DNA & Protein Synthesis

A double-stranded DNA molecule is found to contain 30% adenine. According to Chargaff's rules, what percentage of the molecule is guanine?

• A20%✓
• B30%
• C40%
• D70%

Answer: In double-stranded DNA, A pairs with T and C pairs with G, so %A = %T and %C = %G. If A = 30%, then T = 30%, leaving 40% for C+G combined, so G = 20% (and C = 20%).

Which set of features correctly distinguishes an RNA nucleotide from a DNA nucleotide?

• ARNA contains deoxyribose and the base uracil
• BRNA contains ribose and the base thymine
• CRNA contains deoxyribose and the base thymine
• DRNA contains ribose and the base uracil instead of thymine✓

Answer: RNA nucleotides contain the pentose sugar ribose and use the base uracil in place of thymine. DNA nucleotides contain deoxyribose and use thymine. Both share adenine, guanine and cytosine.

The Meselson–Stahl experiment demonstrated that DNA replication is semi-conservative. What does 'semi-conservative' mean in this context?

• AEach new DNA molecule contains two newly synthesised strands
• BEach new DNA molecule contains one original (template) strand and one newly synthesised strand✓
• CThe original DNA molecule is fully conserved and a completely new copy is made separately
• DFragments of old and new DNA are randomly mixed within each strand

Answer: Semi-conservative replication means each daughter DNA molecule consists of one parental strand acting as a template and one newly synthesised complementary strand. The 'dispersive' model (random mixing within a strand) and the 'conservative' model were both ruled out by Meselson and Stahl.

Which enzyme joins adjacent nucleotides together by catalysing the formation of phosphodiester bonds on the new strand during DNA replication?

• ADNA helicase
• BRNA polymerase
• CDNA ligase
• DDNA polymerase✓

Answer: DNA polymerase catalyses the formation of phosphodiester bonds between the sugar and phosphate of adjacent nucleotides on the new strand. DNA helicase unwinds and separates the strands by breaking hydrogen bonds; RNA polymerase is used in transcription.

During transcription in a eukaryotic cell, which statement is correct?

• ARNA polymerase uses the template (antisense) strand to build a complementary mRNA strand✓
• BDNA polymerase copies both strands into mRNA
• CThe mRNA produced is identical in base sequence to the template strand
• DTranscription incorporates thymine opposite adenine in the mRNA

Answer: RNA polymerase reads the template (antisense) strand and assembles a complementary mRNA strand, inserting uracil (not thymine) opposite adenine. The mRNA is therefore complementary to the template and identical (except U for T) to the coding/sense strand.

The genetic code is described as degenerate. What does this mean?

• AEach codon can code for more than one amino acid
• BMost amino acids can be coded for by more than one codon✓
• CSome codons do not code for any amino acid and are never used
• DThe code is different in every organism

Answer: Degenerate means most amino acids are specified by more than one codon (e.g. leucine has six codons). This can reduce the effect of some point mutations. The code is non-overlapping and almost universal, and each codon specifies only one amino acid.

What is the role of transfer RNA (tRNA) during translation?

• AIt carries the genetic code from the nucleus to the ribosome
• BIt forms the structure of the ribosome
• CIt carries a specific amino acid and has an anticodon that binds to a complementary mRNA codon✓
• DIt catalyses the unwinding of the DNA double helix

Answer: Each tRNA carries a specific amino acid and bears an anticodon that base-pairs with the complementary codon on the mRNA, ensuring amino acids are added in the correct order. mRNA carries the code from the nucleus, and rRNA forms part of the ribosome.

A single base in a gene is substituted, but the amino acid sequence of the resulting protein is unchanged. This is best explained by which property of the genetic code?

• AThe code is overlapping
• BThe code is degenerate✓
• CThe code is non-universal
• DThe mutation must have been a deletion

Answer: Because the code is degenerate, a substitution can change a codon to another codon that still specifies the same amino acid (a synonymous or 'silent' mutation), leaving the protein unchanged. A deletion would usually cause a frameshift, altering many amino acids.

Why does the addition or deletion of a single nucleotide (an indel) usually have a more severe effect on a protein than a single base substitution?

• AIt always introduces a disulfide bond
• BIt causes a frameshift, altering every codon downstream of the mutation✓
• CIt changes the sugar in the DNA backbone from deoxyribose to ribose
• DIt prevents DNA helicase from binding

Answer: An insertion or deletion shifts the reading frame, so all codons after the mutation are read differently, typically changing many amino acids and often introducing a premature stop codon. A substitution affects at most one codon and may even be silent.

A length of mRNA contains 1500 nucleotides, of which a region is non-coding. If 1200 nucleotides code for amino acids, what is the maximum number of amino acids in the polypeptide produced (ignoring the stop codon)?

• A1200 amino acids
• B600 amino acids
• C400 amino acids✓
• D3600 amino acids

Answer: Each amino acid is coded for by a triplet of three bases (a codon), so 1200 coding nucleotides ÷ 3 = 400 amino acids. Only the coding region is translated, and a stop codon does not specify an amino acid.