Draw labelled diagrams to show the structure of RNA nucleotides and how they are linked together to form a molecule of RNA.

Explain transcription.

Distinguish between continuous and discrete variation, using examples.

a. ribose drawn as pentagon and labelled sugar/ribose;
b. base drawn with correct link to (C₁ of) ribose and labelled base/nitrogenous base;
c. phosphate drawn with correct link to (C₅ of) ribose and labelled P/phosphate;
d. two (or more) ribonucleotides drawn with correct link (C₃ to C₅)

a. synthesis of RNA/mRNA / transcription of DNA to RNA;
b. RNA nucleotides linked together to form a strand/chain;
c. RNA strand assembled on DNA template/antisense strand / copy made of sense strand;
d. RNA polymerase carries out transcription/links RNA nucleotides;
e. uncoiling/separation of DNA strands;
f.  5’ end of nucleotides linked to 3’ end of (growing RNA) strand;
g. complementary base pairing (is the basis of copying the base sequence);
h. uracil instead of thymine in RNA;
i. starts at/RNA polymerase binds to a promoter;
j. regulated by transcription factors/DNA binding proteins/nucleosomes;

Annotated diagrams can be used.

Many candidates were able to draw the structure of an RNA nucleotide and link it correctly by a 5’ to 3’ bond to another nucleotide. The commonest error was to show two strands of nucleotides linked by base pairing, indicating confusion between RNA and DNA.

Answers were mixed. Strong candidates had no difficulty in describing transcription in detail but weaker ones tended to get confused with replication and/or translation. Helicase was often stated as the enzyme that uncoils and splits the double helix, rather than RNA polymerase. Marks were not awarded merely for stating that transcription is 5’ to 3’ unless it was clear that the candidate understood that the 5’ terminal of a free nucleotide is linked to the 3’ terminal of the chain of nucleotides already linked up. Candidates are expected to show understanding in their answers, rather than just state memorized phrases.

Differences between continuous and discrete variation were not well known and the average performance for this question was the lowest for any part of Section B. A general fault was to describe the two types of variation separately and in consequence forget to include both sides of a distinction. For example, some candidates stated that the environment can cause continuous variation, but most did not then also state that discrete variation is generally unaffected by environment. A mark was awarded for examples of the two types of variation, but in some cases an inappropriate example was chosen, such as hair colour for discrete variation or eye colour for continuous variation. The latter example was best avoided entirely as there are aspects of both continuous and discrete variation in the pigmentation of the iris.

Cells go through a repeating cycle of events in growth regions such as plant root tips and animal embryos. Outline this cell cycle.

Draw a labelled diagram of the formation of a chiasma by crossing over.

Explain the control of gene expression in eukaryotes.

a. mitosis is the division of a nucleus to produce two genetically identical daughter nuclei 

b. consists of four phases: prophase, metaphase, anaphase, telophase 

c. cytokinesis occurs after mitosis 

d. interphase is the metabolically active phase between cell divisions  OWTTE

e. the interphase consists of the S phase, G1 and G2 

f. DNA replicates in the S phase 

g. cell growth
OR
preparation for mitosis
OR
duplication of organelles in G1 and G2

a. «crossing over/chiasmata shown between» homologous chromosomes  

b. centromere drawn and labelled  

c. single strand break «SSB»/DNA cut between homologous chromosomes 

d. non-sister chromatids labelled
OR
sister chromatids labelled 

e. chiasma between homologous chromosomes labelled «shown forming after SSB»

Homologous chromosomes must be labelled and correctly drawn.

