DNA and PCR edexcel A2

I read a website and I am confused .

"Further, each strand has a directionality. Individual nucleotides are not really like round beads on a string, because each nucleotide has two distinct sites where it connects to other nucleotides. These are the 5' (five prime) and 3' (three prime) carbons, located in the deoxyribose (sugar) part of the nucleotide.


DNA is produced in living organisms by enzymes called DNA polymerases. Most of these work by adding nucleoside triphosphates on to the 3' end of an existing DNA strand. This means that polymerases construct DNA strands in the 5' to 3' direction. We use our half-arrowhead to indicate the direction in which a strand would grow, if it were to be extended by polymerase."


what does the sentence " Most of these work by adding nucleoside triphosphates on to the 3' end of an existing DNA strand. This means that polymerases construct DNA strands in the 5' to 3' direction." means? arent the two sentences contradict each other??

thank you!!!

WAHH, I dont think you should look at this stuff in detail. Just now how PCR is carried out and why its carried out. Its in the specification. if you need any help in understanding the PCR method, I'll be happy to help

I read a website and I am confused .

"Further, each strand has a directionality. Individual nucleotides are not really like round beads on a string, because each nucleotide has two distinct sites where it connects to other nucleotides. These are the 5' (five prime) and 3' (three prime) carbons, located in the deoxyribose (sugar) part of the nucleotide.


DNA is produced in living organisms by enzymes called DNA polymerases. Most of these work by adding nucleoside triphosphates on to the 3' end of an existing DNA strand. This means that polymerases construct DNA strands in the 5' to 3' direction. We use our half-arrowhead to indicate the direction in which a strand would grow, if it were to be extended by polymerase."


what does the sentence " Most of these work by adding nucleoside triphosphates on to the 3' end of an existing DNA strand. This means that polymerases construct DNA strands in the 5' to 3' direction." means? arent the two sentences contradict each other??

thank you!!!

in 2014Jan alternative 6B paper the answer corresponding to the properties of Taq DNA polymerase is that The Taq DNA polymerase needs a primer/ 3'-OH to begin synthesis of the complementary strand.



what is that sentence means ?@@

No, this isn't contradictory. We actually consider the direction of synthesis and the point of nucleotide addition to be different. Thus, you can visualize that if I add a nucelotide on the 3' end of the DNA molecule, I am going away from the 5' prime end of the nucelotide to which the new one was added. Thus my direction is actually away from the 5' end.
Also if I continue adding nucleotides in this way, the last nucleotide will have its 3' end exposed.

Thus my direction is technically 5'-3', which means I am going away from the 5' end and towards the 3' end.


That means that Taq polymerase, like all DNA polymerases, needs a free 3' end to add a new nucleotide to it. For this, it first needs a sequence, called a primer, that can bind to specific sites on DNA and allow the Taq polymerase to elongate the DNA strand.

I hope that helps.

Am I correct ?


Also must it be a hydroxyl group added to the dna for elongation of dna to start???

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Yes. To both.