Download Topic guide 7.7: Genes and evolution

Unit 7: Molecular biology and genetics
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77
Genes and evolution
What are your perceptions about mutations? Are mutations always harmful?
Can some be useful? Are some neutral?
Changes to genetic material have enabled evolution to occur.
On successful completion of this topic you will:
•• understand how changes in gene frequencies result in changes in
populations (LO5).
To achieve a Pass in this unit you will need to show that you can:
•• explain how mutations provide the variation necessary for evolution to
occur within a given species (5.1)
•• assess the significance of the Hardy–Weinberg principle as it relates to
evolution (5.2)
•• explain how in genetic drift random events change allele frequencies (5.3)
•• discuss factors influencing natural selection using appropriate
examples (5.4)
•• discuss the Human Genome Project (5.5).
1
Unit 7: Molecular biology and genetics
1Mutations
Key terms
Mutation: Change to genetic
material.
Mutagen: Agent that causes
a mutation – some are physical
(ionising radiation), chemical
(benzopyrene in tobacco tar) or
biological (retroviruses, transposons).
Mutations are changes to genetic material, either DNA or chromosomes. While
some mutations are copying errors during nucleic acid replication, mutations may
also be caused by mutagens. Point mutations alter the sequence of DNA bases by
substitution, insertion or deletion.
Some DNA sequences are highly susceptible to mutation and have a higher than
usual frequency of mutations. Such sequences are called hot spots.
Substitutions
Substitutions involve the alteration of a single nucleotide base pair in a piece
of DNA. Some are silent mutations as, due to the degenerate nature of the genetic
code, many amino acids are coded for by more than one base triplet. However,
molecular biologists now know that not all DNA codes for proteins and they are
now finding that silent mutations may be involved with certain genetic diseases,
such as Marfan’s syndrome, if they occur in a regulatory portion of DNA and lead
to changes in splicing or the structure of a regulatory length of RNA.
If the base substitution changes the triplet making it code for a different amino
acid, this is a missense mutation. It alters the primary protein structure and may
prevent it forming its functioning tertiary structure. If the substitution changes the
base triplet to a stop code, this is a nonsense mutation and leads to a truncated
protein that cannot function.
Sickle cell anaemia, phenylketonuria and some mutations to the CFTR gene are
the result of base substitutions. However, whereas sickle cell anaemia is always the
result of the same point mutation (A replaced by T on the 17th nucleotide so GAG
changing to GTG codes for valine instead of glutamic acid at the sixth amino acid
in the 159 amino acid beta haemoglobin chain), cystic fibrosis can be the result
of several different mutations. One of these is when T replaces C at nucleotide
1609, changing the CAG code for glutamine to TAG, which says ‘stop’, leading to a
truncated protein with only 493 instead of the normal 1480 amino acids.
Indels
An insertion is where an extra base pair is inserted and a deletion is loss of a base
pair. Both types of indel cause a frameshift and, during translation, the ribosome
reads the mRNA code differently as it still reads bases in triplets. Such a frameshift
can alter the whole structure of a protein (see Figure 7.7.1).
However, deletion does not always lead to a frameshift. Seventy % of cases of
cystic fibrosis are caused by deletion of a whole base triplet and the subsequent
loss of one amino acid from the polypeptide.
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Unit 7: Molecular biology and genetics
Figure 7.7.1: Some examples of
possible mutations and their effects
on a protein’s primary structure.
Normal
Point mutation
Missense
Point mutation
TTA
CGC AAT CCC
DNA
Met
Gln
Phe
Leu
Arg
Polypeptide
ATG CAG CAG CAG
TTT
TCA
CGC AAT CCC
DNA
Met
Phe
Ser
Arg
Polypeptide
CGC AAT CCC
Gln
Gln
Gln
Gln
Gln
Asn
Asn
Pro
Pro
TTT
TAA
Met
Gln
Phe
Stop
Point mutation
ATG CAG CAG CAG
TTT
TTG
CGC AAT CCC
DNA
Silent mutation
Met
Gln
Phe
Leu
Arg
Asn
Pro
Polypeptide
Point mutation
ATG CAG CAG CAG
TTT
TAC
GCA ATC
CC
DNA
Met
Phe
Tyr
Val
Frameshift
See Topic guide 7.6, section 1 for more
information on cystic fibrosis, sickle
cell anaemia, PKU and Huntington’s
disease.