It is likely that more than one diagram will need to be included to demonstrate the stages.

a. mRNA conveys genetic information from DNA to the ribosomes «where it guides polypeptide production» 

b. gene expression requires the production of specific mRNA «through transcription» 

c. most genes are turned off/not being transcribed at any one time/regulated
OR
some genes are only expressed at certain times 

d. some genes are only expressed in certain cells/tissues
OR
«cell» differentiation involves changes in gene expression 

e. transcription factors/proteins can increase/decrease transcription 

f. hormones/chemical environment of cell can affect gene expression 

g. example of cell environment
eg: auxin/insulin/cytoplasmic gradient in embryo

h. transcription factors/proteins may prevent or enhance the binding of RNA polymerase 

i. nucleosomes limit access of transcription factors to DNA/regulate gene expression/transcription
OR
activate or silence genes 

j. DNA methylation/acetylation appears to control gene expression «as epigenetic factor»
OR
methylated genes are silenced 

k. «some» DNA methylation patterns are inherited 

l. introns may contain positive or negative gene regulators
OR
gene expression can be regulated by post-transcriptional modification/splicing/mRNA processing

Describe the structure of nucleosomes.

Explain how the two strands of the DNA double helix are held together.

There are multiple tandem repeats of nucleotide segments of DNA found in the non-coding DNA between genes. Outline how tandem repeats are used for DNA profiling.

Explain what happens in a person when they develop type I diabetes.

State how type I diabetes should be treated to avoid harmful health consequences of the condition.

a unit of DNA wound/coiled around 8 histone proteins / octamer;

1. hydrogen bonding between nucleotides / bases;
2. complementary base pairs;
3. adenine-thymine and cytosine-guanine form base pairs (between the two strands with H-bonding);
4. 2 bonds between A and T, while 3 bonds between C and G;

OWTTE

Full names required for c and d though use ecf.

1. tandem repeats (at one locus) vary in number of times sequence repeats / represent different alleles for one locus;
2. DNA sample cut by restriction enzymes into fragments;
3. samples of DNA are amplified at specific genetic sites with PCR;
4. the fragments are separated by their size/number of repeats with gel electrophoresis;
5. fluorescent/radioactive label attached to different tandem repeats;
6. data from several loci at one time uniquely identify individuals / like a fingerprint, combinations of alleles are specific to an individual;
7. comparisons/similarities between fragment patterns to determine paternity/evidence match to a suspect’s profile / other example of comparison/similarity;

OWTTE

1. insulin is necessary to control/regulate blood glucose concentrations
   OR
   insulin is necessary for the cells to take up glucose (for energy);
2. insufficient insulin is made by the pancreas
   OR
   autoimmune response/antibodies destroy the (β) cells of the pancreas that make insulin;
3. reduced uptake of glucose from the blood / glucose accumulates in the blood / elevated blood glucose levels;

“sugar” is NOT accepted in place of glucose. However, this should only be penalized once; i.e., utiltize ECF.

1. inject insulin / monitoring blood glucose / devices that release insulin;
2. decrease consumption of sugars/CHO / diet modification;
3. increase exercise;
4. keep weight in healthy range;

The structure of a nucleosome was correctly described for many, but some did not mention the number of histones.

There was a noticeable number of blank answers with respect to this question about using tandem repeats as the foundation of DNA profiling. Others had a good idea about PCR and gel electrophoresis, but many didn't mention restriction enzymes and did not clearly understand that patterns are compared.

This question about blood sugar regulation was well done by most. This was a question where the use of general terms rather than specific terms led to lost marks. For example, sugars instead of glucose and ‘the body’ instead of ‘the pancreas’

Identify the stage of mitosis labelled X in the image, giving a reason.

[Source: Copyright 2002, The Trustees of Indiana University]

Outline what is indicated by the mitotic index of tissue taken from a tumour.

DNA has regions that do not code for proteins. State two functions of these regions.

1.

2.

Outline the difference in methylation pattern between tumorous and normal tissue samples.

