Download Chapter 2 DNA to end Extended Response

Chapter 2 DNA extended response
[108 marks]
1a. Describe the genetic code and its relationship to polypeptides and proteins.
[5 marks]
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
a. (the genetic code is based on) sets of three nucleotides/triplets of bases called codons;
b. bases include adenine, guanine, cytosine and thymine in DNA / adenine, guanine, cytosine and uracil in RNA; (do
not accept ATCG)
c. each codon is code for one amino acid;
d. some codons are (start or) stop codons;
e. DNA is transcribed into mRNA by base-pair matching/complementary base pairing;
f. mRNA is translated into a sequence of amino acids/polypeptide;
g. each gene codes for a polypeptide;
h. polypeptides may be joined/modified to form proteins;
1b. Outline the role of proteins in active and passive transport of molecules through membranes.
[5 marks]
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
a. channel proteins allow diffusion/osmosis/passive transport;
b. large/polar molecules cannot cross the (hydrophobic) membrane freely;
c. facilitated diffusion involves moving molecules through proteins down their concentration gradient/without requiring
ATP;
d. aquaporins (specific integral membrane proteins) facilitate the movement of water molecules/osmosis;
e. some proteins (for facilitated diffusion) are specific to molecule/ions;
f. active transport involves moving molecules through proteins against their concentration gradient/requiring ATP;
g. (some) proteins in the membrane are pumps / pumps perform active transport / sodium potassium pump;
1c. Many cell functions, like synthesis of macromolecules and transport, require energy in the form of ATP. Explain
how ATP is generated in animal cells.
[8 marks]
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
a. ATP is a form of energy currency/immediately available for use;
b. ATP is generated in cells by cell respiration (from organic compounds);
c. aerobic (cell respiration) requires oxygen;
d. anaerobic (cell respiration) does not require oxygen;
e. glycolysis breaks down glucose into pyruvate;
f. glycolysis occurs in cytoplasm;
g. (by glycolysis) a small amount of ATP is released;
h. ADP changes into ATP with the addition of a phosphate group/phosphoric acid / accept as chemical equation;
i. in mitochondria/aerobic respiration produces large amount of ATP / 38 mols (for the cell, per glucose molecule);
j. oxygen/aerobic respiration is required for mitochondrial production of ATP;
k. in mitochondria/aerobic respiration pyruvate is broken down into carbon dioxide and water;
2a.
Draw a labelled diagram of a section of DNA showing four nucleotides.
[5 marks]
Markscheme
Award [1] for each labelled item shown correctly connected.
2b.
Outline a technique used for gene transfer.
[5 marks]
Markscheme
a. plasmid used for gene transfer/removed from bacteria;
b. plasmid is a small/extra circle of DNA;
c. restriction enzymes/endonucleases cut/cleave DNA (of plasmid);
d. each restriction enzyme cuts at specific base sequence/creates sticky ends;
e. same (restriction) enzyme used to cut DNA with (desired) gene;
f. DNA/gene can be added to the open plasmid/sticky ends join gene and plasmid;
g. (DNA) ligase used to splice/join together/seal nicks;
h. recombinant DNA/plasmids inserted into host cell/bacterium/yeast;
2c.
Explain how evolution may happen in response to an environmental change.
[8 marks]
Markscheme
a. (genetic) variation in population;
b. (variation is) due to mutation / sexual reproduction;
c. valid example of variation in a specific population;
d. more offspring are produced than can survive / populations over-populate;
e. competition / struggle for resources/survival;
f. example of competition/struggle for resources;
g. survival of fittest/best adapted (to the changed environment)/those with beneficial adaptations / converse;
h. example of changed environment and adaptation to it;
i. favourable genes/alleles passed on / best adapted reproduce (more) / converse;
j. example of reproduction of individuals better adapted to changed environment;
k. alleles for adaptations to the changed environment increase in the population;
l. example of genes/alleles for adaptations increasing in a population;
m. evolution by natural selection;
n. evolution is (cumulative) change in population/species over time / change in allele frequency;
Suitable examples are antibiotic resistance and the peppered moth but any genuine evidence-based example of
adaptation to environmental change can be credited.
3a.
Distinguish between RNA and DNA.
[3 marks]
Markscheme
DNA is double-stranded while RNA is single-stranded;
DNA contains deoxyribose while RNA contains ribose;
the base thymine found in DNA is replaced by uracil in RNA;
one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);
3b. Explain the process of DNA replication.
[8 marks]
Markscheme
occurs during (S phase of) interphase/in preparation for mitosis/cell division;
DNA replication is semi-conservative;
unwinding of double helix / separation of strands by helicase (at replication origin);
hydrogen bonds between two strands are broken;
each strand of parent DNA used as template for synthesis;
synthesis continuous on leading strand but not continuous on lagging strand;
leading to formation of Okazaki fragments (on lagging strand);
synthesis occurs in 5'→3' direction;
RNA primer synthesized on parent DNA using RNA primase;
DNA polymerase III adds the nucleotides (to the 3' end)
added according to complementary base pairing;
adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.)
DNA polymerase I removes the RNA primers and replaces them with DNA;
DNA ligase joins Okazaki fragments;
as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates broken off releasing energy to form
bond;
Accept any of the points above shown on an annotated diagram.
3c.
Outline how enzymes catalyse reactions.
