3.4 Genetic information, variation and relationships between organisms
Biological diversity – biodiversity – is reflected in the vast number of species of organisms, in the variation of individual characteristics within a single species and in the variation of cell types within a single multicellular organism.
Differences between species reflect genetic differences. Differences between individuals within a species could be the result of genetic factors, of environmental factors, or a combination of both.
A gene is a section of DNA located at a particular site on a DNA molecule, called its locus. The base sequence of each gene carries the coded genetic information that determines the sequence of amino acids during protein synthesis. The genetic code used is the same in all organisms, providing indirect evidence for evolution.
Genetic diversity within a species can be caused by gene mutation, chromosome mutation or random factors associated with meiosis and fertilisation. This genetic diversity is acted upon by natural selection, resulting in species becoming better adapted to their environment.
Variation within a species can be measured using differences in the base sequence of DNA or in the amino acid sequence of proteins.
Biodiversity within a community can be measured using species richness and an index of diversity.
DNA, genes and chromosomes
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In prokaryotic cells, DNA molecules are short, circular and not associated with proteins. In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome. The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein. A gene is a base sequence of DNA that codes for:
A gene occupies a fixed position, called a locus, on a particular DNA molecule. A sequence of three DNA bases, called a triplet, codes for a specific amino acid. The genetic code is universal, non-overlapping and degenerate. In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are, for example, non-coding multiple repeats of base sequences between genes. Even within a gene only some sequences, called exons, code for amino acid sequences. Within the gene, these exons are separated by one or more non-coding sequences, called introns. |
DNA and protein synthesis
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The concept of the genome as the complete set of genes in a cell and of the proteome as the full range of proteins that a cell is able to produce. The structure of molecules of messenger RNA (mRNA) and of transfer RNA (tRNA). Transcription as the production of mRNA from DNA. The role of RNA polymerase in joining mRNA nucleotides.
Translation as the production of polypeptides from the sequence of codons carried by mRNA. The roles of ribosomes, tRNA and ATP. Students should be able to:
Students will not be required to recall in written papers specific codons and the amino acids for which they code. |
Genetic diversity can arise as a result of mutation or during meiosis
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Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution. Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids. Mutagenic agents can increase the rate of gene mutation. Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis. Meiosis produces daughter cells that are genetically different from each other. The process of meiosis only in sufficient detail to show how:
Students should be able to:
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AT d
Students could examine meiosis in prepared slides
of suitable plant or animal tissue.
MS 0.5
Students could:
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Genetic diversity and adaptation
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Genetic diversity as the number of different alleles of genes in a population. Genetic diversity is a factor enabling natural selection to occur. The principles of natural selection in the evolution of populations.
Directional selection, exemplified by antibiotic resistance in bacteria, and stabilising selection, exemplified by human birth weights. Natural selection results in species that are better adapted to their environment. These adaptations may be anatomical, physiological or behavioural. Students should be able to:
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MS 2.5 Students could use a logarithmic scale when dealing with data relating to large numbers of bacteria in a culture. |
Required practical 6: Use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth. | AT i |
Species and taxonomy
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Two organisms belong to the same species if they are able to produce fertile offspring. Courtship behaviour as a necessary precursor to successful mating. The role of courtship in species recognition. A phylogenetic classification system attempts to arrange species into groups based on their evolutionary origins and relationships. It uses a hierarchy in which smaller groups are placed within larger groups, with no overlap between groups. Each group is called a taxon (plural taxa). One hierarchy comprises the taxa: domain, kingdom, phylum, class, order, family, genus and species. Each species is universally identified by a binomial consisting of the name of its genus and species, eg, Homo sapiens. Recall of different taxonomic systems, such as the three domain or five kingdom systems, will not be required. Students should be able to appreciate that advances in immunology and genome sequencing help to clarify evolutionary relationships between organisms. |
Biodiversity within a community
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Biodiversity can relate to a range of habitats, from a small local habitat to the Earth. Species richness is a measure of the number of different species in a community. An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species. Calculation of an index of diversity ( ) from the formula
where = total number of organisms of all species and = total number of organisms of each species. Farming techniques reduce biodiversity. The balance between conservation and farming. |
MS 2.3 Students could be given data from which to calculate an index of diversity and interpret the significance of the calculated value of the index. |
Investigating diversity
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Genetic diversity within, or between species, can be made by comparing:
Quantitative investigations of variation within a species involve:
Students will not be required to calculate standard deviations in written papers. |
AT k Students could:
MS 1.2 Students could use standard scientific calculators to calculate the mean values of data they have collected or have been given. MS 1.10 Students could calculate, and interpret the values of, the standard deviations of their mean values. |