+

Hershey-Chase Experiment

Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study the structure of DNA

James Watson and Francis Crick used this information to deduce the double helical structure of DNA

Chargaff’s Rule

# of A’s = # of T’s # of C’s = # of G’s

If it’s determined that a sample of DNA consists of 23% Adenine, how much of the sample is Guanine?

27%

DNA strands are antiparallel

Numbering of strands is based on position of deoxyribose sugars

DNA is wrapped tightly around proteins & folded

DNA must unwind for replication to occur

DNA Replication occurs during the S phase of Interphase is semi-conservative (Meselson & Stahl) Used 14N and 15N – different densities

Overview of DNA Replication: Unreplicated DNA. Strands “unzip” at

several points creating replication forks.

Each strand serves as template for complementary nucleotides to H-bond.

New nucleotides of each daughter strand are linked.

Steps in DNA Replication:

Helicase breaks hydrogen bonds.

Binding proteins stabilize strands; prevent them from rejoining.

Primase makes an RNA primer.

Free nucleotides move in & H-bond; DNA polymerase links nucleotides to each other starting at primer & working in the 5’ to 3’ direction

DNA polymerase “proofreads” new strand (replaces incorrect bases); leaves errors 1/1,000,000,000 base pairings

DNA replication is continuous on one strand (leading strand)

DNA replication is discontinuous on other strand (lagging strand), producing Okazaki fragments

Repair enzymes remove RNA primers; Ligase connects Okazaki fragments.

Ligase

Origin of replication Replication forks Replication bubbles

Okazaki fragments Leading vs. Lagging strands

The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells

1/10,000 base pairings is an error; only 1/1,000,000,000 errors left after proofreading