Chapter 12~ The Molecular Basis of Inheritance. 12.1 The Genetic Material Frederick Griffith investigated virulence of Streptococcus pneumoniae Concluded

• Chapter 12~ The

Molecular Basis of Inheritance

12.1 The Genetic Material

• Frederick Griffith investigated virulence of Streptococcus pneumoniae Concluded that virulence could be passed

from a dead strain to a nonvirulent living strain Transformation

• Further research by Avery et al. Discovered that DNA is the transforming

substance DNA from dead cells was being incorporated

into the genome of living cells2

The Genetic Material

• Griffith’s Transformation Experiment Mice were injected with two strains of

pneumococcus: an encapsulated (S) strain and a non-encapsulated (R) strain.

• The S strain is virulent (the mice died); it has a mucous capsule and forms “shiny” colonies.

• The R strain is not virulent (the mice lived); it has no capsule and forms “dull” colonies.

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Griffith’s Transformation Experiment

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

capsule

Injected live S strain hascapsule and

causes mice to die.

a. b.

Injected live R strain hasno capsule

and mice do not die.

c.

Injected heat-killed S strain does not cause

mice to die.

d.

Injected heat-killed S strain plus liveR strain causes

mice to die. Live S strain iswithdrawn from

dead mice.

Searching for Genetic Material

• Hershey and Chase (1952)• bacteriophages (phages)• DNA, not protein, is the hereditary material• Experiment: sulfur(S) is in protein,

phosphorus (P) is in DNA; only P was found in host cell


• Transformation of organisms today: Result is the so-called genetically modified

organisms (GMOs) • Invaluable tool in modern biotechnology today• Commercial products that are currently much used • Green fluorescent protein (GFP) can be used as a

marker – A jellyfish gene codes for GFP

– The jellyfish gene is isolated and then transferred to a bacterium, or the embryo of a plant, pig, or mouse.

– When this gene is transferred to another organism, the organism glows in the dark

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Animation

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D:\ImageLibrary1-17\16-MolecularBasisInheritance\16-02-PhageT2Reproduction.mov


• DNA contains: Two Nucleotides with purine bases

• Adenine (A)• Guanine (G)

Two Nucleotides with pyrimidine bases• Thymine (T)• Cytosine (C)

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The Genetic Material• Chargaff’s Rules:

The amounts of A, T, G, and C in DNA:• Are constant among members of the same species• Vary from species to species

In each species, there are equal amounts of:• A and T• G and C

All this suggests that DNA uses complementary base pairing to store genetic information

Each human chromosome contains, on average, about 140 million base pairs

The number of possible nucleotide sequences is 4140,000,000

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Nucleotide Composition of DNA

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O

N

N

CH

CH

C

C

NH2

cytosine(C)

3C C2

C1

OHO P O

O

H

HH

HH

OH

CH3

O

HN

N

C

CH

C

C

OHO P O

O

H

HH

HH

OH

HN

N

N

CCH

O

C

CC

NH2N

C2

C2

C1

C1

OHO P O

O

guanine(G)

phosphate

H

HH

HH

OH

N

N

N

HCCH

NH2

C

CC

N

4

3C

2

C1

5 O

O

O

O

O

O

H

HH

HH

OH

c. Chargaff’s data

DNA Composition in Various Species (%)

Species

Homo sapiens (human)

Drosophila melanogaster (fruit fly)

Zea mays (corn)

Neurospora crassa (fungus)

Escherichia coli (bacterium)

Bacillus subtilis (bacterium)

31.0

27.3

25.6

23.0

24.6

28.4

31.5

27.6

25.3

23.3

24.3

29.0

19.1

22.5

24.5

27.1

25.5

21.0

18.4

22.5

24.6

26.6

25.6

21.6

A T G C

a. Purine nucleotides b. Pyrimidine nucleotides

nitrogen-containingbase

sugar = deoxyribose

thymine(T)

adenine(A)

HO P O CH2

5CH2

5CH2

5CH2

C

4C

4C

4C

C

3C

3C

O


• X-Ray diffraction: Rosalind Franklin studied the structure of DNA using

X-rays. She found that if a concentrated, viscous solution of

DNA is made, it can be separated into fibers. Under the right conditions, the fibers can produce an

X-ray diffraction pattern• She produced X-ray diffraction photographs.• This provided evidence that DNA had the following features:

– DNA is a helix.– Some portion of the helix is repeated.

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X-Ray Diffraction of DNA

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© Photo Researchers, Inc.; c: © Science Source/Photo Researchers, Inc.

X-ray beam

b.c.

Rosalind Franklin

diffraction pattern

CrystallineDNA

diffractedX-rays

a.


• The Watson and Crick Model (1953)

Double helix model is similar to a twisted ladder

• Sugar-phosphate backbones make up the sides

• Hydrogen-bonded bases make up the rungs

Complementary base pairing ensures that a purine is always bonded to a pyrimidine (A with T, G with C)

Received a Nobel Prize in 1962

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Watson and Crick Model of DNA

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P

P

P

P

c.

