GENETICS, BIODIVERSITY AND CLASSIFICATION

• DNA, GENES AND CHROMOSOMES

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

  Locus/loci;
• 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)

  1. (Because) base/nucleotide sequence;

  2. (In) triplet(s);

  3. (Determines) order/sequence of amino acid sequence/primary

  structure (in polypeptide);
• Define the term exon (1)

  Base/nucleotide/triplet sequence coding for polypeptide/sequence of amino

  acids/primary structure;
• Describe how a phosphodiester bond is formed between two nucleotides

  within a DNA molecule. (2)

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

  Histone;
• 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)

  ) 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'.
• 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)


  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
• 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)

  Deoxyribose in DNA and ribose in RNA;
• The diagram shows a tRNA molecule.

  (c) Name the structures labelled W and X in the diagram.(1)

  W = amino acid binding site and X = anticodon;

  W Idea of binding site needed
• Not all mutations in the nucleotide sequence of a gene cause a change in

  the structure of a polypeptide.

  Give two reasons why. (2)

  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
• Compare and contrast the DNA in eukaryotic cells with the DNA in

  prokaryotic cells. (5)

  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;
• The dark stain used on the chromosomes binds more to some areas of the

  chromosomes than others, giving the chromosomes a striped appearance.

  Suggest one way the structure of the chromosome could differ along its

  length to result in the stain binding more in some areas. (1)

  Differences in base sequences

  OR

  Differences in histones/interaction with histones

  OR

  Differences in condensation/(super)coiling;

  Answer must be in context of differences in

  arrangement of chromosomes not just related to the

  properties of the stain.

  Accept spec section 8 ideas e.g. different

  methylation/acetylation

  Accept different genes

  Reject different alleles
• In Figure 2 the chromosomes are arranged in homologous pairs.

  What is a homologous pair of chromosomes? (1)


  (Two chromosomes that) carry the same genes;

  Reject 'same alleles'

  Accept 'same loci' (plural) or 'genes for the same

  characteristics
• TRANSCRIPTION AND TRANSLATION

  Name the type of bond between:

  complementary base pairs

  adjacent nucleotides in a DNA strand (2)

  1. Hydrogen (bonds);

  2. Phosphodiester (bonds);

  Accept ester/covalent bond
• Describe two differences between the structure of a tRNA molecule and

  the structure of an mRNA molecule. (2)

  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;
• 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)

  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;
• Describe how one amino acid is added to a polypeptide that is being

  formed at a ribosome during translation. (3)

  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);
• Describe how mRNA is produced from an exposed template strand of

  DNA.

  Do not include DNA helicase or splicing in your answer. (3)


  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;
• Describe how mRNA is formed by transcription in eukaryotes.(5)

  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);
• Describe how a polypeptide is formed by translation of mRNA. (6)

  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;
• What is the proteome of a cell? (1)

  (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
• Give two structural differences between a molecule of messenger RNA

  (mRNA) and a molecule of transfer RNA (tRNA)(2)

  ) 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
• 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)


  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)
• The genetic code is described as degenerate.

  What is meant by this? Use an example from Table 1 to illustrate your

  answer(2)

  1. More than one codon codes for a single amino acid;

  Accept 'triplet' or 'sequence of 3 bases/nucleotides'

  for 'codon'.

  Reject 'production/produces' for 'codes for'.

  Do not infer mp1 from mp2.

  2. Suitable example selected from Table 1;
• Describe how mRNA is produced in a plant cell. (5)

  1. The DNA strands separate by breaking the H bonds;

  OR

  H bonds broken between (complementary) (DNA) bases;

  2. (Only) one of the strands/template strand is used (to make

  mRNA/is transcribed);

  3. (Complementary) base pairing so A ⟶ U, T ⟶ A, C ⟶ G, G

  ⟶ C;

  4. (RNA) nucleotides joined by RNA polymerase;

  5. pre-mRNA formed;

  6. Splicing / introns removed to form mRNA;

  1. Ignore 'hydrolysis' of bonds

  1. Accept DNA "unzips" by breaking the H bonds

  6. Accept 'non-coding' sections for introns
• Give the two types of molecule from which a ribosome is made.(1)


  One of RNA / ribonucleic acid(s) / nucleotide(s)/nucleic acid(s) /

  rRNA / ribosomal RNA / ribosomal ribonucleic acid

  and

  one of protein(s) / polypeptide(s) / amino acid(s) / peptide(s) /

  ribosomal protein;

  Reject DNA, deoxyribonucleic acid, tRNA, transfer

  RNA, transfer ribonucleic acid, mRNA, messenger

  RNA, messenger ribonucleic acid.

