Gene expression and regulation VCAA MCQ

The codon table can be used to determine amino acids coded for by a nucleotide sequence.

It is correct to state that

• A - identical amino acid sequences are found in all organisms

• B - the genetic code is degenerate with respect to Met.

• C - the codon GGU adds Trp to a polypeptide chain.

• D - the DNA template sequence GAA codes for Leu.

Answer explanation

D is correct - The DNA template sequence is GAA, this means the mRNA sequence for this codon is CUU. This codes for the amino acid Leu.

A is incorrect as the same amino acids are found in all organisms, but not the same sequences.

B is incorrect as the genetic code is NOT degenerate with respect with Met. This amino acid (the start amino acid) is always codes by AUG only.

C is incorrect as GGU codes for Gly.

The genetic code is described as a degenerate code. This means that

• A - in almost all organisms the same DNA triplet is transcribed to the same amino acid.

• B - some amino acids may be encoded by more than one codon

• C - a single nucleotide cannot be part of two adjacent codons

• D - three codons are needed to specify one amino acids

Consider the following sequence of six amino acids that make up part of a polypeptide:

phe --- leu --- pro --- val --- tyr --- ala

Another mutation within the original resulted in a shortened protein. The protein was truncated (cut short) after the amino acid valine, as shown below:

phe --- leu --- pro --- val

This truncated protein resulted from the codon

• A - UAU changing to UAA

• B - UAC changing to UGC

• C - GUA changing to UGA

• GUG changing to UGG

In humans, Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin-encoding DMD gene. The DMD gene contains 79 exons. In some patients, duplication of one exon occurs.

If the number of nucleotides in the duplicated exon is divisible by three, the

• A - transcribed mRNA will contain many stop codons.

• B - length of each of the 79 exons will increase by three nucleotides.

• C - translated protein will be longer than the dystrophin protein found in a person without DMD.

• D - dystrophin of these patients will show one amino acid change in the sequence compared to normal dystrophin.

The codon table can be used to determine the sequence of amino acids coded for by a nucleotide sequence.

The following nucleotide occurs on the template strand of a DNA molecule.

AAA GCT ACC TAT CGG TTA

In a mutation, the eighth nucleotide in this sequence was changed from C to T. What would be the result of this mutation?

• A - The peptide chain would be shortened.

• B - The third amino acid would change from Thr to Ile.

• C - The fourth amino acid would change from Ile to Tyr.

• D - There would be no change in the amino acid sequence.

Answer explanation

The change would occur on the third codon.

On the DNA template strand this would change from:

ACC to ATC

This means the mRNA codons would change from:

UGG to UAG

This changes the amino acid from Cys to STOP, therefore shortening the length of the peptide chain.

The following nucleotide sequence occurs on the template strand of a DNA molecule.

AAA GCT ACC TAT CGG TTA

In a mutation, a T was inserted after the fourth nucleotide on the given template strand. The result of this mutation would be that

• A - all amino acids in this entire sequence would change.

• B - only the first amino acid in the sequence would change.

• C - only the second amino acid in the sequence would change.

• D - all amino acids after the first in the sequence would change.

Answer explanation

Inserting a T after the fourth nucleotide would change the sequence from:

AAA GCT ACC TAT CGG TTA

to

AAA GTC TAC CTA TCG GTT A

This would change every amino acid in this sequence from the point of the mutation. All amino acids would be different, except for the first one.

Transcription of a structural gene can be initiated by the

• A - presence of DNA polymerase.

• B - arrival of mRNA at a ribosome.

• C - increase in concentration of amino acids in the cytosol of a cell.

• D - absence of a protein that is usually attached to an operator for that structural gene.

Answer explanation

A is incorrect as RNA polymerase is needed to initiate transcription (NOT DNA polymerase).

B is incorrect as the arrival of mRNA at a ribosome initiates translation.

C is incorrect as the concentration of amino acids in the cytosol has no effect on starting transcription.

D is the correct answer. Absence of a repressor protein attached to an operator allows trasncription to go ahead for a structural gene. This is specific to prokaryotic cells.

Ricin is a naturally occurring, powerful poison that affects eukaryotic organisms. Large mammals that ingest small amounts of ricin can die within three to five days. Studies have concluded that ricin stops the movement of a ribosome along a specific molecule. This specific molecule has been labelled Molecule X in the diagram.

What is the name of molecule X?

• A - rRNA

• B - tRNA

• C - mRNA

• D - pre-mRNA

Answer explanation

The image shows a ribosome move along a molecule. This must be an image of translation occurring, therefore the molecule must be mRNA.

Ricin is a naturally occurring, powerful poison that affects eukaryotic organisms. Large mammals that ingest small amounts of ricin can die within three to five days. Studies have concluded that ricin stops the movement of a ribosome along a specific molecule. This specific molecule has been labelled Molecule X in the diagram.

Eukaryotic organisms die from ricin poisoning because the ricin immediately causes

• A - nerve damage to motor neurons

• B - the process of mitosis in the cells to stop

• C - irreversible damage to DNA in the nuclei of cells

• D - the process of protein synthesis in cells to stop

Answer explanation

The image shows translation occurring. Therefore ricin affects the process of protein synthesis.

Tryptophan is an amino acid that is produced by many bacteria. Genes that code for the production of tryptophan are found on bacterial DNA and together are called the trp operon.

In the process of gene regulation, a repressor protein (trp repressor) binds to the trp operon. When such binding occurs, the process of tryptophan production stops. This is illustrated in the diagram.

When the trp repressor binds to the trp operon, which one of the following enzymes would be blocked from functioning normally?

• A - RNA polymerase

• B - DNA polymerase

• C - endonuclease

• D - DNA ligase

Answer explanation

RNA polymerase is the enzyme associated with gene transcription.

We haven't covered these yet other types of enzyme yet, but their functions are listed below:

- DNA polymerase is used for copying both strands of DNA for processes such as mitosis and meiosis.

- Endonucleases are enzymes that cut strands of DNA at specific recognition sites.

- DNA ligase is an enzyme that joins two cut fragments of DNA or DNA together.