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I am confused. I know that a DNA molecule is made of two polynucleotides.

But does each polynucleotide represent one nucleic acid? If so, a DNA molecule is made of two nucleic acids, right?

Or do the two polynucleotides (that form a molecule of DNA) make up only one nucleic acid? If so, a DNA molecule is only made of one nucleic acid, right?

Mark
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  • What do you mean by "two polynucleotides"? I would say "two polynucleotide strands" - start with a nucleotide, connect some of them together and you have a nucleic acid (strand), join two of those and you get double-stranded DNA. Is this not covered in wikipedia pages like : https://en.wikipedia.org/wiki/Nucleic_acid ? – gilleain Nov 14 '17 at 13:08
  • Well, each strand of a DNA molecule makes up one polynucleotide... Right? – Mark Nov 14 '17 at 13:18
  • I would just say it "is" a polynucleotide, but yes - double stranded DNA is two polynucleotide strands wrapped into a helix. – gilleain Nov 14 '17 at 13:40
  • See that's the problem... You're talking about "polynucleotide strands". But how many polynucleotides is there in one strand of DNA: a) one b) many? – Mark Nov 14 '17 at 14:02
  • One. A polynucleotide is made of multiple nucleotides - it is a strand. – gilleain Nov 14 '17 at 14:14
  • Oh ok. So, there are two types of nucleic acids: 1) DNA --> which is double-stranded, so it's made of only two polynucleotides 2) RNA --> which is single-stranded so it's made of only one polynucleotide. So if we get back to my original question "Is a single polynucleotide necessarily and always a nucleic acid?" --> the answer is no, for DNA is a nucleic acid and it is made of two polynucleotides. Am I right? – Mark Nov 14 '17 at 14:27
  • Yes, that's right. – gilleain Nov 14 '17 at 14:35
  • Hallelujah! I finally understand! Thanks a lot, man!! – Mark Nov 14 '17 at 14:43
  • Mark: No, you don't. See my comment to your answer. –  Nov 14 '17 at 16:45
  • Possible duplicate of Is a DNA molecule a single strand of polynucleotide or two of them linked together? – David Nov 14 '17 at 22:27
  • I would advise you to focus on ideas in molecular biology rather than questions of nomenclature, which betray a chemical attitude to the topic. Counting strands is not where the action is, but if you do so at least get it right. – David Nov 14 '17 at 22:32

2 Answers2

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There are two types of nucleic acids: 1) DNA --> which is double-stranded, so it's made of only two polynucleotides 2) RNA --> which is single-stranded so it's made of only one polynucleotide. So if we get back to my original question "Is a single polynucleotide necessarily and always a nucleic acid?" --> the answer is no, for DNA is a nucleic acid and it is made of two polynucleotides.

Mark
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    Wrong. Some viruses have single-stranded DNA (ssDNA). One polynucleotide = one DNA molecule (always). Two polynucleotides = two DNA molecules (always). (Note: two strands of DNA are two molecules; two strands in double-stranded DNA are still two molecules, because there is no covalent bond between them. –  Nov 14 '17 at 16:43
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    ...and some RNA viruses are double-stranded. As I commented previously, I suggest you stop counting the strands in nucleic acids and study their biology. – David Nov 14 '17 at 22:34
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Your question arrangement is wrong, DNA is made up of 2 polynucleotide strand indeed. And these polynucleotides are the polymers of nucleotides, where nucleotides consist of deoxyribose sugar in case of DNA and ribose sugar in case of RNA+nitrogenous base+phosphate. Yes it is always a nucleic acid.

user37769
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