TTT
ATG CAG CAG CAG
Nonsense
Link
ATG CAG CAG CAG
Gln
Gln
Gln
Gln
Gln
Gln
Gln
DNA
Polypeptide
Thr
Polypeptide
Besides point mutations, a length of DNA can be duplicated. Huntington’s disease
is one example caused by an expanding triple nucleotide repeat.
Sometimes whole genes become duplicated. It is very likely that duplication of
a single Hox complex in annelids, during the Precambrian period, allowed more
complex arthropods to evolve.
Chromosomal abnormalities
These include deletion, deficiency, duplication, translocation and inversion.
Sometimes whole sets of chromosomes are duplicated (polyploidy) and this is
responsible for many cultivated varieties of crop plants being larger than their wild
ancestors. Details of these chromosomal abnormalities are as follows:
•• Deficiency – loss of an end fragment of a chromosome.
•• Deletion – loss of an inner fragment of chromosome.
•• Duplication – the doubling of one or several chromosome fragments.
•• Inversion – where a segment of chromosome is turned around and, as
transcription happens in one particular direction, may result in the gene being
read ‘back to front’ giving a different protein.
•• Translocation – the transfer of a chromosomal segment onto a nonhomologous chromosome. One example is the 14/21 translocation
responsible for some types (heritable) of Down’s syndrome. It may seem that
this should not make any difference as all the genetic material is still present
in cells but the translocation can interfere with the regulatory genes;
for example, a translocated gene may now be near an inappropriate
promoter region.
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Unit 7: Molecular biology and genetics
Case study: Modern bread wheat
Macief works for the Campden and Chorleywood
Food Research Association, formerly called The Flour
Milling and Baking Research Association. He surveys
the wheat varieties grown throughout the UK and
classifies them according to their suitability for
baking bread or for use as animal feed.
Key term
Species: Reproductively isolated
populations. Members of the same
species have similar genes, anatomy,
physiology and behaviour and can
interbreed to give fertile offspring.
Einkorn wheat is a diploid species and is grown in
some parts of the world for animal feed. Modern
bread wheat, Triticum sp., is a hexaploid species.
Plant breeders carry out selection programmes,
focusing on characteristics such as resistance to
fungi and aphids, high protein content, straw
stiffness, resistance to lodging (stems bending over
in wind or rain) and increased yield, to produce
improved varieties. Some carry out traditional
selective breeding programmes and some use
genetic modification.
Wild einkorn
AUAU
2n = 14
Domestication and artificial
selection, which altered the
phenotype but not the
chromosome number
Einkorn
AUAU
2n = 14
Wild grass
BB
2n = 14
Sterile hybrid P
AUB
Mutation that
doubled the
chromosome number
Emmer wheat
AUAUBB
4n = 28
Figure 7.7.2: How artificial selection has
produced modern bread wheat from wild
ancestors. The letters AU, B and D denote
sets of chromosomes (genomes).
Goat grass
DD
2= 5 14
Sterile hybrid Q
AUBD
Mutation that
doubled the
chromosome
number
Common wheat
AUAUBBDD
6n = 42
Activity: Deletion of sections of chromosome 15
Two genetic diseases of humans, Prader–Willi syndrome (PWS; 1 in 20 000 births) and Angelman
syndrome (AS: 1 in 10–20 000 births) have totally distinct sets of symptoms but they are both
caused by the loss of an identical piece of chromosome 15 and loss of the same genes. However,
PWS (infants are small with floppy muscles but develop overeating and obesity later, with mild
mental retardation and temper outbursts) is caused when the paternal chromosome suffers
a deletion, whereas AS (severe mental retardation, small brain size, speech deficiencies and
spontaneous laughter for no reason) is caused by loss of part of the maternal chromosome 15.
In some cases of PWS the children have two intact copies of chromosome 15 but both have come
from the mother and some cases of AS have two intact copies of the paternal chromosome 15.
