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LearnAQA7402First teaching 2015Paper 1 · Paper 1: Topics 1–4Genetic information, variation and relationships between organisms

Paper 1 · Module

Genetic information, variation and relationships between organisms

3 subtopics≈ 150 min in total

Start the module: Meiosis and variation Open graphing calculator

Meiosis and variation
Meiosis makes four genetically different haploid cells from one diploid cell, by two divisions. Meiosis I separates homologous chromosomes (pairs carrying the same genes): in prophase I homologues pair as bivalents and cross over at chiasmata, swapping sections of chromatids; in metaphase I bivalents line up randomly (independent assortment); in anaphase I homologues separate. Meiosis II separates sister chromatids, like mitosis. Variation comes from crossing over, independent assortment ( $2^n$ combinations) and random fertilisation. Non-disjunction (failure to separate) gives gametes with an extra or missing chromosome, e.g. Down syndrome (trisomy 21). Meiosis halves the chromosome number so fertilisation restores diploidy.
≈ 50 min
Classification and biodiversity
Organisms are classified into three domains (Bacteria, Archaea, Eukarya) and a hierarchy: domain, kingdom, phylum, class, order, family, genus, species; named binomially (Homo sapiens). Eukarya kingdoms: protoctists, fungi, plants, animals. Biodiversity includes ecosystem, species (richness and evenness) and genetic diversity. Measure with random sampling (quadrats at random coordinates) or systematic sampling (transects across a gradient). Mobile animals: mark–release–recapture, $N=\frac{n_1\times n_2}{m_2}$ . Simpson's index $D=1-\sum\left(\frac nN\right)^2$ (0 to 1; higher = more diverse). Spearman's rank tests correlation between two variables: $r_s=1-\frac{6\sum d^2}{n(n^2-1)}$ .
≈ 50 min
Transcription and translation
A gene is a sequence of DNA bases coding for a polypeptide. The code is triplet (3 bases = 1 amino acid), degenerate (most amino acids have several codons), non-overlapping and universal. Transcription (nucleus): RNA polymerase reads the template strand, building pre-mRNA; introns are spliced out, leaving exons. Translation (ribosome): mRNA codons pair with anticodons on tRNA carrying specific amino acids; peptide bonds form; the ribosome moves along until a stop codon. Gene mutations: substitution (may be silent, missense or nonsense), insertion or deletion (frameshift, usually serious). Sickle cell: one base substitution changes glutamic acid to valine in the β chain.
≈ 50 min