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Gene mutation
Any change to the structure of DNA is known as a mutation
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Mutations occurring during the formation of
gametes may be inherited
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Substitution of bases
This is a type of gene mutation where a nucleotide in a DNA molecule is replaced by another
nucleotide
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1
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The
final protein would almost certainly be significantly different and he protein could
not perform its normal function
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A mis-sense mutation:- arises when the base change results in a different amino acid
being coded for
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The
significance of this change will depend upon the precise role of the original amino
acid
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The protein may
therefore have a different shape not function properly
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A silent mutation:-occurs when the substituted base, although different, still codes
for the same amino acid as before
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In which most amino acids have more than one codon
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The genetic code is read in units of three with no overlaps
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This gene is now read in the wrong
three-base groups and the genetic message is altered
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The basic mutation rate
of 1 or two mutations per every 100 000 genes can be increased by outside factors known
as mutagenic agents or mutagens
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High energy radiation that can disrupt the DNA molecule
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Chemicals that alter the DNA structure or interfere with transcription
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One benefit is that it produces genetic diversity
which is necessary for natural selection and speciation
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In addition mutations that occur in
body cells rather than gametes can disrupt normal cellular activities, such as cell division
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In normal cells
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 Proto-oncogenes that stimulate cell division
 Tumour suppressor genes that slow cell division
Role of proto-oncogenes
In a normal cell, growth factors attach to a receptor protein on the cell-surface membrane
and, via relay protein in the cytoplasm ‘switch on’ the genes necessary for DNA replication
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These oncogenes
can effect cell division in two ways:1
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2
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The result is that cells divide too rapidly and a tumour or cancer develops
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A normal tumour suppressor gene will therefore maintain normal rates of cell division and
prevent the formation of tumours
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It stops inhibiting cell division, which therefore increases
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Not all
tumours are harmful (malignant) some are harmless (benign)
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Every gene is capable of making everything that the
body can produce
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 Some genes are permanently expressed in all cells – such as those of enzymes
needed in cells for respiration
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 Differentiated cells differ from one another because they each produce different
proteins
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 An organism develops from a single fertilised egg, which has the ability to give rise to
all types of cells
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 The early cells that are derived from the fertilised egg are also totipotent
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The ways in which genes are prevented from expressing themselves include:1
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Breaking down mRNA before its genetic code can be translated
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The nucleus contains the gene so if the nucleus is broken
down or lost once they are matured they can no longer develop into other cells
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Stem cells are undifferentiated dividing cells that occur in adult animal tissues and need to be
constantly replaced
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Under certain conditions the stem cells can develop into any other type of cell
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As well as being found in mature
animals, stem cells are found at the earliest stage of development and these cells are called
embryonic stem cells
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Human embryonic stem cells and the treatment of diseases
1
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3
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5
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These molecules are called
transcriptional factors
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 When it binds, it stimulates this region of DNA to begin the process of transcription
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 When a gene is not being expressed, the site on the transcription factor that binds to
DNA is blocked by an inhibitor molecule
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