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 the pentose deoxyribose and the base uracil in place of thymine
• BRNA contains the pentose ribose and the base thymine in place of uracil
• CRNA contains the pentose deoxyribose and the base thymine, exactly as DNA does
• 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 and no parental DNA
• 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 unwinds the double helix and copies both strands into two complementary mRNA molecules
• CThe mRNA produced is identical in base sequence to the template (antisense) strand it was copied from
• DTranscription incorporates thymine opposite adenine in the mRNA, just as DNA replication does

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 so that the ribosome can read the gene directly

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 introduces an extra disulfide bond that distorts the tertiary structure
• 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, so the gene can no longer be transcribed

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.

What are the components of a DNA nucleotide?

• ARibose, a phosphate group and a base
• BDeoxyribose, a phosphate group and a base✓
• CDeoxyribose, an amino group and a base
• DRibose, an amino group and a base

Answer: A DNA nucleotide is a deoxyribose sugar, a phosphate and one of adenine, thymine, cytosine or guanine. RNA uses ribose and replaces thymine with uracil; amino groups belong to amino acids.

Why is DNA replication described as semi-conservative?

• AEach new molecule keeps one original strand✓
• BEach new molecule keeps both original strands
• CEach new molecule contains no original strands
• DHalf of the new molecules are entirely original

Answer: The double helix unwinds and each strand templates a new partner, so every daughter molecule is one old strand and one new. Meselson and Stahl demonstrated this using nitrogen isotopes of different densities.

What is the role of DNA helicase in replication?

• ABreaking phosphodiester bonds along each strand
• BJoining nucleotides together on the new strand
• CBreaking hydrogen bonds between the two strands✓
• DJoining fragments of the lagging strand together

Answer: Helicase unwinds the helix by breaking the hydrogen bonds between complementary bases. DNA polymerase then forms the phosphodiester bonds of the new strand, and ligase seals the fragments.

What happens during transcription?

• AmRNA is made from a DNA template in the cytoplasm
• BProtein is made from an mRNA template in the nucleus
• CmRNA is made from a DNA template in the nucleus✓
• DProtein is made from an mRNA template in the cytoplasm

Answer: RNA polymerase builds a complementary mRNA copy of one DNA strand inside the nucleus. Translation — building the protein from that mRNA — happens afterwards on ribosomes in the cytoplasm.

What is a codon?

• AThree tRNA bases coding for one amino acid
• BThree mRNA bases coding for one amino acid✓
• CThree DNA bases coding for one whole protein
• DOne mRNA base coding for one whole amino acid

Answer: The genetic code is read in non-overlapping triplets, each specifying one amino acid or a stop signal. The complementary triplet on tRNA is the anticodon — same three bases, opposite molecule.

Why must pre-mRNA be spliced before translation can occur in a eukaryotic cell?

• AThe introns must be removed and the exons joined together✓
• BThe ribosome can only bind a fully double-stranded strand
• CThe bases must be converted from thymine into uracil first
• DThe molecule must be shortened to fit through a nuclear pore

Answer: Eukaryotic genes contain non-coding introns. These are cut out of the pre-mRNA and the coding exons joined, giving mature mRNA that leaves the nucleus. Prokaryotes have no introns, which is why they cannot express a eukaryotic gene taken straight from DNA.

What is meant by the term codon?

• AA sequence of three bases on mRNA coding for one amino acid✓
• BA sequence of three bases on tRNA that binds to the ribosome
• CThe complete sequence of bases coding for a functional protein
• DA sequence of three amino acids joined by two peptide bonds

Answer: A codon is a triplet of mRNA bases specifying one amino acid or a stop signal. The complementary triplet carried on tRNA is the anticodon, and a full coding sequence for a protein is a gene.

What is an intron?

