Dna the Molecule of Heredity1

7/27/2019 Dna the Molecule of Heredity1

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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures forBio logy, Seventh Edit ion

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 16

The Molecular Basis of

Inheritance


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Overview: Lifes Operating Instructions

In 1953, James Watson and Francis Crickintroduced an elegant double-helical model for the

structure of deoxyribonucleic acid, or DNA

DNA, the substance of inheritance, is the mostcelebrated molecule of our time

Hereditary information is encoded in DNA and

reproduced in all cells of the body

This DNA program directs the development of

biochemical, anatomical, physiological, and (to

some extent) behavioral traits


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Concept 16.1: DNA is the genetic material

Early in the 20th century, the identification of themolecules of inheritance loomed as a major

challenge to biologists


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The Search for the Genetic Material: ScientificInquiry

When Morgans group showed that genes arelocated on chromosomes, the two components of

chromosomesDNA and proteinbecame

candidates for the genetic material

The key factor in determining the genetic material

was choosing appropriate experimental organisms

The role of DNA in heredity was first discovered bystudying bacteria and the viruses that infect them


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Evidence That DNA Can Transform Bacter ia

The discovery of the genetic role of DNA beganwith research by Frederick Griffith in 1928

Griffith worked with two strains of a bacterium, a

pathogenic S strain and a harmless R strain

When he mixed heat-killed remains of the

pathogenic strain with living cells of the harmless

strain, some living cells became pathogenic

He called this phenomenon transformation, nowdefined as a change in genotype and phenotypedue to assimilation of foreign DNA


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LE 16-2

Living S cells

(control)

Living R cells

(control)

Heat-killed

S cells (control)

Mixture of heat-killed

S cells and living

R cells

Mouse dies

Living S cells

are found in

blood sample

Mouse healthy Mouse healthy Mouse dies

RESULTS


8/62Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

In 1944, Oswald Avery, Maclyn McCarty, andColin MacLeod announced that the transformingsubstance was DNA

Their conclusion was based on experimental

evidence that only DNA worked in transformingharmless bacteria into pathogenic bacteria

Many biologists remained skeptical, mainly

because little was known about DNA



Evidence That Vi ral DNA Can Program Cells

More evidence for DNA as the genetic materialcame from studies of a virus that infects bacteria

Such viruses, called bacteriophages (or phages),

are widely used in molecular genetics research

Animation: Phage T2 Reproductive Cycle
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LE 16-3

Bacterialcell

Phage

head

Tail

Tail fiber

DNA

100nm



In 1952, Alfred Hershey and Martha Chaseperformed experiments showing that DNA is the

genetic material of a phage known as T2

To determine the source of genetic material in thephage, they designed an experiment showing thatonly one of the two components of T2 (DNA orprotein) enters an E. colicell during infection

They concluded that the injected DNA of thephage provides the genetic information

Animation: Hershey-Chase Experiment

LE 16 4
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LE 16-4

Bacterial cell

Phage

DNA

Radioactive

protein

Emptyprotein shell

PhageDNA

Radioactivity

(phage protein)

in liquid

Batch 1:

Sulfur (35S)

RadioactiveDNA

Centrifuge

Pellet (bacterialcells and contents)

PelletRadioactivity

(phage DNA)

in pellet

Centrifuge

Batch 2:Phosphorus (32P)



Additional Evidence That DNA I s the Genetic

Material

In 1947, Erwin Chargaff reported that DNAcomposition varies from one species to the next

This evidence of diversity made DNA a more

credible candidate for the genetic material

By the 1950s, it was already known that DNA is a

polymer of nucleotides, each consisting of a

nitrogenous base, a sugar, and a phosphate group

Animation: DNA and RNA Structure

LE 16 5
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LE 16-5Sugarphosphate

backbone

5 end

Nitrogenousbases

Thymine (T)

Adenine (A)

Cytosine (C)