They have all the right genes in the correct amounts but, because they lack certain genes from
both parents (from imprinted chromosomes), they develop severe disorders. One gene, UBE3A on
chromosome 15, is needed for brain function but is only expressed by the maternal chromosome
of a homologous pair. Hence a child lacking this gene on the maternal chromosome or lacking a
maternal chromosome 15 will develop AS.
1 Suggest a mechanism for development of PWS in a child that has two maternal copies of
chromosome 15.
2 In all cases of PW and AS, the parents are unaffected. Suggest how/when the chromosomal
abnormalities occur.
Transposons
Transposons are sometimes called ‘jumping genes’. Up to 20% of any organism’s
genome may consist of transposons. Sometimes they translocate and disrupt
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Unit 7: Molecular biology and genetics
gene expression or cause insertions in the genome, hence causing mutations.
Retroviruses insert their genomes into their host genome when they use reverse
transcriptase to copy their RNA into DNA. In bacteria, transposons aid the transfer
of antibiotic resistance genes and they can also cause mutations in laboratory
populations of yeast or fruit flies.
Key term
Population: Group of individuals
of the same species, living and
interbreeding in the same place at
the same time.
Retrotransposons
Retrotransposons are a type of transposon that replicate and translocate via
RNA intermediates – they copy themselves to RNA and then back to DNA. Like
transposons they can induce mutations by inserting near to genes or their
regulatory sequences. The yellow phenotype (see Figure 7.7.3) in mice (see Topic
guide 7.5, section 3) is caused by a retrotransposon, inserted just before the agouti
gene, that produces abnormal RNA and keeps the agouti gene permanently
switched on, so hairs do not have the black bands at each end. However, if the
retrotransposon is methylated, it does not express much abnormal RNA and the
agouti gene is not continuously switched on allowing black tips to form and the
agouti fur to be seen.
Figure 7.7.3: How variation in the
expression of the retrotransposons
affects the expression of the agouti
gene in mice, leading to phenotypic
variability (coat colour variability)
between genetically identical individuals.
Agouti gene
Gene switched on
and off cyclically
Normal
Cycles of agouti protein
and banded hair
Agouti gene
Inserted retrotransposon
expresses an abnormal
RNA and agouti is switched
on permanently
Avy unmethylated
Yellow hair
Agouti gene
Inserted retrotransposon
is methylated, doesn’t express
much abnormal RNA and agouti
is produced more normally
Avy methylated
Banded hair
About 42% of the human genome is retrotransposons and about 3% is
transposons.
2 The Hardy–Weinberg principle
Individuals have genomes – the sum total of their entire DNA. Within any
population there is a pool of genetic diversity – the gene pool.
The Hardy–Weinberg principle, developed by a British mathematician, Godfrey
Hardy, and a German doctor, Wilhelm Weinberg, states that allele and genotype
frequencies within a population remain constant, in genetic equilibrium, from
generation to generation. This assumes that:
1 the population is large
2 within the population all mating is random
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3 there is no mutation or genetic drift
4 there is no migration and hence no exchange of alleles between populations
5 there is no selection pressure acting on any genotype.
In reality this is not the case as factors such as genetic drift, natural selection, nonrandom mating and mutation can alter the genetic variation within a population.
Therefore the Hardy–Weinberg equilibrium can be used as a baseline against
which changes in population genetics can be measured over time. It can be
used to calculate allele frequencies, in populations, for traits having dominant or
recessive inheritance patterns.
Application of the Hardy–Weinberg principle
Case study: Genetic epidemiologist
Aled is a genetic epidemiologist and, during the course of his work, he may need to apply the
Hardy–Weinberg principle. Below is an example.
We know that the incidence of cystic fibrosis within the UK population is 1 in 2500. Cystic fibrosis
has a recessive inheritance pattern and, at a simple level, we can consider that there are two
alleles for the CFTR gene, CF and cf.
We want to know how many of the population are carriers, genotype CFcf.
The frequency of the dominant allele, CF, is denoted by p.
The frequency of the recessive allele, cf , is denoted by q.
Therefore:
q2 denotes the frequency of the genotype cfcf
p2 denotes the frequency of the genotype CFCF
2pq denotes the frequency of the genotype CFcf. In any random mating for this monohybrid
cross, the ratio of genotypes is 1 CFCF (P × P = P2) : 2 CFcf (2 × p × q) : 1cfcf (q × q = q2).
