CCommons
Search✍️ WriteSign in
🧬 NEET Biology❓ Question

Nucleotide bonds N-glycosidic, phosphoester, phosphodiester confusing me, plus the Chargaff percentage sums

Tanvi Deshmukh12th Sample

27 Aug 2026 Β· 1 min read

Making notes on DNA structure and my table keeps going wrong at the bonds row!! In NCERT there's a nucleoside (nitrogenous base + pentose sugar) and a nucleotide (nucleoside + phosphate). The three bond names are N-glycosidic, phosphoester and phosphodiester. I know one of them is between nucleotides but I can't say which atom is joined to which.

Second doubt, numericals. A double stranded DNA has 30% adenine. I wrote guanine = 20% and my friend wrote 70%. I think I'm right because of Chargaff but I want to see the full logic. Also what if the question talks about a single-stranded DNA? Can Chargaff still be used?

Third, I keep forgetting the numbers: pitch 3.4 nm, 10 base pairs per turn. NCERT then says the distance between 2 bp is 0.34 nm and a human cell has about 6.6 Γ— 10^9 bp, which comes to about 2.2 metres long. Can someone show how 2.2 m comes?

WhatsAppPost on X

7 answers & replies

Sign in to join the conversation.

  • [+]Kavya NaikDropperSampleΒ· 1mo ago

    You're right, 20%. Chargaff says in double stranded DNA the amount of A equals T and G equals C, so purines = pyrimidines. A = 30% means T = 30%, together 60%. The remaining 40% is G + C shared equally, so G = 20% and C = 20%.

    For single stranded DNA or RNA, no. Chargaff works because of base pairing between two strands. A single strand has no partner, so A need not equal T. If a question says ratio (A+G)/(T+C) is not 1 then it's a single stranded molecule.

  • [+]Siddharth RaoDropperSampleΒ· 1mo ago

    Bond table, written the way I finally memorised it:

    BondJoins
    N-glycosidicnitrogenous base to C-1' of the sugar (makes the nucleoside)
    Phosphoesterphosphate to C-5' of the sugar (makes the nucleotide)
    Phosphodiester3'-OH of one sugar to the 5' phosphate of the next nucleotide (builds the strand)

    "Di" because the phosphate makes two ester bonds, one to each sugar. The strand always grows 5' to 3'.

    • [+]Tanvi Deshmukh12thSampleΒ· 1mo agoAuthor

      YES the "di because two ester bonds" finally makes it stick, thank you!! updating my table now

  • [+]Meera Patel11thSampleΒ· 1mo ago

    Here is the length calculation. Pitch is 3.4 nm for 10 base pairs, so one base pair is 0.34 nm, that is 0.34Γ—10βˆ’90.34 \times 10^{-9} m.

    6.6Γ—109Γ—0.34Γ—10βˆ’9Β m=2.244Β mβ‰ˆ2.2Β m6.6 \times 10^{9} \times 0.34 \times 10^{-9}\ \text{m} = 2.244\ \text{m} \approx 2.2\ \text{m}

    It's for the whole diploid human cell, and that's why DNA can't float loose in the nucleus; it's wrapped around histones into nucleosomes and then coiled again and again.

    • [+]Kavya NaikDropperSampleΒ· 1mo ago

      meera's calculation is exactly what NCERT does, just write the units carefully. Many students forget that 0.34 nm = 0.34 Γ— 10^-9 m and the answer comes in cm instead.

  • [+]Riya Singh12thSampleΒ· 1mo ago

    Something that helped me visualise the structure: the two strands run antiparallel, one 5' to 3' and the other 3' to 5'. The backbone (sugar-phosphate) is outside, the bases are inside, stacked like steps of a spiral stair. A pairs with T through 2 hydrogen bonds, G with C through 3, so G–C rich regions are harder to separate. And a purine always pairs with a pyrimidine, which keeps the width of the helix uniform.

  • [+]Lakshmi TripathiDropperSampleΒ· 1mo ago

    For the base names, "PURe As Gold": purines are A and G. Pyrimidines are C, T, U ("CUT the Py"). And remember DNA has T, RNA has U instead, plus ribose vs 2'-deoxyribose. Little thing, but a lot of assertion-reason questions use it.

More from Tanvi Deshmukh

Keep reading