DNA nucleotidePhosphate

3 endGuanine (G)

Sugar (deoxyribose)


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Building a Structural Model of DNA: Scientif ic I nquiry

After most biologists became convinced that DNAwas the genetic material, the challenge was to

determine how its structure accounts for its role

Maurice Wilkins and Rosalind Franklin were usinga technique called X-ray crystallography to study

molecular structure

Franklin produced a picture of the DNA moleculeusing this technique

LE 16 6


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LE 16-6

Franklins X-ray diffraction

photograph of DNA

Rosalind Franklin


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Franklins X-ray crystallographic images of DNAenabled 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 thenitrogenous bases

The width suggested that the DNA molecule was

made up of two strands, forming a double helix

Animation: DNA Double Helix

LE 16 7
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LE 16-7

5 end

3 end

5 end

3 end

Space-filling modelPartial chemical structure

Hydrogen bond

Key features of DNA structure

0.34 nm

3.4 nm

1 nm


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Watson and Crick built models of a double helix toconform to the X-rays and chemistry of DNA

Franklin had concluded that there were two

antiparallel sugar-phosphate backbones, with the

nitrogenous bases paired in the molecules interior

At first, Watson and Crick thought the bases

paired like with like (A with A, and so on), but such

pairings did not result in a uniform width

Instead, pairing a purine with a pyrimidine resulted

in a uniform width consistent with the X-ray

LE 16 UN298


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LE 16-UN298

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: width

consistent with X-ray data


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Watson and Crick reasoned that the pairing wasmore specific, dictated by the base structures

They determined that adenine paired only with

thymine, and guanine paired only with cytosine

LE 16-8


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LE 16-8

Adenine (A) Thymine (T)

Guanine (G) Cytosine (C)

Sugar

Sugar

Sugar

Sugar


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Concept 16.2: Many proteins work together inDNA replication and repair

The relationship between structure and function ismanifest in the double helix

Watson and Crick noted that the specific base

pairing suggested a possible copying mechanismfor genetic material

i i i i i S


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The Basic Principle: Base Pairing to a Template Strand

Since the two strands of DNA are complementary,each strand acts as a template for building a new

strand in replication

In DNA replication, the parent molecule unwinds,and two new daughter strands are built based on

base-pairing rules

Animation: DNA Replication Overview

LE 16-9 1
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LE 16 9_1

The parent molecule hastwo complementarystrands of DNA. Each baseis paired by hydrogenbonding with its specificpartner, A with T and G withC.

LE 16-9 2


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LE 16 9_2

The parent molecule hastwo complementarystrands of DNA. Each baseis paired by hydrogenbonding with its specificpartner, A with T and G withC.

The first step in replicationis separation of the twoDNA strands.

LE 16-9 3


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LE 16 9_3

The parent molecule hastwo complementarystrands of DNA. Each baseis paired by hydrogenbonding with its specificpartner, A with T and Gwith C.


Each parental strand nowserves as a template thatdetermines the order ofnucleotides along a new,complementary strand.

LE 16-9 4


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_

The parent molecule hastwo complementarystrands of DNA. Each baseis paired by hydrogenbonding with its specificpartner, A with T and Gwith C.


Each parental strand nowserves as a template thatdetermines the order ofnucleotides along a new,complementary strand.

The nucleotides areconnected to form thesugar-phosphate back-bones of the new strands.Each daughter DNAmolecule consists of oneparental strand and onenew strand.


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Watson and Cricks semiconservative model ofreplication predicts that when a double helixreplicates, each daughter molecule will have oneold strand (derived or conserved from the parent

molecule) and one newly made strand Competing models were the conservative model

and the dispersive model

LE 16-10


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Conservative

model. The two

parental strands

reassociate after

acting as

templates for

new strands,thus restoring

the parental

double helix.

Semiconservative

model. The two

strands of the

parentalmolecule

separate, and

each functions as

a template for

synthesis of a

new, comple-

mentary strand.

