genetics exam questions

Question 1

Describe two differences between the structure of a tRNA molecule and
the structure of an mRNA molecule

Answer 1

• tRNA molecule is a clover leaf shape, mRNA is linear
• mRNA has no amino acid binding point while tRnA does
• mRNA has codons while tRNA has anti codons
  ( max 2)

Question 2

In a eukaryotic cell, the structure of the mRNA used in translation is
different from the structure of the pre-mRNA produced by transcription.
Describe and explain a difference in the structure of these mRNA
molecules ( 2 marks)

Answer 2

mRNA has no introns/has (only) exons
because of splicing

Question 3

Describe how one amino acid is added to a polypeptide that is being
formed at a ribosome during translation.

Answer 3

1. tRNA brings specific amino acid (to ribosome);
2. Anticodon (on tRNA) binds to codon (on mRNA);
3. Amino acids join by peptide bond (using ATP)

Question 4

Describe how mRNA is formed by transcription in eukaryotes.

Answer 4

-Hydrogen bonds (between DNA bases) break;
-one DNA strand acts as a template;
-(Free) RNA nucleotides align by complementary base pairing;
-RNA polymerase joins (adjacent RNA) nucleotides;
- (By) phosphodiester bonds (between adjacent nucleotides);
Pre-mRNA is spliced to form mRNA

( 5 max)

Question 5

Describe how a polypeptide is formed by translation of mRNA.

Answer 5

(mRNA attaches) to ribosomes
OR
(mRNA attaches) to rough endoplasmic reticulum;
(tRNA) anticodons (bind to) complementary (mRNA) codons;
tRNA brings a specific amino acid;
Amino acids join by peptide bonds;
(Amino acids join together) with the use of ATP;
tRNA released (after amino acid joined to polypeptide)

Question 6

Give the two types of molecule from which a ribosome is made

Answer 6

rRNA and ribosomal protein

Question 7

The fruit fly has a gene that codes for an enzyme called alcohol dehydrogenase ). AD catalyses the breakdown of alcohol when alcohol is in the insects’ food.

The gene coding for AD has two alleles, ADF and ADS.
(a) The enzyme encoded by the ADF allele catalyses the breakdown of alcohol
faster
than the enzyme encoded by the ADS allele. Suggest why.

Answer 7

Different primary structure/amino acid sequence;

Different tertiary structure/shape of active site;

Enzyme-substrate complexes more likely (with enzyme from ADF
allele);

Question 8

Alcohol is toxic to fruit flies. Suggest and explain why the frequency of the
ADF allele increased during 45 generations.

Answer 8

Flies with ADF/allele have selective advantage (in the presence of
alcohol)
So insects (with ADF more likely to) reproduce;
Pass on ADF (allele/gene);
(So) allele frequency increases;

Question 9

In recent years, these farm animals have not been given tetracycline in
their food. Despite this, the percentage of bacteria resistant to tetracycline
has remained constant.
Suggest one reason why.

Answer 9

Bacteria resistant to tetracycline are passed on from one generation of
farm animals to the next (probably via faeces);

Question 10

On some farms, animals are routinely given antibiotics in their food.
Scientists investigated whether these farm animals had antibiotic-resistant
bacteria in their intestines. They tested the bacteria for resistance to two
antibiotics, tetracycline and streptomycin.

Suggest and explain one reason why bacteria resistant to tetracycline are
more common than bacteria resistant to streptomycin in these farm
animals.

Answer 10

Tetracycline used more often / in higher doses;
so a higher frequency of mutations has occurred for tetracycline resistance

Question 11

When are chromosomes referred to as chromatids ?

Answer 11

During anaphase where they are being pulled apart to opposite poles of the cell