chapter 16 the molecular basis of inheritance copyright © 2002 pearson education, inc., publishing...

CHAPTER 16 THE MOLECULAR BASIS OF

INHERITANCE

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

DNA as the Genetic Material

1. The search for genetic material led to DNA

2. Watson and Crick discovered the double helix by building models to

conform to X-ray data

http://profiles.nlm.nih.gov/CC/A/A/L/P/

DNA STRUCTURE; DNA STRUCTURE; REPLICATION ANIMATION REPLICATION ANIMATION

STEPSSTEPS DNA makes DNA makes DNA.mhtDNA.mht

T.H. Morgan’s group showed that genes are T.H. Morgan’s group showed that genes are located on chromosomes.located on chromosomes.

But which component? Protein or DNA? But which component? Protein or DNA?

The search for genetic material led to The search for genetic material led to DNADNA


The discovery of the genetic role of DNA The discovery of the genetic role of DNA began with research by began with research by Frederick Griffith in Frederick Griffith in 1928.1928.

He studied He studied StreptococcusStreptococcus pneumoniaepneumoniae, a , a bacterium that causes pneumonia in mammals.bacterium that causes pneumonia in mammals. One strain, the One strain, the R strain, was harmless.R strain, was harmless. The other strain, the S strain, was pathogenic.The other strain, the S strain, was pathogenic.

Griffith mixed heat-killed S strain with live R Griffith mixed heat-killed S strain with live R strain bacteria and injected this into a mouse.strain bacteria and injected this into a mouse.

The mouse The mouse dieddied and he recovered the and he recovered the pathogenic strain from the mouse’s blood.pathogenic strain from the mouse’s blood.


Griffith called this phenomenon Griffith called this phenomenon transformationtransformation, a change in genotype and , a change in genotype and phenotype due to the assimilation of a foreign phenotype due to the assimilation of a foreign substance (now known to be DNA) by a cell.substance (now known to be DNA) by a cell.

(he did not know what was assimilated)(he did not know what was assimilated)


Fig. 16.1

1944, 1944, Oswald Avery, Maclyn McCarty and Oswald Avery, Maclyn McCarty and Colin MacLeodColin MacLeod expanded on Griffith’s expanded on Griffith’s research.research.

Announced that the transforming substance was Announced that the transforming substance was DNA.DNA.

Scientific community still skeptical!!Scientific community still skeptical!!


Alfred Hershey & Martha Chase:Alfred Hershey & Martha Chase: 1952: The “blender” experiment1952: The “blender” experiment DNA functions as genetic material.DNA functions as genetic material. Used bacteriophages (phages); Phage2Used bacteriophages (phages); Phage2

Viruses structure: DNA (sometimes RNA) and a protein Viruses structure: DNA (sometimes RNA) and a protein coat.coat.


phage T2.phage T2.


Fig. 16.2a

virus particle labeled with 35S

virus particle labeled with 32P

bacterial cell (cutaway view)

label outside cell

label inside cell

They Infected They Infected bacteria with bacteria with

Labeled Viruses Labeled Viruses

(radioisotopes of (radioisotopes of Sulfur and Sulfur and

Phosphorous)Phosphorous)

They allowed each batch to infect separate They allowed each batch to infect separate E. coliE. coli cultures.cultures.

Shortly after the onset of infection, they spun the Shortly after the onset of infection, they spun the cultured infected cells in a blender, shaking loose any cultured infected cells in a blender, shaking loose any parts of the phage that remained outside the bacteria.parts of the phage that remained outside the bacteria.


The mixtures were spun in a centrifuge which separated The mixtures were spun in a centrifuge which separated the heavier bacterial cells in the pellet from lighter free the heavier bacterial cells in the pellet from lighter free phages and parts of phage in the liquid supernatant. phages and parts of phage in the liquid supernatant.

Tested the pellet and supernatant of the separate Tested the pellet and supernatant of the separate treatments for the presence of radioactivity.treatments for the presence of radioactivity.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 16.2b

1947: Erwin Chargaff 1947: Erwin Chargaff DNA composition varies from species to DNA composition varies from species to

species.species. In any one species, the four bases are found in In any one species, the four bases are found in

characteristic, but not necessarily equal, ratios.characteristic, but not necessarily equal, ratios. Regularity in these proportions consistent in all Regularity in these proportions consistent in all

organismsorganisms


Chargaff’s base-pairing rulesChargaff’s base-pairing rules::

Number of Thymine = number of Adenine.Number of Thymine = number of Adenine. Number of guanines = the number of cytosines. Number of guanines = the number of cytosines.