Dispersive model.

Each strand ofbothdaughter

molecules

contains

a mixture of

old and newly

synthesized

DNA.

Parent cellFirstreplication

Secondreplication


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Experiments by Meselson and Stahl supported thesemiconservative model

They labeled the nucleotides of the old strands

with a heavy isotope of nitrogen, while any newnucleotides were labeled with a lighter isotope

The first replication produced a band of hybrid

DNA, eliminating the conservative model A second replication produced both light and

hybrid DNA, eliminating the dispersive model and

supporting the semiconservative model

LE 16-11


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Bacteria

cultured in

medium

containing15N

DNA sample

centrifuged

after 20 min

(after first

replication)

DNA sample

centrifuged

after 40 min

(after second

replication)

Bacteria

transferred to

medium

containing14N

Less

dense

More

dense

Conservative

model

First replication

Semiconservative

model

Second replication

Dispersive

model

DNA R li ti A Cl L k


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DNA Replication: A Closer Look

The copying of DNA is remarkable in its speedand accuracy

More than a dozen enzymes and other proteins

participate in DNA replication

Getting Started: Origins of Replication


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Getting Started: Origins of Replication

Replication begins at special sites called origins ofreplication, where the two DNA strands are

separated, opening up a replication bubble

A eukaryotic chromosome may have hundreds oreven 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

replication fork, a Y-shaped region where new

DNA strands are elongating

LE 16-12


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In eukaryotes, DNA replication begins at may sitesalong the giant DNA molecule of each chromosome.

Two daughter DNA molecules

Parental (template) strand

Daughter (new) strand0.25 m

Replication fork

Origin of replication

Bubble

In this micrograph, three replicationbubbles are visible along the DNAof a cultured Chinese hamster cell(TEM).


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Animation: Origins of Replication

Elongating a New DNA Strand
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Elongating a New DNA Strand

Enzymes called DNA polymerases catalyze theelongation of new DNA at a replication fork

Each nucleotide that is added to a growing DNAstrand is a nucleoside triphosphate

The rate of elongation is about 500 nucleotidesper second in bacteria and 50 per second inhuman cells


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Antiparal lel Elongation


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Antiparal lel Elongation

The antiparallel structure of the double helix (twostrands oriented in opposite directions) affects

replication

DNA polymerases add nucleotides only to the free3end of a growing strand; therefore, a new DNA

strand can elongate only in the 5to3direction


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Along one template strand of DNA, called theleading strand, DNA polymerase can synthesize a

complementary strand continuously, moving

toward the replication fork

To elongate the other new strand, called the

lagging strand, DNA polymerase must work in the

direction away from the replication fork

The lagging strand is synthesized as a series of

segments called Okazaki fragments, which are

joined together by DNA ligase

LE 16-143


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Parental DNA

5

3

Leading strand

3

5

3

5

Okazaki

fragments

Lagging strand

DNA pol III

Template

strand

Leading strand

Lagging strand

DNA ligaseTemplate

strand

Overall direction of replication


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Animation: Leading Strand

Priming DNA Synthesis
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Priming DNA Synthesis

DNA polymerases cannot initiate synthesis of a

polynucleotide; they can only add nucleotides to

the 3 end

The initial nucleotide strand is a short one called

an RNA or DNA primer

An enzyme called primase can start an RNA chain

from scratch

Only one primer is needed to synthesize the

leading strand, but for the lagging strand each

Okazaki fragment must be primed separately


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LE 16-15_2Primase joins RNA


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53

nucleotides into a primer.

Template

strand

5 3


RNA primer3

5

35

DNA pol III addsDNA nucleotides tothe primer, formingan Okazaki fragment.



