The Molecular Basis of Inheritance · 2016-11-23 · •DNA, the substance of inheritance, is the most celebrated molecule of our time •Hereditary information is encoded in DNA

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for

Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 16

The Molecular Basis of

Inheritance


Overview: Life’s Operating Instructions

• In 1953, James Watson and Francis Crick

introduced an elegant double-helical model for the

structure of deoxyribonucleic acid, or DNA

• DNA, the substance of inheritance, is the most

celebrated 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



Concept 16.1: DNA is the genetic material

• Early in the 20th century, the identification of the

molecules of inheritance loomed as a major

challenge to biologists


The Search for the Genetic Material: Scientific Inquiry

• When Morgan’s group showed that genes are

located on chromosomes, the two components of

chromosomes—DNA and protein—became

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 by

studying bacteria and the viruses that infect them


Evidence That DNA Can Transform Bacteria

• The discovery of the genetic role of DNA began with 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, now defined as a change in genotype and phenotype due to assimilation of foreign DNA

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


• In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA

• Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria

• Many biologists remained skeptical, mainly because little was known about DNA


Evidence That Viral DNA Can Program Cells

• More evidence for DNA as the genetic material

came 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

media/16_03PhageT2Reproduction_A.html

LE 16-3

Bacterialcell

Phage

head

Tail

Tail fiber

DNA

100 n

m


• In 1952, Alfred Hershey and Martha Chase

performed experiments showing that DNA is the

genetic material of a phage known as T2

• To determine the source of genetic material in the phage, they designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection

• They concluded that the injected DNA of the phage provides the genetic information

Animation: Hershey-Chase Experiment

media/16_04HersheyChaseExp_A.html

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 Is the Genetic Material

• In 1947, Erwin Chargaff reported that DNA

composition 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

media/16_05DNAandRNAStructure_A.html

LE 16-5Sugar–phosphate

backbone

5 end

Nitrogenousbases

Thymine (T)

Adenine (A)

Cytosine (C)

DNA nucleotidePhosphate

3 endGuanine (G)

Sugar (deoxyribose)


Building a Structural Model of DNA: Scientific Inquiry

• After most biologists became convinced that DNA

was the genetic material, the challenge was to

determine how its structure accounts for its role

• Maurice Wilkins and Rosalind Franklin were using

a technique called X-ray crystallography to study

molecular structure

• Franklin produced a picture of the DNA molecule

using this technique

LE 16-6

Franklin’s X-ray diffraction

photograph of DNA

Rosalind Franklin


• Franklin’s X-ray crystallographic images of DNA

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

Animation: DNA Double Helix

media/16_07DNADoubleHelix_A.html

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


• Watson and Crick built models of a double helix to

conform 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 molecule’s 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

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: width

consistent with X-ray data


• Watson and Crick reasoned that the pairing was

more specific, dictated by the base structures

• They determined that adenine paired only with

thymine, and guanine paired only with cytosine

LE 16-8

Adenine (A) Thymine (T)

Guanine (G) Cytosine (C)

Sugar

Sugar

Sugar

Sugar


Concept 16.2: Many proteins work together in DNA replication and repair

• The relationship between structure and function is

manifest in the double helix

• Watson and Crick noted that the specific base

pairing suggested a possible copying mechanism

for genetic material


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

media/16_09DNAReplicatOverview_A.html

LE 16-9_1

The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

LE 16-9_2


The first step in replication is separation of the two DNA strands.

LE 16-9_3



Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

LE 16-9_4



Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

The nucleotides are connected to form the sugar-phosphate back-bones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.


• Watson and Crick’s semiconservative model of replication predicts that 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

• Competing models were the conservative model and the dispersive model

LE 16-10

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

parental

molecule

separate, and

each functions as

a template for

synthesis of a

new, comple-

mentary strand.

Dispersive model.

Each strand of

both daughter

molecules

contains

a mixture of

old and newly

synthesized

DNA.

