DNA Structure & Replication

Chapter 15 continued

Bedford County Public Schools – Jami N. Key

The Structure of DNADouble 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 DNABase-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 ReplicationBig Picture:

A new and identical molecule of DNA is made, using the old one as a template

Occurs in the nucleus

DNA ReplicationDNA 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 ReplicationAt 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 ProofreadingDNA 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 ReplicationHelicases- 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 ReplicationDNA replication is semi-conservative

Each new DNA molecule has one old strand and one new strand