Investigating Diversity (AQA AS Biology): Revision Note

Measuring genetic diversity

• A species can be defined as

  • a group of organisms that can interbreed and produce fertile offspring

• Members of one species are reproductively isolated from members of another species

• In reality, it is quite hard to define ‘species’, and the determination of whether two organisms belong to the same species is dependent on investigation

• Individuals of the same species have similar behavioural, morphological (structural) and physiological (metabolic) features

• A common example used to illustrate this concept is mules: the infertile offspring produced when a male donkey and a female horse mate

Genetic diversity

• Genetic diversity is the number of different alleles of genes

• Genetic diversity within and between species can be measured by looking at the following:

  • Displays of measurable characteristics

  • The nucleotide base sequence of DNA

  • The nucleotide base sequence of mRNA

  • The amino acid sequence of proteins

Measurable & observable characteristics

• Comparing characteristics of different individuals is usually the quickest but least reliable form of determining genetic diversity

  • The genetic differences between individuals can only be implied using this technique

• This method was used successfully to classify organisms into the taxonomic hierarchy for hundreds of years before DNA sequencing

• Characteristics that could be measured include:

  • number of legs

  • number of seeds in a berry

  • number of petals

  • number of leaf indentations

• Characteristics that could be observed include:

  • colour

  • patterns on fur/scales/feathers

  • habitat

  • presence of hair/wings/fins

• The problem with this method is that it is not precise enough if only one characteristic is looked at, for example, many animals have four legs

• It can be useful if a species has unique characteristics

• Often, if two species cannot be distinguished from their observable characteristics, measurable characteristics will give a better understanding of the similarities and differences

DNA analysis & comparison

• DNA sequence analysis has replaced using characteristics as a means of determining genetic diversity

• DNA is extracted from the nuclei of cells taken from an organism

  • DNA can be extracted from blood or skin samples from living organisms or fossils

• The extracted DNA is processed, analysed, and the base sequence is obtained

• The base sequence is compared to that of other organisms to determine evolutionary relationships

  • The more similarities there are in the DNA base sequence, the more closely related (in that the less distant the species separation) members of different species are

  • Computers can be used to highlight matches between the DNA samples

  • Two groups of organisms with very similar DNA will have separated into separate species more recently than two groups with less similarity in their DNA sequences

• DNA sequence analysis and comparison can also be used to create family trees that show the evolutionary relationships between species

mRNA analysis & comparison

• mRNA is often easier to isolate from cells than DNA, as it is found in the cytoplasm and there are usually multiple copies of the same mRNA

• Collected mRNA from an individual can be used as a template to produce cDNA (complementary DNA)

  • The first strand of cDNA produced is complementary to the mRNA (the same as the template strand of the DNA)

  • The first strand is then used to produce a second cDNA strand, which is the same as the coding strand of DNA

  • Unlike the original DNA in the nucleus, the cDNA contains only the coding regions of the gene (exons) and no introns

• It is important to compare the same mRNA between samples

  • mRNA for a known, universal protein is often used and compared, for example, cytochrome-c (from the electron transport chain)

  • Primers can be used that bind to specific sequences

Amino acid sequence analysis & Comparison

• Similarly to mRNA, proteins are often easier to isolate from the cell than DNA

• The sequence of amino acids of the same protein can be compared between individuals

  • The protein chosen must be found in all the individuals/species being compared e.g. haemoglobin is used for many animals

• Amino acid sequences can also be determined from mRNA sequencing if the 'frame' is known (the correct start codon is determined)

• Amino acid sequences of proteins evolve much more slowly than DNA, especially if the protein is of high importance

  • Therefore, it is likely that closely related species (e.g. humans and chimpanzees) will have the same amino acid sequence even though these species split from their common ancestors millions of years ago

  • This is because the shape, and therefore function and specificity, of a protein is determined by the amino acid sequence, as the position of amino acids determines the intermolecular forces between R groups