The Control of Gene Expression
The Control of Gene Expression is the A-level Biology topic covering mutations, transcription factors, epigenetics, cancer, and gene technologies including electrophoresis, PCR, genetic engineering and gene probes. It is the most synoptic biology topic, drawing on DNA structure, protein structure and cell biology from earlier topics, and includes log-graph and exponential (PCR) calculations.
Before you start
Make sure you're comfortable with these topics first:
Method
- Classify a mutation as silent, missense, nonsense or frameshift by comparing the original and mutated codon/amino acid sequence, using the degenerate nature of the genetic code to explain silent mutations.
- For transcription factor questions, describe the pathway in order: a signal molecule (e.g. a hormone) binds a transcription factor, the complex enters the nucleus, and it binds a specific DNA sequence to stimulate or inhibit transcription.
- For epigenetics, link the mechanism (DNA methylation or histone acetylation/deacetylation) directly to whether chromatin becomes more condensed (transcription reduced) or more open (transcription increased).
- For PCR calculations, remember the amount of DNA doubles every cycle, so final mass = starting mass x 2^(number of cycles); take logs to find the number of cycles needed for a target mass.
- For gel electrophoresis calibration graphs, remember the relationship between log(fragment size) and migration distance is linear even though fragment size and migration distance are not, so always work in log10(size) when reading values from the line.
- For genetic engineering questions, work through the process in order: cut the gene and vector with the same restriction enzyme, join them with DNA ligase, insert the vector into a host cell, then identify successfully transformed cells (e.g. using marker genes).
Worked example
A scientist set up a PCR reaction starting with 4.00 x 10^-3 ng of target DNA. Assuming 100% amplification efficiency, so the amount of DNA doubles at every cycle, show that the mass of DNA present after 18 complete cycles is approximately 1050 ng.
- After n cycles, the mass of DNA = starting mass x 2^n.
- Substitute the values given: mass = 4.00 x 10^-3 x 2^18.
- Calculate 2^18 = 262144.
- Multiply: 4.00 x 10^-3 x 262144 = 1048.576.
- Final answer: approximately 1050 ng (to 3 significant figures), confirming the value given.
Practice questions
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Q1Give a definition of the term gene silencing.Show answer
Answer: The prevention or reduction of the expression of a specific gene, so that little or no functional protein is produced from it.
Q2Name the enzyme used to join a human gene into a cut plasmid, and state the type of bond it forms.Show answer
Answer: DNA ligase; forms phosphodiester bonds.
Q3What term describes the specific DNA sequence that a transcription factor binds to in order to switch on transcription of a gene?Show answer
Answer: Promoter (region).
Q4A single base substitution changes a DNA triplet, but because of the degenerate genetic code the amino acid coded for does not change. Name this type of mutation.Show answer
Answer: A silent mutation.
Q5Two calibration points on a gel electrophoresis calibration graph are: a 200 bp fragment migrated 46 mm, and a 2000 bp fragment migrated 10 mm. Calculate the gradient of the line plotting log10(fragment size/bp) against migration distance (mm), to 3 significant figures.Show answer
Answer: -0.0278 (log10(200) = 2.301, log10(2000) = 3.301; gradient = (3.301 - 2.301) / (10 - 46) = 1 / -36 = -0.0278)
Q6A forensic scientist starts a PCR reaction with 5.00 x 10^-3 ng of target DNA. Calculate the minimum number of complete cycles needed to produce at least 640 ng of DNA, assuming the amount of DNA doubles every cycle.Show answer
Answer: 17 cycles (640 / 0.005 = 128000; 2^17 = 131072 which is at least 128000, while 2^16 = 65536 is not enough)
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
A gene can undergo either a single base deletion or a single base substitution. Explain which of these two mutation types is more likely to have a significant effect on the amino acid sequence of the polypeptide produced, and why.
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Explain how increased methylation of DNA can silence a gene.
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A genetic engineer produces recombinant bacteria that express a human gene. The human gene is inserted into a bacterial plasmid carrying a kanamycin-resistance gene and a separate tetracycline-resistance gene, with the human gene cloned into the middle of the tetracycline-resistance gene so that it disrupts it. Describe how replica plating bacterial colonies onto kanamycin-only and tetracycline-only agar plates would allow colonies that have taken up the recombinant (human-gene-containing) plasmid to be distinguished from colonies with no plasmid or colonies containing a re-formed (non-recombinant) plasmid.
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See real A Level Science past-paper questions, with official mark schemes →
Free printable worksheet
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