DNA Polymerase Chain Reaction Question (1 Viewer)

[Dreamerish*~]

Being a non-biology student, I feel like I'm missing an arm and a leg.

Quoted from CC:

Because a person has two types of DNA - one from each parent - each primer pair will have grown two DNA fragments - one from each original type of DNA. These fragments may be the same or different.

Remember that there were two synthesised fragments per pair of primers because there were two types of DNA - one from each parent.

I thought that during PCR Amplification, only one fragment of DNA is copied. Why is there two? Isn't it the same fragment of DNA that is being copied multiple times?

Pleas explain. Ta.

 

[Survivor39]

Hmm... I think the auother of CC is confused as well.

The double helix unwind to form TWO strands (in which the authors called it "two types" from different parent - not really true. He is confused with homologous chromosomes that are derived from each parent)

One primer attach to one strand and add bases onto it - form one double-stranded DNA
Another primer attach to one strand and add bases onto it - form the second double-stranded DNA.

Hence two double-stranded DNA, identical to the parent DNA, unless wrong base-paring occurs.

I thought that during PCR Amplification, only one fragment of DNA is copied. Why is there two? Isn't it the same fragment of DNA that is being copied multiple times?

The author meant 2 newly-synthesised DNA or fragments, not the original fragment.

 

[Dreamerish*~]

Thank you so much for clearing that up.

The CC diagrams are confusing too.

 

[thejosiekiller]

is there another text you can use?

 

[Dreamerish*~]

I was about to look through Chemistry in Context but I thought it's easier just to ask BoS.

 

[Dreamerish*~]

Another question:

Are primers placed at the beginning AND the end of the fragment we want to copy? When the DNA double helix unzips into two strands, a primer is placed on each strand, at different locations. Diagrams and animations I've seen did not indicate another pair of primers placed at the end of the desired fragment, but CC tells me there are.

 

[Survivor39]

The primer will be placed at the beginning of each strand. You don't need to know what the primer does but if you're interested, the primer is needed so that the polymerse can add bases onto the primer, as it cannot just synthesise a new strand from scratch.

Because the two original strands are running opposite each other from a 5' to 3' position, one primer will attach one strand at the 5' position (beginning), while the other primer will also attach at the 5' end, but giving the appearance that it is attached to the end of the strand, but it's not.

e.g.

Strand 1: 5'-------------- 3'
Primer attach to the 5' end, indicated in bold

Strand 2: 3' ---------------5'
Primer attach to the 5' end, indicated in bold, but it's giving the appreance that it is attach to the end of the strand, but it's not really because remember this strand is running in the opposite of the other strand.

Then the addition of bases will occur from the 5' to 3' direction for both strand.

Hence one strand needs ONE primer at the BEGINNING of the strand (the 5' end).

 

[Dreamerish*~]

Thank you so much.

If you don't mind me asking, are you planning to transfer to another course from med science? I was thinking of doing med science but I've been told by too many people that the only reason people do it is to transfer to another course at the end of one year.

 

[Survivor39]

Although a lot of people likes to transfer out of medical science, I am one of those who are going to stay still the end. I am even going to do Honours and a higher degree if I am given the opportunity. I just want to be in medical research, which is what B Med science has to offer.

 

[Survivor39]

I haven't done this modules so I don't know what depth you need to know. But sometimes you need to know more thazn you need to in order to understand certain concepts. But generally you learn the 5', 3' of the DNA and stuff like this in 1st year. Then in seconmd year, you understand the whole processes of how DNA replicate, the transcription, translation of it and how different enzymes and different factors and control help.

 

[Dreamerish*~]

Is this 1st Year MBLG(molecular biology and genetics) ? . Sounds complicated for Yr12 chemistry o_o

Yeah, it doesn't sound like chemistry, period.

I don't think this is in the syllabus, but if I can understand PCR as well as I can, it would really help with answering some of the questions.