P

S SCG

PP

S

SAT

G

C

C

G

T

T

A

A

C

G

P

0.34 nm3.4 nm

sugar-phosphatebackbone

a.

b.

d.

5′ end 3′ end

3′ end 5′ endcomplementarybase pairing

hydrogen bondssugar

2 nm

a: © Photodisk Red/Getty RF; d: © A. Barrington Brown/Photo Researchers

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Animation

12.2 Replication of DNA

• DNA replication is the process of copying a DNA molecule.

• Semiconservative replication - each strand of the original double helix (parental molecule) serves as a template (mold or model) for a new strand in a daughter molecule.

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Semiconservative Replication

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oldstrand

newstrand

A

A

A

A

A

A

A

A

AA

A

A

AA

A

A

A

A

A

A

T

T

T

T

T

TT

T

T

T

TT

T

G

G

G

G

GG

GG

G

G

G

G

GG

G

G

C

C

C

CC

C

C

C

C

C C

C

C

A

T

TG

C

A

T

T

GC

C

region of parentalDNA double helix

region ofreplication:new nucleotidesare pairingwith those ofparental strands

region ofcompletedreplication

oldstrand

newstrand

daughter DNA double helix

daughter DNA double helix

DNA polymeraseenzyme

5′ 3′

3′

5′


Animation

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Replication of DNA

• Replication requires the following steps:

Unwinding, or separation of the two strands of the parental DNA molecule

Complementary base pairing between a new nucleotide and a nucleotide on the template strand

Joining of nucleotides to form the new strand

• Each daughter DNA molecule contains one old strand and one new strand

DNA Replication: a closer look

• Origin of replication (“bubbles”): beginning of replication• Replication fork: ‘Y’-shaped region where new strands of

DNA are elongating• Helicase:catalyzes the untwisting of the DNA at the

replication fork• DNA polymerase:catalyzes the elongation of new DNA


Animation

Replication of DNA• Eukaryotic Replication

DNA replication begins at numerous points along each linear chromosome

DNA unwinds and unzips into two strands

Each old strand of DNA serves as a template for a new strand

Complementary base-pairing forms a new strand paired with each old strand

• Requires enzyme DNA polymerase

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Replication of DNA• Eukaryotic Replication

Replication bubbles spread bidirectionally until they meet

The complementary nucleotides are joined to form new strands. Each daughter DNA molecule contains an old strand and a new strand.

Replication is semiconservative:

• One original strand is conserved in each daughter molecule, i.e., each daughter double helix has one parental strand and one new strand.

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Aspects of DNA Replication

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GC

AT

T

GC

G C

AP

P

P

P

P

P

P

P

P

P

P

P

5′

4′C

3′ C C 2′

C1′

H H

H

H H

O

5′ end

3′ end

Deoxyribose molecule

3′

3′

3′

5′

5′

3′

5′

3

5

2

4

6

7

1

OH Is attached herebase is attached here

helicase at replication fork

DNA polymerase

templatestrand

Okazaki fragmenttemplatestrandlagging

strand

Direction of replication

template strand

parental DNA helix

DNA polymerase

Replication fork introduces complications

DNA ligase

RNA primer

3 ′end

OHCH2

OH

5′ end

new strand

leadingnew strand

DNA polymeraseattaches a newnucleotide to the3 ′ carbon of theprevious nucleotide.

Replication of DNA

• Accuracy of Replication

DNA polymerase is very accurate, yet makes a mistake about once per 100,000 base pairs.

• Capable of identifying and correcting errors

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DNA Replication, II

• Antiparallel nature: sugar/phosphate backbone runs in opposite directions (Crick);

• one strand runs 5’ to 3’, while the other runs 3’ to 5’;

• DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA

strands in the 5’ to 3’ direction only

Figure 16.12 The two strands of DNA are antiparallel

DNA Replication, III• Leading strand: synthesis toward the

replication fork (only in a 5’ to 3’ direction from the 3’ to 5’ master strand)

• Lagging strand: synthesis away from the replication fork (Okazaki fragments); joined by DNA ligase (must wait for 3’ end to open; again in a 5’ to 3’ direction)

• Initiation: Primer (short RNA sequence~w/primase enzyme), begins the replication process

DNA Replication: the leading and lagging strand

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Figure 16.15 The main proteins of DNA replication and their functions

DNA Repair

• Mismatch repair: DNA polymerase

• Excision repair:Nuclease

• Telomere ends:telomerase

Figure 16.17 Nucleotide excision repair of DNA damage

Figure 16.18 The end-replication problem

Figure 16.17 Nucleotide excision repair of DNA damage

Figure 16.19b Telomeres and telomerase

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Chapter 12~ The Molecular Basis of Inheritance. 12.1 The Genetic Material Frederick Griffith investigated virulence of Streptococcus pneumoniae Concluded