4. Genetic Information, Variation & Relationships Between Organisms: Definitions (AQA A-Level Biology): Revision Notes
4. Genetic Information, Variation & Relationships Between Organisms: Definitions
Adaptation: A characteristic of an organism that enhances its chances of surviving in its particular environment. These features can be structural, physiological, or behavioural in nature.
Allele: An alternative form or version of a particular gene.
Anticodon: A three-nucleotide sequence found at one end of a transfer RNA molecule that corresponds to a specific messenger RNA codon.
Arithmetic mean: The calculated average of a collection of numerical values, determined by adding all values together and dividing by the total count of values.
Artificial classification: A method of grouping organisms based on analogous features such as leaf structure, limb number, and wing type.
Binomial system: The universal two-part naming system for organisms, consisting of a genus name followed by a species name, for example Homo sapiens.
Biodiversity: The total variety of genes, species, and habitats found within a specific geographical area.
Cellular proteome: The complete set of proteins that are produced within a particular cell type.
Chromatid: Each of the two identical strands that make up a replicated chromosome.
Chromosome: A structure formed from a long, coiled DNA molecule and its associated proteins, responsible for transmitting genetic information between generations.
Chromosome mutation: An alteration in the number or structural arrangement of chromosomes that can happen spontaneously.
Classification: The systematic organisation of organisms into groups. This can be done using either artificial or phylogenetic methods.
Codon: A sequence of three nucleotide bases in messenger RNA that specifies a particular amino acid.
Conservation: Human efforts to maintain ecosystems and biodiversity in order to protect Earth's natural resources.
Courtship: Behavioural patterns that members of a species use to select mating partners. These behaviours help organisms recognise members of their own species, identify suitable mates, form pair bonds, synchronise mating behaviour, and become reproductively ready.
Crossing over: The process during the first stage of meiosis where homologous chromosomes pair together, their chromatids wrap around each other, and equivalent portions of chromatids exchange alleles, creating genetic variation.
Degenerate: A characteristic of the genetic code whereby multiple triplets can code for the same amino acid.
Deletion: A type of gene mutation where one or more nucleotide bases are lost from a DNA sequence, potentially altering all subsequent amino acids and making the resulting protein non-functional.
Directional selection: A form of natural selection that favours individuals whose characteristics differ in a particular direction from the population average, thereby shifting the population's characteristics.
Ecosystem diversity: A measurement of the variety of different habitats present within a particular geographical area.
Eukaryotic DNA: Linear DNA molecules that combine with histones to form chromosomes. The DNA found in mitochondria and chloroplasts of eukaryotic cells is circular and lacks associated proteins.
Exon: A DNA sequence that contains the code for an amino acid sequence.
Fertilisation: The random combination of haploid gametes during reproduction, producing a diploid zygote and mixing genetic information to create genetic variation.
Gene: A section of DNA on a chromosome that contains the instructions for producing one or more polypeptide chains and functional RNA molecules.
Gene mutation: An alteration to at least one nucleotide base in DNA or a rearrangement of bases, which can occur spontaneously during DNA replication.
Generic name: The first part of an organism's scientific name, indicating its genus, with the first letter capitalised (e.g., Homo).
Genetic code: The system by which triplets in a DNA base sequence determine the sequence of amino acids in a polypeptide chain. This code is degenerate, universal, and non-overlapping.
Genetic diversity: The total number of different alleles present in a population. This can be studied by examining observable characteristics, DNA sequences, messenger RNA base sequences, and amino acid sequences.
Genome: The complete collection of genes present in a cell.
Histones: Proteins that combine with DNA to form chromosomes in the nuclei of eukaryotic cells.
Homologous chromosomes: A matching pair of chromosomes, one inherited from each parent, that carry genes for the same characteristics at corresponding positions.
Independent segregation: The random separation of homologous chromosomes during the first division of meiosis, which contributes to genetic variation.
Index of diversity (d): A measure that describes the relationship between the number of different species and the relative abundance of individuals in each species within a community. It is calculated using the formula:
where represents the diversity index, is the total number of organisms of all species, and is the total number of organisms of each individual species.
Intron: A section of DNA that does not code for proteins.
Locus: The specific position where a gene is located on a chromosome.