A=T and G=CA=T and G=C


Base PairingBase Pairing

C with GC with G

A with TA with T

ALWAYS TRUEALWAYS TRUE

NOVA Secret of Photo 51 NOVA Secret of Photo 51 PBS.mhtPBS.mht

http://www.pbs.org/wgbh/nova/minutes/q_3009.html

Watson and Crick discovered the Watson and Crick discovered the double helix by building models to double helix by building models to

conform to X-ray dataconform to X-ray data 1962 Nobel Prize1962 Nobel Prize James Watson Francis James Watson Francis

Crick,Maurice WilkinsCrick,Maurice Wilkins

Watson & Crick Watson & Crick Developed Developed

Accurate ModelAccurate Model

Double helix shapeDouble helix shape

Two strands of nucleotidesTwo strands of nucleotides

Like a spiral staircaseLike a spiral staircase

Watson-Crick Watson-Crick Model: 1953Model: 1953

2 Strands of nucleotides2 Strands of nucleotides Sugar and phosphates covalently bonded to one Sugar and phosphates covalently bonded to one

another. another.

2 Strands held together by 2 Strands held together by

hydrogen bondshydrogen bonds between bases (rungs on a ladder) between bases (rungs on a ladder)

ShapeShape of a double helix of a double helix

Twisted like a coiled spring.Twisted like a coiled spring.

The molecule coils into this The molecule coils into this shape as a result of theshape as a result of the base bonding.base bonding.

Rosalind Franklin’s Photo 51 Rosalind Franklin’s Photo 51

Expert in x-ray imagesExpert in x-ray images X-ray crystallographyX-ray crystallography Watson saw her photoWatson saw her photo

while visiting her labwhile visiting her lab

Consistent with images of Consistent with images of

Other spiral-shaped molecules.Other spiral-shaped molecules.


Fig. 16.3

• The phosphate group of one nucleotide is attached to the sugar of the next nucleotide in line.

• The result is a “backbone” of alternating phosphates and sugars, from which the bases project.

The sugar-phosphate chains of each strand are like The sugar-phosphate chains of each strand are like the side ropes of a rope ladder.the side ropes of a rope ladder.

Pairs of nitrogen bases, one from each strand, form Pairs of nitrogen bases, one from each strand, form rungs.rungs.

The ladder forms a twist every ten bases.The ladder forms a twist every ten bases.



Fig. 16.5

Adenine and Guanine are purines (larger)Adenine and Guanine are purines (larger) Cytosine and Thymine are pyrimidinesCytosine and Thymine are pyrimidines

Hydrogen bondingHydrogen bonding Confirmed Chargoff’s rules:Confirmed Chargoff’s rules: C GC G

A TA T



DNA Replication and Repair

1. During DNA replication, base pairing enables existing DNA strands to

serve as templates for new complementary strands

2. A large team of enzymes and other proteins carries out DNA replication

3. Enzymes proofread DNA during its replication and repair damage to

existing DNA

4. Telomeres

In a second paper Watson and Crick published In a second paper Watson and Crick published their hypothesis for how DNA replicates.their hypothesis for how DNA replicates. Essentially, because each strand is complementary to Essentially, because each strand is complementary to

the other, each can form a template when separated.the other, each can form a template when separated. The order of bases on one strand can be used to add The order of bases on one strand can be used to add

in complementary bases and therefore duplicate the in complementary bases and therefore duplicate the pairs of bases exactly.pairs of bases exactly.


When a cell copies a DNA molecule, each When a cell copies a DNA molecule, each strand serves as a template for ordering strand serves as a template for ordering nucleotides into a new complementary strand.nucleotides into a new complementary strand. One at a time, nucleotides line up along the One at a time, nucleotides line up along the

template strand according to the base-pairing rules.template strand according to the base-pairing rules. The nucleotides are linked to form new strands.The nucleotides are linked to form new strands.


