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AQA A - level biology

Please might someone be able to explain to me the meaning/ significance of the 3-5 and 5-3 directions in DNA and what these mean in order that I might be able to understand, please?
Hi,

As you will know, DNA is a polymer of nucleotides; each nucleotide is a combination of three chemical constituents: a pentose sugar (deoxyribose in DNA), a nitrogen base and a phophate group.

look at the pic below to understand the answer to your Q.

You may be aware that organic compounds [=compounds of carbon], especially those with several carbon atoms have their carbon atoms numbered in sequence from one end.

In the case of a nucleotide, the 5 crbon atoms of the pentose (cf, pentagon =5-sided polygon] are numbered from 1-5.

The 5' end has the phosphate attached to it, and the 3' carbon is linearly opposite it (along one of the DNA strands). - the other strand of the double helix will have the 3' and 5' ends interposed.

3'-5' ends of DNA.jpg

The strand with the actual genetic triplet code sequence is known as the sense strand.

So, as in the diagram below, eaxh strand of the DNA double helix will have a 3' end and a 5' end.

Quite easy once you know, isn't it?

Best of luck!

M (specialist A level biology tutor)

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