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CraftExam
medium difficulty6 min

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.

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

Concept Practice Coming Soon

Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1If cytosine is 18 percent in double-stranded DNA, guanine is 18 percent.
  2. 2A + T is 64 percent, so adenine and thymine are each 32 percent.

Method you can reuse

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

Sketch it in your answer

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why are the two DNA strands called complementary and antiparallel?

Think first, then show answer
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'.

Common wrong answer

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

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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

Answer plan

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    A nucleotide has a nitrogenous base, pentose sugar, and phosphate group.

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