DNA: The Stuff of Life

Griffith and Transformation • Griffith and Transformation

• In 1928, British scientist Fredrick Griffith was trying to

learn how certain types of bacteria caused pneumonia.

• He isolated two different strains of pneumonia bacteria

from mice and grew them in his lab.

Griffith and Transformation Performed the first major experiment that led to the

discovery of DNA as the genetic material

Griffith and Transformation

– Transformation

• Griffith determined that bacteria could pass genetic

information from one to another.

• Griffith called this process transformation because

one strain of bacteria (the harmless strain) had

changed permanently into another (the disease-

causing strain).

Oswald Avery

1931 - Oswald Avery repeated Griffith’s work

to determine which molecule was most

important for transformation.

Identified the molecule that transformed the

R strain of bacteria into the S strain

Concluded that when the S cells were killed,

DNA was released

R bacteria incorporated this DNA into their

cells and changed into S cells.

The Hershey-Chase

Experiment • The Hershey-Chase Experiment

– Alfred Hershey and Martha Chase studied

viruses—nonliving particles smaller than a

cell that can infect living organisms.

• Bacteriophages

• A virus that infects bacteria is known as a

bacteriophage.

• Bacteriophages are composed of a DNA or RNA core

and a protein coat.

The Hershey-Chase

Experiment

The Hershey-Chase

Experiment

• If Hershey and Chase could determine which part of

the virus entered an infected cell, they would learn

whether genes were made of protein or DNA.

• They grew viruses in cultures containing radioactive

isotopes of phosphorus-32 (32P) and sulfur-35 (35S).

The Hershey-Chase

Experiment

Hershey and Chase Used radioactive labeling to

trace the DNA and protein

Concluded that the viral DNA

was injected into the cell and

provided the genetic

information needed to produce

new viruses

Chargaff Chargaff’s rule: C = G and T = A

•The percentages of

guanine [G] and

cytosine [C] bases are

almost equal in any

sample of DNA.

•The percentages of

adenine [A] and thymine

[T] bases are almost

equal in any sample of

DNA.

Two Groups of Bases in DNA

• Pyrimidines are

single ring bases.

– Thymine & Cytosine

• Purines are double ring bases.

– Adenine & Guanine

C

C

C

C

N

N

O

N

C C

C C

N

N

N

N

N

C

X-ray Diffraction

Rosalind Franklin’s X-ray diffraction data

helped solve the structure of DNA

Indicated that DNA was a double helix

Watson and Crick

Built a model of the double

helix that conformed to the

others’ research

1. Model was a double helix

2. Backbone made of sugars and

phosphates

3. Base pairs attached to deoxyribose with A

& T in equal amounts and C & G in equal

amounts.

DNA Structure

DNA often is compared to a twisted ladder.

Rails of the ladder are

represented by the

alternating deoxyribose

and phosphate.

The pairs of bases

(cytosine–guanine or

thymine–adenine) form the

steps.

The Components and

Structure of DNA • Watson and Crick discovered that hydrogen

bonds can form only between certain base

pairs—adenine and thymine, and guanine

and cytosine.

• This principle is called base pairing.

Copyright Pearson Prentice Hall

• DNA Double Helix

DNA Structure

Nucleotides

Consist of a five-carbon sugar, a phosphate

group, and a nitrogenous base

DNA Replication DNA DNA

DNA Replication

– Duplicating DNA

• Before a cell divides, it

duplicates its DNA in a

process called

replication.

• Replication ensures that

each resulting cell will

have a complete set of

DNA.

• Occurs in the S phase of

Interphase.

DNA Replication • DNA must be copied in order for cells to

multiply in number and maintain genetic information.

• The DNA molecule produces 2 IDENTICAL new complementary strands following the rules of base pairing:

A-T, G-C

•Each strand of the original DNA serves as a

template for the new strand

DNA Replication

• Semiconservative

Model:

1. Watson and Crick

showed: the two strands

of the parental molecule

separate, and each

functions as a template

for synthesis of a new

complementary strand.

. Parental DNA

DNA Template

New DNA

DNA Replication

• DNA Replication • Each strand of the DNA double helix has all the

information needed to reconstruct the other half by

the mechanism of base pairing.

• In most prokaryotes, DNA replication begins at a

single point and continues in two directions.

• In eukaryotic chromosomes, DNA replication

occurs at hundreds of places. Replication proceeds

in both directions until each chromosome is

completely copied

DNA Replication

The sites where separation

and replication occur are

called replication forks.

Nitrogen Bases

Replication Fork

DNA Polymerase

Replication Fork

Original strand New Strand

DNA Replication • DNA must first unwind and “unzip”

• DNA helicase, an enzyme, is responsible for

unwinding and unzipping the double helix.

– Single stranded binding proteins hold the DNA open

DNA Replication • DNA primase adds a short segment of RNA, called

an RNA primer, on each DNA strand.

• DNA polymerase continues adding appropriate

nucleotides to the chain by adding to the 3′ end of

the new DNA strand.

• This creates Okazaki fragments.

DNA Replication • Okazaki fragments, sections of nucleotides

created by replication in small segments, must be

joined together.

• Finally, DNA ligase links the fragments together

to form the full complementary strand.

DNA Replication • Give the complementary sequence for the

following strand of DNA:

– DNA 5’ A T C C G A A G C T T 3’

– DNA 3’ T A G G C T T C G A A 5’