Describe the characteristics of stem cells that make them potentially useful in medicine.

Outline a technique of gene transfer resulting in genetically modified organisms.

Explain the use of karyotyping in human genetics.

(stem cells) have/retain the capacity to divide;
can be used to produce cell cultures/large number of identical cells;
can be used to repair/replace damaged/lost cells/tissue;
(stem cells) are undifferentiated / have not yet differentiated/specialized;
can differentiate/specialize in different ways / are pluripotent/totipotent;
can be used to form a variety of different tissues / form organs;
used in medical research;
used in treatment of (named) disease;

gene transfer takes a gene from one species/organism and inserts it into another;
using plasmid/viral vector/ballistic impregnation/electroporation;
use of reverse transcriptase to obtain gene from mRNA;
restriction enzyme/endonuclease used to cut out/excise gene;
(same) restriction enzyme used to cut open plasmid;
sticky ends used to link DNA/link gene to plasmid;
DNA ligase used to seal nicks/splice;
bacterium takes in plasmid / plasmid transferred to bacterium/plant/host cell;
valid documented example (e.g. human insulin from bacterium/yeast / salt-tolerant tomato plant / carotene/vitamin A in rice /
herbicide/Roundup/glyphosate resistance in crop plants / factor IX/clotting factor in sheep milk / low phosphate feces in pigs;

Definition and construction of karyotypes:
karyotype is the number and type / image of chromosomes in a cell;
cells collected from chorionic villus / by amniocentesis;
requires cells in metaphase / stimulate cells to divide and reach metaphase;
burst cells and spread chromosomes / photo taken of chromosomes;
chromosomes are arranged in pairs;
according to size/structure/position of centromere/banding pattern;

Uses for karyotypes:
karyotypes used to identify sex/gender;
male is XY and female XX;
used to identify chromosome mutations/abnormal numbers/non-disjunction;
Down syndrome due to extra chromosome 21 / other trisomy/aneuploidy example;
used for pre-natal diagnosis of chromosome abnormalities;
may lead to a decision to abort the fetus;
prepare for consequences of abnormality in offspring;

The characteristics of stem cells and their uses were generally well known. Almost all candidates mentioned that they are undifferentiated cells and that they can differentiate in different ways. Some distinguished pluripotent from totipotent stem cells which was impressive. Fewer candidates than expected mentioned the ability of stem cells to divide repeatedly. Some candidates who were struggling to find much to include in their answer wrote extensively about the ethics of stem cell research, which was not required.

This was answered more poorly than expected. The examining team was anticipating thorough accounts of gene transfer using reverse transcriptase, restriction enzymes, plasmids, sticky ends and plasmids but few of these were seen. In many cases the techniques were not well understood, with errors and omissions in candidates‟ answers. The word splice was often used to mean slice or cut, when it actually means linking together. One use of this word before the days of molecular biology was the act of joining the ends of ropes by weaving together their strands –this image might help candidates see the word splice is used for joining together fragments of DNA using sticky ends and DNA ligase.

Despite an apparently narrow focus, this was one of the better answered questions in Section B. Most candidates at least knew that Down syndrome is due to trisomy of chromosome 21. The best answers included details both of how karyotypes are prepared and also what can be learned from them. The weakest candidates mostly wrote in vague terms about testing for genetic defects and did not appreciate the difference between gene and chromosome mutations.