Suggest a way methylation may affect tumour cell genes.

telophase because the chromosomes/chromatids have reached the poles
OR
«late» anaphase as some chromosomes/chromatids are still moving/tails visible ✔

OWTTE

a. mitotic index is an indication of the ratio/percentage of cells undergoing mitosis/cell division ✔

b. cancer cells «generally» divide much more than normal «somatic» cells ✔

c. a high/elevated mitotic index in tumours / possible diagnosis of cancer /measure of how aggressive/fast growing the tumour is ✔

a. promoters / operators / regulation of gene expression/transcription ✔

b. telomeres/give protection to the end of chromosomes «during cell division» ✔

c. genes for tRNA/rRNA production ✔

d. other valid function for non-coding sequence ✔

Do not accept stop codon, accept centromeres (connecting sister chromatids).

a. «overall» much more methylation in the colon tumour samples than normal ✔

b. tumour and normal samples the markers 258 and 269 similar degree of methylation/fewer differences ✔

c. degree of methylation on certain markers may correlate with the presence of cancer / correct example of a marker only methylated in tumour cells eg marker 32 ✔

a. «DNA» methylation may inhibit transcription of genes that would prevent cancer/tumor formation ✔

b. «DNA» methylation may increase mitosis/cell division leading to tumor formation ✔

Do not accept discussion of histone methylation.

Both anaphase and telophase were accepted as answers, but students had difficulty providing a reason that was an appropriate justification; for example, students might say telophase and then support this with the statement because chromatids are seen moving to opposite poles. Students sometimes referred to chromatids as homologous chromosomes. As this is an image of plant cells, students would be mistaken by referring to a cleavage furrow. The mechanism of cytokinesis involves the formation of a cell plate.

Many students could accurately define the term mitotic index. An area of misunderstanding was to categorize the mitotic index as a rate rather than a ratio.

This question was well answered. The occasional answer made reference to processes associated with translation.

Many students detected that greater degrees of methylation in tumour samples. Fewer could accurately summarize the specific differences between methylation in the two samples.

Students lacked the understanding of epigenetics necessary to make reasonable suggestions.

Label a peptide bond in the diagram of a polypeptide.

Outline the primary structure of proteins.

Identify two hydrolysis reactions that occur in the small intestine.

Award [1] for any one of the four peptide bonds identified in this markscheme.

number/sequence/order of amino acids «in a protein/polypeptide chain» ✔

a. polysaccharides to disaccharides/monosaccharides
OR
starch/glycogen to maltose ✔

b. sucrose to glucose AND fructose ✔

c. maltose to glucose ✔

d. lactose to glucose AND galactose ✔

e. proteins/peptides/polypeptides to shorter peptides/amino acids
OR
dipeptides to amino acids ✔

f. triglycerides/lipids/fats/oils to glycerol AND fatty acids ✔

Draw molecular diagrams to show the condensation reaction between two amino acids to form a dipeptide.

Outline the roles of the different binding sites for tRNA on ribosomes during translation.

Explain the production of antibodies.

a. each amino acid with a COO–/COOH group at one end AND a NH₂/NH₃⁺ at the other  
Both needed.
mp a requires the double bond to be shown between the C and O.

b. CH in middle with H or R group attached 

c. peptide bond correctly drawn between N and C=0 

d. COO–/COOH group at one end of dipeptide AND NH₂/NH₃⁺ at other end  
Both needed.

e. loss of water

a. A, P and E binding sites are on the large subunit of the ribosome 

b. initiation of translation starts with binding of met-tRNA to the start codon 

c. large sub-unit binds with «start» tRNA in the P site 

d. A binding site holds the tRNA with the next amino acid to be added 

e. peptide bond is formed between the amino acids of the A site and the polypeptide at the P site 

f. polypeptide is transferred to the tRNA in the A site 

g. the tRNA «with polypeptide» in A site then moves to P site
OR
P binding site holds the tRNA attached to the growing polypeptide 

h. E binding site «exit» is where the tRNA «from P site without amino acid» leaves the ribosome

Accept annotated diagrams of the sites.

