TOPIC 4: AQA Biology Mark scheme Questions

How do two amino acids differ from one another?

different R chemical groups

describe how mRNA is produced in the nucleus of a cell (6 marks)

-DNA helicase

-breaks hydrogen bonds

-template strand

-free nucleotides

-complementary base pairing

-DNA polymerase

-joins adjacent nucleotides

-pre-mRNA

-spliced, to remove introns

-mRNA

describe how proteins are digested in the gut (4 marks)

-proteases are produced by the stomach, they hydrolyse peptide bonds, producing amino acids and shorter polypeptides from polypeptides

-endopeptidases hydrolyse peptide bonds in the middle of polypeptides

-exopeptidases, terminal peptide bonds

-peptidases, hydrolyse dipeptides into amino acids

-absorbed through the ileum, via co-transport

how can the production of protein with one amino acids lead to disease (2 marks)

-non functional protein

-different tertiary structure

explain how a gene codes for a protein (2 marks)

-triplet codes

-each three bases codes for a specific amino acid

-primary structure

what is meant by degenerate? (1 mark)

some amino acids are coded for by more than one amino acid

what is a codon? (2 marks)

-three bases on mRNA

-complementary to an anticodon on tRNA

explain how a change in the DNA base sequence for a protein may result in a change in the structure of the protein (3 marks)

-change in amino acids sequence

-change to tertiary structure

-change to ionic and hydrogen bonds

at what phase of the cell cycle is mutation more likely to occur? explain you answer (2 mark)

-interphase

-when DNA divides

why does the percentages of bases in the middle of a chromosome differ from the percentage of bases at the end of the chromosome (2 marks)`

-different DNA base sequence

-code for a different protein

what are the two subunits a protein is made from? (1 marks)

-prtien

-rRNA

describe the role of a ribosome in protein synthesis (3 marks)

-translation

-mRNA attaches to ribosome, AT THE START CODON

-joins complementary codons to anticodon

-ribosome moves along mRNA

-allows two tRNA molecules to attach at one time

-forms peptide bond between

explain the difference in the percentage of bases in different species of bacteria (2 marks)

-different species have different genes

-produce different proteins

-different sequence of bases

Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells.(5)

1. Nucleotide structure is identical;

2. Nucleotides joined by phosphodiester bond;

3. DNA in mitochondria has a similar (structure) to DNA in prokaryotes

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;

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

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)

5. RNA polymerase joins (adjacent RNA) nucleotides;

6. (By) phosphodiester bonds (between adjacent nucleotides) via condensation reactions

7. Pre-mRNA is spliced (to form mRNA)

Describe how mRNA is produced in a plant cell. (5)

Breaks hydrogen bonds;

Only one DNA strand acts as template;

RNA nucleotides attracted to exposed bases;

(Attraction) according to base pairing rule;

RNA polymerase joins (RNA) nucleotides together;

Pre-mRNA spliced to remove introns.

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

1. (mRNA attaches) to ribosomes

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;

Define 'gene mutation' and explain how a gene mutation can have:

• no effect on an individual

• a positive effect on an individual. (4)

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);

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)

7. May result in increased reproductive success

Mutation can result in an increase in genetic variation within a species.

Describe and explain the other processes that result in increases in genetic variation within a species.

1. Independent segregation of homologous chromosomes/pairs;

2. Crossing over between homologous chromosomes/pairs;

3. Random fertilisation of gametes;

4. (Produces) new combinations of alleles;

(Produces) new combinations of maternal and paternal chromosomes

Penicillin has been the antibiotic of choice for the treatment of bacterial meningitis. Since the year 2000, strains of Neisseria meningitidis that are resistant to penicillin, sulfonamides and rifampin have been discovered in the UK.

Describe how a population of Neisseria meningitidis (Nm) can become resistant to these antibiotics.

(4)

1. Mutation leads to new allele / variation

Allow horizontal gene transfer

2. Results in Nm cell with allele for resistance to one antibiotic / to named antibiotic

3. (This) cell survives and passes the allele for resistance to offspring; 2. and 3. If gene for resistance, penalise once

4. Process repeated with different genes conferring resistance to each of the other (two) antibiotics

An environmental scientist investigated a possible relationship between air pollution and the size of seeds produced by one species of tree.

