Ch. 12.2: Replication of DNA Section objective: Summarize DNA replication

Replication of DNA Before a cell can divide by mitosis or meiosis, it must first make a copy of its chromosomes. The DNA in the chromosomes is copied in a process called DNA replication. Without DNA replication, new cells would have only half the DNA of their parents. DNA is copied during interphase prior to mitosis and meiosis. It is important that the new copies are exactly like the original molecules.

Replication of DNA DNA replication depends on specific base pairing In DNA replication, the strands separate Enzymes use each strand as a template to assemble the new strands Nucleotides Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA

Replication of DNA Semiconservative replication: Parental strands of DNA separate, serve as templates and produce 2 molecules of DNA that have one strand of parental DNA and one strand of new DNA

Copying DNA Matching bases allows DNA to be easily copied

DNA replication DNA Helicase: enzyme responsible for uncoiling the double helix and unzipping the weak hydrogen bonds between the base pairs

DNA replication Enzyme DNA polymerase adds new bases DNA bases in nucleus Enzyme DNA polymerase adds new bases DNA polymerase

Copying DNA Build daughter DNA strand use original parent strand as “template” add new matching bases synthesis enzyme = DNA polymerase DNA Polymerase

Replication Adding bases 5 3 energy DNA Polymerase III Adding bases can only add nucleotides to 3 end of a growing DNA strand need a “starter” nucleotide to bond to strand only grows 53 energy DNA Polymerase III DNA Polymerase III energy DNA Polymerase III The energy rules the process. energy 3 5

Leading strand- elongates as DNA unwinds Lagging strand –elongates in opposite direction . Synthesized discontinuously into small segments called Okazaki fragments DNA ligase (an enzyme)links these sections

Leading & Lagging strands Okazaki Limits of DNA polymerase III can only build onto 3 end of an existing DNA strand  5 Okazaki fragments 5 5 3 5 3 5 3 ligase Lagging strand 3 growing replication fork 3 5 Leading strand  3 5 Lagging strand Okazaki fragments joined by ligase “spot welder” enzyme 3 DNA polymerase III Leading strand continuous synthesis

New copies of DNA Get 2 exact copies of DNA to split between new cells polymerase DNA polymerase