Question
(a) Draw the structure of a section of DNA showing all possible bases [4]
(b) Outline the structural and genetic characteristics of eukaryotic chromosomes [4]
(c) Explain how a polypeptide chain is synthesized in a eukaryotic cell [7]
▶️Answer/Explanation
Ans:
a
a sugar, phosphate and base linked correctly to form at least one nucleotide
b deoxyribose, phosphate and base/named base labelled at least once
c adenine paired with thymine and cytosine paired with guanine
d two antiparallel chains of nucleotides linked by hydrogen bonds with all sugar-phosphate bonds in correct position
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. For mpc, full names of all four bases are required, but not relative sizes of the purine and pyrimidine bases. For mpd, a bond should connect the C3 of deoxyribose on one nucleotide to thephosphate on the adjacent nucleotide. Two nucleotides in each strand is sufficient.
b
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
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
i supercoiled/condensed «during mitosis/meiosis»
c
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
Question
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.
▶️Answer/Explanation
Markscheme
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
Question
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.
▶️Answer/Explanation
Markscheme
DNA and histone
methylation/acetylation/phosphorylation/epigenetic tags/modification of nucleosome tails/N-terminal tails
Question
Outline the structure and functions of nucleosomes.
Explain how DNA is used to pass on genetic information to offspring accurately but also produce variation in species.
Accurate transmission of base sequences to offspring depends on successful gamete production. Describe how spermatogenesis occurs in humans.
▶️Answer/Explanation
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
a. found in eukaryotes;
b. consists of DNA wrapped around proteins/histones;
c. histones are in an octamer/group of eight;
d. are held together by another histone/protein;
e. in linker region;
f. help to supercoil chromosomes / to facilitate DNA packing;
g. (function is to) regulate transcription / gene expression;
Remember, up to TWO “quality of construction” marks per essay.
a. DNA is replicated/copied semi-conservatively/from a template;
b. mutations can be a source of variation / resulting protein has new or different functions;
c. mutations/changes in the DNA may not result in changes in the amino acid for which the triplet codes;
d. genetic code is redundant;
e. genes occur as paired alleles which can be different;
f. crossing-over occurs;
g. recombines linked alleles producing new combinations;
h. random orientation of bivalents / homologous chromosomes (in metaphase I);
i. large genetic variation in (haploid) gametes / 2n / 223;
j. random recombination of alleles during fertilization (leads to variation);
k. different phenotypes among members of the same population;
l. natural selection may lead to enhanced survival of recombinants;
Remember, up to TWO “quality of construction” marks per essay.
a. germinal cells / spermatogonia undergo mitosis to keep a supply of germinal cells present;
b. some germinal cells / spermatogonia grow larger to become primary spermatocytes;
c. primary spermatocytes go through meiosis I;
d. to form secondary spermatocytes;
e. these secondary spermatocytes go through meiosis II;
f. to produce spermatids;
g. spermatids differentiate/grow a tail and reduce their cytoplasm
h. spermatids associated with nurse cells (Sertoli cells);
i. sperm detach from Sertoli cells and enter lumen of the seminiferous tubule;
j. testosterone stimulates sperm production;
Question
Draw a labelled diagram of Escherichia coli as an example of a prokaryote.
Explain the process of transcription in prokaryotes.
Some prokaryotes cause infectious diseases which stimulate the body’s immune system. Outline the principles that form the basis of immunity.
▶️Answer/Explanation
Markscheme
Award [1] for each structure clearly drawn and correctly labeled.
cell wall; (with some thickness)
plasma membrane; (shown as single line or very thin)
cytoplasm;
pilus/pili; (shown as single lines coming from the cell wall)
flagellum/flagella; (thicker and longer than pili and embedded in cell wall)
70S ribosomes; (shown as small dots)
nucleoid / naked DNA;
approximate width 0.5 μm / approximate length 2.0 μm;
Award [3 max] if the bacterium drawn does not have the shape of a bacillus (rounded-corner rectangle with length approximately twice its width).
Award [3 max] if any eukaryotic structures included.
transcription, synthesis of RNA identical to one strand/coding strand of DNA;
antisense stand acts as template/is transcribed;
RNA polymerase attaches to sequence of DNA known as promoter (region);
RNA polymerase separates the two strands of DNA;
(unwinding) exposes (10–20) DNA bases for pairing with RNA nucleotides;
RNA nucleotides matched to complementary bases;
adenine with uracil and cytosine with guanine / uracil replaces thymine;
H bonds between RNA nucleotide and complementary base on DNA strand;
(RNA) nucleoside triphosphates used;
hydrolysis of (two) phosphate molecules provides energy for reaction;
adds nucleotides to the 3′ end of RNA molecule/in 5′ → 3′ direction;
terminator is sequence of DNA signaling end of transcription;
RNA molecule separates completely from DNA;
Award any of the above points for a clearly drawn correctly annotated diagram.
skin and mucous membranes form barriers to pathogens as first line of defence;
macrophage recognizes antigens and ingests pathogen (in blood/body tissues);
presents antigen/MHC on cell surface;
macrophage activates helper T-cells that are complementary to antigen;
complementary B-cell becomes activated/stimulated by T-helper cells;
activated B-cell increases in size and divides by mitosis / creates clone of B-cells;
B-cells differentiate into plasma cells and memory cells; (both needed) plasma cells secrete specific antibodies;
memory cells remain/form basis of long-term immunity;
polyclonal response / multiple B-cells activated by different molecules of antigen;
Award any of the above points for a clearly drawn correctly annotated diagram.
(Plus up to [2] for quality)