Exercises: Gene expression

Explain in your own words what it means when a cell "expresses a gene", and why the final result of this expression is a protein.

Look at the diagram of a eukaryotic cell representing gene expression: labels ?1 and ?2 show where transcription and translation take place, without indicating which is which. State which process occurs at each labelled point and where in the cell it takes place.

What is the genetic code? Explain what a codon is and how many bases make it up.

State two differences between transcription and translation: the molecule produced in each process and the location where it occurs.

The template strand of a gene has the sequence 3'-TAC GGA TTT-5'. Write the sequence of the messenger RNA synthesised from it, keeping in mind that in RNA, thymine is replaced by uracil.

Explain the role of the ribosome and transfer RNA (tRNA) during translation: where does each bind and what does tRNA transport?

Two patients with point mutations in the same codon of their mature mRNA are analyzed; the original codon was GAA (encodes glutamic acid):

• In patient A, the codon mutates to GAG (encodes glutamic acid).

• In patient B, it mutates to GUA (encodes valine).

1. a) Based on the properties of the genetic code, justify why patient A shows no alteration while patient B presents a non-functional enzyme.
2. b) Could a "silent mutation" like the one in patient A exist if the genetic code were not degenerate? Justify your answer by deducing what consequences mutations would have in a hypothetical code with only one codon per amino acid.

A bacterium synthesizes an oligopeptide from the following mature mRNA fragment:

5'-... AUG GCC UUU UAA ...-3'

A mutagenic agent causes the deletion (loss) of the first nucleotide of the codon GCC (the first cytosine), resulting in the sequence: 5'-... AUG GCU UUU AA...-3'.

Given that:

• AUG = Methionine (start)

• GCC = Alanine; GCU = Alanine; UUU = Phenylalanine; UUA = Leucine

• UAA = Stop (termination)

1. a) Write the resulting amino acid sequence before and after the mutation.
2. b) Justify why a reading frameshift mutation (deletion or insertion) usually has much more severe consequences for protein function than a single base substitution mutation.

A student claims: "Since a neuron and a skin cell have completely different functions and shapes, their DNA molecules must be different; for example, the neuron has eliminated the genes for skin proteins that it does not need."

1. a) Refute the student's statement by explaining what we would actually find if we sequenced the genomic DNA of both cell types.
2. b) If the DNA is identical, predict and justify what differences you would expect to find when analyzing the types of mRNA and proteins present in the cytoplasm of both cells.