DNA Structure and Semi-Conservative Replication
Learn how DNA's double helix is built and how semi-conservative replication works, including complementary base pairing, DNA helicase and DNA polymerase.
DNA is a double helix made of two polynucleotide strands. During semi-conservative replication, the strands separate and each acts as a template for a new strand. Each resulting DNA molecule therefore contains one original strand and one newly made strand.
DNA structure
A nucleotide contains a deoxyribose sugar, a phosphate group and one nitrogen-containing base: adenine, thymine, cytosine or guanine. Nucleotides are joined within each strand by phosphodiester bonds, forming a sugar-phosphate backbone.
The two strands are held together by hydrogen bonds between complementary bases: adenine pairs with thymine, and cytosine pairs with guanine. This pairing means that the base sequence on one strand determines the sequence on the other. The sequence of bases carries genetic information. The strands run in opposite directions, with the paired bases facing one another inside the helix and the sugar-phosphate backbones on the outside.
How replication works
- DNA helicase breaks the hydrogen bonds between the paired bases, separating the two strands.
- Each original strand acts as a template. Free DNA nucleotides pair with its exposed bases according to the complementary base-pairing rules.
- DNA polymerase joins adjacent nucleotides in each new strand by condensation reactions, forming phosphodiester bonds.
Because each exposed base pairs with its complementary partner, both original strands guide the formation of a matching new strand. This allows the base sequence to be copied accurately before a cell divides.
For example, if a section of a template strand has the bases adenine, cytosine, guanine, its new complementary strand will have thymine, guanine, cytosine. The result is two DNA molecules with the same base sequence as the original, provided no copying error occurs.
Exam mistake to avoid: semi-conservative does not mean that each new molecule contains half of each original strand. Each contains one whole original strand and one whole new strand. Also, helicase separates the strands; polymerase joins nucleotides to build the new strands.
