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The universality of DNA and protein structures
Despite the incredible complexity of life, the building
components of living organisms are not only simple in structure
but are also universal.
DNA, contained within all living organisms, consists of 4 bases
in different sequences.
Proteins are made up of the same 20 amino acids in all living
organisms – if a gene is transferred from one organism to
another it will produce the same polypeptide (if the introns have
been removed).
So what is a clade?
A Clade is a group of organisms that have evolved from a
common ancestor. Cladograms are diagrams which show us
how clades have diverged over time.
How can organisms be grouped into clades?
The evidence - DNA
Comparisons can be made between DNA base
sequences of different organisms.
Scientists often use the cytochrome C (protein
involved in respiration) gene for comparison.
If these sequences are very similar, it suggests that the
organisms concerned are closely related and originated
from a common ancestor.
The evidence – DNA
Hybridisation
• Reliant upon breaking of H-bonds when DNA
is heated
• When cooled, the original strands recombine
and reform the original strand
DNA hybridisation animation
DNA Hybridisation
Purines –
adenine and
guanine
Single species
Hybrid DNA – two
closely related
species
Hybrid species – not
closely related
Pyrimidines –
Cytosine and
Thymine
• DNA is extracted and
cut into short pieces
• One set of DNA is
labelled with a
radioactive marker
• DNA from each
species is mixed
• Mixture is heated to
separate strands
• Mixture is cooled
• Strands recombine with
other strands with
complementary base
sequences
• Hybridisation – a strand
from one organism is
joined to a strand from
another.
• How do we know which strands are hybrid?
• Hybrid strands are separated out and
temperature is increased in stages
• Explain how the temperature needed to
separate the strands indicates how related
the DNA molecules are…
The higher the temperature at which the hybrid
strands separate (hydrogen bonds break), the
more closely related the species.
The 50% labelled strands are separated and the
temperature is increased in stages until the hybrid
strands separate
DNA from one species
is labelled with a
radioactive marker
DNA is extracted,
purified and cut into
short pieces.
Some of the reformed strands will
contain one strand of each species. This
is hybridisation
Labelled and unlabelled DNA
are mixed together and
heated
Mixture is cooled and strands combine
with other strands with
complementary bases
More recently, biologists have used DNA hybridisation to confirm
the relationships between different species of crane. They made
samples of hybrid DNA from the same and from different species.
They measured the percentage of hybridisation of each sample. The
results are shown in the table.
Species of crane from which hybrid DNA was made
Percentage DNA
hybridisation
Grus americana
and
Grus monachus
97.4
Grus monachus
and
Grus rubicunda
95.7
Grus americana
and
Grus rubicunda
95.5
Grus rubicunda
and
Grus rubicunda
99.9
Grus americana
and
Grus americana
99.9
Grus monachus
and
Grus monachus
99.8
Which two species seem to be the most closely related? Explain
your answer. (2)
G. americana and G. monachus;
Highest percentage (DNA hybridisation) / more bases are
similar/complementary / more hydrogen bonds / more base pairings;
The evidence –
Immunological Comparisons
• Antibodies of one species will respond to
antigens in the blood serum of another
• Draw a flow chart to show how this process is
carried out.
• How can you tell which species are more or
less closely related?
Immunological comparisons of
proteins
Albumin is a protein found in blood plasma.
There are differences in amino acid sequences of
albumin from different species of animals.
Scientists wanted a quick and reliable way of
measuring the differences between albumin from
different species.
Scientists were able to do this based on their
understanding of the principles of immunology.
1. Human albumin is removed and injected into a
rabbit, in which it acts as an antigen.
2. Rabbit creates antibodies to human albumin
3. Anti-human albumin antibodies are removed from
rabbit and then added to albumin of other animals to
create a precipitation reaction.
If the species are closely related, they will have similar
albumin. In humans, all of the anti-human antibodies
from the rabbit will bind to the human albumin.
Humans = 100% precipitation
Chimpanzees = 97% precipitation
Rabbits = 8% precipitation
The evidence – amino acid sequences
Similarities in amino acid sequences tell us that the
genes coding for these proteins are similar.
Similar genes are due to shared ancestry i.e. the
organisms have evolved from a common ancestor in
the past.
Differences in amino acid sequence tell us that
mutations have occurred in the genes since the
organisms separated from the common ancestor.
The bigger the differences, the longer the organisms
have been separated.
Comparison of amino acid
sequences
A comparison between species can be made by either;
Counting the number of similarities in each sequence
Counting the number of differences in each sequence
Comparison of amino acid
sequences
Haemoglobin polypeptide
Biologists can also use protein structure to investigate the
relationship between different species of crane. Explain
why. (2)
1. More closely related (species) have more similarities in amino
acid sequence/primary structure;
2. In same protein / named protein e.g. albumin;
3. Amino acid sequence is related to (DNA) base/triplet sequence;
OR
4. Similar species have a similar immune response to a
protein/named protein;
5. More closely related (species) produce more ‘precipitate’ /
antibody-antigen (complexes) / agglutination;
Accept: ‘Similar species have similarities in amino acid sequence’ for
first marking point.
Accept: Converse for marking points 1, 4 and 5.
Marking point 5 is for measuring the extent of the immune
response.
Comparing the base sequence of a gene provides more
information than comparing the amino acid sequence for which
the gene codes. Explain why. (2)
Reference to base triplet/triplet code/more bases than amino
acids/longer base sequence than amino acid sequence;
Introns/non-coding DNA;
Same amino acid may be coded for/DNA code is degenerate;
Different (base) triplets code for same amino acid = 2 marks
Reject different amino acids are formed/produced.
Ignore reference to codon.
2 max
Why do biologists need cladistics?
There are three important reasons for using
cladistics to organise and discuss organisms:
• It is useful for creating systems of classification so
that biologists can communicate their ideas about
species and the history of life.
• Cladograms are used to predict the properties of
organisms.
• Cladistics can help to explain and clarify the
mechanisms of evolution by looking at similarities
between the DNA and proteins of different species.
Phylogeny
• The evolutionary relationship between
organisms
• Reflects the evolutionary branch that led up
to it
• Used to establish clades and draw cladograms
(phylogenetic trees)
• The oldest species is at the base; the newest
at the ends of the branches.
Speciation is the evolutionary process by which
new biological species arise.
Cladograms
Cladograms show the evolutionary relationship
between certain groups of organisms. The closer the
branches, the closer the evolutionary relationship.
present
Speciation
past
Draw a cladogram of apes and humans. Include:
Humans, gibbons, chimpanzees, gorillas and orang utans
Phylogenetic tree – a bit more detail….
Phylogenetic trees show the evolutionary relationship between
certain groups of organisms. The closer the branches, the closer the
evolutionary relationship.
This phylogenetic tree
shows the evolutionary
relationships of four
species (A,B,C,D).
They are all descended
from an ancestor with five
(1,2,3,4,5) traits.
How many speciation events have occurred?
Describe the relationship between the different species.
Main: Complete the past paper questions
Success Criteria: What I’m looking for…..
Complete the past paper questions
How successful were we this lesson?
Learning Objective
We were learning…..
Keywords:
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