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53


Template

strand

5 3


RNA primer3

5

35


Okazaki

fragment3

5

5

3

After reaching thenext RNA primer (not

shown), DNA pol IIIfalls off.


l tid i t i


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53


Template

strand

5 3


RNA primer3

5

35


Okazaki

fragment3

5

5

3



33

5

5

After the second fragment isprimed, DNA pol III adds DNAnucleotides until it reaches thefirst primer and falls off.


l tid i t i


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53


Template

strand

5 3


RNA primer3

5

35


Okazaki

fragment3

5

5

3



33

5

5


33

5

5

DNA pol I replacesthe RNA with DNA,adding to the 3 endof fragment 2.


nucleotides into a primer


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53


Template

strand

5 3


RNA primer3

5

35


Okazaki

fragment3

5

5

3



33

5

5


33

5

5

DNA pol I replacesthe RNA with DNA,adding to the 3 endof fragment 2.

3

3

5

5

DNA ligase forms abond between the newestDNA and the adjacent DNAof fragment 1.

The laggingstrand in the regionis now complete.


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Animation: Lagging Strand

Other Proteins That Assist DNA Replication
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Other Proteins That Assist DNA Replication

Helicase untwists the double helix and separatesthe template DNA strands at the replication fork

Single-strand binding protein binds to andstabilizes single-stranded DNA until it can be used

as a template

Topoisomerase corrects overwinding ahead ofreplication forks by breaking, swiveling, and

rejoining DNA strands


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Primase synthesizes an RNA primer at the 5 ends

of the leading strand and the Okazaki fragments

DNA pol III continuously synthesizes the leading

strand and elongates Okazaki fragments

DNA pol I removes primer from the 5 ends of the

leading strand and Okazaki fragments, replacing

primer with DNA and adding to adjacent 3 ends

DNA ligase joins the 3 end of the DNA that

replaces the primer to the rest of the leading

strand and also joins the lagging strand fragments


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Animation: DNA Replication Review

The DNA Replication Machine as a Stationary
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p y

Complex The proteins that participate in DNA replication

form a large complex, a DNA replication machine

The DNA replication machine is probably

stationary during the replication process

Recent studies support a model in which DNApolymerase molecules reel in parental DNA andextrude newly made daughter DNA molecules

Proofreading and Repairing DNA


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g p g

DNA polymerases proofread newly made DNA,replacing any incorrect nucleotides

In mismatch repair of DNA, repair enzymes correct

errors in base pairing In nucleotide excision repair, enzymes cut out and

replace damaged stretches of DNA

LE 16-17A thymine dimer


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DNA

ligase

DNA

polymerase

DNA ligase seals the

free end of the new DNA

to the old DNA, making the

strand complete.

Repair synthesis by

a DNA polymerase

fills in the missing

nucleotides.

A nuclease enzyme cuts

the damaged DNA strandat two points and the

damaged section is

removed.

Nuclease

distorts the DNA molecule.

Replicating the Ends of DNA Molecules


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Limitations of DNA polymerase create problemsfor the linear DNA of eukaryotic chromosomes

The usual replication machinery provides no wayto complete the 5 ends, so repeated rounds of

replication produce shorter DNA molecules

LE 16-185


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End of parental

DNA strands

3

Lagging strand 5

3

Last fragment

RNA primer

Leading strand

Lagging strand

Previous fragment

Primer removed but

cannot be replaced

with DNA because

no 3end available

for DNA polymerase5

3

Removal of primers and

replacement with DNA

where a 3 end is available

Second round

of replication

5

3

5

3

Further rounds

of replication

New leading strandNew leading strand

Shorter and shorter

daughter molecules


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Eukaryotic chromosomal DNA molecules have attheir ends nucleotide sequences called telomeres

Telomeres do not prevent the shortening of DNA

molecules, but they do postpone the erosion of

genes near the ends of DNA molecules


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If chromosomes of germ cells became shorter inevery cell cycle, essential genes would eventually

be missing from the gametes they produce

An enzyme called telomerase catalyzes the

lengthening of telomeres in germ cells

Documents

Dna the Molecule of Heredity1