Core — the two reasons bacteria are used
Core asks you to state why bacteria are useful in biotechnology and genetic modification. (1) Rapid reproduction rate — they divide by binary fission, roughly every 20-30 minutes in ideal conditions, so a culture scales to enormous numbers within hours. (2) Ability to make complex molecules — bacterial ribosomes and enzymes synthesise proteins such as human insulin once the gene is supplied. Name both when two are asked for.
Extended — ethics and plasmids add two more
The Supplement statement asks you to discuss, adding two reasons to the Core pair. (3) Fewer ethical concerns — bacteria are single-celled, non-sentient and not reared as food, so modifying them attracts fewer objections than modifying plants or animals. (4) Presence of plasmids — a plasmid can be cut open with a restriction enzyme, have a foreign gene sealed in with DNA ligase to form a recombinant plasmid, then be taken up by cells that multiply and express it.
Plasmid — the vector that makes GM possible
A plasmid is a small, circular loop of deoxyribonucleic acid (DNA) inside a bacterial cell, separate from the main bacterial chromosome. Both halves of that definition are needed for full marks. Being small, it can be extracted, opened and re-inserted far more easily than the main chromosome — which is why it, not the chromosome, is the vector carrying a foreign gene into a bacterium.
Drawn from real examiner reports.
"Found in the human body" scores zero
A recurring wrong answer to "state why bacteria are useful in biotechnology" is "bacteria are found inside the human body" or "bacteria are easy to find". Where bacteria live has nothing to do with why they are chosen. The mark scheme wants the properties that make them efficient to use: rapid reproduction, and making complex molecules.
Flagged s23 P12/13 Q38/Q40 — "bacteria are found inside the human body" was a common wrong answer
Plasmid ≠ bacterial chromosome
"The human gene is inserted into the bacterial chromosome" is not credited. The foreign gene goes into a plasmid — a small circular loop of DNA separate from the main chromosome — forming a recombinant plasmid the cell then takes up. Naming the chromosome describes the wrong piece of DNA. Memorise: "bacteria have plasmids that can take up genes."
Flagged s23 P23 Q39 — many wrote "bacterial chromosomes" or omitted the word plasmid
Genetic modification ≠ selective breeding
Genetic modification directly changes an organism's genetic material by removing, changing or inserting individual genes, often between species. Selective breeding chooses parents with the most desirable characteristics already in a population and breeds them over many generations — no gene is directly edited. Naming it when asked for genetic modification loses the mark.
Flagged s22 P42 Q6d; w22 P43 Q3vi — candidates named selective breeding when asked for genetic modification
"Biotechnology" alone is too vague
Biotechnology is the broad use of living organisms or their processes to make useful products — it covers fermentation, enzyme use and genetic modification. So when a question asks specifically about genetic modification, answering "biotechnology" names the whole family instead of the process, and is not credited alone. Give the gene-level definition.
Flagged s22 P42 Q6d; w22 P43 Q3vi — "biotechnology" was judged too vague and not credited alone
Bacterial growth is exponential, not linear
In population calculations, candidates multiply the starting number by the number of doublings instead of by 2 raised to that power — writing 200 × 4 = 800 where the answer is 200 × 2⁴ = 3200. Each division doubles the whole population. Work out the doublings first (total time ÷ generation time), then raise 2 to that power.
Binary fission is not mitosis
Bacteria are prokaryotes with no nucleus, so they reproduce by binary fission — one cell dividing into two genetically identical daughter cells. Mitosis is nuclear division in eukaryotic cells and is the wrong term here. Write "binary fission" whenever you explain how a bacterial culture multiplies rapidly in a fermenter.
Use the four-reason scaffold
Hold four reasons ready: reproduce rapidly; make complex molecules (Core); fewer ethical concerns; have plasmids that take up genes (Extended). Every 21.1 "why bacteria" question is answered from this list — nothing else is in this sub-topic.
Let the command word set the length
"State" (Core, 1-2 marks) wants brief named points with no explanation — one or two of the Core pair is enough. "Discuss" (Extended, 3-4 marks) signals that several points are expected, so bring in the ethical and plasmid reasons as well and add a short justification to each.
Honour "other than" exclusions
Questions often say "other than rapid reproduction, state two reasons". Repeating the excluded reason wastes the mark. Cross it off the four-reason list and answer from the remaining three — this is exactly why keeping all four separate in your head is worth the effort.
Keep the four reasons independent
In "evaluate the claim" questions, the trap is letting one reason cancel another — e.g. "bacteria reproduce quickly, so ethics need not be considered". Speed is a practical advantage; ethics is a separate reason. Say the claim is overstated, then treat each reason on its own.
Biotechnology is the use of living organisms (or their biological processes) to make useful products.
Genetic modification (genetic engineering) is changing the genetic material of an organism by removing, changing, or inserting individual genes.
Plasmid is a small, circular loop of deoxyribonucleic acid (DNA) found in a bacterial cell, separate from the main bacterial chromosome, that can be extracted, modified, and taken back up by bacterial cells.
Binary fission is the asexual method of reproduction bacteria use: one bacterial cell divides into two genetically identical daughter cells.
(Extended) Define a plasmid.
(a) State two reasons why bacteria are useful in biotechnology and genetic modification. [2]
(b) Define a plasmid. [2]
