DNA Replication

Why Does DNA Need to Replicate? Your cells are constantly dying and being replaced! White blood cells live for about a year Red blood cells live about 4 months Skin cells last 2-3 weeks Sperm cells die after 3 days Every time your cells divide to produce new cells, DNA must be copied.

` “It has not escaped our notice that the specific [base] pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” -Watson and Crick

Replication Nitrogenous bases pair only in one way (A-T, C-G) One DNA strand serves as a template for a new strand Replication- the process by which DNA is copied Ensures that all cells in your body have the same set of genetic information

How is Replication Carried Out? Enzymes and other proteins help replicate DNA molecules Replication overview: Double helix is “unzipped” Single strand is used as template to build a new, complementary strand DNA polymerases attach nucleotides to the DNA strand Results in two identical copies of the DNA molecule

Replication Process DNA helicase (an enzyme) unzips the DNA strands at multiple places along the molecule. These are called replication forks or origins of replication

Replication Process Another enzyme called DNA polymerase bond free-floating nucleotides in the nucleus to their complementary bases on the DNA template strand.

Result of Replication Two identical double-stranded DNA molecules Replication is semiconservative Each new molecule contains one original strand and one new strand

Replication Process Replication is fast and accurate Multiple replication forks in eukaryotes allows replication to be completed in just a few hours

Do Mistakes Ever Occur? DNA polymerase “proofreads” as it goes Occasionally, it adds the wrong base but can usually detect the error and correct it before moving on. Replication errors happen only about 1 time per billion nucleotides.