AQA BIOL5 Biology Unit 5 Exam - 22nd June 2011

Yeah! I saw that yesterday! I reckon it's a mistake in the mark scheme, cause all the text books say it's the movement of potassium ions out of the axon that causes repolarisation.

Thanks.

But what I want to know is what happens after the virus enters the body cells.

Hi

siRNA is a method which cells use to control gene expression (the other being the second messenger model)

Double-stranded RNA is cut into short fragments by an enzyme (name not required for the exam)

The short strands of RNA are called siRNA.

The double-stranded siRNA unwinds (as it is quiet short the H-bonds break easily)

One of the strands degrades. The other forms a RNA-induced silencing complex -RISC (name not required for the exam). RISC is a collection of one of the strands of siRNA with a complex of protein

This siRNA strand now binds with its complementary mRNA molecule in the cell. The proteins in the RISC complex break down the mRNA. Translation of gene is prevented = no gene expression.

Very useful in research if you wanted to prove that a gene was responsible for a particular phenotype. Design siRNA complementary to the gene, this "knocks out" gene function = change in phenotype indicates that gene was responsible for phenotype.

haha eats it!?!



1.Double stranded siRNA is broken up into smaller double stranded siRNA by an enzyme

2. The siRNA binds to the RISC protein forming a RISC-siRNA complex - this breaks the two strands - with one strand being the RISC-siRNA complex (with the siRNA being single stranded) and the other strand of siRNA being rapidly degraded

3. The RISC-siRNA complex binds to the mRNA molecules in the cytoplasm by comp base pairing

4. This binding causes the mRNA molcule to break into two strands - hence the cleaved mRNA can no-longer be translated

This occurs after transcription, but before translation.

siRNA (small interfering RNA) is a double stranded RNA molecule which interferes with gene expression. DNA goes through the process of translation and produces a single strand of mRNA. The siRNA molecule will have one strand which is complimentary. It releases the other strand, binds to the mRNA and proteins/enzymes digest the mRNA, meaning it does not get to translation.

That is what I remember. I think the siRNA molecule is attatched to the protein/enzyme which may digest at the end.

Hope that helps

Please could someone help explain to me the difference between totipotent cells and stem cells. Aren't both unspecialised?

I know this has been discussed a little before, but I really can't get my head around kineses. I understand that it is non-directional and effected by the intensity of the stimulus, but I keep reading that in unfavourable conditions, the organism will move more quickly and change direction more often. But surely walking fast in a straight line is more likely to result in finding more favourable conditions? If anyone can find it online, question 1 on the June 06 paper illustrates the point I'm trying to make, the body-louse turns more often at 35 degrees to stay there longer.

I know it's a minor point that will probably only come up as a one marker, if at all, but it's really annoying me so a quick clarification would be much appreciated

its asking how the resting potential is created. It uses the ATP for the sodium-potassium pump, active transport needs ATP, so resting potential is when sodium is pumped out and potassium back into the axon.

Think of it like the wood-louse is panicking and trying to find a way out quickly of the conditions, so it moves faster and makes more turns. In the favorable conditions the wood-louse is more relaxed, moves more slowly and doesn't turn more often. Bit silly, and not very biologically accurate, but it helps me remember

Also, I think one of the answers from the past exam questions is wrong, I asked my teacher and she just said to ignore it

All I can think of is to use organic fertiliser, and only use the exact amount you need.

A way of remembering the Parnicius Corpuscle is by applying pressure to your thumb cos it looks like it and it stretches.

Ok, i'll try and make them as AQA-like as possible :P

Describe PCR
If I start off with 10 strands of DNA and end up with 2560 strands, how many cycles of PCR did the DNA go through?
A child has his genetic fingerprint taken. Half of the bands match with the mother. Explain why all of the bands do not match with the mother.
What are the disadvantages of treating SCID with gene therapy?
Explain why dna fragments move different distances in gel electropphoresis.

Would appreciate it if you asked me questions on homeostasis/oestrus cycle