Flashcards · H420 · Topic 6

Genetics, evolution and ecosystems flashcards

29 cards on Topic 6, subtopics 6.1.1 to 6.3.2. Pick a subtopic or study them all.

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6.1.1 Cellular control

Silent, missense and nonsense mutations: Silent: the altered codon codes for the same amino acid (the code is degenerate). Missense: one amino acid is changed. Nonsense: a stop codon forms, giving a shorter polypeptide.
When does an insertion or deletion cause a frameshift?: When the number of nucleotides added or lost is not a multiple of three. Adding or losing 3, 6, 9… nucleotides keeps the frame, adding or losing amino acids at that site only.
The lac operon: lactose absent vs present: Absent: the repressor (from lacI) binds the operator, blocking RNA polymerase, so lacZ and lacY are not transcribed. Present: lactose binds the repressor, changing its shape so it leaves the operator; lacZ (β-galactosidase) and lacY (permease) are transcribed.
Homeobox sequences vs Hox genes: A homeobox is a conserved sequence of about 180 base pairs coding for a DNA-binding domain of about 60 amino acids, found in plants, animals and fungi; the proteins act as transcription factors. Hox genes are a subset in animals, in clusters whose order matches the head-to-tail order in which they act.
Apoptosis and its role in development: Programmed cell death: the cytoskeleton breaks down, the chromatin condenses and the cell breaks into vesicles that phagocytes take up. It removes unwanted cells, such as the tissue between developing fingers and toes, and is controlled by cell signals.

6.1.2 Patterns of inheritance

Linkage vs epistasis: Linkage: genes on the same chromosome are inherited together, so parental classes are commonest. Epistasis: one gene masks another because their products interact, giving a modified 9 : 3 : 3 : 1 ratio.
Hardy–Weinberg equations: p + q = 1 (allele frequencies). p² + 2pq + q² = 1 (genotype frequencies). Dominant phenotype = p² + 2pq; recessive phenotype = q².
Genetic bottleneck vs founder effect: Bottleneck: an existing population falls sharply in number, so the survivors carry a reduced, random sample of alleles. Founder effect: a new population is started by a few migrants carrying only some of the alleles.
Chi-squared test: what it tells you: Whether the difference between observed and expected results is significant. Compare the calculated χ² with the critical value at p = 0.05 for (number of categories − 1) degrees of freedom: if it is larger, the difference is significant and not due to chance.
Stabilising vs directional selection: Stabilising: the average phenotype is favoured and both extremes are selected against, so variation narrows (human birth mass). Directional: one extreme phenotype is favoured, so the mean shifts (antibiotic resistance in bacteria).
Allopatric vs sympatric speciation: Allopatric: a geographical barrier separates populations, which then evolve separately until they cannot interbreed. Sympatric: populations in the same area become reproductively isolated by reproductive mechanisms, such as different courtship or breeding seasons.

6.1.3 Manipulating genomes

Why does a dideoxynucleotide stop DNA synthesis in sequencing?: It has no 3′ hydroxyl group, so no further nucleotide can be joined to it and the growing strand ends.
DNA profiling: what is compared?: Short tandem repeats (STRs): short base sequences repeated head to tail, with a variable number of repeats. A DNA profile is the combination of repeat lengths at several loci, separated by PCR and electrophoresis.
Somatic vs germ line gene therapy: Somatic: body cells are treated; the change is not inherited. Germ line: gametes, a zygote or early embryo are changed; the change is inherited.
PCR: the three steps and temperatures: Denaturation at about 95 °C separates the strands; annealing at about 55 °C lets primers bind; extension at about 72 °C, where Taq polymerase adds nucleotides. Each cycle doubles the amount of DNA.
Steps in genetic engineering: Cut out the gene with restriction enzymes (or make it from mRNA using reverse transcriptase); cut a plasmid vector with the same enzyme and join the sticky ends with DNA ligase; put the recombinant plasmid into host cells (for example by electroporation); identify transformed cells using marker genes.

6.2.1 Cloning and biotechnology

Embryo twinning vs somatic cell nuclear transfer (SCNT): Embryo twinning splits an embryo from a sexual cross, so the adult features are unknown. SCNT puts the nucleus of an adult body cell into an enucleated egg, so it copies an existing adult.
Batch vs continuous culture: Batch: a closed system that passes through all four growth phases (lag, exponential, stationary, death). Continuous: nutrients added and products removed, holding the culture in the exponential phase.
Natural cloning in plants (vegetative propagation): Plants reproduce asexually by structures such as runners (strawberry), rhizomes, bulbs and suckers (elm). Gardeners take cuttings, which grow roots and form genetically identical plants.
Micropropagation (tissue culture): Small pieces of tissue (explants), often from meristems, are sterilised and grown on a sterile medium with nutrients and plant hormones (auxin and cytokinin); they form a callus, which is divided and grows into many identical plantlets.
Advantages of immobilised enzymes: The enzyme does not contaminate the product, can be recovered and reused, and is often more stable over a wider range of temperature and pH. Example: immobilised lactase used to make lactose-free milk.

6.3.1 Ecosystems

Nitrogen fixation vs nitrification: Fixation: nitrogen gas → ammonium, by Azotobacter (free-living) and Rhizobium (in root nodules). Nitrification: ammonium → nitrite (Nitrosomonas) → nitrate (Nitrobacter).
Efficiency of transfer between trophic levels: Efficiency = energy (or biomass) in the higher trophic level ÷ energy (or biomass) in the lower one × 100. Energy is lost as heat from respiration, in parts not eaten, in faeces and in excretory products.
Primary succession: Pioneer species such as lichens colonise bare rock; their decay forms a thin soil; mosses, then grasses, shrubs and trees follow, until a stable climax community forms. Deflected succession: human activity, such as grazing, holds the community at a plagioclimax.
Abiotic vs biotic factors in an ecosystem: Abiotic: non-living, such as light, temperature, water availability, pH and soil type. Biotic: living, such as predation, competition, parasites and disease.

6.3.2 Populations and sustainability

Density-dependent vs density-independent factors, and carrying capacity: Density-dependent: effect strengthens as density rises (food, disease, predation, competition). Density-independent: effect is unrelated to density (frost, flood, drought). Carrying capacity: the maximum population an environment can sustain long term.
Interspecific vs intraspecific competition: Interspecific: between different species for the same resource; one species may outcompete the other. Intraspecific: between individuals of the same species; it keeps the population near the carrying capacity.
Conservation vs preservation: Conservation: active management to maintain biodiversity, which can include sustainable use of the resource. Preservation: protecting an area by keeping it untouched by human activity.
Sustainable management of timber and fish stocks: Timber: coppicing, selective felling and replanting. Fish: quotas, minimum mesh sizes, closed seasons and fish farming.