DNA mutations can occur during DNA replication either by mitosis or meiosis just before
nuclear division
When a mutation occurs as a cell divides by mitosis the mutation is not passed on, but the
mutation may contribute to the ageing process or lead to cancer (Known as SOMATIC
MUTATIONS)
Mutations within the DNA may be inherited from cells that have divided by mitosis
Certain substances can also cause mutations (MUTAGENS) which include: tar, UV light, X-rays
and gamma rays
A gene mutation is a change to DNA and it could be by base deletion, addition or substitution,
inversion or repeat of a triplet.
A chromosome mutation is a change to the structure of a chromosome, such as deletion, inversion
or translocation or a change to the number of chromosomes in a nucleus.
DNA mutations are changes to the genes due to changes in nucleotide base sequences
Types of DNA mutations
substitutions (also known as point mutations) involve one base pair replacing another, so
only one base has changed
insertions occur where an extra nucleotide base is inserted into the genetic code
deletions occur where one base is removed from the genetic code
Insertions and deletions cause a frameshift.
A frameshift is where the inserted or deleted base pair changes the entire coding of the DNA
molecule as the triplet codons all change
Types of substitution mutations
Missense mutations: different amino acid is formed
Nonsense mutations: stop codon initiated and protein chain is shorter
Silent mutations: there is a change in the DNA base sequence, but the amino acid coded for
remains the same (so there is no effect).
Many genetic diseases are the result of DNA mutations such as sickle-cell anaemia and cystic
fibrosis
in 70% of cases of Cystic Fibrosis, the mutation is the deletion of a triplet of base pairs, deleting
an amino acids from the sequence of 1480 amino acids in the normal polypeptide.
Sickle cell anaemia results from point mutation on triplet 6 of the gene for the -polypeptide
chains of haemoglobin
this causes the amino acid valine to be inserted at this position of the polypeptide chain, in
place of glutamic acid
growth promoting genes (proto-oncogenes) can be changed into oncogenes by a point
mutation that alters the ability of the proto-oncogene to be switched off. They remain
permanently switched on, oncogenes promote unregulated cell division which leads to a tumour

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Huntington disease results from an expanded nucleotide repeat-a stutter. The normal gene for
Huntington protein has repeating CAG sequences. If these expand to above a threshold number,
the protein is altered sufficiently to cause Huntington disease.…read more

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