Parent cellFirstreplication

Secondreplication


• Experiments by Meselson and Stahl 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

• 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

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 Replication: A Closer Look

• The copying of DNA is remarkable in its speed and

accuracy

• More than a dozen enzymes and other proteins

participate in DNA replication


Getting Started: Origins of Replication

• Replication begins at special sites called origins of

replication, 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

replication fork, a Y-shaped region where new

DNA strands are elongating

LE 16-12

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 µm

Replication fork

Origin of replication

Bubble

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


Animation: Origins of Replication

media/16_12OriginsOfReplication_A.html


Elongating a New DNA Strand

• Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork

• Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate

• The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells

LE 16-13

New strand

5 end

PhosphateBase

Sugar

Template strand

3 end 5 end 3 end

5 end

3 end

5 end

3 end

Nucleoside

triphosphate

DNA polymerase

Pyrophosphate


Antiparallel Elongation

• The antiparallel structure of the double helix (two

strands oriented in opposite directions) affects

replication

• DNA polymerases add nucleotides only to the free

3end of a growing strand; therefore, a new DNA

strand can elongate only in the 5 to 3direction


• Along one template strand of DNA, called the

leading 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 16-14

Parental DNA

5

3

Leading strand

3

5

3

5

Okazaki

fragments

Lagging strand

DNA pol III

Template

strand

Leading strand

Lagging strand

DNA ligase Template

strand

Overall direction of replication


Animation: Leading Strand

media/16_14LeadingStrand_A.html


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

LE 16-15_1

53

Primase joins RNA

nucleotides into a primer.

Template

strand

5 3


LE 16-15_2

53

Primase joins RNA


Template

strand

5 3


RNA primer3

5

35

DNA pol III addsDNA nucleotides to the primer, formingan Okazaki fragment.

LE 16-15_3

53

Primase joins RNA


Template

strand

5 3


RNA primer3

5

35


Okazaki

fragment3

5

5

3

After reaching thenext RNA primer (not

shown), DNA pol IIIfalls off.

LE 16-15_4

53

Primase joins RNA


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.

LE 16-15_5

53

Primase joins RNA


Template

strand

5 3


RNA primer3

5

35


Okazaki

fragment3

5

5

3



33

5

5


33

5

5

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

LE 16-15_6

53

Primase joins RNA


Template

strand

5 3


RNA primer3

5

35


Okazaki

fragment3

5

5

3



33

5

5


33

5

5

DNA pol I replaces the 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 lagging strand in the regionis now complete.


Animation: Lagging Strand

media/16_15LaggingStrand_A.html


Other Proteins That Assist DNA Replication

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

• Single-strand binding protein binds to and stabilizes single-stranded DNA until it can be used as a template

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


• 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

LE 16-16

5

3

Parental DNA

3

5


DNA pol III

Replication fork

Leading

strand

DNA ligase

Primase

OVERVIEW

PrimerDNA pol III

DNA pol I

Lagging

strand

Lagging

strand

Leading

strand

Leading

strand

Lagging

strandOrigin of replication


Animation: DNA Replication Review

media/16_16DNAReplicatReview_A.html


The DNA Replication Machine as a Stationary 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 DNA polymerase molecules “reel in” parental DNA and “extrude” newly made daughter DNA molecules


Proofreading and Repairing DNA

• 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 16-17

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 strand

at two points and the

damaged section is

removed.

Nuclease

A thymine dimer

distorts the DNA molecule.


Replicating the Ends of DNA Molecules

• Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes

• The usual replication machinery provides no way to complete the 5 ends, so repeated rounds of replication produce shorter DNA molecules

LE 16-18

End of parental

DNA strands

5

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 3 end 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 strand

New leading strand

Shorter and shorter

daughter molecules


• Eukaryotic chromosomal DNA molecules have at

their 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

LE 16-19

1 µm


• If chromosomes of germ cells became shorter in

every 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

The Molecular Basis of Inheritance · 2016-11-23 · •DNA, the substance of inheritance, is the most celebrated molecule of our time •Hereditary information is encoded in DNA