Within a population the frequency of alleles for a particular gene adds up to 1, so p + q = 1.
Within that population the frequency of genotypes adds up to 1 (100%).
So p2 + 2pq + q2 = 1.
If 1 in 2500 has cystic fibrosis, genotype cfcf, then
q2 = 0.0004
Therefore q = 0.02
So p = 1 − q = 0.98
Therefore p2 = 0.9604
and 2pq = 1 − (p2 + q2) = 0.039 or 3.9%
This indicates that about 4% or 1 in 25 people in the UK are symptomless carriers of cystic fibrosis.
•• Suggest why this may not be a true figure for the whole UK population.
Aled does not need to use this principle to estimate the frequency of inherited traits determined
solely by codominant alleles. For example, for MN blood grouping there are two codominant
alleles. Consider a population of 1000 people where 360 are blood group M and therefore
genotype MM, 480 are blood group MN and therefore genotype MN and 160 are blood group NN
and therefore genotype NN. In this case we know how many heterozygotes there are because
both alleles M and N are expressed in their phenotypes. So in this population there are 2000 alleles
in the gene pool for this trait, with 720 + 480 = 1200 being M, and 480 + 320 = 800 being N.
7.7: Genes and evolution
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Unit 7: Molecular biology and genetics
Activity: Using the Hardy–Weinberg equation
Within the human population some people can taste PTC (phenylthiocarbamide) – a bitter-tasting
poisonous chemical. Ability to taste PTC is governed by a single gene with two alleles T/t. The letter
t denotes inability to taste and is recessive.
Thirty % of the UK population are non-tasters.
•• Calculate the genotype frequency of:
(a) homozygous tasters
(b)heterozygous tasters.
3 Genetics and evolution
Natural selection
Migrations and mutation introduce new alleles into populations. Some mutations
are beneficial, some neutral and some harmful. The same mutation could be
any of these depending on the environment. We see evolution in action when
populations of bacteria become resistant to antibiotics. The antibiotic is the
selection pressure. If a random mutation gives a bacterium an advantage, by
making an altered protein that is an enzyme to break down the antibiotic, over
others in the population, then this bacterium will survive where others die. It then
has no competition for nutrients and will divide, passing the antibiotic resistance
allele to its offspring. This happens over many generations causing a shift in allele
frequency in the population. Eventually the whole population becomes resistant
to that antibiotic.
Link
There are three main types of natural
selection: directional, stabilising
and disruptive.
Natural selection leads to changes in allele frequencies, which lead to different
populations and may eventually lead to new species arising from existing species,
which is evolution.
Frequency-dependent selection can lead to stable polymorphisms. One
example is the higher than expected frequency for the sickle cell allele in
populations where malignant malaria is endemic. This is due to heterozygotes,
HbNHbS, having a selective advantage by being resistant to malaria, and therefore
being selected for.
Investigations have shown that both mice and humans prefer to mate with a
partner whose smell is different from the smell of family members. This is likely to
promote heterozygosity among the resulting offspring.
Charles Darwin developed the theory of natural selection as a mechanism for
evolution and it is a widely accepted theory, although it may not be the whole
story as genetic drift also plays a part.
7.7: Genes and evolution
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Unit 7: Molecular biology and genetics
Activity: Other examples of frequency-dependent selection
The gene for determining ABO blood group is on chromosome 9. It has six exons (expressed
lengths of DNA) and five introns (inexpressed lengths of DNA) and codes for an enzyme, galactosyl
transferase, that catalyses the formation of the glycolipid markers on red blood cell surface
membranes. The A allele differs from the B allele at letters 523, 700, 793 and 800 of the genetic
code. A has C, G, C, G and B has G, A, A, C. The O allele has a deletion, the G at letter 258 is missing,
causing a frameshift and no functioning enzyme, so no glycolipids are made. There appears to be
no selective advantage or disadvantage to having blood group O but, by the late 1980s, a scientist
had discovered that children of blood group A were susceptible to some types of infant diarrhoea
and not to others; those of blood group B were susceptible to diarrhoea caused by a different
strain of bacterium and those of blood group O were much more susceptible to cholera infection,
whereas those of group AB were resistant to cholera. The survival of heterozygotes, AB, maintains
the rarer allele B in the population, although group B people are susceptible to infant diarrhoea.
