4: Genetic information, variation and relationships between organisms

Question 1

Define the term exon.

Answer 1

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

Question 2

Describe Transcription in Eukaryotes [5]

Answer 2

1. Hydrogen bonds (between DNA bases) break;
2. (Only) one DNA strand acts as a template;
3. (Free) RNA nucleotides align by complementary base pairing;
4. (In RNA) Uracil base pairs with adenine (on DNA) OR (In RNA) Uracil is used in place of thymine;
5. RNA polymerase joins (adjacent RNA) nucleotides;
6. (By) phosphodiester bonds (between adjacent nucleotides);
7. Pre-mRNA is spliced (to form mRNA) OR Introns are removed (to form mRNA);

Question 3

Describe Translation [5]

Answer 3

1. (mRNA attaches) to ribosomes OR (mRNA attaches) to rough endoplasmic reticulum;
2. (tRNA) anticodons (bind to) complementary (mRNA) codons;
3. tRNA brings a specific amino acid;
4. Amino acids join by peptide bonds via condensation reaction;
5. (Amino acids join together) with the use of ATP;
6. tRNA released (after amino acid joined to polypeptide);
7. The ribosome moves along the mRNA to form the polypeptide;

Question 4

Describe the role of ATP in the process of translation in protein synthesis.

Answer 4

1. Releases energy;
2. (So) peptide bonds form between amino acids OR (So) amino acid joins to tRNA;

Question 5

Give two structural differences between a molecule of messenger RNA (mRNA) and a molecule of transfer RNA (tRNA).

Answer 5

1. mRNA does not have hydrogen bonds / base pairing, tRNA does; OR mRNA is linear / straight chain, tRNA is cloverleaf;
2. mRNA does not have an amino acid binding site, tRNA does;
3. mRNA has more nucleotides;
4. (Different) mRNAs have different lengths, all tRNAs are similar / same length;
5. mRNA has codons, tRNA has an anticodon;

Question 6

What is an allele?

Answer 6

(Different) form/type/version of a gene / different base sequence of a gene;

Formed via mutation.

Question 7

What is a gene?

Answer 7

• A sequence of DNA (nucleotide) bases that codes for a polypeptide;
• ‘codes for a functional RNA’
• ‘codes for rRNA/tRNAs’
• ‘codes for a sequence of amino acids/primary structure’

Question 8

Define degenerate code

Answer 8

More than one codon for one amino acid

Question 9

Explain what the term non-overlapping DNA means.

Answer 9

A base from one triplet cannot be used in an adjacent triplet

Question 10

Explain how selection occurs in living organisms [5]

Answer 10

1. Variation due to mutation;
2. Different environmental/abiotic/biotic conditions / selection pressures;
3. Selection for different/advantageous, features/characteristics/mutation/ /allele;
4. Differential reproductive success / (selected) organisms survive and reproduce;
5. Leads to change in allele frequency;
6. Occurs over a long period of time;

Question 11

A mutation in a gene coding for an enzyme
could lead to the production of a non-functional enzyme. Explain how [5]

Answer 11

1. Change in base sequence (of DNA/gene);
2. Change in amino acid sequence / primary structure (of enzyme);
3. Change in hydrogen/ionic/ disulphide bonds;
4. Change in the tertiary structure/active site (of enzyme);
5. Substrate not complementary/cannot bind (to enzyme / active site) / no enzyme-substrate complexes form;

Question 12

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

Answer 12

1. Triplets code for same amino acid
2. Occurs in introns /non-coding sequence;

Question 13

Define genome

Answer 13

(All) the DNA in a cell/organism;
(all) the ‘genes’/alleles’ ‘genetic material/code’ in a cell/organism/ person’

Question 14

Define proteome

Answer 14

(The proteome is the full) range of / number of different proteins that a cell is able to produce (at a given time);

OR

(The proteome is the full) range of / number of different proteins the genome / DNA is able to code for;

Question 15

Define ‘gene mutation’ and explain how a gene mutation can have:
* no effect on an individual
* a positive effect on an individual.

[6]

Answer 15

(Definition of gene mutation)
1. Change in the base/nucleotide (sequence of chromosomes/DNA);
2. Results in the formation of new allele;
(Has no effect because)
3. Genetic code is degenerate (so amino acid sequence may not change); OR
Mutation is in an intron (so amino acid sequence may not change);
Accept description of ‘degenerate’, eg some amino acids have more than one triplet/codon.
4. Does change amino acid but no effect on tertiary structure;
5. (New allele) is recessive so does not influence phenotype;
(Has positive effect because)
6. Results in change in polypeptide that positively changes the properties (of the protein) OR Results in change in polypeptide that positively changes a named protein; For ‘polypeptide’ accept ‘amino acid sequence’ or ‘protein’.
7. May result in increased reproductive success OR May result in increased survival (chances);

Question 16

Describe two aseptic techniques she would have used when transferring a sample of broth culture on to an agar plate.
Explain why each was important.

Answer 16

1. Keep lid on Petri dish OR Open lid of Petri dish as little as possible.
2. To prevent unwanted bacteria contaminating the dish.
   OR
3. Wear gloves / Wear mask / Wash hands;
4. To prevent contamination from bacteria on hands / mouth / Prevent spread of bacteria outside the lab;
   OR
5. Use sterile pipette / Flame the loop / Flame the neck of the container of the culture;
6. To maintain a pure culture of bacteria