T4 Genetic information

Question 1

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

Answer 1

  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 2

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

Answer 2

Histone

Question 3

In the process of semi-conservative DNA replication, the two strands within a DNA
molecule are separated. Each then acts as a template for the formation of a new
complementary strand.
Describe how the separation of strands occurs. 2 marks

Answer 3

1. DNA helicase;
2. Breaks hydrogen bonds between base pairs/ AT and GC/complementary bases
   OR
   Breaks hydrogen bonds between polynucleotide strands;
   Reject hydrolysis of hydrogen bonds

Question 4

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 4

  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 5

Some DNA nucleotides have the organic base thymine, but RNA nucleotides do not have
thymine. RNA nucleotides have uracil instead of thymine.
Give one other difference between the structure of a DNA nucleotide and the structure of an RNA nucleotide. 1 mark

Answer 5

Deoxyribose in DNA and ribose in RNA

Question 6

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 6

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 7

DNA is a polymer of nucleotides. Each nucleotide contains an organic base.
Explain how the organic bases help to stabilise the structure of DNA. 2 marks

Answer 7

1. Hydrogen bonds between the base pairs holds two strands together
2. Many hydrogen bonds provides strength
   Reject strong hydrogen bonds

Question 8

Suggest one advantage of showing the genetic code as base sequences on mRNA, rather
than triplets on DNA. 1 mark

Answer 8

(Because) ribosomes assemble polypeptides using mRNA code
OR
DNA has two strands each with a different (complementary) base sequen

Question 9

What name is given to a group of three bases on mRNA that codes for an amino acid? 1 mark

Answer 9

Codon

Question 10

Name the type of bond between:
1. complementary base pairs
2. adjacent nucleotides in a DNA strand

Answer 10

1. Hydrogen (bonds);
2. Phosphodiester (bonds);
   Accept ester/covalent bond

Question 11

Describe two differences between the structure of a tRNA molecule and the structure of an mRNA molecule. 2 marks

Answer 11

1. tRNA is ‘clover leaf shape’, mRNA is linear;
   Must be a comparison
   Reject tRNA is double stranded
   Accept tRNA is folded for tRNA is ‘clover leaf shaped’
2. tRNA has hydrogen bonds, mRNA does not;
3. tRNA has an amino acid binding site, mRNA does not;
   Accept ‘CCA end’ for amino acid binding site
4. tRNA has anticodon, mRNA has codon;

Question 12

In a eukaryotic cell, the structure of the mRNA used in translation is different from the
structure of the pre-mRNA produced by transcription.
Describe and explain a difference in the structure of these mRNA molecules 2 marks

Answer 12

1. mRNA fewer nucleotides
   OR
   Pre-mRNA more nucleotides
   OR
   mRNA has no introns/has (only) exons
   OR
   Pre-mRNA has (exons and) introns;
   Accept mRNA is shorter OR pre-mRNA is longer
2. (Because of) splicing;

Question 13

Why is the genetic code described as being universal? 1 mark

Answer 13

(In all organisms / DNA,) the same triplet codes for the same amino acid;
Accept codon / same three bases / nucleotides
Accept plurals if both triplets and amino acids
Reject triplets code for an amino acid
Reject reference to producing amino acid

Question 14

The genetic code uses four different DNA bases. What is the maximum number of
different DNA triplets that can be made using these four bases? 1 mark

Answer 14

64

Question 15

Transcription of a gene produces pre-mRNA.
(b) Name the process that removes base sequences from pre-mRNA to form mRNA. 1 mark

Answer 15

Splicing;
Ignore deletion references
Accept RNA splicing

Question 16

Explain the difference between pre-mRNA and mRNA. 1 mark

Answer 16

(pre-mRNA) contains introns / mRNA contains only exons;
Assume that “it” refers to pre-mRNA.
Accept non-coding as equivalent to intron.

Question 17

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

Answer 17

1. tRNA brings specific amino acid (to ribosome);
2. Anticodon (on tRNA) binds to codon (on mRNA);
3. Amino acids join by condensation reaction (using ATP)
   OR
   Amino acids join to form a peptide bond (using ATP);

Question 18

Describe how mRNA is produced from an exposed template strand of DNA.
Do not include DNA helicase or splicing in your answer. 3 marks

Answer 18

  1. (Free RNA) nucleotides form complementary base pairs;
Accept A-U, G-C OR combination of those pairs
2. Phosphodiester bonds form;
Accept linkages for ‘bonds’
3. By (action of) RNA polymerase;