Fig. 16.7

3 Hypothesis for DNA Replication:3 Hypothesis for DNA Replication: 1.Watson/Crick: semiconservative: each new molecule 1.Watson/Crick: semiconservative: each new molecule

is half old, half new.is half old, half new. 2.Cconservative model : parental double helix remains 2.Cconservative model : parental double helix remains

intact; new made from all new material.intact; new made from all new material. 3.The dispersive model: Both strands of each new DNA 3.The dispersive model: Both strands of each new DNA

contain mix of old and new.contain mix of old and new.


Fig. 16.8

Experiments in the late 1950s by Matthew Experiments in the late 1950s by Matthew Meselson and Meselson and FranklinFranklin Stahl Stahl supported the supported the semiconservative modelsemiconservative model.. In their experiments, they labeled the nucleotides of In their experiments, they labeled the nucleotides of

the old strands with a heavy isotope of nitrogen the old strands with a heavy isotope of nitrogen ((1515N), while any new nucleotides were indicated by N), while any new nucleotides were indicated by a lighter isotope (a lighter isotope (1414N).N).

Replicated strands could be separated by density in Replicated strands could be separated by density in a centrifuge.a centrifuge.



Fig. 16.9

• The first replication in the 14N medium produced a band of hybrid (15N-14N) DNA, eliminating the conservative model.

• A second replication produced both light and hybrid DNA, eliminating the dispersive model and supporting the semiconservative model.

• Then he took these bacteria that had hybrid DNA, grew them in more 14 N medium.

• This second replication produced both light and hybrid DNA, eliminating the dispersive model and supporting the semiconservative model.

• BECAUSE: the hybrid DNA replicated;• the 2 heavy strands incorporated light complementary bases ,

thus ½ of the DNA was hybrid.

• the 2 light strands incorporated light—thus the other half of the new DNA were ALL light)

DNA REPLICATION –THE DNA REPLICATION –THE BASICSBASICS

Semi-conservativeSemi-conservative Basic Structure of nucleotidesBasic Structure of nucleotides SugarSugar

DeoxyriboseDeoxyribose PhosphatePhosphate Nitrogeneous BasesNitrogeneous Bases

Purines: larger, double ring structurePurines: larger, double ring structure AdeAdenine, nine, guaguanineninePyrimidinesPyrimidines smaller, single ring structuresmaller, single ring structure thymine, cytosinethymine, cytosine

base pairing: A to T; C to Gbase pairing: A to T; C to G

Figure 16.5 The double helixFigure 16.5 The double helix

Antiparallel strandsAntiparallel strands: 2 separate strands of : 2 separate strands of nucleotides running in opposite directions.nucleotides running in opposite directions.

5' end and 3' end 5' end and 3' end 5' end has a Phosphate group attached to the 5' end has a Phosphate group attached to the

5th carbon in the sugar comprising the first 5th carbon in the sugar comprising the first nucleotide in the strand.nucleotide in the strand.

The 3' end has a hydroxyl (OH) attached to the The 3' end has a hydroxyl (OH) attached to the 3rd carbon in the sugar at the terminal end of 3rd carbon in the sugar at the terminal end of one strand.one strand.

DNA Synthesis proceeds in a 5' to 3 ' DNA Synthesis proceeds in a 5' to 3 ' directiondirection

Figure 16.12 The two strands of DNA are antiparallelFigure 16.12 The two strands of DNA are antiparallel

Leading StrandLeading Strand Lagging strandLagging strand Okazaki FragmentsOkazaki Fragments

The enzymes involved:The enzymes involved: HelicaseHelicase PrimasePrimase DNA PolymeraseDNA Polymerase LigaseLigase

ORIGIN OF REPLICATIONORIGIN OF REPLICATION WHERE DOES DNA REPLICATION WHERE DOES DNA REPLICATION

ACTUALLY BEGINACTUALLY BEGIN SPECIFIC PROTEINS ARE NEEDED TO INITIATE SPECIFIC PROTEINS ARE NEEDED TO INITIATE

THIS PROCESSTHIS PROCESS FORMATION OF REPLICATION FORKSFORMATION OF REPLICATION FORKS

A ‘Y’ SHAPED REGION OF REPLICATING DNA A ‘Y’ SHAPED REGION OF REPLICATING DNA WHERE THE NEW STRANDS WILL GROWWHERE THE NEW STRANDS WILL GROW

