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petethepiratearr
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#3521
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#3521
(Original post by medlou)
Attachment 227208
Can someone explain why the answer to cii) is AC?
Surely it would be TGGTC?


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because the terminator nucleotide in tube 2 is cytosine, so adenine pairs with thymine and then terminator cytosine pairs with guanine
the strand produced is complementary, that might be where you've gone wrong!
so producing AC as paired with TG
hope this helps :s!

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marleyxd
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#3522
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#3522
Anyone else finding the past papers really hard?? Swear 90% of the questions are interpreting graphs and "suggesting" why experiments where done a certain way!?!?!
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DrewYouTwo
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#3523
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#3523
(Original post by Lyont)
I think the essay may be on either ions and their role in biology or ATP, and it tends to be:

A* - 75%
A- 70%
B - 64%
C- 60%
D - 55%
E - 50%

At least that was what it was last year
**** theyre pretty close together those values...?...?.!!,.
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ahmmm
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#3524
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#3524
(Original post by Lyont)
I think the essay may be on either ions and their role in biology or ATP, and it tends to be:

A* - 75%
A- 70%
B - 64%
C- 60%
D - 55%
E - 50%

At least that was what it was last year
role of ions? you got a plan for that?
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medlou
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#3525
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#3525
(Original post by laser174572)
I'm guessing this is using the Sanger method to sequence a gene and that tube 2 has the cytosine terminator nucleotide in it? If so then it's because the complementary nucleotides to the DNA sequence will join together to form a complementary strand, but as in this tube some of the cytosine bases are terminator nucleotides, there is a probability that at each C (so G on the original sequence) that a terminator nucleotide will be added, stopping the synthesis of that strand as the next nucleotide can't join. As the second base in the DNA sequence is G then there is a probability of this happening when the strand is only two bases long, therefore the shortest sequence is AC.
Sorry if this isn't very clear, if you don't understand this have a look over the section on Sanger sequencing in the textbook, it's pretty good


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Ah I see, Thanks! I know where I went wrong now, I wasn't thinking of complementary bases so thought it would stop at the C on the orginal strand, silly me
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amelia95
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#3526
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#3526
(Original post by mariposaa)
I'm not sure if anyone has answered you yet, but I did a past paper where this was a question and the mark scheme was very vague (1. Vague description of positive feedback 2. Links to LH and oestrogen). I went back over my notes and what I think is that an increase in oestrogen causes an increase in LH. The peak in LH stimulates ovulation. Corpus luteum forms. Corpus luteum secretes progesterone and small amounts of oestrogen. Therefore more oestrogen = more LH = more oestrogen

Hope this helps!
Yes I did this in a past paper too. The mark scheme INSISTED that you couldn't simply put "as oestrogen increases, LH increases". You had to say "Oestrogen increases then LH increases AFTER" x


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2013leaver
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#3527
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#3527
(Original post by Blob2491)
is DNA ligase used in PCR?
No, as vectors are not involved. Only in in vivo where the cut DNA and plasmid are joined
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Simonthegreat
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#3528
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#3528
Hi guys,

With the essays, do you have to have info from each of the units?

Also on the june 2011 paper question 5 b I don't understand how they go the answer 2? Can someone please explain?

thanks
Si
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DELETED ACCOUNT
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#3529
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#3529
(Original post by Blob2491)
is DNA ligase used in PCR?
Pretty sure its not. Its DNA polymerase which joins the complimentary bases on the DNA molecule. DNA ligase is used when you're trying to produce recombinant DNA using single stranded DNA from 2 sources.
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aWildPidgey
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#3530
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#3530
(Original post by medlou)
Attachment 227208
Can someone explain why the answer to cii) is AC?
Surely it would be TGGTC?


Posted from TSR Mobile
Cytosine terminator nucleotide used, as C* (C term. nuc.) would bind to the G (second nuc.) and give the fragment AC.

* * * REMEMBER * * *

The Sagner method gives the COMPLEMENTARY sequence to the base sequence we want to find.

So if the Sagner method gives us:

TGCCCGAATGATTCGAA

Then the DNA which is being sequenced has base sequence:

ACGGGCTTACTAAGCTT
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rm.xo
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#3531
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#3531
Just wanted to say, if you've ran out of questions to test yourself, the Human Biology unit 4 aqa papers have a similar specification so some of the questions overlap.
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TauMuon
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#3532
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#3532
(Original post by noreaction)
can someone please outline

pcr ?