People of blood group O appear to be more resistant to malaria and to some types of cancers than
people of other blood groups.
Another example is that heterozygous carriers of the cystic fibrosis allele are resistant to typhoid.
1 Explain why two mating individuals, both resistant to cholera because they have group AB
blood, do not always produce children with group AB blood.
2 Discuss how endemic diseases act as frequency-dependent selecting agents. Include some
other examples in your discussion. You can research and find out about non-secretors of
blood group immunoglobulins and their resistance to cold and flu viruses but susceptibility to
meningitis and urinary tract infection.
Genetic drift
Genetic drift is when a population’s allele frequencies change due to random
events, rather than in response to a selection pressure. In small populations,
chance plays a bigger part in causing fluctuations in allele frequencies and,
in extreme cases, one allele may be lost altogether. The case study about the
Pingelap atoll describes an example of genetic drift.
About 80 000 years ago the human population went through a population
genetic bottleneck when a huge supervolcano erupted. This means that all
modern humans are similar genetically as they have arisen from the shrunken
gene pool after this catastrophic event.
Case study: The Pingelap atoll
In 1775, on an atoll in the Pacific Ocean, only 30 people survived following a storm and severe
famine, and today’s 2000 inhabitants are descended from them. About 5% of them have a
recessive disorder – achromatopsia, a form of colour blindness, which is extremely rare in other
human populations. One of the 30 survivors was a chief who had this condition. The frequency of
this allele in the population today is 0.23.
7.7: Genes and evolution
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Unit 7: Molecular biology and genetics
Case studies: Founder effect
Some small isolated populations always tend to marry within their population. They have assortative rather than random mating.
•• Among the old order Dunkers people of the USA there is a higher than usual incidence of hitchhiker’s thumb and free rather than attached
earlobes. Neither of these differences is adaptive so there is no selection pressure for or against them and the increase in their incidence is due to
the founder effect – the few people who founded this order had those characteristics.
•• Among the Amish people descended from a few who settled in the US during the reformation period, there is a higher than usual incidence of
hexadactyly (see Figure 7.7.4).
•• The gene pool in Iceland is small and there is a higher than worldwide average rate of breast cancer among the population.
•• In Martha’s Vineyard, USA, there is a higher than usual frequency of genetic deafness within the population.
•• Among Ashkenazi Jews in the US there is a higher than normal frequency of the allele for Tay–Sachs disease. This is partly due to founder effect
as small groups of Jews originally from eastern Europe and Jews tend to carry out endogamy (marrying within their community). It is also likely
that being heterozygous for Tay–Sachs conferred resistance to TB and gave a selective advantage at a time when TB was the biggest cause of
death among humans.
1. In the US when two Ashkenazi Jews intend to marry they are
screened using a blood test to see if they carry the recessive
allele for Tay–Sachs disease. There is no known cure for this
disease. An accumulation of cell membrane components, called
gangliosides, in brain neurones cause progressive mental
deterioration and death, usually by the age of 2 years. This
screening began in 1971. Orthodox Jews are advised not to
marry that person or not to have children.
(a)What other options are open to non-Orthodox Jews?
(b)Why do you think these options are not available to
Orthodox Jews?
Figure 7.7.4: The incidence of hexadactyly is
higher than usual in Amish communities.
Conservation park warden
Key term
Hexadactyly: Presence of an extra
digit (finger and/or toe).
Brianne is a warden at a conservation park in the UK where animals normally found on African or
Asian continents are kept in open enclosures. This park is open to the public who can drive through
as this helps finance the conservation work and educates the public. Brianne spends much of her
time looking after the big cats and this year the park has introduced a breeding programme for
cheetahs as they are an endangered species. Other conservation parks in the UK are doing the
same and, because cheetahs have been through a population bottleneck, their genetic diversity is
low. Each park has a small cheetah population and one of Brianne’s jobs is to liaise with other parks
and arrange regular swapping of cheetahs to introduce new alleles into each breeding population
and reduce the inbreeding. This is important as reducing genetic diversity can lead to an increase
in phenotypes showing maladaptive genetic mutations.
