TOPIC 4 MARK SCHEME SHIT

Question 1

Suggest how the change in the anticodon of a tRNA leads to mitochondrial disease (3)

Answer 1

1. Change to tRNA leads to wrong amino acid being incorporated into protein
2. Tertiary structure changed
3. Protein required for oxidative phosphorylation/Krebs cycle, so less/no ATP made

Question 2

Describe how a gene is a code for the production of a polypeptide. DO NOT INCLUDE INFO ABOUT TRANSCRIPTION OR TRANSLATION (3)

Answer 2

1. Base/nucleotide sequence
2. (In) triplet(s)
3. (Determines) order/sequence of amino acids/primary structure (in polypeptide)

Question 3

Define an exon (1)

Answer 3

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

NOTICE HOW “PROTEIN” ISN’T ALLOWED

Question 4

Define ‘non-coding base sequences’ and describe where the non-coding multiple repeats are positioned in the genome (2)

Answer 4

1. DNA that doesn’t code for protein/polypeptides/(sequences of) amino acids/tRNA/rRNA
2. (Positioned) between genes

Question 5

Give 2 reasons why not all mutations in the nucleotide sequence of a gene causes a change in the structure of a polypeptide (2)

Answer 5

1. Triplets code for same amino acid
   Accept: DNA/code/triplets are degenerate
2. Occurs in introns/non-coding sequence

Question 6

Suggest how a mutation can lead to the production of a protein with one missing amino acid (2)

Answer 6

Loss of codon/3 bases/triplet

Question 7

Describe how mRNA is produced in the nucleus of a cell

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 complementary base pair rule
6. RNA polymerase joins (RNA) nucleotides together via phosphodiester bonds
7. Pre-mRNA spliced to remove introns

Question 8

Describe how a polypeptide is formed by translation of mRNA

Answer 8

1. (mRNA attaches to) ribosomes/rough endoplasmic reticulum
2. ribosome moves to start codon
3. (tRNA) anticodons (bind to) complementary (mRNA) codons
4. tRNA brings a specific amino acid
5. Amino acids join by peptide bonds
6. (Amino acids join together) using ATP
7. tRNA released (after amino acid joined to polypeptide)
8. The ribosome moves along the mRNA to form the polypeptide

Question 9

Suggest why the scientist took his sample from the population at random (1)

Answer 9

1. Avoids bias
2. Results (likely to be) reliable/repeatable

Question 10

Describe how to make a 1 in 10 dilution and then use this to make a 1 in 1000 dilution of the original liquid culture of bacteria (3)

Answer 10

1. Add 1 part culture to 9 parts liquid
2. Mix
3. Repeat using 9 parts fresh liquid and 1 part of 10^-1 and 10^-2 dilutions to make 10^-3 dilution

Question 11

Suggest why student decided not to use 1 in 10 dilution for determining the number of cells in the undiluted liquid culture (2)

Answer 11

1. COUNT unlikely to be accurate/reproducible/repeatable/reliable
2. Because too many cells/cells overlapping/not spread out

Question 12

Suggest & explain one reason why bacteria resistant to A are more common than bacteria resistant to B in farm animals (2)

Answer 12

1. A used more often/in higher doses
2. Resistant bacteria more likely to pass on allele/gene for resistance
3. More/higher frequency of mutations
4. Gene passed on to more bacteria
5. A used over longer time period
6. More time for (chance) mutation to occur/for selection to occur

Question 13

Explain what is meant by a hierarchy (2)

Answer 13

1. Groups within groups
2. No overlap

Question 14

Explain what is meant by a phylogenetic group (1)

Answer 14

(Grouped according to) evolutionary links/history/relationships/common ancestry

Question 15

Explain why having fewer differences in amino acid sequences compared with humans shows that it’s the most closely related to humans (2)

Answer 15

1. Similar primary structure
2. So similar DNA sequence/base sequence

Question 16

Explain why diversity of insects in the area decreased after the forest was cleared (3)

Answer 16

1. Fewer niches
2. Fewer food sources
3. Fewer plant species

Question 17

Explain the role of independent segregation in meiosis (2)

Answer 17

1. Genetic variation
2. Different combinations of maternal and paternal chromosomes/alleles

Question 18

Suggest why an animal crossed from a zebra (32 chromosomes) and a donkey (62 chromosomes) is infertile (2)

Answer 18

1. 47/odd/uneven number of chromosomes
2. Chromosomes cannot pair/are not homologous/chromosome number can’t be halved/meiosis can’t occur/sex cells/haploid cells aren’t produced

Question 19

Define genetic diversity (1)

Answer 19

Number of different alleles OF EACH GENE

Question 20

Give 3 ways in which courtship behaviour increases probability of successful mating (3)

Answer 20

1. Recognise/identify/attract same species
2. Stimulates/synchronises mating/production/release of gametes
3. Recognition/attraction of mate/opposite sex
4. Indication of maturity/fertility/readiness to mate
5. Formation of a pair bond

Question 21

Describe 2 differences between the structure of a tRNA molecule and the structure of a mRNA molecule (2)

Answer 21

1. tRNA is clover-leaf shaped, mRNA is linear
2. tRNA has H bonds, mRNA doesn’t
3. tRNA has an amino acid binding site, mRNA doesn’t
4. tRNA has anticodon, mRNA has codon

Question 22

Describe how one amino acid is added to a polypeptide that is being formed at a ribosome during translation (3)

Answer 22

1. tRNA bring specific amino acid (to ribosome)
2. anticodon (on tRNA) binds to codon (on mRNA)
3. amino acids join by condensation reaction/join to form a peptide bond (using ATP)

Question 23

Describe the role of a ribosome in the production of a polypeptide. DO NOT INCLUDE TRANSCRIPTION (3)

Answer 23

1. mRNA binds to ribosome
2. idea of TWO codons/binding sites
3. (allows) tRNA with ANTICODONS to bind/associate
4. (catalyses) formation of peptide bond between amino acids
5. moves along (mRNA to the next codon)

Question 24

EXPLAIN one other way genetic variation within a species is increased apart from mutation (2)

Answer 24

1. Random fertilisation
2. Produces new allele combinations

Question 25

Define ‘gene mutation’ and explain how a gene mutation can have • no effect on an individual • a positive effect on an individual (4)

Answer 25

1. Change in the base/nucleotide (sequence of DNA) 2. Results in the formation of a new ALLELE 1. Genetic code is degenerate/mutation is in an intron 2. Changes AA but no effect on 3º structure 3. (New allele) is recessive so doesn’t influence phenotype 1. Results in change in polypeptide that positively changes the properties (of the protein) 2. May result in increased survival/reproductive success

Question 26

Describe crossing over and explain how it increases genetic diversity (4)

Answer 26

1. Homologous pairs of chromosomes associate/form a bivalent 2. Chiasma(ta) form 3. (Equal) lengths of (non-sister) chromatids/alleles are exchanged 4. Producing new combinations of alleles

Question 27

Explain why it is more useful to calculate index of diversity than to record species richness (2)

Answer 27

1. Measures abundance/number/population of EACH species 2. May be many/few of some species

Question 28

Give one way in which sampling procedures could be standardised

Answer 28

Same size of area Size size net/mesh Same sampling time Samples taken at same time of day/on the same day