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Structure of DNA and RNA

DNA and RNA are polynucleotides made of nucleotide units; DNA usually forms a double helix with two antiparallel complementary strands, while RNA is usually single-stranded and contains ribose sugar and uracil.

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Student-friendly explanation

A nucleotide has a nitrogenous base, pentose sugar, and phosphate group. In DNA, deoxyribose sugar joins phosphate by phosphodiester bonds to form a sugar-phosphate backbone. Bases project inward and pair specifically: adenine with thymine, and guanine with cytosine. This explains Chargaff's rule that A equals T and G equals C in double-stranded DNA. The two strands run in opposite directions, one 5' to 3' and the other 3' to 5'. RNA differs by having ribose, uracil instead of thymine, and usually a single strand, making it suitable for information transfer and protein synthesis.

How to write this in exams

  1. 1

    Start with the exact idea

    DNA and RNA are polynucleotides made of nucleotide units; DNA usually forms a double helix with two antiparallel complementary strands, while RNA is usually single-stranded and contains ribose sugar and uracil.

  2. 2

    Then show how to use it

    Identify nucleotide parts, state bond linking nucleotides, explain base pairing, state strand polarity, then contrast DNA with RNA if asked.

  3. 3

    Add one concrete example

    In a DNA sample, if adenine is 20 percent, thymine will also be 20 percent. The remaining 60 percent is shared equally by guanine and cytosine, so each is 30 percent.

  4. 4

    Avoid this incomplete answer

    Writing A pairs with G or saying RNA contains thymine as its normal base.

Definition

DNA and RNA are polynucleotides made of nucleotide units; DNA usually forms a double helix with two antiparallel complementary strands, while RNA is usually single-stranded and contains ribose sugar and uracil.

Example

In a DNA sample, if adenine is 20 percent, thymine will also be 20 percent. The remaining 60 percent is shared equally by guanine and cytosine, so each is 30 percent.

Rule to remember

Chargaff's rule: in double-stranded DNA, A = T and G = C; purines pair with pyrimidines to keep helix width uniform.

Memory hook

DNA has T for thymine and two strands; RNA has U for uracil and is usually one strand.

Examples and method

Worked example

If cytosine is 18 percent in double-stranded DNA, guanine is 18 percent. A + T is 64 percent, so adenine and thymine are each 32 percent.

Method to apply

Identify nucleotide parts, state bond linking nucleotides, explain base pairing, state strand polarity, then contrast DNA with RNA if asked.

Diagram support

Draw two sugar-phosphate backbones, label 5' and 3' ends, A-T with two hydrogen bonds, G-C with three hydrogen bonds, major groove, minor groove, and antiparallel strands.

How CBSE asks it

Usually asked as a diagram-label question, Chargaff percentage problem, or short answer comparing DNA and RNA.

Avoid common mistakes

Common confusion

Students often write that all nucleic acids are double-stranded. Most cellular RNA is single-stranded, though it can fold by internal base pairing.

Common wrong answer

Writing A pairs with G or saying RNA contains thymine as its normal base.

Exam tip

In structure questions, mention nucleotide components, phosphodiester bonds, complementary base pairing, antiparallel polarity, and Chargaff's rule.

Quick check

Why are the two DNA strands called complementary and antiparallel?

They are complementary because adenine pairs only with thymine and guanine pairs only with cytosine. They are antiparallel because one strand runs 5' to 3' while the other runs 3' to 5'.

Answer writing and exam use

1-mark answer

Define nucleic acid as a polynucleotide, list nucleotide parts, describe DNA double helix with antiparallel strands and complementary pairing, add Chargaff's rule, and give one DNA-RNA contrast.

2-mark answer

Define nucleic acid as a polynucleotide, list nucleotide parts, describe DNA double helix with antiparallel strands and complementary pairing, add Chargaff's rule, and give one DNA-RNA contrast.

3-mark answer

Define nucleic acid as a polynucleotide, list nucleotide parts, describe DNA double helix with antiparallel strands and complementary pairing, add Chargaff's rule, and give one DNA-RNA contrast.
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