[7 marks]
Markscheme
they increase rate of (chemical) reaction;
remains unused/unchanged at the end of the reaction;
lower activation energy;
activation energy is energy needed to overcome energy barrier that prevents reaction;
annotated graph showing reaction with and without enzyme;
substrate joins with enzyme at active site;
to form enzyme-substrate complex;
active site/enzyme (usually) specific for a particular substrate;
enzyme binding with substrate brings reactants closer together to facilitate chemical reactions (such as electron
transfer);
induced fit model / change in enzyme conformation (when enzyme-substrate/ES complex forms);
making the substrate more reactive;
4a.
Draw a labelled diagram to show how two nucleotides are joined together in a single strand of DNA.
[3 marks]
Markscheme
Award [1] for each labelled item shown above.
Award [2 max] if the two nucleotides are not shown in a single strand.
4b.
Outline a basic technique for gene transfer.
[6 marks]
Markscheme
plasmid removed from bacteria;
plasmid cleaved/cut open by restriction enzymes;
desired gene/DNA extracted from donor;
DNA from donor cleaved using same restriction enzyme;
results in sticky ends;
with complementary base sequences;
pieces of DNA from two organisms mixed;
ligase used to splice pieces (DNA);
recombinant plasmids formed;
insertion into host cells;
4c.
Explain the process of translation.
[9 marks]
Markscheme
translation is the synthesis of proteins/polypeptide chain/specific sequence of amino acids;
translation occurs in cytoplasm/ribosomes;
uses information on the mRNA;
mRNA carries the genetic information of DNA;
mRNA binds to ribosome;
mRNA contains series of codons/base triplets;
tRNA binds with an amino acid and carries it to the ribosome;
tRNA has the anticodon that is complementary to the codon on the mRNA;
two tRNAs bind to a ribosome/mRNA at the same time;
(peptide) bond forms between two amino acids (carried by tRNA molecules to the ribosome);
the first tRNA detaches, ribosome moves along mRNA and another tRNA carrying an amino acid binds;
process repeats forming chain of amino acids/polypeptides;
5a.
Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive
sequences in nuclear DNA.
[5 marks]
Markscheme
Award [1] for each pair of statements in the table and [1] for any statement below the table.
satellite DNA is repetitive;
repetitive sequences are used for profiling;
prokaryotes do not (usually) contain repetitive sequences
5b.
Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two
strands.
[5 marks]
Markscheme
Award [1] for each of these structures clearly drawn and labelled.
four nucleotides shown in diagram with one nucleotide clearly labelled;
base, phosphate and deoxyribose (shown as pentagon) connected between the correct carbons and labelled at least
once;
backbone labelled as covalent bond between nucleotides correctly shown as 3' to 5' bond;
two base pairs linked by hydrogen bonds drawn as dotted lines and labelled;
two H bonds between A and T and three H bonds between C and G;
adenine to thymine and cytosine to guanine; do not accept initials of bases
antiparallel orientation shown;
5c.
Explain the methods and aims of DNA profiling.
[8 marks]
Markscheme
DNA sample obtained;
from hair/blood/semen/human tissue;
DNA amplified / quantities of DNA increased by PCR/polymerase chain reaction;
satellite DNA/highly repetitive sequences are used/amplified;
DNA cut into fragments;
using restriction enzymes/restriction endonucleases;
gel electrophoresis is used to separate DNA fragments;
using electric field / fragments separated by size;
number of repeats varies between individuals / pattern of bands is unique to the individual/unlikely to be shared;
Award [5 max] for methods
forensic use / crime scene investigation;
example of forensic use e.g. DNA obtained from the crime scene/victim compared to DNA of suspect / other example
of forensic use;
paternity testing use e.g. DNA obtained from parents in paternity cases;
biological father if one half of all bands in the child are found in the father;
genetic screening;
presence of particular bands correlates with probability of certain phenotype / allele;
other example;
brief description of other example;
Award [4 max] for aims
6a.
Explain why DNA must be replicated before mitosis and the role of helicase in DNA replication.
[4 marks]
Markscheme
two genetically identical nuclei/daughter cells formed during mitosis (so hereditary information in DNA can be passed
on);
two copies of each chromosome/DNA molecule/chromatid needed;
helicase unwinds the DNA/double helix;
to allow the strands to be separated;
helicase separates the two (complementary) strands of DNA;
by breaking hydrogen bonds between bases;
6b.
Explain how the base sequence of DNA is conserved during replication.
[5 marks]
Markscheme
DNA replication is semi-conservative;
DNA is split into two single/template strands;
nucleotides are assembled on/attached to each single/template strand;
by complementary base pairing;
adenine with thymine and cytosine with guanine / A with T and C with G;
strand newly formed on each template strand is identical to other template strand;
DNA polymerase used;
Marks may be awarded for any of the above points if clearly presented in a well-annotated diagram.
6c.
Describe the events that occur during mitosis.
[9 marks]
Markscheme
sequence of stages is prophase → metaphase → anaphase → telophase;
chromosomes condense/supercoil/become shorter and fatter in prophase;
spindle microtubules grow (from poles to equator) in prophase/metaphase;
nuclear membrane breaks down in prophase/metaphase;
spindle microtubules attach to the centromeres/chromosomes in metaphase;
chromosomes line up at equator in metaphase;
centromeres divide / (paired) chromatids separate / chromosomes separate into two chromatids in
metaphase/anaphase;
(sister) chromatids/chromosomes pulled to opposite poles in anaphase;
spindle microtubules disappear in telophase;
nuclear membrane reforms around chromosomes/chromatids in telophase;
chromosomes/chromatids decondense in telophase;
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