This free quiz has 20 multiple choice questions on cellular control and manipulating genomes, all taken from OCR A Level Biology A exam papers. It marks itself, so you get your score as soon as you finish.
What the quiz covers (OCR A spec 6.1.1 and 6.1.3)
Types of gene mutation, such as deletions of a single base
Levels at which gene expression is controlled, from chromatin packing to after transcription
The lac operon: its parts and what an operon is
Hox and homeobox genes and how they control body plan
Cell processes involved in shaping the body plan
DNA sequencing methods, what they need and what sequencing is used for
Gel electrophoresis procedure and how it separates DNA fragments, plus the advantages of PCR
Concerns about genetic modification, and somatic versus germ line gene therapy
How to use it
Many questions ask which statement is not correct, or which numbered statements are correct, so judge each statement on its own before choosing a combination. Read every option before committing, as several look plausible at first glance.
Where students lose marks (from OCR examiners' reports and mark schemes)
Each point below is followed by wording that gained marks.
Stop codons and shorter chains: Many students did not convert the DNA triplets into mRNA codons first, called a mutation that creates an early stop codon a frameshift, or said the stop codon ends transcription early.
✔ Gains credit: Explaining that a premature stop codon (a nonsense mutation) ends translation, so the polypeptide chain is shorter with fewer amino acids; and that because the genetic code is degenerate, a silent mutation leaves the amino acid sequence unchanged.
Weighing DNA profile evidence: Many students only said the profiles matched and filled the space with examples, without considering what might weaken the evidence; others described bands as genes or bases, or mentioned a twin without saying identical.
✔ Gains credit: A match means the chance of two people sharing the profile is very low, but only a small number of loci were compared, so a close relative or an identical (monozygotic) twin could produce the same profile.
Bioinformatics, not bench methods: Many students described PCR, electrophoresis or Southern blotting, or talked about sequencing amino acids, when asked how genomes are compared or how bioinformatics and computational biology are used; definitions of DNA sequencing using 'base pairs' gained no credit.
✔ Gains credit: Sequencing the genomes, storing the sequence data in databases, and using software, algorithms, statistical tests or models to identify similarities and differences between DNA sequences; DNA sequencing defined as working out the order of nucleotides or bases.
Explaining electrophoresis changes: Many students suggested a buffer, a stain or a longer run without explaining why, linked extra time to DNA moving further rather than separating more, or confused the anode and cathode.
✔ Gains credit: Each change paired with its purpose, such as a buffer solution to carry the current and control pH, a dye, stain or labelled probe to make the bands visible, reference DNA fragments of known size for comparison, and a longer run so the fragments separate more.
Naming how DNA enters cells: Many students wrote 'vector' or 'injection' with no further detail, confused electroporation with electrofusion, or gave somatic cell nuclear transfer or restriction enzymes as a way of inserting a gene.
✔ Gains credit: Heat shock with calcium ions, or electroporation, to get plasmids into bacteria; and a named vector such as a virus, liposome or plasmid to deliver a gene into human cells in gene therapy.
How inserted genes disrupt others: Many students wrote 'mutation' without saying what kind, or discussed epistasis or meiosis, missing that the question was about other genes on the same chromosome.
✔ Gains credit: An inserted gene landing inside a functioning gene or its promoter can disable it or cause a frameshift that alters the triplets, and nearby genes on the same chromosome can be switched on or off.
Why PCR falls short: Many students blamed DNA being 'lost', 'not replicated correctly' or the enzyme denaturing, and some named RNA polymerase, ligase or helicase as the enzyme that builds new strands in PCR.
✔ Gains credit: Running out of primers or free nucleotides, primers failing to anneal, separated strands rejoining each other, and (Taq) DNA polymerase as the enzyme, which comes from thermophilic organisms and withstands higher temperatures than normal DNA polymerase.
Keep practising
Go on to the Genetics and Evolution quiz, the DNA and Protein Synthesis quiz or the Cloning and Biotechnology quiz, or browse all our OCR A Level Biology quizzes. Our OCR A masterclasses (Year 12 on Wednesdays at 5.15pm, Year 13 on Tuesdays at 6.30pm) work through real exam questions on one topic each week.