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What is gene mutation?
Any change to the quantity or structure of the DNA of an organism is known as a mutation.
Any change to one or mole nucleotide bases, or any rearrangement of the bases, in DNA is
known as a gene mutation.
Causes of Mutations
Gene mutations can arise spontaneously.
However, they arise with a set frequency - typically around one or two mutations per
100,000 genes per generation.
This basic mutation rate is increased by mutagenic agents or mutagens, including:
high-energy radiation that can disrupt the DNA molecule
chemicals that alter the DNA structure or interfere with transcription
Substitution of Bases
The type of gene mutation in which a nucleotide in a DNA molecule is replaced by another
nucleotide that has a different base is known as a substitution.
Deletion of Bases
A gene mutation by deletion arises when a nucleotide is lost from the normal DNA
sequence. One deleted nucleotide creates a 'frame-shift' because the reading frame that
contains each three letters of the code has been shifted to the left by one letter. The gene
is now read in the wrong three-base groups and the genetic message is altered.
Genetic Control of Cell Division
Proto-oncogenes stimulate cell division. In a normal cell, growth factors attach to a receptor
protein on the cell-surface membrane and, via relay proteins in the cytoplasm, 'switch on'
the genes necessary for DNA replication. A gene mutation can cause proto-oncogenes to
mutate into oncogenes. These oncogenes can affect cell division in two ways:
The receptor protein on the cell-surface membrane can be permanently activated, so
that cell division is switched on even in the absense of growth factors
The oncogene may code for a growth factor that is then produced in excessive
amountsQ , again stimulating excessive cell division.


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