BIOLOGY 12 DNA Replication

Genetic Information the order of the nucleotide bases in the DNA makes the genetic code a gene is a sequence of bases along a DNA molecules each sequence codes for a protein, which in turn codes for a function or a trait

DNA Replication Why does DNA have to replicate? because when cells divide (reproduce), the new cells produced must have a full set of genes to function properly each DNA strand acts as a template to build a complementary strand

DNA Replication DNA is always synthesized in the 5’ to 3’ direction (the new strand starts at 5’) replication is semiconservative – each new DNA molecule consists of one parent strand and one new strand replication does not have parent strands together and new strands together

Replication Enzymes DNA helicase – unwinds the double helix by breaking the hydrogen bonds DNA polymerase III – responsible for synthesizing new DNA strands (adds the bases) RNA primase – binds to lagging strand and lays a primer, allows polymerase to lay in 5’ to 3’ direction

Replication Enzymes DNA polymerase I – replaces nucleotides from the primer on the lagging strand DNA ligase – joins DNA okazaki fragments together (on the lagging strand) Exonuclease – molecular “spellchecker” that repairs mistakes made during replication DNA polymerase I and DNA polymerase III are exonucleases; they also act as spellcheckers

DNA Replication synthesis of the new strand is always 5’ to 3’ the new strand that uses the 3’ to 5’ parent strand as a template is the leading strand and is built continuously towards the replication fork

DNA Replication the lagging strand is built discontinuously is short fragments in the opposite direction the short fragments of DNA along the lagging strand are called Okazaki Fragments

DNA Replication 1. a protein binds to the DNA at the origins 2. - DNA helicase breaks the hydrogen bonds between the bases - the double helix begins to unzip - the junction where the strands are still joined is called the replication fork

DNA Replication Leading Strand 3. as the nucleotides separate, DNA polymerase III binds to the parent strand 4. - the enzyme moves along the chain, bonding new nucleotides to the nucleotides of the parent strand - base pairing must occur, so the new chain is a complement to the parent chain

DNA Replication Lagging Strand 3. RNA primase lays a primer on the lagging strand 4. - DNA polymerase III can now bind to the parent strand and add nucleotides in the 5’ to 3’ direction - DNA polymerase I then replaces the primer - DNA ligase joins the Okazaki fragments into one strand by a phosphodiester bond

DNA Replication 5. exonuclease proof reads the new strands to ensure no mistakes occurred 6. - the nucleotides in the chains bond together - the chains will automatically twist into a helix

DNA Replication each new DNA molecule consists of one parent and one new strand they are exact replicas of the parent molecule

See figure on page 222

DNA Replication Process

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