DNA is copied before a cell divides, during interphase, so that each daughter cell receives an identical copy. Replication is semi-conservative.The steps are:1. The enzyme DNA helicase breaks the hydrogen bonds between complementary bases, so the double helix unwinds and the two strands separate.
2. Each original strand acts as a template.
3. Free, activated DNA nucleotides in the nucleus are attracted to their complementary exposed bases (A to T, C to G) and are held by hydrogen bonds.
4. DNA polymerase joins adjacent nucleotides by forming phosphodiester bonds in condensation reactions. It adds nucleotides to the 3' end of the growing strand only.
5. Because the strands are antiparallel, one new strand is made in short sections; these are joined by DNA ligase.
6. Each new molecule winds back into a double helix.
Each new DNA molecule contains one original strand and one newly made strand. Complementary base pairing means the two molecules are identical to each other and to the original, apart from occasional copying errors (mutations).Worked example: a template strand reads 5'-ATGC-3'. The new strand is complementary and antiparallel, so it reads 3'-TACG-5'.
2. Each original strand acts as a template.
3. Free, activated DNA nucleotides in the nucleus are attracted to their complementary exposed bases (A to T, C to G) and are held by hydrogen bonds.
4. DNA polymerase joins adjacent nucleotides by forming phosphodiester bonds in condensation reactions. It adds nucleotides to the 3' end of the growing strand only.
5. Because the strands are antiparallel, one new strand is made in short sections; these are joined by DNA ligase.
6. Each new molecule winds back into a double helix.
Each new DNA molecule contains one original strand and one newly made strand. Complementary base pairing means the two molecules are identical to each other and to the original, apart from occasional copying errors (mutations).Worked example: a template strand reads 5'-ATGC-3'. The new strand is complementary and antiparallel, so it reads 3'-TACG-5'.