He was provided with a very large number of seeds collected from a population of trees in the centre of a city and also a very large number of seeds collected from a population of trees in the countryside.

Describe how he should collect and process data from these seeds to investigate whether there is a difference in seed size between these two populations of trees. (5)

1. Use random sample of seeds (from each population);

2. Use (large enough) sample to be representative of whole population;

3. Indication of what size was measured e.g. mass;

4. Calculate a mean and standard deviation (for each population);

5. Use the (Student's) t-test;

6. Analyse whether there is a significant difference between (the means of) the two populations;

Haemoglobins are chemically similar molecules found in many different species.

Differences in the primary structure of haemoglobin molecules can provide evidence of phylogenetic (evolutionary) relationships between species.

Explain how.

1. Mutations change base / nucleotide (sequence);

Reject if mutation in amino acid

2. (Causing) change in amino acid sequence;

3. Mutations build up over time;

4. More mutations / more differences (in amino acid / base / nucleotide sequence / primary structure) between distantly related species;

5. Closely related species have recent common ancestor;

Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells.(5)

1. Nucleotide structure is identical;

2. Nucleotides joined by phosphodiester bond;

3. DNA in mitochondria has a similar (structure) to DNA in prokaryotes

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;

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

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)

5. RNA polymerase joins (adjacent RNA) nucleotides;

6. (By) phosphodiester bonds (between adjacent nucleotides) via condensation reactions

7. Pre-mRNA is spliced (to form mRNA)

Describe how mRNA is produced in a plant cell. (5)

Breaks hydrogen bonds;

Only one DNA strand acts as template;

RNA nucleotides attracted to exposed bases;

(Attraction) according to base pairing rule;

RNA polymerase joins (RNA) nucleotides together;

Pre-mRNA spliced to remove introns.

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

1. (mRNA attaches) to ribosomes

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;

Define 'gene mutation' and explain how a gene mutation can have:

• no effect on an individual

• a positive effect on an individual. (4)

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);

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)

7. May result in increased reproductive success

Mutation can result in an increase in genetic variation within a species.

Describe and explain the other processes that result in increases in genetic variation within a species.

1. Independent segregation of homologous chromosomes/pairs;

2. Crossing over between homologous chromosomes/pairs;

3. Random fertilisation of gametes;

4. (Produces) new combinations of alleles;

(Produces) new combinations of maternal and paternal chromosomes

Penicillin has been the antibiotic of choice for the treatment of bacterial meningitis. Since the year 2000, strains of Neisseria meningitidis that are resistant to penicillin, sulfonamides and rifampin have been discovered in the UK.

Describe how a population of Neisseria meningitidis (Nm) can become resistant to these antibiotics.

(4)

1. Mutation leads to new allele / variation

Allow horizontal gene transfer

2. Results in Nm cell with allele for resistance to one antibiotic / to named antibiotic

3. (This) cell survives and passes the allele for resistance to offspring; 2. and 3. If gene for resistance, penalise once

4. Process repeated with different genes conferring resistance to each of the other (two) antibiotics

An environmental scientist investigated a possible relationship between air pollution and the size of seeds produced by one species of tree.

He was provided with a very large number of seeds collected from a population of trees in the centre of a city and also a very large number of seeds collected from a population of trees in the countryside.

Describe how he should collect and process data from these seeds to investigate whether there is a difference in seed size between these two populations of trees. (5)

1. Use random sample of seeds (from each population);

2. Use (large enough) sample to be representative of whole population;

3. Indication of what size was measured e.g. mass;

4. Calculate a mean and standard deviation (for each population);

5. Use the (Student's) t-test;

6. Analyse whether there is a significant difference between (the means of) the two populations;

Haemoglobins are chemically similar molecules found in many different species.

Differences in the primary structure of haemoglobin molecules can provide evidence of phylogenetic (evolutionary) relationships between species.

Explain how.

1. Mutations change base / nucleotide (sequence);

Reject if mutation in amino acid

2. (Causing) change in amino acid sequence;

3. Mutations build up over time;

4. More mutations / more differences (in amino acid / base / nucleotide sequence / primary structure) between distantly related species;

5. Closely related species have recent common ancestor;