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.

If these sequences are very similar, it suggests that the organisms concerned are closely related and originated from a

common ancestor.

Scientists often use the cytochrome C (protein involved in respiration) gene for comparison.

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

Single species

Hybrid DNA – two closely related

species

Hybrid species – not closely related

Purines – adenine and

guanine

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.

DNA is extracted, purified and cut

into short pieces.

DNA from one species is labelled with a radioactive

marker

Labelled and unlabelled DNA are mixed together

and heated

Mixture is cooled and strands combine with other strands with

complementary bases

Some of the reformed strands will contain one strand of each species.

This is hybridisation

The 50% labelled strands are separated and the temperature is increased in stages until the

hybrid strands separate

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

madePercentage 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% precipitationChimpanzees = 97% precipitation

Rabbits = 8% precipitation

The evidence – amino acid sequencesSimilarities 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 sequenceCounting 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;OR4. 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 aminoacids/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.

Draw a cladogram of apes and humans. Include: Humans, gibbons, chimpanzees, gorillas and orang utans

present

past

Speciation

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

Learning Objective

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