Molecular Biology of the Gene

Chapter 10

Viruses are biological saboteurs Hijacking the genetic

material of host cells in order to reproduce themselves

Viruses provided some of the earliest evidence That genes are made of

DNA

Griffith Discovers Transformation

1928 Attempting to develop a vaccine he isolated

two strains of pneumonia Rough strain was harmless Smooth strain was pathogenic

Griffith Discovers Transformation

Transformation

The harmless R cells had been transformed by material from the dead S cells

Descendents of the transformed cells were also pathogenic

10.1 Experiments showed that DNA is the genetic material

The Hershey-Chase experiment showed that certain viruses reprogram host cells To produce more viruses by injecting their DNA

Head

Tail

Tail fiber

DNA

300

,00

0

Bacteriophages

Viruses that infect bacteria

Consist of protein and DNA

Inject their hereditary material into bacteria

The Hershey-Chase experiment

10.2 DNA and RNA are polymers of nucleotides

DNA is a nucleic acid Made of long chains of nucleotide monomers

DNA polynucleotide

A

C

T

G

T

Sugar-phosphate backbone

Phosphate group

Nitrogenous base

SugarA

C

T

G

T

Phosphategroup

O

O–

OO P CH2

H3CC

C

C

CN

C

N

H

H

O

O

C

O

O

H

C H H

H

C

H

Nitrogenous base(A, G, C, or T)

Thymine (T)

Sugar(deoxyribose)

DNA nucleotide

DNA nucleotide

DNA has four kinds of nitrogenous bases A, T, C, and G

CC

C

CC

C

O

N

C

H

H

ONH

H3C

H H

H

H

N

N

N

H

OC

H HN

H C

N

N N

N

C

CC

C

H

H

N

N

H

C

CN

C HN

CN

H C

O

H

H

Thymine (T) Cytosine (C) Adenine (A) Guanine (G)

PurinesPyrimidines

• RNA is also a nucleic acidBut has a slightly different

sugarAnd has U instead of T

Nitrogenous base (A, G, C, or U)

Phosphategroup

O

O–

OO P CH2

HC

C

C

C

N

C

N

H

H

O

O

C

O

O

H

C H H

OH

C

H

Uracil (U)

Sugar(ribose)

Key

Hydrogen atom

Carbon atom

Nitrogen atom

Oxygen atom

Phosphorus atom

DNA is a double-stranded helix

James Watson and Francis Crick Worked out the three-dimensional structure of DNA,

based on work by Rosalind

G C

T A

A T

G

G

C

C

A T

GC

T A

T A

A T

A T

G C

A T

O

O

OH

–OP

OO

–OPO

OO

P– O

– O OP

OO

O

OH

H2C

H2C

H2C

H2C

O

O

O

O

O

O

O

O

PO–

O–

O–

O–

OH

HO

O

O

O

P

P

P

O

O

O

O

O

O

O

O

T A

G C

C G

A T

CH2

CH2

CH2

CH2

Hydrogen bond

Basepair

Ribbon model Partial chemical structure Computer model

Hydrogen bonds between basesHold the strands together

Each base pairs with a complementary partnerA with T, and G with C

DNA Replication DNA replication depends on specific base pairing

DNA replication Starts with the separation of DNA strands

Then enzymes use each strand as a template To assemble new nucleotides into complementary strands

A T

C G

G C

A T

T A

A T

C G

G C

A T

T A

A T

C G

G C

A T

T A

A T

C G

G C

A

T

A T

CG

A

C

T

A

Parental moleculeof DNA

Both parental strands serve as templates

Two identical daughtermolecules of DNA

DNA Replication DNA replication is a complex process

Due in part to the fact that some of the helical DNA molecule must untwist

G C

A T

G C

A T

C G

AGA

CG

C

GC

G

TA

G

C

TAT

AA

TT

A

CG

CG

CG

T

AG

C

T

A

T

A

AT

T

A

TC

T

DNA Replication DNA replication

Begins at specific sites on the double helix

Origin of replication

Two daughter DNA molecules

Parental strand

Daughter strand

Bubble

DNA Replication

Each strand of the double helix Is oriented in the

opposite direction (antiparallel)

DNA Replication

Using the enzyme DNA polymeraseThe cell synthesizes one daughter strand

as a continuous piece The other strand is synthesized as a series

of short piecesWhich are then connected by the enzyme

DNA ligase

DNA Replication