  Ignore enzyme(s), base(s).
• Describe the role of a ribosome in the production of a polypeptide. Do not

  include transcription in your answer. (3)

  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

  (held by tRNA molecules);

  5. Moves along (mRNA to the next codon) / translocation

  described;

  Assume 'it' refers to ribosome.
• In a eukaryotic cell, the base sequence of the mRNA might be different

  from the sequence of the pre-mRNA.

  Explain why. (2)

  1. Introns (in pre-mRNA);

  2. Removal of sections of (pre-mRNA) / splicing;

  Introns removed' scores 2 marks.

  Reference to 'introns present in mRNA' disqualifies

  mp1 but allow ECF for mp2.

  Accept for 1 mark mRNA contains only exons.
• MUTATION AND MEIOSIS

  Define the term mutagenic agent. (1)


  (A factor that) increases (the rate of) mutations;
• Name the type of mutation that changed the number of chromosomes in S.

  townsendii to produce S. anglica. Explain your answer.

  Name of mutation

  Explanation (3)

  Name of mutation

  1. Non-disjunction;

  Explanation

  Ignore homologous

  2. (In) meiosis;

  Accept reference to first division or second division

  as indicating meiosis

  Ignore mitosis

  3. Chromosomes not separated
• 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)

  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
• Give one reason why trout eggs produced by meiosis are genetically

  different. (1)

  Independent segregation

  OR

  Crossing over;

  Accept labelled diagram

  Accept (eggs produced) have different

  combinations of maternal and paternal

  chromosomes
• The offspring produced from farmed trout are sterile. Suggest and explain

  why (2)

  1. Too many/extra set/three copies of chromosomes;

  Accept 1 paternal, 2 maternal

  2. (Homologous) chromosomes do not pair

  OR

  (Homologous) chromosomes do not separate (evenly);

  Accept divide for separate

  3. (So) no meiosis;
• Define 'gene mutation' and explain how a gene mutation can have:

  • no effect on an individual

  • a positive effect on an individual (4)


  (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);
• Give two differences between mitosis and meiosis (2)

  Mitosis given first

  Differences must be given as comparisons

  Ignore references to asexual / sexual

  reproduction, growth, repair &

  replacement

  1. One division, two divisions in meiosis;

  2. (Daughter) cells genetically identical,

  daughter cells genetically different in

  meiosis;

  Reference to 'genetically' needed once

  3. Two cells produced, (usually) four cells produced in meiosis;

  4. Diploid to diploid/haploid to haploid, diploid to haploid in

  meiosis;

  Accept same number chromosomes in

  mitosis, but half the number in meiosis

  5. Separation of homologous chromosomes only in meiosis;

  6. Crossing over only in meiosis;

  7. Independent segregation only in meiosis;
• Explain how the chromosome number is halved during meiosis. (2)


  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
• What is the haploid number of chromosomes for this species of moss? (1)

  6;
• Crossing over greatly increases genetic diversity in this species of moss.

  Describe the process of crossing over and explain how it increases genetic

  diversity (4)

  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
• Describe how the change in chromosome number in Patau syndrome was

  produced. (2)

  1. In meiosis; 2. Homologous chromosomes / sister chromatids do not separate; 2. Accept non-disjunction
• Explain why all the cells of the body will have this mutation (2)

  1. Mutation / extra chromosome in gamete / egg / sperm (that

  formed zygote);

  2. All cells derived (from a single cell / zygote) by mitosis;

  OR

  3. All cells derived from a single cell / zygote by mitosis;

  4. Mitosis produces genetically identical cells / a clone;

  Mark points 1 and 2 OR 3 and 4

  4. Accept: have same DNA / same alleles