topic 4

• Adaptation: A feature of an organism that increases its chance of survival in its environment. An adaptation may be anatomical, physiological or behavioural.
• Allele: A version of a gene.
• Anticodon: A sequence of three nucleotide bases at one end of a tRNA molecule that is specific to an mRNA codon.
• Arithmetic mean: The average of a set of numbers calculated by dividing the sum of the values by the number of values.
• Artificial classification: A type of classification that divides organisms into groups based on analogous characteristics such as leaf shape, number of legs and type of wing.
• Binomial system: A universal system of naming organisms that consists of two parts: the generic name and the specific name, e.g. Homo sapiens.
• Biodiversity: The variety of genes, species and habitats within a particular area.
• Cellular proteome: The proteins expressed in a given type of cell.
• Chromatid: One strand of a replicated chromosome.
• Chromosome: A structure consisting of a long, coiled molecule of DNA and its associated proteins, by which genetic information is passed from generation to generation.
• Chromosome mutation: A change to the number or structure of chromosomes that can occur spontaneously.
• Classification: The organisation of organisms into groups. There are two types of classification: artificial and phylogenetic.
• Codon: A sequence of three bases on mRNA that codes for a specific amino acid.
• Conservation: The maintenance of ecosystems and biodiversity by humans in order to preserve the Earth’s resources.
• Courtship: The behaviour by which members of a species select reproductive partners. It enables organisms to recognise their own species, identify a mate with a capacity to breed, form a pair bond, synchronise mating and become able to breed themselves.
• Crossing over: The process in meiosis 1 in which homologous chromosomes pair up, their chromatids wrap around one another and their alleles are exchanged at equivalent portions of chromatids. This creates genetic variation.
• Degenerate: A feature of the genetic code; more than one triplet can code for a particular amino acid.
• Deletion: A form of gene mutation in which one or more nucleotide bases are removed from a DNA sequence. This may change all amino acids in a sequence, rendering the protein non-functional.
• Directional selection: A type of selection that favours individuals that differ in one direction (fall to the left or the right) from the population mean. This changes the traits of the population.
• Ecosystem diversity: A measure of the range of different habitats in a particular area.
• Eukaryotic DNA: Linear molecules of DNA which, together with histones, form chromosomes. DNA in the mitochondria and chloroplasts of eukaryotic cells is circular and does not have associated proteins.
• Exon: A sequence of DNA that codes for an amino acid sequence.
• Fertilisation: The random fusion of haploid gametes during fertilisation to produce a diploid zygote. Genetic information is mixed, creating genetic variation.
• Gene: A length of DNA on a chromosome that codes for the production of one or more polypeptide chains and functional RNA.
• Gene mutation: A change to at least one nucleotide base in DNA or the arrangement of bases. Gene mutations can occur spontaneously during DNA replication.
• Generic name: Denotes the organism’s genus. The first letter is written in upper case, e.g. Homo.
• Genetic code: The rules by which triplets in a DNA base sequence code for the sequence of amino acids in a polypeptide chain. The genetic code is degenerate, universal and non-overlapping.
• Genetic diversity: The number of different alleles in a population. Genetic diversity between organisms can be investigated by comparing observable characteristics, DNA and mRNA base sequences and amino acid sequences.
• Genome: The entire set of genes in a cell.
• Histones: Proteins that, together with DNA, form chromosomes in the nuclei of eukaryotic cells.
• Homologous chromosomes: A chromosome pair, one paternal and one maternal, with the same gene loci.
• Independent segregation: The random separation of homologous chromosomes in meiosis 1 that produces genetic variation.
• Index of diversity (d): Describes the relationship between the number of different species and the abundance of individuals in each of these species within a community. It is calculated using the formula: d = (number of different species) / (total abundance of all species) where d is the index of diversity, N is the total number of organisms of all species and n is the total number of organisms of each species.

  $d=$$N(N-1)/∑ n(n-1)$
• Intron: A non-coding sequence of DNA.
• Locus: The position of a gene on a chromosome.
• Mean (normal distribution curve): A measure of the maximum height of a normal distribution curve.
• Meiosis: A type of cell division that produces four genetically different daughter cells (gametes) with a haploid number of chromosomes. It involves two divisions.
• Messenger RNA (mRNA): A type of RNA that carries genetic information from the nucleus to the ribosomes for protein synthesis. It is a single helix consisting of thousands of mononucleotides.
• Mitosis: A form of cell division that produces two genetically identical diploid daughter cells.
• Mutagenic agent: An agent that increases the rate of gene mutations above normal level.