A-Level Biology Flashcards

What is the difference between mRNA, tRNA, and rRNA?

Answer: mRNA carries genetic code from DNA to ribosomes; tRNA brings specific amino acids; rRNA is a structural component of ribosomes

mRNA (messenger RNA) carries the genetic code from DNA to the ribosome. tRNA (transfer RNA) has an anticodon and carries a specific amino acid to the ribosome during translation. rRNA (ribosomal RNA) forms part of the ribosome structure.

What is photorespiration and why is it a problem for plants?

Answer: A wasteful process in C3 plants where RuBisCO fixes oxygen instead of CO₂, reducing photosynthetic efficiency

Photorespiration occurs in C3 plants when RuBisCO, the main CO₂-fixing enzyme, reacts with O₂ instead of CO₂ (especially at high temperatures or low CO₂). This produces no useful sugar, wasting energy and reducing yield.

What is the significance of the lac operon in bacteria?

Answer: It is an example of gene regulation where lactose-metabolising genes are switched on only when lactose is present and glucose is absent

The lac operon (E. coli) demonstrates inducible gene regulation. In the absence of lactose, a repressor binds to the operator, blocking transcription. Lactose (as allolactose) binds the repressor, removing it and allowing transcription of lactase genes.

What is the role of FSH in the human menstrual cycle?

Answer: To stimulate the development of a follicle in the ovary and oestrogen production

FSH (follicle-stimulating hormone) from the anterior pituitary stimulates follicle development in the ovary. The developing follicle produces oestrogen, which feeds back to stimulate LH surge and eventually inhibit FSH.

What is convergent evolution?

Answer: When unrelated species independently evolve similar traits in response to similar environmental pressures

Convergent evolution occurs when unrelated organisms independently evolve similar features due to similar selective pressures. Example: wings in birds and insects; streamlined body shape in sharks (fish) and dolphins (mammals).

What is the role of oestrogen in controlling the menstrual cycle?

Answer: At low concentrations it inhibits LH/FSH; at high concentrations it stimulates the LH surge that triggers ovulation; it also repairs the uterine lining

Oestrogen has complex feedback effects: at low levels it negatively feeds back to inhibit FSH/LH. At high levels near ovulation, it provides positive feedback triggering the LH surge. It also repairs and thickens the endometrium after menstruation.