DNA Structure & Replication Chapter 15 continued Bedford County Public Schools – Jami N. Key

The Structure of DNA  Double helix  Each nucleotide is made up of:  Deoxyribose (sugar)  A phosphate group  A nitrogenous base  Adenine, guanine, cytosine, thymine  A, G = purines  2 carbon rings  C, T = pyrimidines  1 carbon ring

The Structure of DNA  Base-Pairing Rules:  Chargaff’s Rules  Guanine pairs with cytosine  Thymine pairs with adenine  DNA strands are anti-parallel  They run in opposite directions  5’ and 3’ ends

DNA Replication  Big Picture:  A new and identical molecule of DNA is made, using the old one as a template  Occurs in the nucleus

DNA Replication  DNA replication begins at the origin of replication, a special sequence of DNA  2 strands are separated by helicase, forming a replication bubble  Replication fork is formed at each end of the replication bubble

DNA Replication  At replication fork, nucleotides “line up” with their complementary mates, according to the base- pairing rules  DNA polymerase III attaches the nucleotides to the exposed bases of the DNA strand

DNA Replication: A Summary

Leading Strand  DNA replication is different on the 2 strands  DNA polymerase can only add nucleotides to the 3’ end of a DNA strand  Along one template strand, the leading strand, DNA polymerase III just follows the replication fork (replicates continuously in one strand)

Lagging Strand  On the other strand of DNA, the lagging strand – DNA polymerase must work in the opposite direction of the replication fork  Short segments of DNA– Okazaki fragments – are made  Okazaki fragments are joined by DNA ligase

DNA Proofreading  DNA polymerase I proofreads each nucleotide as it is added to the DNA strand  If there’s a mistake…  it backs up  removes the wrong nucleotide  adds the right nucleotide

Enzymes & Their Job in Replication  Helicases- unwind the DNA strand  Single strand binding protein- holds the single strands apart for replication.  Primase- inserts RNA primer to begin replication process.  DNA Polymerase III- adds complementary bases to 3’ end of primer or new DNA strand.  DNA Polymerase I- removes RNA primer & inserts DNA nucleotides. (also proofreads)  DNA Ligase- “sews” Okasaki fragments of lagging strand together with covalent bonds.

Semi-Conservative Replication  DNA replication is semi-conservative  Each new DNA molecule has one old strand and one new strand