a. each antibody corresponds to a specific antigen 

b. antibodies are necessary for immunity/resistance to «infectious» disease 

c. macrophage/phagocyte ingests/engulfs pathogen 

d. macrophage/phagocyte digests pathogen 

e. macrophage/phagocyte displays antigen from pathogen 

f. antigens of a pathogen correspond to a specific T lymphocytes/cells
OR
T lymphocytes/cells are activated by antigen binding 

g. T lymphocytes/cells activate B lymphocytes/cells 

h. «B cells» divide by mitosis to form many/clones of plasma cells 

i. plasma cells secrete specific antibody 

j. some «activated» B lymphocytes/cells act as memory cells

Accept annotated diagrams of the process

Sickle cell anemia is due to a single base substitution in DNA. Using the diagram, outline how this translates into the production of an abnormal hemoglobin molecule by completing the spaces labelled I, II and III.

DNA codes for the amino acid sequence of polypeptides. List two other functions of DNA.

Distinguish between the purpose of free and bound ribosomes.

a. I: GUG
AND
II: CAC 

b. III: Val/valine

Both required. Do not accept GAG to GTC.

a. promotion/repression/regulation of gene expression 

b. introns (have functions in regulation of gene expression) 

c. telomeres (form caps at the end of chromosomes) 

d. coding for tRNAs/rRNAs 

e. allows genes/traits/heritable characteristics to be passed to offspring

free ribosomes produce proteins for use inside the cell/cytoplasm whereas bound ribosomes produce proteins for secretion/for use outside the cell/for use in lysosomes

Describe the transport of organic compounds in vascular plants.

The flowers of angiospermophyta are used for sexual reproduction. Outline three processes required for successful reproduction of angiospermophyta.

Growth in living organisms includes replication of DNA. Explain DNA replication.

a. phloem transports organic compounds/sucrose

b. from sources/leaves/where produced to sinks/roots/where used

c. through sieve tubes/columns of cells with sieve plates/perforated end walls

d. loading of organic compounds/sucrose into /H⁺ ions out of phloem/sieve tubes by active transport/using ATP

e. high solute concentration causes water to enter by osmosis (at source)

f. high (hydrostatic) pressure causes flow (from source to sink)

g. companion cells help with loading / plasmodesmata provide a path between sieve tubes and companion cell

h. translocation/mass flow

a. meiosis / production of male and female gametes

b. pollination / transfer of pollen from anther to stigma

c. fertilization happens after pollination / fertilisation is joining of gametes

d. seed dispersal / spread of seeds to new locations

Reject fruit dispersal.

a. helicase unwinds the double helix

b. gyrase/topoisomerase relieves strains during uncoiling

c. helicase separates the two strands of DNA/breaks hydrogen bonds

Accept unzips here but not for mark point a.

d. each single strand acts as a template for a new strand / process is semi-conservative

e. DNA polymerase III can only add nucleotides to the end of an existing chain/to a primer 

f. (DNA) primase adds RNA primer/short length of RNA nucleotides

g. DNA polymerase (III) adds nucleotides in a 5’ to 3’ direction

h. complementary base pairing / adenine to thymine and cytosine to guanine

Do not accept letters.

i. DNA polymerase (III) moves towards the replication fork on one strand and away from it on the other strand

j. continuous on the leading strand and discontinuous/fragments formed on the lagging strand

k. DNA polymerase I replaces primers/RNA with DNA

l. ligase joins the fragments together/seals the nicks

Draw a molecular diagram to show the formation of a peptide bond.

Outline how proteins are digested and the products of protein digestion absorbed in humans.

