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.

DNA Replication depends on specific base pairing DNA replication follows a semiconservative model – The two DNA strands separate – Each strand is used as a pattern to produce a complementary strand, using specific base pairing – Each new DNA helix has one old strand with one new strand Parental molecule of DNA Nucleotides Both parental strands serve as templates Two identical daughter molecules of DNA

DNA REPLICATION II Untwisting and replication of DNA

Copying DNA Matching bases allows DNA to be easily copied

Making new DNA Copying DNA replication DNA starts as a double-stranded molecule matching bases (A:T, C:G) then it unzips…

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

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

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

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 DNA Polymerase III DNA Polymerase III DNA Polymerase III DNA Polymerase III energy Replication energy

DNA replication begins at the origins of replication – DNA unwinds at the origin to produce a “bubble” – Replication proceeds in both directions from the origin – Replication ends when products from the bubbles merge with each other DNA replication occurs in the 5’ 3’ direction – Replication is continuous on the 3’ 5’ template – Replication is discontinuous on the 5’ 3’ template, forming short segments 10.5 DNA replication proceeds in two directions at many sites simultaneously Copyright © 2009 Pearson Education, Inc.

DNA replication 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

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

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

Copied & Paired Up Chromosomes centromere