REPLICATION BUBBLE – POSITION WHERE THE REPLICATION BUBBLE – POSITION WHERE THE HELIX OPENS, CREATED BY THE FORMATION OF THE HELIX OPENS, CREATED BY THE FORMATION OF THE REPLICATION FORKREPLICATION FORK

1. INITIATION:1. INITIATION:Starts at specials point called the ORIGINS OF Starts at specials point called the ORIGINS OF REPLICATIONREPLICATION

Helicase cuases DNA to unwind, exposing the template.Helicase cuases DNA to unwind, exposing the template. This unwinding causes a BUBBLE to form, and is known as a This unwinding causes a BUBBLE to form, and is known as a

REPLICATION BUBBLE with replication forks at each end.REPLICATION BUBBLE with replication forks at each end. the DNA strands separate forming a replication “bubble” with the DNA strands separate forming a replication “bubble” with

replication forksreplication forks at each end. at each end. The replication bubbles elongate as the DNA is replicated and The replication bubbles elongate as the DNA is replicated and

eventually fuse.eventually fuse.


Fig. 16.10

Origins of ReplicationOrigins of Replication

EUKARYOTIC CHROMOSOMES HAVE EUKARYOTIC CHROMOSOMES HAVE HUNDREDS OR EVEN THOUSANDS OF HUNDREDS OR EVEN THOUSANDS OF REPLICATING ORIGINS.REPLICATING ORIGINS.

VIRAL OR BACTERIAL DNA HAVE VIRAL OR BACTERIAL DNA HAVE ONLY ONE REPLICATION ORIGINONLY ONE REPLICATION ORIGIN

ELONGATION OF A NEW DNA ELONGATION OF A NEW DNA STRANDSTRAND

FIRST THE STRAND WILL SEPARATE,FIRST THE STRAND WILL SEPARATE, 2 PROTEINS ARE INVOLVED2 PROTEINS ARE INVOLVED

HELICASESHELICASES – WHICH ARE ENZYMES THAT WILL – WHICH ARE ENZYMES THAT WILL CATALYZE THE UNWINDING OF PARENTAL CATALYZE THE UNWINDING OF PARENTAL DOUBLE HELIX TO EXPOSE THE TEMPLATEDOUBLE HELIX TO EXPOSE THE TEMPLATE

SINGLE STRAND BINDING PROTEIN,SINGLE STRAND BINDING PROTEIN, WHICH WHICH KEEP THE STRANDS SEPARATE AND STABALIZE KEEP THE STRANDS SEPARATE AND STABALIZE THE UNWOUND DNA, UNTIL NEW THE UNWOUND DNA, UNTIL NEW COMPLIMENTARY STRANDS ARE FORMEDCOMPLIMENTARY STRANDS ARE FORMED

ELONGATION OF A NEW ELONGATION OF A NEW STRAND OF DNASTRAND OF DNA

DNA POLYMERASEDNA POLYMERASE – THE FUNCTIONAL – THE FUNCTIONAL ENZYMEENZYME

LINKS THE NUCLEOTIDES TO THE GROWING LINKS THE NUCLEOTIDES TO THE GROWING STANDSTAND

THE NEW NUCLEOTIDES ARE THE NEW NUCLEOTIDES ARE ADDED TO ADDED TO THE 3’ END OF THE GROWING STRAND.THE 3’ END OF THE GROWING STRAND.

NUCLEOSIDE TRIPHOSPHATENUCLEOSIDE TRIPHOSPHATE DRIVES THIS DRIVES THIS REACTION (ENERGY SOURCE SIMILAR TO REACTION (ENERGY SOURCE SIMILAR TO ATP)ATP)

ANTIPARRALLELANTIPARRALLEL SUGAR PHOSPHATE BONDS IN DNA RUN IN SUGAR PHOSPHATE BONDS IN DNA RUN IN

OPPOSITE DIRECTIONSOPPOSITE DIRECTIONS DNA CAN ONLY ELONGATE STRANDS IN THE 5’ DNA CAN ONLY ELONGATE STRANDS IN THE 5’