Migrations of individuals carry new alleles into populations and this is called
gene flow. See the Case study below for an example of gene flow.
Case study: Gene flow
In West Africa many people carry the Duffy antigen on their red blood cells. This is a result of
natural selection as it makes them resistant to malaria. Due to interbreeding between immigrant
West Africans and resident Caucasians in the US, many white Americans now have the Duffy
antigen, although there is no selective advantage to this as malaria is not endemic in the US.
7.7: Genes and evolution
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Unit 7: Molecular biology and genetics
4 The Human Genome Project
Figure 7.7.5: Map of the
human X chromosome.
xp22.33
xp22.32
xp22.31
xp22.2
xp22.13
xp22.12
xp22.11
xp21.3
xp21.2
xp21.1
xp11.4
xp11.3
Fanconi-Anämie B
Wiskott-Aldrich Syn.
The techniques used included DNA sequencing, PCR, electrophoresis, yeast
and bacterial artificial chromosomes and use of restriction fragment-length
polymorphisms, RFLPs.
xp11.23
xp11.22
xp11.21
xq11.1
xq11.1
xq11.2
xq12
xq13.1
xq13.2
xq13.3
xq21.1
xq21.2
xq21.31
xq21.32
xq21.33
xq22.1
xq22.2
xq22.3
xq23
The Human Genome Project (HGP), which began in 1990 and was completed
in 2003, was an international collaborative research project to map the human
genome’s 3 billion DNA base pairs. When it began scientists postulated that we
would have about 100 000 genes but the HGP has shown we have about 20 500.
Other genomes have been sequenced and mice have 30 000 genes – about 10 000
of which are concerned with their whiskers and sense of smell! However, scientists
have discovered much about the regulatory genes that code for RNA and the
subtle and complex gene/gene and gene/protein interactions that make us so
complex.
Pelizeuz-Merzbacher
Krankheit
xq24
xq25
xq26.1
xq26.2
xq26.3
xq27.1
xq27.2
xq27.3
Fragile X-Syndrom
xq27.3
G6PD-Mangel
It is hoped that applications of this project will include better diagnosis and
treatments for genetic disorders as well as ‘tailor-made’ medicines that can target
people of different genotypes, be more efficient and not produce side effects.
However, there have been fears that it could create a genetic underclass – people
who know they have a genetic predisposition to certain chronic illnesses may
not be able to obtain life insurance, jobs or mortgages. Therefore, there is a
large department of the Human Genome Research Institute (HGRI) concerned
with ethical, social and legal issues and the development of policy options for
consideration by the public.
A very large amount of data has been produced by this project and by other gene
sequencing projects and that has led to the rediscovery and use of a branch of
science concerned with storing, retrieving and analysing this complex biological
data. Figure 7.7.5 shows the information we have about just one chromosome.
5Bioinformatics
Bioinformatics stores and analyses data on RNA and DNA sequences, protein
structure, metabolic pathways, genetic interactions and evolutionary relationships,
structural biology and modelling. It generates new knowledge in fields such as
drug design and also deals with algorithms (mathematical formulae), databases,
web technologies, pattern recognition and computing.
Bioinformatics aims to increase our understanding of biological systems. It uses
special software for gene-prediction, sequence alignment, protein structure
prediction, prediction of gene expression and of interaction between proteins,
and for modelling evolution. BLAST programmes are used for analysing DNA
sequences of many thousands of organisms whose genomes are being sequenced.
These programmes can compensate for point mutations in DNA sequences and can
identify related, although not identical, sequences. As scientists discover more about
the roles of DNA that was previously thought not to have a function, the analytical
programmes being used will be developed and improved. Already it is helping
scientists understand gene regulation and gene expression. This will also help in the
understanding of the impact of mutations on development of cancers.
7.7: Genes and evolution
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Unit 7: Molecular biology and genetics
Bioinformatics aims to help evolutionary biologists and may also help to produce
a better evolutionary tree as comparisons between different species’ DNA
establishes better-understood evolutionary relationships (see Figure 7.7.6).
