Molec and Cell 5

bacteria and the viruses that infect them

1928: worked with two strains of a bacterium, one pathogenic and one harmless

a change in genotype and phenotype due to assimilation of foreign DNA

When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic

1944: announced that the transforming substance was DNA based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria

viruses that infect bacteria

1952: experiments showing that DNA is the genetic material of a phage known as T2 they designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection

1950: DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group DNA composition varies from one species to the next

that in any species there is an equal number of A and T bases, and an equal number of G and C bases

X-ray crystallography to study molecular structure Franklin produced a picture of the DNA molecule using this technique

enabled Watson to deduce that DNA was helical

The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases The width suggested that the DNA molecule was made up of two strands, forming a double helix

two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior

Determined pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C) (Consistent with Chargaff’s rule)

that the specific base pairing suggested a possible copying mechanism for genetic material Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication

when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand

supported the semiconservative model They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope

where the two DNA strands are separated, opening up a replication “bubble” A eukaryotic chromosome may have hundreds or even thousands of origins of replication Replication proceeds in both directions from each origin, until the entire molecule is copied

At the end of each replication bubble is a Y-shaped region where new DNA strands are elongating

enzymes that untwist the double helix at the replication forks

binds to and stabilizes single-stranded DNA until it can be used as a template

corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands

they can only add nucleotides to the 3’ end

short RNA initial nucleotide strand for DNA elongation