Mean (normal distribution curve): A statistical measure representing the peak point of a normal distribution curve.
Meiosis: A form of cell division that generates four genetically distinct daughter cells (gametes) containing half the chromosome number of the parent cell. This process involves two consecutive divisions.
Messenger RNA (mRNA): A form of RNA that transports genetic information from the nucleus to the ribosomes for protein construction. It consists of a single helix made up of thousands of individual nucleotides.
Mitosis: A type of cell division that generates two genetically identical diploid daughter cells.
The key difference between meiosis and mitosis is that meiosis produces genetically different gametes for sexual reproduction, while mitosis produces identical cells for growth and repair.
Mutagenic agent: A substance that increases the frequency of gene mutations beyond the normal rate.
Natural selection: The evolutionary process through which the frequency of beneficial alleles gradually increases within a population's gene pool over successive generations.
Non-coding sequence: DNA sequences that do not code for amino acid sequences, including repetitive base sequences and introns. These sequences constitute a substantial portion of eukaryotic nuclear DNA.
Non-disjunction: An error in chromosome separation during meiosis where homologous chromosomes fail to separate properly, potentially resulting in gametes with abnormal chromosome numbers.
Non-overlapping: A feature of the genetic code ensuring that each base in a sequence is read only once and belongs to just one triplet.
Phylogenetic classification: A classification system that groups organisms based on their evolutionary relationships and homologous characteristics, using a hierarchical structure where smaller groups nest within larger ones without overlap.
Phylogeny: The evolutionary relationships that exist between individuals or groups of organisms.
Prokaryotic DNA: Circular DNA molecules that exist without associated proteins.
Random sampling: A technique used to eliminate bias in data collection, such as creating a grid system and using randomly generated coordinates.
Recombination: The process where broken pieces of chromatid join with chromatid from a different chromosome during crossing over.
Ribosomes: Subcellular structures composed of small and large subunits where protein synthesis occurs.
RNA polymerase: An enzyme that travels along the DNA template strand, connecting adjacent nucleotides to create pre-messenger RNA.
Species: A group of similar organisms capable of interbreeding to produce viable, fertile offspring.
Species diversity: A measure that considers both the number of different species and the relative abundance of individuals within each species in a community.
Species richness: A measure of the total number of different species present in a community at any given time, representing one aspect of species diversity.
Specific name: The second part of an organism's scientific name, indicating its species, written in lowercase letters (e.g., sapiens).
Splicing: The post-transcription process in eukaryotic cells where introns are removed from pre-messenger RNA and exons are connected to form mature messenger RNA.
Stabilising selection: A form of natural selection that favours individuals with characteristics close to the population mean, thereby maintaining the existing traits of the population.
Standard deviation (normal distribution curve): A statistical measure indicating the spread of a normal distribution curve and showing the range of values.
Substitution: A form of gene mutation where one nucleotide base is replaced with another, which may alter an amino acid or, due to the degeneracy of the genetic code, produce the same amino acid.
Taxon: Each individual group within a phylogenetic classification system.
Transcription: The process of creating pre-messenger RNA in eukaryotes and messenger RNA in prokaryotes from a DNA template strand, representing the initial stage of protein synthesis.
Transfer RNA (tRNA): A form of RNA that transports specific amino acids to the ribosomes. It has a single-stranded, clover-leaf structure with one side longer than the other for amino acid attachment, and contains an anticodon at the opposite end that corresponds to a specific amino acid.
Translation: The second stage of protein synthesis occurring in the ribosomes, where messenger RNA serves as a template for attaching transfer RNA molecules with matching anticodons, and the amino acids carried by adjacent transfer RNA molecules link together to form a polypeptide chain.
Triplet: A sequence of three bases that specifies a particular amino acid.
Universal: A characteristic of the genetic code indicating that the same code operates in nearly all organisms, providing evidence for evolutionary relationships.
Variation: The differences observed between individuals resulting from genetic factors, environmental influences, or a combination of both.
Key Points to Remember:
- Meiosis creates genetic variation through independent segregation and crossing over
- Classification can be artificial (based on analogous features) or phylogenetic (based on evolutionary relationships)
- Species diversity considers both the number of species and their relative abundance in a community
- Gene mutations can be substitutions or deletions, with different effects on the resulting proteins