Bioinformatics uses an information-related system to store and analyse data about
biological information that organisms use to build, operate and repair themselves.
An example is shown in Figure 7.7.7 – modelling an ecosystem.
Figure 7.7.6: Circular genome map
showing shared genetic material between
human, chimpanzee, mouse, rat, dog,
chicken and zebra fish chromosomes,
indicating evolutionary divergence.
Figure 7.7.7: Ecological food web
model representing the ecosystem
of a Caribbean coral reef and
relationships between organisms.
Models such as this can be used to
predict ecological outcomes.
7.7: Genes and evolution
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Unit 7: Molecular biology and genetics
Taking it further: Humans as superorganisms
We think of the human body as being made of 10 trillion cells but within our intestines we also
have a microbiome – about 100 trillion bacterial cells of different species, having about 3 million
genes between them. These bacteria are not parasites but rather members of a community and
we should perhaps consider ourselves as part of a superorganism, whose genome consists of our
genes and the bacterial genes. We and our bacteria have coevolved. Ten % of our daily energy
comes from food we eat that only our bacteria can digest for us. Even human milk contains glycans
that humans cannot digest but gut bacteria can as they produce the enzyme glycoside hydrolase.
One reason that babies are born head first with the head passing near the mother’s anus is that
they pick up essential gut bacteria. The biome also makes essential vitamins, K, B2, B12 and folic acid,
for us and can adjust its output to meet its human’s needs. These bacteria also make molecules that
can regulate the activity of human cells.
Our biome helps prevent infection from pathogenic organisms and humans with disruptions
to their biomes are more at risk of developing obesity, heart disease, asthma, eczema, multiple
sclerosis, autism and bowel cancer.
People with autism have changes to genes that regulate sulfur metabolism (sulfur is needed
for brain development) and if they also have too many Clostridia bacteria in their gut producing
phenols that require sulfur to detoxify them, this could make their symptoms worse.
Scientists have also found that our appendix, previously thought of as a useless vestige, is a
reservoir for gut symbionts. You may know that when people in hospital have prolonged antibiotic
treatment, which also kills some of their gut bacteria, one type, Clostridium difficile (C. diff), usually
present in the gut in small numbers, can flourish and cause serious illness. Research has shown that
people who still have their appendix are far less likely to get a C. diff infection following prolonged
antibiotic treatment and this is probably because bacteria from their appendix fill the gap left by
those in the gut killed by antibiotics, and suppress the growth of C. diff.
Checklist
At the end of this topic guide you should be familiar with the following ideas:
 mutations, changes to genetic material, can be beneficial, neutral or harmful
 transposons can cause mutations
 the Hardy–Weinberg principle can be used to estimate allele frequency and genotype
frequency within a population
 natural selection is a mechanism for evolution
 genetic drift also alters allele frequency within a population
 the Human Genome Project has sequenced the whole human genome
 scientists are discovering the complex interactions involved in gene regulation and
expression
 bioinformatics is the branch of science that deals with how to store, retrieve and analyse
the large amounts of complex biological data and allows modelling of complex biological
systems, enabling predictions of outcomes to be made.
7.7: Genes and evolution
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Unit 7: Molecular biology and genetics
Acknowledgements
The publisher would like to thank the following for their kind permission to reproduce their
photographs:
Corbis: MedicalRF.com; Getty Images: Custom Medical Stock Photo 9, Science Photo Library Ltd: Martin
Krzywinski 11, Neo Martinez 12
All other images © Pearson Education
We are grateful to the following for permission to reproduce copyright material:
Figure 7.7.1: Some examples of possible mutations and their effects on a protein primary structure,
from OCR A2 Biology, Pearson, page 109. Used with permission of Pearson Education Ltd; Figure
7.7.2: How artificial selection has produced modern bread wheat from wild ancestors, from OCR
A2 Biology, Pearson, page 145. Used with permission of Pearson Education Ltd; Figure 7.7.3: Carey,
Nessa (2012) The Epigenetics Revolution: How Modern Biology is Rewriting Our Understanding of
Genetics, Disease and Inheritance. London: Icon Books. Used with permission.
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7.7: Genes and evolution
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