Cambridge International A Level Biology Revision Guide

Chapter 16: Inherited change
vision, by rapid, jerky movements of the eyes and by a
tendency to avoid bright light.
Mutations at several loci may be responsible for this
condition, but in its classic form the mutation is an
autosomal recessive and individuals that are homozygous
for the recessive allele show albinism. Such individuals
occur in about 1 in 17 000 births worldwide. However, the
condition is relatively common in some populations such
as the Hopi in Arizona and the Kuna San Blas Indians in
Panama. A different form of albinism, which affects the
eyes, but not the skin, is sex-linked.
A mutation in the gene for the enzyme tyrosinase
results in either the absence of tyrosinase or the presence
of inactive tyrosinase in the cells responsible for melanin
production. In these melanocytes, the first two steps of
the conversion of the amino acid, tyrosine into melanin
cannot take place. Tyrosine cannot be converted into
DOPA and dopaquinone.
tyrosinase
tyrosine → DOPA → dopaquinone → melanin
Tyrosinase is an oxidase and has two copper atoms
in its active site which bind an oxygen molecule. It is a
transmembrane protein and is found in the membrane of
large organelles in the melanocytes called melanosomes.
Most of the protein, including the active site, is inside
the melanosome.
Tyrosinases occur in plant as well as in animal tissues.
The action of the enzyme can be seen in the blackening of
a slice of potato left exposed to the air.
Huntington’s disease
So far, in the examples of the inheritance of human
conditions, the mutations have been inherited as recessive
alleles. Huntington’s disease (HD) provides an example
of a mutation that is inherited as a dominant allele.
This means that most people with the condition are
heterozygous and have a 1 in 2 chance of passing on the
condition to a child.
HD is a neurological disorder resulting in involuntary
movements (chorea) and progressive mental deterioration.
Brain cells are lost and the ventricles of the brain become
larger. The age of onset is variable, but occurs most
commonly in middle age, so that individuals may have
children before they know that they themselves have
the condition.
The mutation is an unstable segment in a gene on
chromosome 4 coding for a protein, huntingtin. In people
who do not have HD, the segment is made up of a small
number of repeats of the triplet of bases CAG. People
with HD have a larger number of repeats of the CAG
triplet. This is called a ‘stutter’. There is a rough inverse
correlation between the number of times the triplet of
bases is repeated and the age of onset of the condition: the
more stutters, the earlier the condition appears.
Gene control in prokaryotes
In both prokaryotes and eukaryotes, transcription of
a gene is controlled by transcription factors. These are
proteins that bind to a specific DNA sequence and control
the flow of information from DNA to RNA by controlling
the formation of mRNA.
To understand how gene expression in bacteria is
controlled, you must distinguish between structural genes
and regulatory genes.
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Genes that code for proteins required by a cell are
called structural genes. Such proteins may literally
form part of a cellular structure, but they may also
have some other role, such as acting as an enzyme.
Genes that code for proteins that regulate the
expression of other genes are called regulatory genes.
You must also distinguish between repressible and
inducible enzymes.
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The synthesis of a repressible enzyme can be prevented
by binding a repressor protein to a specific site, called
an operator, on a bacterium’s DNA.
The synthesis of an inducible enzyme occurs only when
its substrate is present. Transcription of the gene occurs
as a result of the inducer (the enzyme’s substrate)
interacting with the protein produced by the
regulatory gene.
The different roles of structural and regulatory genes can
be seen by looking at the control of gene expression in a
prokaryote using the lac operon. An operon is a length of
DNA making up a unit of gene expression in a bacterium.
It consists of one or more structural genes and also control
regions of DNA that are recognised by the products of
regulatory genes.
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