Explain how polypeptides are produced by the process of translation.

a. two amino acids correctly drawn;

b. removal/production of H₂O molecule shown;

c. peptide bond labelled between C of C=O and N of N-H;

Accept specific examples of amino acids correctly drawn for example glycine with H instead of R.

mpc can be awarded if the peptide bond is shown as in the dipeptide diagram but there are errors elsewhere.

a. digested by peptidases/proteases;

b. pepsin/pepsinogen/endopeptidase secreted by stomach (lining)/digests proteins in stomach;

c. pancreas secretes/pancreatic juice contains endopeptidase/trypsin/peptidase;

d. endopeptidase digest proteins/polypeptides to shorter chains of amino acids/shorter peptides;

e. amino acids absorbed by active uptake/transport;

f. in small intestine/ileum;

g. villi increase the surface area for absorption;

h. absorbed into bloodstream/into capillaries;

Peptidase can be accepted instead of endopeptidase in mpc, but not in mpb or mpd.

a. mRNA is translated;

b. mRNA binds with ribosome/with small subunit of ribosome;

c. tRNA-activating enzymes/aminoacyl tRNA synthetases attach specific amino acid to tRNA;

d. anticodon of 3 bases/nucleotides on tRNA;

e. start codon/AUG on mRNA;

f. tRNA carrying first amino acid/methionine binds to P/peptidyl site (when large subunit binds);

g. anticodon (on tRNA) binds to codon (on mRNA);

h. complementary base pairing (between codon and anticodon);

i. tRNA for next codon binds to A site/amino acyl site;

j. peptide bond forms between amino acids (on tRNAs) at P and A sites;

k. ribosome moves along mRNA to next codon/by three bases/in 5’ to 3’ direction;

l. tRNA released from E/exit site;

m. process/cycle repeats to elongate the polypeptide/until stop codon is reached;

n. release of polypeptide and mRNA/disassembly of ribosome complex at stop codon;

Marks can be awarded in an annotated diagram.

Accept UAA, UAG or UGA instead of stop codon in mpm or mpn but do not accept terminator sequence.

Do not award mpk for the ribosome moving to the start codon in a 5’ to 3’ direction.

Mostly full marks were gained although some had the correct diagrams but failed to label the peptide bond.

Nearly all candidates had some knowledge of protein digestion and many scored 3 or 4 of the 4-mark total. The relatively low mark allocation reflects that the program does not require detailed study of protein digestion or absorption. Few students referred to endopeptidases, which are stipulated in guidance for sub-topic 6.1 and also few stated that villi in the small intestine increase the surface area for absorption.

Explanations of translation were generally good, with many of the significant events included. Fewer candidates than in the past included unnecessary information about transcription. A common omission was the presence of a three-base (or nucleotide) anticodon on tRNA and a frequent small error was the idea that during initiation the first tRNA binds to the A site and then moves to the P site, rather than binding to the P site immediately.

Nucleosomes help to regulate transcription in eukaryotes.

State the components of a nucleosome.

Nucleosomes help to regulate transcription in eukaryotes.

State a chemical modification of a nucleosome that could impact gene expression.

DNA and histone

methylation/acetylation/phosphorylation/epigenetic tags/modification of nucleosome tails/N-terminal tails

Outline the roles of helicase and ligase in DNA replication.

Explain how natural selection can lead to speciation.

Outline the features of ecosystems that make them sustainable.

helicase:

a. unwinds/uncoils the DNA «double helix» ✔

b. breaks hydrogen bonds «between bases» ✔

c. separates the «two» strands/unzips the DNA/creates replication fork ✔

ligase:

d. seals nicks/forms a continuous «sugar-phosphate» backbone/strand ✔

e. makes sugar-phosphate bonds/covalent bonds between adjacent nucleotides ✔

f. after «RNA» primers are removed/where an «RNA» primer was replaced by DNA ✔

g. «helps to» join Okazaki fragments ✔

a. variation is required for natural selection/evolution/variation in species/populations ✔

b. mutation/meiosis/sexual reproduction is a source of variation ✔

c. competition/more offspring than the environment can support ✔

d. adaptations make individuals suited to their environment/way of life ✔

e. survival of better adapted «individuals)/survival of fittest/converse ✔

f. inheritance of traits/passing on genes of better adapted «individuals»
OR
reproduction/more reproduction of better adapted/fittest «individuals» ✔

g. speciation is formation of a new species/splitting of a species/one population becoming a separate species ✔

