Exam-style Questions: DNA and the Genetic Code

1. a) Draw and label a simple diagram of an RNA nucleotide containing uracil.

In 1961 biologists made synthetic mRNA. When they produced mRNA containing only uracil nucleotides, it coded for one type of amino acid, phenylalanine. When mRNA was produced with alternating uracil and guanine nucleotides, two types of amino acid were coded for, valine and cysteine. This is summarised in the table.

Nucleotide sequence in mRNA

Amino acids coded for

UUUUUUUUUUUUUUU

phenylalanine

UGUGUGUGUGUGUGU

valine and cysteine

b) For the amino acid phenylalanine what is

(i) the corresponding DNA base sequence,

(ii) the tRNA anticodon?

c) Explain how the information in the table supports the idea of a triplet code.

(Marks available: 8)

Answer

Answer outline and marking scheme for question: 1

Give yourself marks for the following:

a) Drawing shows phosphate, pentose and uracil;
arranged in correct sequence;
details of molecular structure correct;

(3 marks)

b) (i) AAA

(ii) AAA

(2 marks)

c) Single base coding for 4 amino acids;
double base coding for 42 amino acids;
GUG/UGU for valine;
UGU/GUG for cysteine;
only possible code UUU for phenylalanine.

(3 marks)

(Marks available: 8)

2. The diagram shows the structure of a tRNA molecule.

a) Give two ways in which the structure of a tRNA molecule differs from that of a DNA molecule.

b) Explain how the specific shape of the tRNA molecule shown in the diagram is determined by the pattern of bonding.

c) (i) Give the base sequence of the anticodon of this tRNA molecule.

(ii) Which mRNA codon would correspond to this anticodon?

(Marks available: 6)

Answer

Answer outline and marking scheme for question: 2

Give yourself marks for mentioning any of the points below:

a) ribose/deoxyribose;
single strand/double strand/helix;
uracil/thymine

(max 2 marks)

(b) bases pair by hydrogen bonding;
complementary pairing/GC/UA;
clover leaf shapes have no bonds/not compatible.

(max 2 marks)

c) (i) GAA

(ii) CUU

(2 marks)

(Marks available: 6)

3. Write an essay on:

Now nucleic acids are suited to their functions in living organisms.

(Marks available: 25)

Answer

Answer outline and marking scheme for question: 3

You can give yourself up to 2 marks for discussing any of the following issues in A, B, C and D up to a maximum of 16:

a) Types and structure of nucleic acid:

1. The types of nucleic acids - DNA, mRNA, tRNA;

2. The basic structure of a nucleic acid (e.g. DNA) -nucleotide chain, sugar phosphate backbone;

3. How DNA and RNA are different (to include both U vs T and ribose vs deoxyribose).

b) Storage of genetic information:

1. DNA is stable - because it has a double-stranded structure/is retained in the (eukaryote) nucleus - so is less likely to be corrupted;

2. mRNA is labile, (because it is single-stranded) so it is 'around' to promote (specific) protein synthesis for only a limited amount of time;

3. Explanation of general structure of tRNA/need for having a range of tRNA molecules.

c) Genetic information is able to be copied/transcribed:

1. Explanation of complementary base pairing - principle and detail - A=T(U), G=C;

2. Nature of replication of DNA (idea of 'accuracy' in mitosis/semi-conservative nature);

3. Nature of transcription of mRNA (location, enzymic process).

d) Genetic information is used in protein svnthesis:

1. Principle of coding/information transfer/'central dogma' (DNA to RNA to protein);

2. Details of the genetic code - triplet per amino acid, idea of 'dictionary' , sense and nonsense;

3. Way in which the sequence of nucleotides in DNA controls/is translated into - the amino acid
sequence in a polypeptide gene product;

4. Way in which a change in DNA sequence may alter the gene product/its potential activity.

Breadth (max 3 marks)

Does the answer appropriately:

  • deal with each of DNA, mRNA and tRNA?
  • locate the different processes in the cell?
  • Consider the breadth of functions (C & D)?

Selection of appropriate material ('Depth') (max 3 marks)

Does the answer appropriately:

  • describe/ give diagram of NA structure?
  • distinguish transcription & translation?
  • and fully explain the process of translation?

(Award one mark within each category for each point to maximum of 3)

In the real exam you will also be awarded marks for quality of language (in this case, there were 3 marks available). It may be difficult for you to judge your own text, however you may be able to get someone else to give you an opinion.

(Marks available: 25)