Download Chapter 2 DNA to end Short Answer

Chapter 2 DNA Stuff Short Answer
[59 marks]
1a. Outline primary and quaternary protein structures.
[2 marks]
Primary protein structure:
Quaternary protein structure:
Markscheme
a. (primary structure) is sequence of amino acids;
b. (quaternary structure) is the linking of two or more polypeptides to form one protein;
1b. List three limiting factors of photosynthesis.
[3 marks]
Markscheme
a. temperature;
b. pH;
c. light;
d. CO2;
2a. Draw a labelled diagram of the structure of DNA, showing the arrangement of subunits.
[3 marks]
Markscheme
correctly shows two antiparallel sugar-phosphate strands/backbones with linkages between phosphates and sugars
connected through bases; (phosphate and simple names such as sugar and base are acceptable labels. They must be
given at least once.)
correctly labeled phosphate and deoxyribose and base;
sugar linked to phosphates through correct pentagon corners/(5’–3’) linkages;
shows complementary base pairs of A-T/Adenine–Thymine and G-C/Guanine–Cytosine;
correctly indicates both covalent/phosphodiester and hydrogen bonds.
2b. Explain DNA replication.
[3 marks]
Markscheme
DNA replication is semi-conservative/daughter DNA molecule contains one parent strand and one new strand;
unwinding of double helix/separation of two strands by helicase;
separated (parent) strands become templates for new strands;
free/single nucleotides join (parent/template) strands through complementary base pairing;
DNA polymerase joins nucleotides in new strands;
Award [3] for the above points clearly shown in an annotated diagram.
3a.
Identify the chemical group labelled I.
[1 mark]
Markscheme
phosphate
3b. State the type of bond labelled II.
[1 mark]
Markscheme
covalent / phosphodiester
3c. Distinguish between the sense and antisense strands of DNA during transcription.
[1 mark]
Markscheme
only the antisense strand is transcribed / the antisense strand is transcribed to mRNA and the sense strand is not
transcribed/has the same base sequence as mRNA (with uracil instead of thymine)
To award [1], reference must be made to both strands and transcription.
3d. Compare the DNA found in prokaryotic cells and eukaryotic cells.
[2 marks]
Markscheme
Award marks for paired statements only. Answers do not need to be shown in a table format.
4a.
DNA replication involves a number of enzymes including DNA polymerase. Identify one other enzyme involved in [1 mark]
DNA replication.
Markscheme
helicase / RNA primase / (DNA) ligase
4b.
Explain the role of Okazaki fragments in DNA replication.
[2 marks]
Markscheme
DNA fragments/sections (formed) on the lagging strand;
because replication must be in the 5' –3' direction;
replication starts repeatedly and moves away from replication fork;
4c.
Label the sense and antisense strands.
[1 mark]
Markscheme
both strands clearly labelled
Check carefully whether the correct strand has been labelled if the labels are shown in helical parts of the DNA.
Reject if the sense strand label points to the mRNA.
4d. Draw an arrow on the diagram to show where the next nucleotide will be added to the growing mRNA strand.
[1 mark]
Markscheme
a clearly drawn arrow pointing at the free 3' end of the mRNA strand or to the first free nucleotide on the antisense
strand to the left of the mRNA or to a nucleotide added by the candidate to the left hand end of the mRNA
5a. The diagram below represents a DNA nucleotide.
Identify the phosphate group and deoxyribose.
Phosphate group: .................................................
Deoxyribose: ........................................................
Markscheme
Phosphate: I
Deoxyribose: III
Both correct for one mark.
[1 mark]
5b.
Draw a labelled diagram to show how four nucleotides are joined together to form a double-stranded DNA
molecule with two base pairs.
[3 marks]
Markscheme
two sugar-phosphate strands shown connected through bases;
a sugar-phosphate bond labeled as a covalent bond;
hydrogen bonds labeled on line between bases;
boxes labeled as (nitrogenous) bases;
complementary base pairing/A–T/G–C;
(5'–3') linkages correctly shown; (no label required)
5c.
State two differences between RNA and DNA nucleotides.
[2 marks]
Markscheme
RNA nucleotides contain ribose and DNA nucleotides contain deoxyribose;
(some) RNA nucleotides contain uracil and (some) DNA nucleotides contain thymine;
6a. Draw a labelled diagram of the molecular structure of DNA including at least four nucleotides.