• AA coding DNA sequence retained within mature mRNA
• BA non-coding DNA sequence found only in prokaryotes
• CA non-coding DNA sequence removed from pre-mRNA✓
• DA coding DNA sequence found only in mitochondria

Answer: Eukaryotic genes contain introns that are spliced out of pre-mRNA, leaving the exons joined as mature mRNA. Prokaryotes generally lack introns, which is one reason their genes can be transcribed and translated simultaneously.

Why can a substitution mutation have no effect on the protein?

• AThe new codon may specify the same amino acid✓
• BThe new codon may be removed during splicing
• CSubstitutions never alter the DNA base sequence
• DSubstitutions always occur within intron regions

Answer: Because the code is degenerate, changing the third base often still codes for the same amino acid — a silent mutation. Deletions and insertions are far more damaging because they shift the whole reading frame.

In one strand of a DNA molecule, 22% of the bases are adenine, 30% are thymine and 28% are guanine. What percentage of the bases in the complementary strand are guanine?

• A28%
• B14%
• C24%
• D20%✓

Answer: Guanine in the complementary strand pairs with cytosine in the first strand. Cytosine in the first strand = 100 − 22 − 30 − 28 = 20%, so the complementary strand is 20% guanine. The tempting 28% copies the first strand's guanine, which pairs with cytosine instead; 24% wrongly assumes G = C within one strand, a rule that holds only for the double-stranded molecule.

Bacteria were grown for many generations in a medium containing only heavy nitrogen (¹⁵N), then transferred to a medium containing only light nitrogen (¹⁴N). DNA was extracted and centrifuged after each round of replication.

If replication is semi-conservative, in what ratio would the DNA molecules form bands after three rounds of replication in ¹⁴N?

• A1 hybrid : 1 light, with no heavy band
• B1 heavy : 7 light, with no hybrid band
• C1 hybrid : 3 light, with no heavy band✓
• DA single hybrid band holding the whole sample

Answer: Each original molecule has two ¹⁵N strands. After three rounds it gives 2³ = 8 molecules; the two original strands end up in two hybrid (¹⁵N/¹⁴N) molecules and the other six are fully light. 2 : 6 = 1 hybrid : 3 light. The tempting 1 : 1 ratio is the result after two rounds; 1 heavy : 7 light is what conservative replication would give, and a single hybrid band appears after one round.

An antibiotic was added to a culture of bacteria. After 30 minutes, compared with an untreated culture: Concentration of mRNA: 98% Rate of amino acid incorporation into protein: 9% DNA content per cell: 100%

Which process is the antibiotic most likely to inhibit?

• ATranscription of genes by RNA polymerase
• BReplication of DNA by DNA polymerase
• CTranslation of mRNA at the ribosomes✓
• DSplicing of introns out of pre-mRNA

Answer: Protein synthesis has fallen to 9% while mRNA and DNA are almost unchanged, so the step after mRNA production is blocked: translation. Blocking transcription would lower mRNA, and blocking replication would affect DNA content. Bacterial genes do not contain introns, so splicing is not a step in bacterial protein synthesis.

The start of the mRNA for a 200-amino-acid polypeptide reads AUG UUC GCA GGU … A mutation deletes bases 5, 6 and 7 of this mRNA (counting the A of AUG as base 1). UUC codes for phenylalanine, GCA for alanine and UCA for serine.

What is the effect on the polypeptide?

• AIt has 199 amino acids: phenylalanine becomes serine, alanine is lost, the rest is unchanged✓
• BIt has 199 amino acids: alanine is lost and each of the other amino acids is unchanged
• CIt has a changed sequence after the first amino acid, as the deletion causes a frameshift
• DIt has 197 amino acids, because removing three bases also removes three of its amino acids

Answer: Deleting bases 5–7 (U, C, G) leaves AUG U|CA GGU…, which reads AUG UCA GGU. The second codon becomes UCA (serine instead of phenylalanine), the alanine codon disappears, and because three bases were lost the reading frame is restored from GGU onwards. The deletion spans two codons, so it is not simply the loss of alanine; three bases make one codon, so only one amino acid is lost, not three.