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ICSE Question Bank
2018Q1(e)(v)Structure of Chromosomes, Cell Cycle and Cell Division
Correct the statementHard1 mark
1(e)

Correct the following statements by changing the underlined words:

NITROGEN bonds are present between the complementary nitrogenous bases of DNA.

Topic: Hydrogen bonds

Previous-year question practice database

Answer

Exam answer

HYDROGEN bonds are present between the complementary nitrogenous bases of DNA.

Explanation

The two strands of the DNA double helix are held together by HYDROGEN BONDS between the paired bases: ADENINE – THYMINE, joined by TWO hydrogen bonds GUANINE – CYTOSINE, joined by THREE hydrogen bonds

WHY HYDROGEN BONDS AND NOT SOMETHING STRONGER: a hydrogen bond is WEAK, and that weakness is essential. The two strands must be able to separate easily so that each can act as a template during REPLICATION. Strong bonds would make the molecule useless as a carrier of heredity.

The bases are called NITROGENOUS because they contain nitrogen — which is what makes "nitrogen bonds" a plausible-sounding error, and exactly why the examiner chose it.

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