A2 Biology OCR June 2015 Revision Thread

I reckon around 85-90. depends on the other grade boundaries. Like one paper was 77 for an A and the one I've just done was 65 for an A

Could someone explain to me where PCR plays a role in sequencing the genome?
I have memorised the steps for sequencing the genome using BACs and the automated process, but I can't really see where does PCR come into either. I mean, I know the automated is based on PCR, but how?
For the BAC method, in the OCR book it says as the cells grow many copies of the DNA strands are produced and cells containing specific BACs are taken; so is PCR even needed for this method?

Any help would be much appreciated.

Could someone explain to me where PCR plays a role in sequencing the genome?
I have memorised the steps for sequencing the genome using BACs and the automated process, but I can't really see where does PCR come into either. I mean, I know the automated is based on PCR, but how?
For the BAC method, in the OCR book it says as the cells grow many copies of the DNA strands are produced and cells containing specific BACs are taken; so is PCR even needed for this method?

Any help would be much appreciated.

outline how the polymerase chain reaction (PCR) can be used to make multiple copies of DNA fragments; The DNA sample is mixed with a supply of DNA nucleotides and DNA polymerase The mixture is heated to 95°C. This breaks the hydrogen bonds holding the strands together, so the samples are now single stranded Primers (short lengths of single stranded DNA) are added The temperature is reduced to 55°C to allow the primers to bind and form small sections of double stranded sections DNA polymerase can bind to these double-stranded sections The temperature is raised to 72°C. The enzyme extends the double stranded section by adding nucleotides to the unwound DNA When the DNA polymerase reaches the other end of the DNA, a new, double stranded DNA molecule is generated The whole process can be repeated many times so the amount of DNA increase exponentially


That's pretty much what the spec wants you to know about PCR

It's used for amplifying DNA when you only have a very small sample, so you can compare it easier, say for example mitochondria DNA (very small amount) could be amplified to match to a sample, in crimes or whatever, i hope that helps?x

outline how the polymerase chain reaction (PCR) can be used to make multiple copies of DNA fragments; The DNA sample is mixed with a supply of DNA nucleotides and DNA polymerase The mixture is heated to 95°C. This breaks the hydrogen bonds holding the strands together, so the samples are now single stranded Primers (short lengths of single stranded DNA) are added The temperature is reduced to 55°C to allow the primers to bind and form small sections of double stranded sections DNA polymerase can bind to these double-stranded sections The temperature is raised to 72°C. The enzyme extends the double stranded section by adding nucleotides to the unwound DNA When the DNA polymerase reaches the other end of the DNA, a new, double stranded DNA molecule is generated The whole process can be repeated many times so the amount of DNA increase exponentially


That's pretty much what the spec wants you to know about PCR

It's used for amplifying DNA when you only have a very small sample, so you can compare it easier, say for example mitochondria DNA (very small amount) could be amplified to match to a sample, in crimes or whatever, i hope that helps?x

I've finally understood it! The automated process is basically PCR but it's interrupted because of fluorescent markers attached to free DNA nucleotides, and the automated process is used in the BAC method to sequence the DNA fragments.

Its interrupted because that the DNA polymerase is thrown off when it reaches a fluorescent/radioactively marked nucleotide, so you end up with DNA of different lengths, which is then separated by electrophoresis and read using a machine. Make sure you state that there are two different types of nucleotides.

Its interrupted because that the DNA polymerase is thrown off when it reaches a fluorescent/radioactively marked nucleotide, so you end up with DNA of different lengths, which is then separated by electrophoresis and read using a machine. Make sure you state that there are two different types of nucleotides.

Thank you
What confuses me though is that in the ocr book, it says electrophoresis occurs before the automated process. So does electrophoresis occur before and after?

It's part of the automated process. PCR, electrophoresis, analysing the base sequence using a computer rogram or whatever. The stands only need to be separated when the strands are being analysed.

Yeah, I think the idea is that the machine basically does both the electrophoresis and analysing. It's safest to talk about all three processes separately imo

Ok thank you!

I think it shouldn't matter too much though if you said electrophoresis slightly earlier in your answer, as if it was a big 9/10 marker, by just explaining all of them you should still secure the marks.

What's the point of separating DNA of the same length before amplifying it? When you talk about electrophoresis, you have to talk about how it separates DNA by length. How do we get our different lengths? Interrupted PCR. It's safer putting it in order.

My teacher never mentioned any of that stuff ;_;

that electrophoresis seperates dna by length?

No, about how you have varied lengths of DNA fragments in the first place