C
CraftExam
medium difficulty6 min

DNA Replication

DNA replication is the semi-conservative process by which one DNA molecule produces two identical DNA molecules, each containing one parental strand and one newly synthesized strand.

In one line: Old strand stays, new strand pairs; one fork side is smooth and the other is fragment-based.

Concept Practice Coming Soon

Understand it

  • Replication begins at specific origin sites where the DNA helix unwinds.
  • Each old strand acts as a template because complementary base pairing determines the new strand sequence.
  • DNA polymerase adds nucleotides only in the 5' to 3' direction, so one strand is synthesized continuously as the leading strand, while the other is synthesized discontinuously as Okazaki fragments on the lagging strand.
  • RNA primers provide a starting point, and DNA ligase joins fragments.
  • Meselson and Stahl supported semi-conservative replication using nitrogen isotopes and density-gradient centrifugation.

Example

If a DNA molecule with both strands labelled with heavy nitrogen replicates once in light nitrogen medium, each daughter DNA molecule has one old heavy strand and one new light strand.

Rule to remember

Replication rule: template strand plus complementary base pairing plus 5' to 3' DNA polymerase activity produces accurate daughter DNA.

See it worked out

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

Worked example

  1. 1A heavy DNA sample grown for one generation in light nitrogen gives hybrid-density DNA.
  2. 2After two generations, both hybrid and light DNA appear, supporting semi-conservative replication.

Method you can reuse

Identify origin, unwind DNA, use each strand as template, place primers, extend new DNA 5' to 3', synthesize leading and lagging strands, remove primers, seal fragments, form two DNA molecules.

Sketch it in your answer

Label origin of replication, replication fork, parental strands, leading strand, lagging strand, RNA primer, DNA polymerase, Okazaki fragments, and DNA ligase.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is DNA replication called semi-conservative?

Think first, then show answer
It is called semi-conservative because each daughter DNA molecule keeps one original parental strand and contains one newly synthesized complementary strand.

Common wrong answer

Saying DNA polymerase starts synthesis without a primer.

Don't mix this up

Students often write that both new strands are synthesized continuously. The lagging strand is made as Okazaki fragments because DNA polymerase works only 5' to 3'.

Exam tip

For replication fork answers, include template strand, primer, DNA polymerase, leading strand, lagging strand, Okazaki fragments, and ligase.

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

Commonly asked as Meselson-Stahl interpretation, replication fork diagram labels, or explanation of leading and lagging strands.

Answer plan

Define semi-conservative replication, explain template-based complementary synthesis, describe leading and lagging strand formation with enzymes, and mention Meselson-Stahl evidence if required.
  1. 1

    Enough for 1 mark

    DNA replication is the semi-conservative process by which one DNA molecule produces two identical DNA molecules, each containing one parental strand and one newly synthesized strand.

  2. 2

    Add for 2 marks

    Example: If a DNA molecule with both strands labelled with heavy nitrogen replicates once in light nitrogen medium, each daughter DNA molecule has one old heavy strand and one new light strand.

  3. 3

    Add for 3 marks

    Replication begins at specific origin sites where the DNA helix unwinds.

Practice

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