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Components of Cells
The Macromolecules
Stages in DNA Replication
Getting started

the two strands of DNA have to be separated before replication can begin

here a replication fork has just formed

DNA replication is semiconservative and proceeds in three major stages:

  1. Unwinding - DNA molecules consists of two individual strands of linked nucleotides coiled around each other in a double helix. Before any form of replication can occur, these two intertwined strands have to be separated.

    This is more difficult than it looks. Imagine two lengths of string twisted round and around each other in a double spiral. Hold each end of this twisted pair firmly and then ask someone to pull the individual pieces of string apart. It cannot be done.

    Before the two strands of DNA can be separated, at least one of the strands must be snapped, or broken to relieve the thermodynamic strain and allow a point of rotation as the two halves of the DNA molecule are pulled apart.

    Special unwinding proteins attach to the DNA. The weak but numerous forces, called hydrogen bonds, that hold the base pairs together are further weakened until the base pairs separate and the strands can be pulled further and further apart. This creates a "Y"-shaped structure called a replication fork, and it is at this fork that the next stage in the replication process takes place.

only one way

a polynucleotide chain can only elongate in one direction

here a new triphosphate is about to be added to the growing chain

  1. Complementary pairing up of the nucleotide bases - Each separate DNA strand now has its sequence of bases exposed and unpaired. Enzymes match up each one of these exposed bases, in turn, with free nucleotide triphosphates; A with T, and G with C.

    It is therefore the sequence of bases on the old, original strand which dictates and specifies the complementary order of bases on the newly created strand.

  2. Completing the joins - Other enzymes, called polymerases, link up the free, matched nucleotide triphosphates by removing the terminal di-phosphate and using energy so released to carry out the very non-spontaneous chemical reaction of joining the phosphate to the deoxyribose sugar.

    New DNA chains, therefore, can only "grow" or elongate in one direction.

continuous and discontinuous

one of the new polynucleotide chains can elongate continuously

the other growing chain does so in short lengths which must later be joined together

  1. Continuous and Discontinuous - At the replicating fork, one of the newly forming DNA strands grows and elongates continuously as more and more of the older, parental DNA molecule unwinds. This can continue indefinitely, or until the replication is complete.

    On the other DNA strand, however, the newly forming half of the molecule must be made in short sections. As the replicating fork opens up, a small section of new DNA strand starts forming right in the fork and moves our away from the join.

    When it has elongated as far as it can, it stops. A new section is made behind it, and then another behind that. Later, the "nicks" or tiny gaps between these short sections are sealed up by other enzymes, called ligases, to make a complete and continuous daughter DNA molecule.

© 2001, Professor John Blamire