Mr. Briner Unit 7.1b DNA Replication HL American School of Milan DP Biology 11 Unit 7.1b DNA Replication HL

DNA Replication

7.1.U3 DNA polymerases can only add nucleotides to the 3’ end of a primer.

DNA Replication Process of DNA Replication DNA replication occurs in a 5’  3’ direction Each nucleotide contains a deoxyribose molecule with 5 carbons Nitrogenous base attached to carbon 1 Phosphate attached to carbon 5

DNA Replication Process of DNA Replication DNA replication occurs in a 5’  3’ direction DNA is built with a repeating backbone sugar - phosphate - sugar – phosphate Thus, DNA has a free 5’ phosphate and a free 3’ carbon Replication only adds nucleotides at the 3’ end

DNA Replication Process of DNA Replication

DNA Replication Process of DNA Replication 5' end of the free nucleotide is added to the 3' end of the exsisting nucleotide chain Bonded covalently to the 3' end on the already formed polynucleotide chain

DNA Replication (a) A free nucleotides in isolation 5 ' is orientated to the 3' so that DNA polymerase enzyme can form the phosphodiester bond between carbon 5 and carbon 3

DNA Replication (b) There is a polynucleotide chain already in place Note the position of the free 3' on the left end of this chain

DNA Replication Process of DNA Replication DNA Polymerase III can only add nucleotides in the 5’ to 3’ direction DNA is antiparallel

DNA Replication Process of DNA Replication DNA polymerase III works in two directions: LEADING STRAND New strand copied in one piece moving towards replication fork LAGGING STRAND New strand copied in many pieces moving away from replication fork

DNA Replication Process of DNA Replication DNA polymerase III works in two directions:

7.1.U5 DNA replication is carried out by a complex system of enzymes.

DNA Replication Enzymes in DNA Replication Helicase: Controls unwinding of coiled DNA Separates complementary strands of DNA, producing a replication fork DNA Gyrase (= Topoisomerase) Reduces the twisting strain created by the unwinding of DNA by helicase

DNA Replication

DNA Replication Enzymes in DNA Replication Helicase:

DNA Replication Enzymes in DNA Replication DNA polymerase III: Can only add nucleotides to a free 3’ end of an existing nucleotide strand Thus, it can only synthesize continuously on one of the two strands of DNA In the direction toward the replication fork This is known as the leading strand

DNA Replication Enzymes in DNA Replication DNA polymerase III: Joins the 5' to 3' of the NEW STRAND Prokaryotic DNA polymerases can work at around 1000 bases per second Whole circular (loop) of DNA can replicated between 20 and 40 minutes

DNA Replication Enzymes in DNA Replication DNA polymerase III:

DNA Replication Enzymes in DNA Replication Hydrolysis of phosphate group provides the energy to form phosphodiester covalent bonds between nucleotides

DNA Replication

DNA Replication Enzymes in DNA Replication

DNA Replication Enzymes in DNA Replication Primase: DNA polymerase III can only add DNA nucleotides to a free 3’ end Primase uses the lagging strand to synthesize a short RNA nucleotide sequence known as an RNA primer DNA polymerase III attaches to the primer to replicate the lagging strand

7.1.U4 DNA replication is continuous on the leading strand and discontinuous on the lagging strand.

DNA Replication Okazaki fragments DNA polymerase III is unable to work directly on the lagging strand  It lacks a free 3’ end on an existing DNA strand So Primase uses the lagging strand to synthesize a short 10 RNA nucleotide RNA primer DNA polymerase III uses the free 3’ end of the RNA primer to synthesize longer sequences of DNA Known as Okazaki fragments

DNA Replication Enzymes in DNA Replication

DNA Replication Enzymes in DNA Replication DNA polymerase I: DNA polymerase I = proofreading enzyme Removes RNA nucleotides of the primer Replaces them with DNA nucleotides This leaves unconnected DNA fragments on the lagging strand Okazaki fragments

DNA Replication Enzymes in DNA Replication DNA ligase: Forms covalent bonds linking together Okazaki fragments Completes DNA synthesis on the lagging strand

DNA Replication Enzymes in DNA Replication

DNA Replication Replication in Eukaryotic organisms Eukaryotic genome is so large it would take days to replicate using a single initiation point Many initiation points are found in each eukaryotic chromosome About 100,000 nucleotides apart With replication forks moving in opposite directions away from each initiation point Until they meet in the middle

DNA Replication Replication in Eukaryotic organisms

DNA Replication Replication in Eukaryotic organisms Eukaryotic DNA polymerase synthesizes about 50 bases per second With up to 80 million bases to replicate the job is achieved in about one hour by having many replication forks

MAJOR SOURCES Thank you to my favorite sources of information when making these lectures! John Burrell (Bangkok, TH) www.click4biology.info Dave Ferguson (Kobe, JA) http://canada.canacad.ac.jp/High/49 Stephen Taylor (Bandung, IN) www.i-biology.net Andrew Allott – Biology for the IB Diploma C. J.Clegg – Biology for the IB Diploma Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate Howard Hugh’s Medical Institute – www.hhmi.org/biointeractive Mr. Hoye’s TOK Website – http://mrhoyestokwebsite.com And all the contributors at www.YouTube.com