h. reproductive isolation of separated populations ✔

i. geographic isolation «of populations can lead to speciation» ✔

j. temporal/behavioral isolation «of populations can lead to speciation» ✔

k. disruptive selection/differences in selection «between populations can lead to speciation» ✔

l. gradual divergence of populations due to natural selection/due to differences in environment ✔

m. changes in the gene pools «of separated populations»/separation of gene pools ✔

n. interbreeding becomes impossible/no fertile offspring «so speciation has happened» ✔

a. recycling of nutrients/elements/components/materials ✔

b. carbon/nitrogen/another example of recycled nutrient/element ✔

c. decomposers/saprotrophs break down organic matter/release «inorganic» nutrients ✔

d. energy supplied by the sun
OR
energy cannot be recycled «so ongoing supply is needed»
OR
energy is lost from ecosystems as heat ✔

e. energy flow along food chains/through food web/through trophic levels ✔

f. photosynthesis/autotrophs make foods/trap energy
OR
autotrophs supply the food that supports primary consumers ✔

g. oxygen «for aerobic respiration» released by autotrophs/photosynthesis/plants ✔

h. carbon dioxide «for photosynthesis» released by respiration ✔

i. populations limited by food supply/predator-prey/interactions/competition
OR
populations regulated by negative feedback
OR
fewer/less of each successive trophic level «along the food chain»/OWTTE ✔

j. supplies of water from rainfall/precipitation/rivers/water cycle ✔

This was generally well answered, with most candidates knowing at least something of the roles of these two enzymes. Most candidates knew that ligase connects Okazaki fragments but some claimed that it creates hydrogen bonds between nucleotides on template and the new strand. Many candidates did not distinguish between unwinding of DNA and separating the strands. Two details that should be more widely known are that helicase separates the two strands of a DNA molecule by encouraging the breakage of hydrogen bonds between bases and that ligase seals nicks by making sugar phosphate bonds.

Most candidates think they understand evolution by natural selection but many do not. Here the focus was on speciation - the splitting of a species into two or more species. Often answers described the evolution of one species over time, rather than speciation itself. An idea central to natural selection that was frequently missing from an answer is adaptation or fitness. Often traits were referred to as ‘favourable’ and therefore likely to lead to survival and reproduction but there is a circularity of argument there. Survival depends on traits fitting the environment, hence being an adaptation to it. The mostly common ideas seen in answers were differential survival and reproduction, due to differences in traits. A common fault was to confuse individuals and species and to refer to a whole species surviving and reproducing more successfully than another species.

There were some vague answers to this question but also some impressive ones that explained ecological processes including nutrient recycling, energy flow and regulation of population sizes.

State one similarity and one difference between the structure of genes and short tandem repeats.

Outline the role of short tandem repeats in DNA profiling.

similarity:

both made of DNA/nucleotides
OR
both have bases/adenine/cytosine/guanine/thymine
OR
neither has uracil ✔

difference: 

genes are longer/have longer DNA/base sequences «without repeats»
OR
genes have introns/exons but tandem repeats do not
OR
genes have base sequences that code for polypeptides/proteins and tandem repeats do not ✔

a. tandem repeats allow individuals to be distinguished/compared/identified
OR
tandem repeats are used to identify the source of a DNA sample ✔

b. unique/different number of repeats/combination of tandem repeats in each individual
OR
unique/different pattern of bands in each individual ✔

c. PCR used for copying/amplifying «tandem repeats» ✔

d. gel electrophoresis used for separation «of tandem repeats»/create pattern of bands ✔

Draw the structure of a section of DNA showing all possible bases.

Outline the structural and genetic characteristics of eukaryotic chromosomes.

