Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA.

Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands.

Explain the methods and aims of DNA profiling.

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

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;

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

Knowledge of the nature of unique and repetitive sequences of DNA was very poor. Very few scored anywhere near full marks. Often odd marks could be picked up by linking widely separated comments, as descriptions of the two types were attempted. Where candidates possessed knowledge, some did not follow the command to distinguish the two types of sequences.

This was a difficult diagram to draw unless it had been well learnt and many showed that this had not been achieved. A few were good enough to get every possible mark and exceed the maximum.  The antiparallel nature of the two strands, arrangement of base, phosphate and deoxyribose and the base pairing relationship were widely known. Individual nucleotides were almost never identified. Hydrogen bonds were indicated with a solid line suggesting that they were equivalent to covalent bonds.  Sometimes the bases were only given as letters.  Commonly, more than four nucleotides were shown.

It was clear that this was a popular section but accounts were still rather vague and unscientific. “Suspects can be identified” and “paternity can be decided” but without any indication of having a DNA sample first and then another with which to compare.  
Very few mentioned using satellite /repetitive sequences in creating a DNA profile. Gel electrophoresis was often outlined but specifics were missing such as the use of restriction enzymes and the creation of a pattern of DNA fragments. Some accounts confused karyotyping and amniocentesis with DNA profiling.