Biology AS AQA Unit 2 Evidence for Relationships between Organisms

Made these notes for my year 12 summer exam to revise and read over. There are spelling mistakes in most of my files but due to the busy exam schedule I had no time to correct them (sorry).

Most files have more information than what is needed but I feel it helps you feel more confident walking into the exam if you have a greater knowledge background and may help when having to apply knowledge to questions. Good luck :)

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  • Created on: 02-09-13 12:45
Preview of Biology AS AQA Unit 2 Evidence for Relationships between Organisms

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Evidence for Relationships between
Organisms
COMPARISON OF DNA BASE SEQUENCES
When one species forms another species during evolution, DNA will be similar.
Due to mutations the sequences of nucleotide bases in new species will change. So over time
as more mutation occurs, closely related generations will have more similar DNA.
DNA HYBRIDISATION: Way of determining similarities between DNA.
When DNA is heated, the strand separates. It reforms as it cools.
1. DNA from two species is extracted, purified and cut into short pieces.
2. DNA from one species is radioactively/fluorescently marked.
3. Mixed with unlabelled DNA.
4. Mixture is cooled which allows strands to combine.
5. Hybridisation occurs where one unmarked and one marked strand form.
6. Hybrids then separate as temperature increases.
The higher the temperature, the more closely the species are related. This is due to the strength of
the hybrid strand. If there are more hydrogen bonds linking the two strands together, the species
have a lot of complementary nucleotide bases. A higher temperature is needed to break strands
with a greater amount of bonds and consequently strength.
USE OF DNA BASE SEQUENCING IN CLASSIFYING PLANTS
Used 565 species to represent all families of plants in the world.
For each plant, DNA sequences were determined and then compared using computer
analysis.
A Phylogenetic tree was produced which showed the evolution into natural groups.
Previously was only MONOCOTYLEDONS (single seed, thin narrowed leaves) and
DICOTYLEDONS (2 seeded and generally broad leaves).
COMPARISON OF AMINO ACID SEQUENCES IN PROTEINS
Sequence of amino acids in the proteins is determined by DNA.
Degree of similarity of the sequence of amino acids shows the relationship.
Can count no. of similarities or differences.
IMMUNOLOGICAL COMPARISON OF PROTEINS
Antibodies of one species will respond to specific antigens on proteins in the blood serum of
another species.
1. Serum albumin from species A is injected into species B.
2. Species B produces antibodies specific to all the antigen sites on the albumin from species A.
3. Serum containing the specific antigens is extracted from species B.
4. Antibodies then respond to their corresponding antigens on the albumin in the serum of
species C.
5. Response is that a precipitate is formed.
Greater number of similar antigens means that a larger amount of precipitate is formed.
Therefore the species are more closely related.

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