Explain how a polypeptide chain is synthesized in a eukaryotic cell.

a. sugar, phosphate and base linked correctly to form at least one nucleotide ✔ For mpa, ignore labelling of the subunits of the nucleotide. Carbon atoms in deoxyribose do not have to be numbered but the phosphate should be linked to C5 and the base to C1. Shapes other than circles and rectangles could be used for the phosphate and base.

b. deoxyribose, phosphate and base/named base labelled at least once ✔

c. adenine paired with thymine and cytosine paired with guanine ✔ For mpc, full names of all four bases are required, but not relative sizes of the purine and pyrimidine bases.

d. two antiparallel chains of nucleotides linked by hydrogen bonds with all sugar-phosphate bonds in correct position ✔ For mpd, a bond should connect the C3 of deoxyribose on one nucleotide to the phosphate on the adjacent nucleotide. Two nucleotides in each strand is sufficient.

a. linear/not circular DNA molecule
OR
one chromosome is one molecule of DNA/one chromosome is two DNA molecules «after replication» ✔

b. associated with histone proteins/nucleosomes ✔

c. centromere joins sister chromatids «after DNA replication» ✔

d. telomeres at the end «of the chromosome/chromatid» ✔

e. carries a sequence of genes / each gene occupies a specific locus ✔ Do not accept ‘sequence of bases’ for mpe.

f. alternative alleles of genes / homologous chromosomes carry same sequence of genes ✔

g. chromosomes in pairs / two «homologous chromosomes» of each type «in a diploid cell» ✔

h. non-coding sequences/example of a non-coding sequence ✔ Do not allow mph if the response states that chromosomes are always condensed.

i. supercoiled/condensed «during mitosis/meiosis» ✔

a. translation occurs on ribosomes ✔

b. tRNA-activating enzymes attach amino acids to tRNAs ✔

c. small and large ribosome units assemble on mRNA
OR
translation/polypeptide synthesis starts at a start codon ✔

d. each tRNA arriving at the ribosome binds to the A site ✔

e. anticodon «on tRNA» binds to codon «on mRNA» ✔

f. according to complementary base pairing/A with U and G with C ✔

g. ribosome moves along the mRNA / mRNA moves over ribosome ✔

h. t-RNA shifts from the A site to P site/from the P to the E site ✔

i. peptide bond between amino acids «on tRNAs at A and P sites» ✔

j. tRNA released from ribosome at E site ✔

k. cycle repeats with other tRNAs / polypeptide grows as tRNAs bring more amino acids ✔

l. until stop codon on mRNA is reached ✔

m. components are disassembled / polypeptide leaves the ribosome ✔

Accept these points in an annotated diagram.
Do not award any marks for events in transcription.

The diagram shows alpha amylase.

Explain the secondary structure of this protein molecule.

Amylase is used in human digestion.

State two sites of production of amylase.

Amylase is used in human digestion.

State the function of amylase.

Explain how enzymes catalyse chemical reactions.

a. secondary structure includes alpha helices/beta pleated sheets 

b. secondary structure «of this protein» consists «mainly» of alpha helices 

c. spiral coils «of polypeptide chain» held together by hydrogen bonds 

d. between oxygen «C=O» and hydrogen atoms «N−H» of amino acids«on backbone» 

e. «some» beta pleated sheets present in this protein

salivary glands AND pancreas

Both needed.

breaks down starch «by hydrolysis» into maltose/disaccharides

a. enzymes work by forming enzyme–substrate complexes 

b. binding of substrate«s» to active site «of enzyme» 

c. «enzyme» changes shape slightly
OR
puts strains on chemical bonds «of substrate» 

d. decreases activation energy / increases rate of reaction 

e. enzymes bind to specific substrates

Can show these points in an annotated diagram.

Nitrogen is part of many important substances in living organisms.

Draw labelled diagrams to show a condensation reaction between two amino acids.

Nitrogen is part of many important substances in living organisms.

Distinguish between transcription and translation.

Nitrogen is part of many important substances in living organisms.

