Flashcards · 7402 · AS Topic 4

Genetic information, variation and relationships between organisms (AS Level) flashcards

25 cards on AS Topic 4, subtopics 3.4.1 to 3.4.7. Pick a subtopic or study them all.

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Gene: A base sequence of DNA that codes for the amino acid sequence of a polypeptide, or for a functional RNA (including ribosomal RNA and tRNAs).
Three properties of the genetic code: Universal: the same triplet codes for the same amino acid in all organisms. Non-overlapping: each base belongs to one triplet only. Degenerate: most amino acids are coded by more than one triplet.
DNA in prokaryotes vs eukaryotes: Prokaryotes: short, circular DNA not associated with proteins. Eukaryotes: very long, linear DNA associated with histone proteins, forming chromosomes. DNA in mitochondria and chloroplasts is like prokaryotic DNA: short, circular, not associated with protein.
Locus, introns and exons: Locus: the fixed position of a gene on a DNA molecule. In eukaryotes much nuclear DNA does not code for polypeptides: genes contain introns (non-coding) separating exons (coding), and there are non-coding multiple repeats between genes.

Splicing of pre-mRNA in eukaryotes: Transcription in the nucleus produces pre-mRNA. Introns (non-coding) are removed and exons joined to give mRNA, which leaves through a nuclear pore. Prokaryotes make mRNA directly, with no splicing.
Genome and proteome: Genome: the complete set of genes in a cell. Proteome: the full range of proteins the cell is able to produce.
Transcription in order: DNA helicase breaks hydrogen bonds so the strands separate; free RNA nucleotides pair with complementary bases on the template strand; RNA polymerase joins them, forming pre-mRNA (in eukaryotes), which is then spliced to mRNA.
Translation in order: mRNA attaches to a ribosome. tRNA molecules, each carrying a specific amino acid, bind by their anticodons to complementary codons; peptide bonds form between adjacent amino acids, using ATP, until a stop codon is reached.

Base substitution compared with base deletion: Substitution changes one triplet: no change (degenerate code), one amino acid changed, or an early stop codon. Deletion shifts the reading frame, changing every triplet after it.
Non-disjunction: Failure of chromosomes (or chromatids) to separate in meiosis, giving gametes with one extra or one missing chromosome.
Two ways meiosis produces genetic variation: Independent segregation: homologous chromosomes go to either pole at random in meiosis I. Crossing over: non-sister chromatids of homologous chromosomes exchange DNA at chiasmata.
A species has 2n = 20. How many genetically different gametes can independent segregation give (no crossing over)?: n = 10 homologous pairs, so 2¹⁰ = 1024. Use the number of pairs, not the diploid number.

Genetic diversity: The number of different alleles of genes in a population.
Directional and stabilising selection: Directional: one extreme phenotype is favoured and the mean shifts (example: antibiotic resistance). Stabilising: phenotypes near the mean are favoured; the mean stays the same and the range narrows (example: human birth weight).
How does natural selection change allele frequency?: Random mutation produces new alleles. If an allele gives an advantage, individuals carrying it are more likely to survive and reproduce, passing it on, so its frequency rises over many generations.
Required practical 6: testing antimicrobial substances: Spread bacteria on agar using aseptic technique; place paper discs soaked in different antimicrobial substances (and a control disc) on the plate; incubate; measure the clear zone of inhibition around each disc.

Species, and the role of courtship behaviour: A species is a group of organisms able to interbreed to produce fertile offspring. Courtship is species-specific, so it allows species recognition and is a necessary precursor to successful mating.
The taxonomic hierarchy: Domain, kingdom, phylum, class, order, family, genus, species. Each species is named by the binomial system: genus then species, for example Homo sapiens.
Phylogenetic classification: Classification that groups organisms according to their evolutionary origins and relationships, often shown as a phylogenetic tree. Each group is placed within a larger group, with no overlap.

Index of diversity: d = N(N − 1) ÷ Σn(n − 1), where N is the total number of organisms of all species and n the number of one species. A higher d means more species and a more even spread.
Species richness: The number of different species in a community. It does not take account of how many individuals of each species there are, unlike the index of diversity.
How does farming reduce biodiversity?: Removing hedgerows, growing monocultures, using pesticides and herbicides, and filling in ponds reduce the number of habitats and species. Conservation schemes, such as keeping field margins, balance this against food production.

How can genetic diversity within or between species be compared?: By comparing the frequency of measurable or observable characteristics, the base sequence of DNA, the base sequence of mRNA, or the amino acid sequence of the proteins encoded by DNA and mRNA.
Why has gene technology changed how diversity is investigated?: It allows direct comparison of DNA base sequences and of proteins, rather than inferring diversity only from observable characteristics, which can be affected by the environment and by several genes.
Sampling for investigating diversity: random sampling, mean and standard deviation: Random sampling avoids bias, and a large sample reduces the effect of chance. The mean summarises the sample; the standard deviation shows the spread of the data about the mean.