[5 marks]
Markscheme
The diagram must show four nucleotides shown with two on each side showing phosphate-sugar backbones and
nitrogen base pairs bonded between them.
Award [1] for each of the following clearly drawn and correctly labelled.
phosphate – shown connected to deoxyribose;
deoxyribose – shown connected to phosphate;
(nitrogenous) bases – shown bonded to deoxyribose;
base pairs – shown with labels adenine/A bonded to thymine/T and cytosine/C bonded to guanine/G;
hydrogen bonds – shown connecting bases;
covalent bonds – shown connecting deoxyribose to phosphates;
nucleotide – clearly identified by the candidate;
Award [4 max] if diagram is not shown double stranded.
6b. A small DNA sample found at a crime scene can be used in an investigation. Describe the steps taken in the
processing of this small sample of DNA.
[6 marks]
Markscheme
DNA samples are taken from crime scene, suspects and victims;
polymerase chain reaction/PCR used to increase the amount of DNA;
restriction enzymes used to cut DNA;
electrophoresis involves electric field/placing sample between electrodes;
used to separate DNA fragments according to size;
creating DNA profiles/unique patterns of bands;
comparison is made between the patterns;
criminals/victims can be identified in this way;
DNA is (quite) stable / DNA can be processed long after the crime;
6c.
Discuss the relationship between one gene and one polypeptide.
[7 marks]
Markscheme
DNA codes for a specific sequence of amino acids/polypeptide;
the DNA code for one polypeptide is a gene;
DNA is transcribed into mRNA;
mRNA moves to a ribosome;
where mRNA is translated into a polypeptide;
originally it was thought that one gene always codes for one polypeptide;
some genes do not code for a polypeptide;
some genes code for transfer RNA/tRNA/ribosomal RNA/rRNA;
some sections of DNA code for regulators that are not polypeptides;
antibody production does not follow this pattern (of simple transcription-translation); (allow other examples)
change in the gene/mutation will affect the primary structure of the polypeptide;
7a.
Outline the bonding between DNA nucleotides.
[2 marks]
Markscheme
hydrogen bonds between nucleotides on opposite strands/ complementary bases/adenine and thymine and cytosine
and guanine;
(reject letters instead of base names)
covalent bonds between nucleotides within strands/between sugar/deoxyribose and phosphate;
7b.
Explain how chemical bonding between water molecules makes water a valuable coolant in living organisms.
[2 marks]
Markscheme
hydrogen bonding between water molecules;
breaking (hydrogen bonds) needs/removes energy/heat;
hydrogen bonds must break when water evaporates/vaporizes;
7c. State a word equation for anaerobic cell respiration in humans.
[1 mark]
Markscheme
pyruvate/pyruvic acid →lactate/lactic acid;
glucose → (pyruvate/pyruvic acid) → lactate/lactic acid;
Accept correct chemical equation with formulae.
8a.
Draw a labelled diagram showing two different complementary pairs of nucleotides in a molecule of DNA.
[4 marks]
Markscheme
The structures underlined must be labelled.
at least one nucleotide with deoxyribose linked to base and phosphate;{ Labels need not be on the same nucleotide.
Do not allow sugar.
phosphate and deoxyribose linked C3 to C5;{ Position required, not label. Straight line from C4 to phosphate is
acceptable. Do not penalize if the second strand is not antiparallel and the bonding is therefore incorrect on it.
(complementary) bases labelled with at least one of each of A, G, T and C correctly linked to C1;
hydrogen bonds between correct complementary bases;{ Bond numbers not required.
correct antiparallel orientation shown; (as seen by shape or orientation of sugar)
8b.
Outline the structure of nucleosomes.
[2 marks]
Markscheme
(eight) histone (proteins);
DNA wrapped around histones/nucleosome;
further histone holding these together;
Do not allow histone wrapped around DNA.
8c.
Explain primary structures and tertiary structures of an enzyme.
[3 marks]
Markscheme
primary structure is (number and) sequence of amino acids;
joined by peptide bonds;
tertiary structure is the folding of the polypeptide/secondary structure/alpha helix;
stabilized by disulfide/ionic/hydrogen bonds/hydrophobic interactions;
tertiary structure gives three dimensional globular shape/shape of active site;
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