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AQA GCSE Biology 8461

1. Cell Biology

Cell division

Bacteria divide by binary fission, and in the right conditions they do it fast enough that the numbers get big quickly. This sub-topic also covers required practical 2 — growing bacteria safely and measuring inhibition zones.

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155 questions4 subtopicsFoundation tier1 required practical

What cell division covers

Every subtopic below has its own questions, mark schemes and notes.

4.1.1.6Binary fission and culturing microorganisms

Bacteria divide by binary fission, and in the right conditions they do it fast enough that the numbers get big quickly. This sub-topic also covers required practical 2 — growing bacteria safely and measuring inhibition zones.

4.1.2.1Chromosomes

Chromosomes are the packaged form of your genetic material. Get the nesting right — DNA is the molecule, a gene is a section of it, a chromosome is a whole coiled-up length carrying many genes — and most of this sub-topic falls into place.

4.1.2.2Mitosis and the cell cycle

The cell cycle is how a body cell turns itself into two identical body cells. Almost every mark here comes from getting the order of events right and from remembering that the two new cells are identical to the parent.

4.1.2.3Stem cells

A stem cell is one that hasn't yet decided what to be. The science is short; most of the marks come from the uses, the risks and the ethical arguments — and from knowing which points belong on which side.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Binary fission — The simple cell division by which prokaryotic cells reproduce.
Mean division time — The average time taken for one bacterial cell to divide into two.
Culture medium — The nutrient broth or agar jelly that microorganisms are grown in or on.
Colony — A visible cluster of microorganisms growing on the surface of an agar plate.
Inhibition zone — The clear area around a disc where bacteria have not been able to grow.
Pathogen — A microorganism that causes disease.
Chromosome — A coiled-up length of DNA, found in the nucleus, that carries many genes.
Gene — A section of DNA that controls the development of a characteristic.
DNA — The molecule that carries genetic information and that chromosomes are made of.
Nucleus — The part of a eukaryotic cell that contains the chromosomes.
Cell cycle — The series of stages a body cell goes through to produce two new cells.
Mitosis — The stage of the cell cycle in which the cell divides to give two genetically identical cells.
Daughter cell — One of the two new cells produced when a cell divides.
Cell fibres — The structures that pull the arms of the chromosomes to opposite ends of the cell during mitosis.
Stem cell — An undifferentiated cell that can divide to make more of itself and can differentiate into other cell types.
Differentiate — To become specialised for a particular job.
Clone — A genetically identical copy.
Therapeutic cloning — Making an embryo with the same genetic information as a patient so that stem cells from it won't be rejected.
Meristem — The region of a plant where growth occurs and where its stem cells are found.

Where marks get lost

The mistakes that come up again and again in this topic.

  • In population calculations, convert the units first and then raise 2 to the power of the number of divisions. Multiplying by 2 instead of using a power is the classic lost mark.
  • Halve the diameter before using area = πr². Putting the diameter in gives an answer four times too big.
  • 25 °C questions want the reason as well as the number: harmful pathogens are more likely to grow above it.
  • Each daughter cell gets exactly one copy of the circular DNA, but a variable number of plasmids. Examiners test that distinction.
  • Don't say a chromosome 'is' a gene, or that it controls one characteristic. One chromosome carries many genes.
  • If a question gives you a total number of chromosomes, the number of pairs is half of it — but the number of copies of any single chromosome is still two.
  • 'Two copies of each chromosome' means one from each parent. That is the mark-scheme answer to 'why are they in pairs'.
  • Mitosis does not halve the chromosome number — the daughter cells have the same number as the parent. Halving happens when sex cells are made, which is a different process.
  • For percentage-of-cells-in-mitosis calculations, add the counts together to get the total first. The number 'not dividing' is not the total.
  • 'Identical' is worth saying explicitly. Questions on why mitosis suits growth and repair usually carry a mark for it.
  • Be precise about which stem cells can become what: embryonic — any cell type; adult — only certain types; plant meristem — any plant cell type, for the plant's whole life.
  • 'Explain how stem cells could treat X' wants two steps: made to differentiate into the right cell type, then used to replace the patient's faulty cells.
  • In ethics questions, give the reasoning and not just the position. 'It's wrong' scores nothing; 'because an embryo is a potential human life' scores the mark.

Other topics

Cell biology Transport in cells Organisation in animals Enzymes and the human digestive system Plant tissues, organs and systems Non-communicable disease Communicable disease and plant defences Preventing and treating disease Monoclonal antibodies Photosynthesis Respiration Homeostasis and the nervous system Hormonal coordination DNA, genes and variation Reproduction Evolution and Classification Adaptations, interdependence and competition Organisation of an ecosystem Humans and biodiversity Working scientifically Practical skills

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