Question 19

Define the term exon. 1 mark

Answer 19

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

Question 20

Describe how mRNA is formed by transcription in eukaryotes. 5 marks

Answer 20

1. Hydrogen bonds (between DNA bases) break;
   Ignore DNA helicase.
   Reject hydrolysing hydrogen bonds.
2. (Only) one DNA strand acts as a template;
3. (Free) RNA nucleotides align by complementary base pairing;
   For ‘align by complementary base pairing’, accept ‘align to
   complementary bases’ or ‘align by base pairing’.
4. (In RNA) Uracil base pairs with adenine (on DNA)
   OR
   (In RNA) Uracil is used in place of thymine;
   Do not credit use of letters alone for bases.
5. RNA polymerase joins (adjacent RNA) nucleotides;
   Reject suggestions that RNA polymerase forms hydrogen bonds or
   joins complementary bases.
6. (By) phosphodiester bonds (between adjacent nucleotides);
7. Pre-mRNA is spliced (to form mRNA)
   OR
   Introns are removed (to form mRNA);

Question 21

Describe how a polypeptide is formed by translation of mRNA 6 marks

Answer 21

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;
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 22

Define ‘gene mutation’ and explain how a gene mutation can have:
* no effect on an individual
* a positive effect on an individual. 4 marks

Answer 22

(Definition of gene mutation)
1. Change in the base/nucleotide (sequence of chromosomes/DNA);
For 4 marks at least one mark must be scored in each section of
the answer.
Accept named mutation for ‘change’.
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 23

What is the proteome of a cell? 1 mark

Answer 23

(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;
Do not accept number of proteins unqualified

Question 24

Give two structural differences between a molecule of messenger RNA (mRNA) and a
molecule of transfer RNA (tRNA). 2 marks

Answer 24

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;
   Accept mRNA cannot carry an amino acid, tRNA can
3. mRNA has more nucleotides;
   Accept mRNA is longer or converse
4. (Different) mRNAs have different lengths, all tRNAs are similar / same length;
5. mRNA has codons, tRNA has an anticodon;
   Statements must be comparative

Question 25

Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide. Do not include descriptions of transcription and splicing in your answer. 5 marks

Answer 25

1. mRNA associates with a ribosome / ribosome attaches to mRNA; Idea of association is required 2. Ribosome moves to / finds the start codon / AUG; 3. tRNA brings / carries (appropriate / specific) amino acid; Must be explicitly stated and not inferred. 4. Anticodon (on tRNA complementary) to codon (on mRNA); 5. Ribosome moves along to next codon; OR Ribosome ‘fits’ around two codons / can fit two tRNAs; Must be explicitly stated and not inferred. 6. (Process repeated and) amino acids join by peptide bonds / condensation reaction (to form polypeptide); OR (Process repeated and) amino acids joined using (energy from) ATP (to form polypeptide);

Question 26

In taxonomy, an organism is identified by referring to the species name and the genus name. What term is used to describe this method of naming organisms? 1 mark

Answer 26

Binomial

Question 27

Define the term mutagenic agent 1 mark

Answer 27

(A factor that) increases (the rate of) mutations;

Question 28

Genetic variation within a species is increased during meiosis by crossing over and the independent segregation of homologous chromosomes. Apart from mutation, explain one other way genetic variation within a species is increased. 2 marks

Answer 28

1. Random fusion of gametes OR Random fertilisation; Accept for ‘gametes’, 2. (Produces) new allele combinations OR (Produces) new maternal and paternal chromosome combinations; Reproductive cells Ignore genes

Question 29

Mitochondrial diseases are caused by faulty mitochondria. All of a person’s mitochondria are inherited from their mother via the egg cell. An egg cell contains approximately 3 × 105 mitochondria. One proposed treatment to prevent passing on faulty mitochondria involves * removing the nucleus from an egg cell donated by a woman with healthy mitochondria * replacing this nucleus with the contents of the polar body from a woman whose egg cells are affected by mitochondrial disease. Suggest how this treatment prevents inheritance of mitochondrial diseases. 2 marks

Answer 29

1. Egg (created) has nucleus / DNA / genes of (affected) woman / mother; Accept ref. to zygote / embryo / child for egg Accept genetic information Ignore references to alleles Reject if nucleus from wrong egg / woman 2. It has mostly / many / lots of normal mitochondria (of unaffected woman) OR There are few faulty mitochondria; Reject ref. to production of healthy mitochondria as result of treatment

Question 30

If most of the mitochondria in a cell are faulty, this prevents many important enzymecatalysed reactions taking place or slows them down. Suggest and explain one reason why. 2 marks

Answer 30

1. Not enough / little ATP produced; One reason asked for, so list rule applies Ignore ref. to no ATP produced 2. ATP provides energy for (enzyme) reactions OR ATP phosphorylates substrates / enzymes, so making them (more) reactive; Accept (leads to) lower activation energy for reaction Reject if mention energy produced

Question 31

Explain how the chromosome number is halved during meiosis 2 marks

Answer 31

1. Homologous chromosomes (pair); 2. One of each (pair) goes to each (daughter) cell / to opposite poles; Ignore descriptions of the second division of meiosis.