Explain how insects excrete nitrogenous wastes.

a. at least one of the amino acid structures completely correct

b. peptide bond shown with N–C and C=O and N–H correct

c. release of water clearly shown

a. DNA is transcribed AND mRNA is translated

Disallow the first mark, if a candidate gets transcription and translation the wrong way round, but allow marks
after that up to [3 max]

b. transcription produces RNA AND translation produces polypeptide/protein

c. RNA polymerase used in only in transcription and ribosomes only in translation

d. transcription in the nucleus «of eukaryotes» and translation in the cytoplasm

e. tRNA needed for translation but not transcription

f. nucleotides linked in transcription and amino acids in translation

   OR

   sugar-phosphate/phosphodiester bonds in transcription and peptide bonds in translation

[Max 4 Marks]

a. excreted as uric acid

b. excretion by Malpighian tubules

c. nitrogenous waste/ammonia «accumulates» in hemolymph

d. nitrogenous waste/ammonia absorbed by Malpighian tubules

e. ammonia converted to uric acid

f. conversion to uric acid requires energy/ATP

g. high solute concentration in Malpighian tubules

    OR

    active transport of ions/Na+/K+ into Malpighian tubules

h. water absorbed by osmosis flushes uric acid/nitrogenous waste to «hind» gut

i. water/ions reabsorbed from the feces and returned to hemolymph

j. uric acid precipitates/becomes solid/forms a paste so can pass out with little water

k. uric acid excreted/egested with the feces

l. water conservation/osmoregulation

   OR

   reduces mass of water «in body»

m. uric acid is non-toxic

[Max 8 Marks]

The figure shows a tripeptide.

Label one peptide bond in this molecule.

Describe the secondary structure of proteins.

Outline the action taken by the diaphragm during inhalation.

circle/bracket around peptide bond / arrow pointing to peptide bond / peptide bond labelled;

Allow either peptide bond

Allow if adjacent C=O and NH groups are included in the circle/bracket, but do not allow if other parts of the molecule are included.

a. polypeptide wound into a helical structure / alpha/α helix
OR
polypeptide folded back on itself forming a pleated sheet / beta/β pleated sheet;
b. stabilized/held in shape by/due to hydrogen bonds (between C=O and N-H groups);
c. secondary structures are regular/unvarying (within polypeptides/proteins);

Allow annotated diagrams

contracts/flattens/becomes less domed/increases volume of thorax;

80 % of candidates indicated one of the two peptide bonds in the diagram correctly.

There were some good answers here describing secondary structures and the role of hydrogen bonding.

80 % of candidates knew that the diaphragm contracts than relaxes during inhalation, or stated that it flattens rather than moves upwards.

State one role of nucleosomes in eukaryotic cells.

Outline how Hershey and Chase’s experiment provided evidence for DNA as the genetic material.

State one function for a region of DNA that does not code for proteins.

Outline the role of the A-site of ribosomes in translation.

Outline the role of tRNA activating enzymes in translation.

a. help to supercoil/pack DNA in chromosomes 

b. help to regulate transcription / gene expression

a. experiment is meant to determine whether DNA or protein is the genetic material 

b. viruses/bacteriophages grown in radioactive S/S³⁵ which enters the protein coat 

c. viruses/bacteriophages grown in radioactive P/P³² which enters the DNA 

d. «radioactive» viral DNA entered the bacterial cell during infection
OR
«radioactive» viral protein did not enter the bacterial cell during infection

regulator of gene expression/introns/telomeres/genes for tRNA / rRNA / promoter / enhancer / silencer / site for primer to bind / codes for mRNA primer

binding/entry of tRNA carrying amino acids/aminoacyl tRNA / charged tRNA / site of transfer of growing polypeptide chains/peptide bond formation

Marks can be awarded to an annotated diagram.

a. ATP «hydrolysis» provides energy for amino acid attachment ✔

b. they attach a specific amino acid to the (3') end / free CCA of a tRNA ✔

c. they do this repeatedly / they attach amino acid to all of the tRNA molecules that have anticodon corresponding to that amino acid