Genetic Information, Variation & Relationships between Organisms Exam Qs

Question 1

A mutation of a tumour suppressor gene can result in the formation of a tumour. Explain how. (2 marks)

Answer 1

1. (Tumour suppressor) gene inactivated / not able to control / slow down cell division;
   Ignore: references to growth
2. Rate of cell division too fast / out of control.
   1 and 2 Accept: mitosis 1 and 2
   Reject: meiosis

Question 2

Not all mutations result in a change to the amino acid sequence of the encoded polypeptide.
Explain why. (1 mark)

Answer 2

1. (Genetic) code degenerate;
   Accept: codon for triplet
   Accept description of degenerate code, e.g. another triplet codes for the same amino acid
2. Mutation in intron.
   Accept: mutation in non-coding DNA

Question 3

Some cancer cells have a receptor protein in their cell-surface membrane that binds to a hormone called growth factor. This stimulates the cancer cells to divide.
Scientists have produced a monoclonal antibody that stops this stimulation.
Use your knowledge of monoclonal antibodies to suggest how this antibody stops the growth of a tumour. (3 marks)

Answer 3

1. Antibody has specific tertiary structure / binding site / variable region;
   Do not accept explanations involving undefined antigen
2. Complementary (shape / fit) to receptor protein / GF / binds to receptor protein / to GF;
   Ignore: same shape as receptor protein / GF
3. Prevents GF binding (to receptor).

Question 4

In the diagram above, the first codon is AUG. Give the base sequence of:
-the complementary DNA base sequence
-the missing anticodon
(2 marks)

Answer 4

TAC
UAC

Question 5

Aspartic acid and proline are both amino acids. Describe how two amino acids differ from one another. You may use a diagram to help your description. (1 mark)

Answer 5

Have different R group
accept in diagram

Question 6

Aspartic acid GAC, GAU
Proline CCA, CCG, CCC, CCU

Deletion of the sixth base (G) in the sequence shown in the diagram above would change the nature of the protein produced but substitution of the same base would not. Use the information in the table and your own knowledge to explain why. (3 marks)

Answer 6

1. Substitution would result in CCA / CCC / CCU;
2. (All) code for same amino acid / proline;
3. Deletion would cause frame shift / change in all following codons / change next codon from UAC to ACC.

Question 7

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell. (6 marks)

Answer 7

1. Helicase;
2. Breaks hydrogen bonds;
3. Only one DNA strand acts as template;
4. RNA nucleotides attracted to exposed bases;
5. (Attraction) according to base pairing rule;
6. RNA polymerase joins (RNA) nucleotides together;
7. Pre-mRNA spliced to remove introns.

Question 8

Describe the structure of proteins (5 marks)

Answer 8

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. Formed by condensation;
4. Primary structure is order of amino acids;
5. Secondary structure is folding of polypeptide chain due to hydrogen bonding;
   Accept alpha helix / pleated sheet
6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds;
7. Quaternary structure is two or more polypeptide chains.

Question 9

Name the fixed position occupied by a gene on a DNA molecule. (1 mark)

Answer 9

Locus/loci;

Question 10

Describe how a gene is a code for the production of a polypeptide. Do not
include information about transcription or translation in your answer. (3 marks)

Answer 10

1. (Because) base/nucleotide sequence;
2. (In) triplet(s);
3. (Determines) order/sequence of amino acid sequence/primary structure (in polypeptide);

Question 11

Define the term exon. (1 mark)

Answer 11

Base/nucleotide/triplet sequence coding for polypeptide/sequence of amino acids/primary structure;

Question 12

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule. (2 marks)

Answer 12

1. Condensation (reaction)/loss of water;
2. (Between) phosphate and deoxyribose;
3. (Catalysed by) DNA polymerase;
   Reject if DNA polymerase joins AT/GC
   OR
   complementary nucleotides/bases
   OR
   forms hydrogen bonds

Question 13

The two DNA strands of a particular gene contain 168 guanine bases between them. The relationship between the numbers of guanine bases (G), adenine bases (A), thymine bases (T) and cytosine bases (C) in these two strands of DNA is shown in the following equation.
G = 4(A + T) – C
Use this information and your understanding of DNA structure to calculate the maximum number of amino acids coded by this gene.
Show your working. (2 marks)

Answer 13

Correct answer for 2 marks = 70;;
Accept for 1 mark,
A = 42 and T = 42
OR
420 (total bases in gene)
OR
210 (bases in template strand)

Question 14

Name the protein associated with DNA in a chromosome. (1 mark)

Answer 14

Histone;

Question 15

Scientists investigated the genetic diversity between several species of sweet potato. They studied non-coding multiple repeats of base sequences.
Define ‘non-coding base sequences’ and describe where the non-coding multiple repeats are positioned in the genome. (2 marks)

Answer 15

1. DNA that does not code for protein/polypeptides
   OR
   DNA that does not code for (sequences of) amino acids
   OR
   DNA that does not code for tRNA/rRNA;
   Accept the idea of not transcribed for ‘does not code for’.
   Do not credit ‘DNA that does not code for an amino acid’.
   Ignore reference to introns.
2. (Positioned) between genes;
   Reject (positioned) ‘in introns’ or ‘between exons’.
   Accept ‘(Positioned) at the end of chromosomes’ or ‘(Positioned) in the telomeres’.

Question 16

The nucleus and a chloroplast of a plant cell both contain DNA.
Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus. (3 marks)

Answer 16

In chloroplasts
Must be comparative statements.
Accept alternatives in context of nuclear DNA

1. DNA shorter;
   Accept smaller
2. Fewer genes;
3. DNA circular not linear;
   Accept DNA in a loop not linear
   Accept no chromosomes (in chloroplast) unlike nucleus
4. Not associated with protein/histones, unlike nuclear DNA;
5. Introns absent but present in nuclear DNA;
   Ignore references to double and single stranded DNA

Question 17

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide.
Give two reasons why. (2 marks)

Answer 17

1.Triplets code for same amino acid
Accept: DNA/code/triplets are degenerate Reject: codons (as question states within genes)

2. Occurs in introns /non-coding sequence;
   Reject: codons (as question states within genes) Ignore junk DNA
   Reject: multiple repeats

Question 18

Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells. (5 marks)

Answer 18

Comparisons
1. Nucleotide structure is identical;
Accept labelled diagram or description of nucleotide as phosphate, deoxyribose and base
2. Nucleotides joined by phosphodiester bond;
OR
Deoxyribose joined to phosphate (in sugar, phosphate backbone);
3. DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes;
Accept shorter than nuclear DNA/is circular not linear/is not associated with protein/histones unlike nuclear DNA;

Contrasts
4. Eukaryotic DNA is longer;
5. Eukaryotic DNA contain introns, prokaryotic DNA does not;
6. Eukaryotic DNA is linear, prokaryotic DNA is circular;
7. Eukaryotic DNA is associated with / bound to protein / histones, prokaryotic DNA is not;

Question 19

In Figure 2 the chromosomes are arranged in homologous pairs. What is a homologous pair of chromosomes? (1 mark)

Answer 19

(Two chromosomes that) carry the same genes;
Reject ‘same alleles’
Accept ‘same loci’ (plural) or ‘genes for the same characteristics’