Question 32

Describe the process of crossing over and explain how it increases genetic diversity. 4 marks

Answer 32

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; 1. Accept descriptions of homologous pairs 2. Accept descriptions of chiasma(ta) e.g. chromatids / chromosomes entangle / twist 2. Neutral Crossing / cross over 3. Reject genes are exchanged 3. Accept lengths of DNA are exchanged 4. Do not accept references to new combinations of genes unless qualified by alleles

Question 33

Name two ways in which meiosis produces genetic variation. 2 marks

Answer 33

1. Independent segregation (of homologous chromosomes); Accept random assortment 2. Crossing over / formation of chiasmata.

Question 34

The fruit fly is a species of small insect. The fruit fly has a gene that codes for an enzyme called alcohol dehydrogenase (AD). AD catalyses the breakdown of alcohol when alcohol is in the insects’ food. The gene coding for AD has two alleles, ADF and ADS. 1.(a)The enzyme encoded by the ADF allele catalyses the breakdown of alcohol faster than the enzyme encoded by the ADS allele. Suggest why. (c). Alcohol is toxic to fruit flies. Suggest and explain why the frequency of the ADF allele changed during the 45 generations.

Answer 34

1. Different primary structure/amino acid sequence; 2. Different tertiary structure/shape of active site; 3. Enzyme-substrate complexes more likely (with enzyme from ADF allele); Accept converse for ADS Accept is more complementary 1. Flies with ADF /allele have selective advantage (in presence of alcohol); Accept converse for ADS Accept description of selective advantage 2. So insects (with ADF more likely to) reproduce; 3. Pass on ADF (allele/gene); 4. (So) allele frequency increases;

Question 35

What is the name of a position of a gene on a chromosome? 1 mark

Answer 35

Locus; Accept: loci

Question 36

What is meant by genetic diversity? 1 mark

Answer 36

Differences in DNA / differences in base sequence of DNA; Accept: number of different alleles / size/variation in gene pool Reject: genes

Question 37

Apart from genetic factors what other type of factor causes variation within a species? 1 mark

Answer 37

Environmental; Accept: Environment

Question 38

The spotted owl is a bird. Numbers of spotted owls have decreased over the past 50 years. Explain how this decrease may affect genetic diversity. 2 marks

Answer 38

Reduced (genetic diversity); As fewer different / varied alleles / genes / reduced gene pool;

Question 39

There are many different species of field mouse in Europe. Using a phylogenetic classification, all of these species have names that start with Apodemus. What information does this give about field mice? 2 marks

Answer 39

1. Same genus; 2. Same evolutionary origin / common ancestor.

Question 40

Name the taxons in order. 1 mark.

Answer 40

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species 1 mark for all answers in correct order

Question 41

Some populations of animals that have never been hunted show very low levels of genetic diversity. Other than hunting, suggest two reasons why populations might show very low levels of genetic diversity. 2 marks

Answer 41

1. Population might have been very small / genetic bottleneck; 2. Population might have started with small number of individuals / by one pregnant female / founder effect; 3. Inbreeding. Allow any two

Question 42

Give three ways in which courtship behaviour increases the probability of successful mating. 3 marks

Answer 42

1. Recognise / identify / attract same species; Ignore: references to letting them produce fertile offspring 2. Stimulates / synchronises mating / production / release of gametes; 3. Recognition / attraction of mate / opposite sex; Accept finding a mate Accept: gender

Question 43

Name the taxonomic group between order and genus. 1 mark

Answer 43

Family

Question 44

What is meant by ‘species richness’? 1 mark

Answer 44

(A measure of) the number of (different) species in a community; For ‘community’ accept ‘habitat/ecosystem/one area/environment’ Reject ‘in a population’

Question 45

The scientists collected bees using a method that was ethical and allowed them to identify accurately the species to which each belonged. In each case, suggest one consideration the scientists had taken into account to make sure their method 1. was ethical. 2. allowed them to identify accurately the species to which each belonged. Suggest and explain two ways in which the scientists could have improved the method used for data collection in this investigation.

Answer 45

1. Must not harm the bees OR Must allow the bee to be released unchanged; 2. Must allow close examination OR Use a key (to identify the species); Accept method that allows close examination Ignore references to DNA sequencing Accept ‘use photographs/specimens (to identify species)' 1. Collect at more times of the year so more points on graph/better line (of best fit) on graph; Both suggestion and explanation is required for each mark point. The explanation must relate to the graph. 2. Counted number of individuals in each species so that they could calculate index of diversity; 3. Collected from more sites/more years to increase accuracy of (mean) data; For ‘accuracy’ accept ‘representative’.

Question 46

Species richness and an index of diversity can be used to measure biodiversity within a community. (a) What is the difference between these two measures of biodiversity?

Answer 46

(a) Species richness measures only number of (different) species / does not measure number of individuals.