And please please please
explain gel electrophoresis, fragmentation, gene sequencing, fingerprinting and gene therapy

I explained PCR earlier on in the thread :P

Gel Electrophoresis:
DNA fragments placed into a well at one end of the gel.
A current is applied across the gel.
Because DNA is negative, the fragments begin to move across the gel towards the positive electrode.
The smaller DNA fragments move furthers (because they interact less with the gel)
The larger DNA fragments move the least distance (because they interact more with the gel)

Fragmentation:
DNA fragments produced by either restriction endonucleases or reverse transcriptase. Restriction endonuclease enzymes 'cut' the DNA at specific recognition sites. The cuts leave either 'blunt' or 'sticky' ends.
Reverse transcriptase is an enzyme which catalyses the formation of DNA from mRNA, forming cDNA. The complementary strand is then added using DNA polymerase, forming a complete DNA strand.

Gene Sequencing:
The Sanger method. Very briefly:
DNA samples mixed with primers, DNA polymerase, nucleotides and small amounts of one terminator nucleotide.
The nucleotides bind randomly with the DNA, producing fragments that vary in length.
Repeating this process with each of the four terminator nucleotides will produce fragments of every possible length.
These are then run through gel electrophoresis, and the order of nucleotides can be determined by reading from the smallest first.

Fingerprinting:
DNA sample extracted and amplified with the PCR. This DNA is then digested with restriction endonucleases. These fragments are then run through gel electrophoresis. The DNA fragments are then transferred to a nylon membrane (southern blotting) and washed with radioactive primers, which bind with the DNA. The resulting membrane is placed under an x-ray film which is then developed, producing a imagine with dark bands. The spacing and thickness of the bands is unique to individuals, due the random nature of 'junk' DNA in each individual.

Gene Therapy:
Gene therapy is the act of replacing or supplementing defective genes.
You can replace the gene in a fertilised egg/embryo; the individual will then be permanently 'cured' from the defective gene.
You can also just target the affected tissues. Because cells constantly die and are replaced, this treatment must be repeated periodically.
Genes delivered in one of two ways:
Using a harmless virus as a vector.
Using a liposome containing the gene which can diffuse through the phospholipid bilayer into cells.
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DELETED ACCOUNT
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#3533
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#3533
(Original post by ahmmm)
role of ions? you got a plan for that?
Not a hard question. Think of these ions and you'll be fine

Fe2+
Cl- in cholera
Na+
K+
NO3- (nitrogen cycle)
NO3- (active transport in plants)
SO4-/PO4- (polyatomic ions in resting potential)
Na+ in glucose-sodium co transport
Cl- in inhibitory synapse

There are lots of ions to talk about
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rm.xo
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#3534
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#3534
(Original post by Simonthegreat)
Hi guys,

With the essays, do you have to have info from each of the units?

Also on the june 2011 paper question 5 b I don't understand how they go the answer 2? Can someone please explain?

thanks
Si
you should try and include something from one of each units, sometimes this is hard though. I would say aim for 3 topics (depending on the question) and also try and include something from all 3 kingdoms; animals/insects, bacteria and plants. This will then ensure you gain all three marks for breadth.

for 5b I'll use the analogy of a rope. If you have a rope and you cut it twice, how many peices do you have? 3. The number of restriction sites on a peice of linear DNA is always one less than the number of peices you obtain.
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pjanoo
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#3535
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#3535
What have people generally been getting with just the questions out of 75 in the past papers?
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Rivastro
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#3536
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#3536
I don't know how you guys revise but probably the best thing to come out of the past few days was my disocvery of this absolute gem http://www.youtube.com/watch?v=x5yPkxCLads

Not sure if its a repost :/
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Lyont
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#3537
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#3537
(Original post by Blob2491)
is DNA ligase used in PCR?
yes it binds the wanted gene to the plasmid at the sticky ends to make a recombinant plasmid
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kingster123
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#3538
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#3538
can any1 help me with muscle structures and adaptations
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Scienceisgood
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#3539
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#3539
(Original post by frogs r everywhere)
Not a hard question. Think of these ions and you'll be fine

Fe2+
Cl- in cholera
Na+
K+
NO3- (nitrogen cycle)
NO3- (active transport in plants)
SO4-/PO4- (polyatomic ions in resting potential)
Na+ in glucose-sodium co transport
Cl- in inhibitory synapse

There are lots of ions to talk about
Don't forget Magnesium in chlorophyll. =)

EDIT:
Also, there was a mention of iron in the June 2013 unit 4 paper in RuBP.
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Lyont
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#3540
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#3540
(Original post by pjanoo)
What have people generally been getting with just the questions out of 75 in the past papers?
ranging from 60 - 65 really, keep making stupid mistakes that's costing me marks /:
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