3.4 DNA Replication
Terms in this set (3)
State that DNA replication is semi-conservative.
DNA replication copies DNA to produce new molecules with the same base sequence. It is semi-conservative because each new molecule formed by replication uses one new strand and one old strand which is conserved from the parent DNA molecule.
Explain DNA replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase.
Stage 1: The DNA double helix is unwound and separated into strands by helicase breaking the hydrogen bonds holding the strands together and creating a replication bubble.
Stage 2: The single strands act as blueprints for new strands. Free nucleotides are present in large numbers. The enzyme primase places primers on the new strand, allowing DNA Polymerase III to add to the strand in a 5-carbon to 3-carbon direction. Several small strands are made within the replication bubble formed by helicase that are known as Okazaki fragments. DNA Polymerase III is then removed, and DNA polymerase I replaces primers with corresponding base pairs. The enzyme ligase then binds these new base pairs to the rest of the strand. The bases of these nucleotides form hydrogen bonds with the bases of the parent strand. The nucleotides are connected to form a new strand.
Stage 3: The daughter DNA molecules each rewind into a double helix.
(The two daughter DNA molecules are identical in base sequence to each other and to the parent molecule, because of complementary base pairing (Adenine pairs with Thymine and Cytosine with Guanine). Each of the new strand is complementary to the template on which it was made and identical to the other template).
Explain the significance of complementary base pairing in the conservation of the base sequence of DNA.
Because the nitrogenous bases that compose DNA can only pair with complementary bases, any two linked strands of DNA are complementary. This ensures that the old base sequence is conserved.
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