DNA structure and replication

Goals

1. To get an idea of how a line of scientific inquiry is pursued. What confused or illuminated the men and women who tried to figure out what material carries genetic information?

2. To understand how the DNA molecule is organized.

3. To understand how it is able to make copies of itself (i.e., replicates).

Outline:

Early ideas about the genetic material

Genes are on chromosomes

Chromosomes are mostly protein with a little DNA

Proteins are more complex and diverse than DNA

Fred Griffith (1928)

Experiments with Streptococcus pneumoniae

Found that an extract from dead bacteria could transform live cells

Oswald Avery (1944)

Purified and identified the transforming factor in S. pneumoniae

Asserted that the transforming factor is DNA (yeah, right...)

Alfred Hershey and Martha Chase (1952)

Radiolabeled DNA (with P) and proteins (with S) in bacteriophages to see which would enter host E. coli cells

Found that DNA carries genetic information in phages

James Watson and Francis Crick (1953)

Used X-ray crystallography and wire models

Came up with the double helix structure of DNA

Structure of double-stranded DNA

• It is composed of two strands of nucleotides
• There is a sugar-phosphate backbone with the nitrogenous bases on the inside
• The bases are H-bonded to each other, Adenine to Thymine only, Cytosine to Guanine only
• They are antiparallel strands: they run in opposite directions
• They form a double helix

Replication of DNA

Initiation

The enzyme helicase separates the strand at an origin of replication

This creates a replication fork

Elongation

DNA polymerase makes new complementary strands using the old strands as templates (only in a 5' to 3' direction)

The leading strand proceeds normally, but the lagging strand grows in segments called Okazaki fragments

Replication must be primed before elongation can proceed, by the enzyme RNA primase and an RNA primer

DNA ligase joins together the ends of each new Okazaki fragment

Errors and their repair

Pairing errors during replication -- mismatch repair done by DNA polymerase proofreading

Later errors, e.g. thymine dimers -- fixed via excision repair

Structure of the chromosome

Prokaryotes

• circular
• small
• naked

Eukaryotes

• linear
• big
• packed with proteins (histones) into nucleosomes, 30 nm thread, etc.

   

Vocabulary

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