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EnglandBiology

3.4 Genetic information, variation and relationships between organisms

6 dot points across 6 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

How is the base sequence of a gene used to build a specific polypeptide through transcription and translation?

The genetic code is universal, non-overlapping and degenerate. Transcription produces mRNA from DNA, in eukaryotes pre-mRNA is spliced to remove introns, and translation at ribosomes uses tRNA and the genetic code to assemble a polypeptide from amino acids.
An exam-focused answer to the AQA A-Level Biology 3.4.2 dot point on protein synthesis. Walks through transcription, splicing of pre-mRNA, the roles of mRNA, tRNA and the ribosome, and translation, with the properties of the genetic code.
11 min answer →

How is genetic information stored differently in prokaryotic and eukaryotic cells, and what does a genome actually contain?

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. A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA. The genome is the complete set of genes in a cell and the proteome is the full range of proteins a cell can produce.
An exam-focused answer to the AQA A-Level Biology 3.4.1 dot point on DNA, genes and chromosomes. Compares prokaryotic and eukaryotic DNA, defines gene, locus, allele, genome and proteome, and explains exons, introns and the triplet code.
10 min answer →

How do gene mutations arise, and why do some change the polypeptide while others have no effect?

Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base substitution and base deletion. Because the genetic code is degenerate, not all mutations result in a change to the amino acid sequence. Mutagens increase the rate of mutation, and mutations are one source of genetic diversity within a gene pool.
An exam-focused answer to the AQA A-Level Biology 3.4.3 dot point on mutation. Explains base substitution and deletion, frameshift effects, why the degenerate code buffers some mutations, the role of mutagens, and how mutation contributes to genetic diversity.
9 min answer →

How does natural selection act on genetic diversity to produce adaptation, and how do the three types of selection differ?

Genetic diversity within a population, expressed as the number of different alleles in a gene pool, is acted on by natural selection. Random mutation produces new alleles, and selection results in changes in allele frequency. Directional and stabilising selection produce different effects, and selection leads to anatomical, physiological and behavioural adaptations that increase the chance of survival and reproduction.
An exam-focused answer to the AQA A-Level Biology 3.4.5 dot point on natural selection. Explains how selection changes allele frequencies, contrasts stabilising, directional and disruptive selection, and covers anatomical, physiological and behavioural adaptations.
11 min answer →

How does meiosis produce gametes that are genetically different from each other and from the parent cell?

Meiosis produces haploid daughter cells from a diploid parent cell, halving the number of chromosomes so that fertilisation restores the diploid number. Genetic variation arises from independent segregation of homologous chromosomes and from crossing over between homologous chromosomes during meiosis, and the number of possible combinations can be calculated.
An exam-focused answer to the AQA A-Level Biology 3.4.4 dot point on meiosis. Explains how two divisions halve the chromosome number, how independent segregation and crossing over generate variation, and how to calculate the number of possible chromosome combinations.
10 min answer →

How are organisms classified into a phylogenetic hierarchy, and how is courtship behaviour used to identify and isolate species?

A species is a group of similar organisms able to reproduce to give fertile offspring. Each species is given a binomial name. Courtship behaviour helps members of a species to recognise each other and is used in classification. Phylogenetic classification arranges species into a hierarchy of groups that share a common ancestor, and the taxa from domain to species reflect evolutionary relationships.
An exam-focused answer to the AQA A-Level Biology 3.4.6 dot point on species and taxonomy. Defines species and the binomial system, explains phylogenetic classification and the taxonomic hierarchy from domain to species, and covers the role of courtship behaviour.
10 min answer →