Back
Home
2.6 compare living primates to hypothesise about relationships between groups of primates using evidence from: karyotype analysis DNA DNA hybridisation comparison of haemoglobins DNA sequencing mitochondrial DNA as a molecular clock
compare living primates to hypothesise about relationships between groups of primates using evidence from:
- karyotype analysis
- DNA–DNA hybridisation
- comparison of haemoglobins
- DNA sequencing
- mitochondrial DNA as a molecular clock
Karyotype analysis
- Karyotype analysis looks at the number, shape, size and banding patterns of the chromosomes in an organism. During mitosis the chromosomes thicken and become visible. At this point a picture can be taken of the chromosomes paired up. A karyotype is defined as the appearance of chromosomes, their size, shape and number. Each organism has a different karyotype. The table below illustrates the different chromosome numbers of a range of organisms.
Organism |
Chromosome number |
human
|
46
|
gorilla
|
48
|
chimpanzee
|
48
|
beans
|
22
|
goldfish
|
94
|
DNA–DNA hybridisation
- DNA is found in almost every living cell. It consists of four bases arranged in a spiral helix. The four bases combine to form the genetic code. From generation to generation there should only be small changes in the genetic make-up of a species. The difference between two species should indicate how long ago they shared a common ancestor. The closer two species are the more similar will be their DNA.
- DNA hybridisation is used to work out how closely two species are related. DNA is extracted from cells. It is heated and the double strands of DNA separate. Restriction enzymes are used to snip the strands into smaller pieces. Then the DNA of two different species is mixed together. As the DNA cools the strands collide into each other and realign to form double stranded DNA. If the DNA is similar then the code will be similar and there will be strong bonds formed. The mixture is then heated and the temperature required to separate the strands again will indicate how closely related the two species are. The table below shows the similarity between different primates based on this technique.
Primate |
% similarity of DNA with human DNA |
human
|
100
|
chimpanzee
|
97.6
|
gibbon
|
94.7
|
rhesus monkey
|
91.1
|
capuchin
|
84.2
|
Comparison of haemoglobins
- Haemoglobins are proteins found in the blood. They are important for the transport of gases in the blood. If two species have similar haemoglobin, their DNA must be similar and they have shared a common ancestor recently. To test for haemoglobin similarity blood serum from humans is injected into rabbits and the rabbits respond by producing antibodies to the human blood proteins. The antibodies are extracted from the rabbits. These antibodies are added to the sample of blood taken from other species. A precipitate forms. The greater the reaction to the human antibodies the more similar to humans is the species. So human blood would have a 100% reaction while a dog may have no reaction. Some results are summarised in the table below.
Species |
Reaction to human antibody (%) |
human
|
100
|
chimpanzee
|
97
|
baboon
|
50
|
dog
|
0
|
DNA sequencing
- The order of bases along a DNA strand is called its sequence. If the sequences are similar then the two organisms have shared a common ancestor in recent times. In DNA sequencing genetic and biochemical tests are used to sequence the bases in a portion of DNA. These are then compared with different species.
Mitochondrial DNA as a molecular clock
- Most of the DNA in a cell is found in the nucleus but a small amount of DNA is found in the mitochondria. The mitochondria were once free-living organisms that have been incorporated into eucaryotic cells. Mitochondrial DNA (mtDNA) is much simpler than nuclear DNA and consists of a single circle of DNA. During fertilisation the female egg cell provides the mitochondria for the new organism. This means that mitochondrial DNA is always through the female line. By comparing the mitochondrial DNA from living primates it is possible to calculate a molecular clock based on a constant rate of mutation. So depending on how different the mtDNA is an indication of how many years has passed since two organisms shared a common ancestor.