Nucleic Acids and Protein Synthesis: Biology 9700 (Cambridge International AS & A Level)
Syllabus 6.1, 6.2 · Strand 2 Genetics, inheritance and evolution
- Questions
- 10
- Total marks
- 49
- Tier mix
- 10 Core
0 of 10 questions completed
Syllabus coverage
- 6.1 6 questions completed
- 6.2 5 questions completed
The information that builds and runs an organism is stored in its nucleic acids, and this topic (syllabus 6.1 and 6.2) explains how that information is held, copied and used. It starts with the nucleotide, the building block of both DNA and RNA, and the phosphorylated nucleotide ATP. DNA is a double helix of two antiparallel strands held together by complementary base pairing, with adenine pairing to thymine and cytosine to guanine, the strands linked along their length by phosphodiester bonds. During the S phase of the cell cycle, DNA is copied by semi-conservative replication, in which DNA polymerase and DNA ligase build new strands, and because polymerase works only in one direction, one strand is made continuously and the other in fragments.
The second half of the topic covers how genes are expressed. The genetic code is a universal set of base triplets, each specifying an amino acid or a start or stop signal. In transcription, RNA polymerase copies a gene into messenger RNA, which in eukaryotes is edited to remove introns. In translation, ribosomes read the mRNA codons while transfer RNA brings the matching amino acids, building a polypeptide. Finally, gene mutations by substitution, deletion or insertion of nucleotides are outlined, along with how each may change the protein produced.
The exam-style questions below are original, written to match these objectives, each with a full worked solution so you can check your reasoning step by step.
Question 1
In a molecule of DNA, which statement about the complementary base pairing between the two polynucleotide strands is correct?
Question 2
A student is comparing a sample of DNA extracted from the nucleus of an onion root tip cell with a sample of mRNA extracted from the same cell.
(a) Describe the structure of a single nucleotide found in a molecule of DNA. [3]
(b) State two structural differences between a molecule of DNA and a molecule of mRNA. [2]
(c) The two strands of the DNA double helix are described as antiparallel. Explain what this means. [2]
Question 3
During the S phase of the cell cycle, a molecule of DNA in a root tip cell of a bean plant is copied by semi-conservative replication before the cell divides.
(a) Describe the role of the enzyme DNA polymerase in this process. [3]
(b) Describe the role of the enzyme DNA ligase in this process. [2]
(c) Explain why this method of DNA replication is described as semi-conservative. [2]
Question 4
Which statement about the genetic code is correct?
Question 5
A short section of a gene in a moss plant has the following sequence on its template (antisense) strand, written from the 3' end to the 5' end:
3'-A T G C G C T T T G G A-5'
(a) State the base sequence, and its polarity (5' to 3' direction), of the mRNA transcribed from this template strand. [2]
(b) Describe how the enzyme RNA polymerase produces this mRNA molecule from the template strand. [3]
(c) Describe how a ribosome and transfer RNA (tRNA) molecules use this mRNA to build part of a polypeptide chain. [3]
(d) A substitution mutation changes the middle base of the second triplet on the template strand from G to T, so the triplet CGC becomes CTC. State the new codon produced in the mRNA at this position, name the amino acid it now specifies, and explain why a substitution mutation such as this does not cause a frameshift. [3]
Question 6
Which statement correctly describes why the genetic code is described as universal?
Question 7
A biochemist is investigating how a single strand of DNA is built up from individual nucleotides, and then works out the sequence of the complementary strand.
(a) Describe how a phosphodiester bond is formed between two DNA nucleotides as a polynucleotide strand is built up, including the type of reaction involved and the small molecule produced. [3]
(b) A new nucleotide can only be added to one particular end of a growing DNA strand. State this end (in terms of 5' and 3'), and explain, in terms of the free hydroxyl group involved, why growth can only happen at this end. [2]
(c) One strand of a short DNA molecule has the sequence 5'-G C T A A C-3'. State the base sequence, written in the 5' to 3' direction, of the complementary strand of this DNA molecule. [2]
Question 8
A gene in a maize (corn) plant is transcribed to produce a pre-mRNA molecule, which is then processed before it is translated at a ribosome.
(a) Explain the difference between the coding (sense) strand and the template (antisense) strand of this gene, and state which of these two strands is used directly by RNA polymerase to synthesise mRNA. [3]
(b) Describe what happens to the pre-mRNA (primary transcript) produced by transcription of this gene, before it leaves the nucleus as mature mRNA. [2]
(c) Explain why the introns must be removed from the pre-mRNA before the mRNA is translated, in terms of the amino acid sequence that would otherwise be produced. [2]
Question 9
An mRNA codon being read at a ribosome has the sequence 5'-G U C-3'. Which of the following is the anticodon, written 5' to 3', of the tRNA molecule that binds to this codon?
Question 10
A microbiologist grows a culture of bacteria for many generations in a growth medium in which the only available nitrogen source is a heavy isotope of nitrogen, so that all the nitrogen atoms in the bacterial DNA become heavy. The bacteria are then transferred to a growth medium containing only the normal, light isotope of nitrogen and allowed to divide once. The DNA is extracted and separated by density-gradient centrifugation, a technique that separates DNA molecules according to their density: denser (heavier) DNA forms a band further down the tube, and less dense (lighter) DNA forms a band further up the tube.
(a) Predict, with an explanation, the position of the band(s) of DNA that would be seen after this single round of replication in the light medium, if DNA replication is semi-conservative. [3]
(b) The bacteria are allowed to divide once more, still in the light medium, and the DNA is again separated by density-gradient centrifugation. Describe how the pattern of bands now seen would differ between the semi-conservative model and the (incorrect) conservative model of DNA replication. [3]