Protein Synthesis and Selection

Question 1

Contrast Eukaryotic DNA and Prokaryotic DNA (3)

Answer 1

1. Eukaryotic DNA is linear whereas Prokaryotic DNA is circular
2. Eukaryotic DNA is associated with histones whereas Prokaryotic DNA is not associated with histones
3. Eukaryotic DNA contains introns whereas Prokaryotic DNA does not contain introns

Question 2

Contrast DNA and mRNA (3)

Answer 2

1. DNA is double stranded whereas RNA is single stranded
2. DNA is longER whereas RNA is shortER
3. Thymine in DNA whereas thymine is replaced with uracil in RNA
4. Deoxyribose in DNA whereas ribose in RNA
5. DNA has hydrogen bonds whereas mRNA doesn’t have hydrogen bonds
6. DNA has introns whereas mRNA doesn’t have introns

Question 3

Define the term exon (1)

Answer 3

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

Question 4

Describe Transcription in Eukaryotes (5)

Answer 4

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
5. RNA polymerase joins (adjacent RNA) nucleotides
6. By phosphodiester bonds and condensation reactions
7. Pre mRNA is spliced

Question 5

Describe Translation (5)

Answer 5

1. mRNA attaches to ribosomes/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
5. with the use of ATP
6. tRNA released (after amino acid joined to polypeptide)
7. Ribosome moves along mRNA to form polypeptide chain

Question 6

Give two structural differences between a molecule of mRNA and tRNA (2)

Answer 6

1. mRNA does not have hydrogen bonds whereas tRNA does have hydrogen bonds
2. mRNA doesn’t have an amino acid binding site whereas tRNA does have amino acid binding site
3. mRNA has codons whereas tRNA has anticodons

Question 7

Define degenerate code (1)

Answer 7

more than one triplet/codon can code for the same amino acid

Question 8

Explain what the term non - overlapping means (1)

Answer 8

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

Question 9

Define what is meant by Universal code

Answer 9

the same three bases on mRNA/DNA code for the same amino acids in all organisms

Question 10

Describe what happens when the third base of a triplet is altered (2)

Answer 10

1. Silent mutation so it will not change the amino acid coded for
2. so will have no effect on the polypeptide chain
3. code is degenerate so more than one codon can code for the same amino acid

Question 11

Describe what happens when the first base of a triplet is altered (3)

Answer 11

1. will change the amino acid coded for
2. alters the sequence of amino acids on polypeptide chain
3. may alter the specific tertiary structure

Question 12

Describe the effect of an addition/deletion mutation towards the end of a sequence of bases. (2)

Answer 12

1. reading frame has been shifted to right/left so causes a frame shift
2. lesser effect but would still alter some amino acids at end of gene

Question 13

Explain the effect of an addition/deletion mutation at the start of a sequence of bases. (2)

Answer 13

1. reading frame has been shifted to right/left so causes a frame shift
2. all triplets coding for polypeptide would be altered so all amino acids would be altered

Question 14

Name 3 types of mutagenic agents and give 2 examples for each

Answer 14

1. High energy ionising radiation - X rays and gamma rays
2. DNA reactive chemicals - benzene and bromine
3. Biological agents - viruses and bacteria

Question 15

Name 3 chromosomal mutations

Answer 15

1. Inversion Mutations
2. Duplication Mutations
3. Translocation Mutations

Question 16

What happens during an inversion mutation? (1)

Answer 16

a segment of bases is reversed end to end

Question 17

What happens during a duplication mutation? (1)

Answer 17

a doubling part of a chromosome, of an entire chromosome, or even the whole genome - a section of DNA is duplicated

Question 18

What happens during a translocation mutation? (1)

Answer 18

groups of base pairs relocate from one area of the genome to another, usually between non-homologous chromosomes

Question 19

Explain how selection occurs in living organisms (5)

Answer 19

1. Variation due to mutation
2. Different abiotic/biotic factors lead to selection pressures
3. Selection will select the most advantageous allele
4. Differential reproductive success so organisms can survive and reproduce
5. Leads to change in allelic frequency
6. Occurs over a long period of time (from one generation to the next)

Question 20

Describe the difference between stabilising selection and directional selection (1)

Answer 20

Stabilising selection favours the average whereas directional selection favours an extreme phenotype

Question 21

What is an allele?

Answer 21

Different version of the same gene

Question 22

Give two types of molecule from which a ribosome is made

Answer 22

rRNA and ribosomal proteins

Question 23

A mutation in a gene coding for an enzyme could lead to the production of a non-functional enzyme. Explain how (3)

Answer 23

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

Question 24

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

Answer 24

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