Toughies

. The use of IVF in conjunction with pre-implantation genetic diagnosis (PGD) to identify single gene disorders and chromosomal abnormalities.

Frame-shift mutations cause all of the codons and all of the amino acids after the mutation to be changed. This has a major effect on the structure of the protein produced.

Contains autorthymic cells that send waves of electrical signals across atria. Located in wall of right atrium and also known as pacemaker. Sets the rate at which the heart contracts.

Peak levels of oestrogen stimulate a surge in the secretion of LH. This surge in LH triggers ovulation.

Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters. Convergent neural pathways can release enough neurotransmitter molecules to reach threshold and trigger an impulses Summation of a series of weak stimuli can release enough neurotransmitter to trigger an impulse.