TO 3’ DIRECTIONTO 3’ DIRECTION PROBLEM OF ANTIPARRALLEL DNA STRANDS PROBLEM OF ANTIPARRALLEL DNA STRANDS

IS SOLVED BY THE CONTINUOUS SYNTHESIS IS SOLVED BY THE CONTINUOUS SYNTHESIS OF THE:OF THE:

LEADING STRANDLEADING STRAND – THIS IS A STRAND THAT IS A – THIS IS A STRAND THAT IS A SINGLE POLYMER IN THE 5’ TO 3’ DIRECTION, SINGLE POLYMER IN THE 5’ TO 3’ DIRECTION, WHICH ISWHICH IS TOWARDS THE REPLICATION FORK TOWARDS THE REPLICATION FORK

OKAZAKI FRAGMENTSOKAZAKI FRAGMENTS

PRODUCE THE PRODUCE THE LAGGING STRANDLAGGING STRAND A SERIES OF A SERIES OF SHORT SEGMENTS SHORT SEGMENTS SYNTHESIZED IN THE 5’ TO 3’ DIRECTIONSYNTHESIZED IN THE 5’ TO 3’ DIRECTION

Away from the Replication ForkAway from the Replication Fork DNA LIGASE DNA LIGASE IS THE ENZYME THAT LINKS IS THE ENZYME THAT LINKS

OKAZAKI FRAGMENTS INTO A NEW DNA OKAZAKI FRAGMENTS INTO A NEW DNA STRANDSTRAND

OKAZAKI FRAGMENTS SOLVE THE OKAZAKI FRAGMENTS SOLVE THE PROBLEM OF THE LAGGING STRANDPROBLEM OF THE LAGGING STRAND

PRIMING DNA SYNTHESISPRIMING DNA SYNTHESIS

RNA PRIMER RNA PRIMER – A SHORT RNA SEGMENT THAT IS – A SHORT RNA SEGMENT THAT IS COMPLIMENTARY TO THE DNA SEGMENT COMPLIMENTARY TO THE DNA SEGMENT NEEDED TO BEGIN DNA REPLICATION.NEEDED TO BEGIN DNA REPLICATION.

PRIMASE PRIMASE – THE ENZYME THAT JOINS RNA – THE ENZYME THAT JOINS RNA NUCLEOTIDES TO FORM THE PRIMERNUCLEOTIDES TO FORM THE PRIMER

ABOUT 10 NUCLEOTIDES LONG (EUKARYOTES)ABOUT 10 NUCLEOTIDES LONG (EUKARYOTES) ONLY 1 PRIMER IS NEEDED FOR THE LEADING STRANDONLY 1 PRIMER IS NEEDED FOR THE LEADING STRAND AN RNA PRIMER IS NEEDED FOR EACH OKAZAKI AN RNA PRIMER IS NEEDED FOR EACH OKAZAKI

FRAGMENT IN THE FORMATION OF THE LAGGING FRAGMENT IN THE FORMATION OF THE LAGGING STRANDSTRAND

To summarize: the leading strand is copied To summarize: the leading strand is copied continuouslycontinuously toward toward the fork from a single primer.the fork from a single primer.

The The lagging strand lagging strand is copiedis copied away away from the fork in short from the fork in short segments, each requiring a new primersegments, each requiring a new primer..

Primers replaced with DNA nucleotides.Primers replaced with DNA nucleotides.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 16.16

DAMAGE REPAIRDAMAGE REPAIR

PROOF READING OF DNAPROOF READING OF DNA DNA REPLICATION IS HIGHLY ACCURATEDNA REPLICATION IS HIGHLY ACCURATE PAIRING ERRORS ARE ONLY ABOUT 1-10,000PAIRING ERRORS ARE ONLY ABOUT 1-10,000 A COMPLETE DNA MOLECULE ERROR IS ONLY A COMPLETE DNA MOLECULE ERROR IS ONLY

ABOUT 1 IN 1 BILLIONABOUT 1 IN 1 BILLION

MISMATCH REPAIRMISMATCH REPAIR

MISTAKES ARE CORRECTED AS THE DNA MISTAKES ARE CORRECTED AS THE DNA IS BEING COPIEDIS BEING COPIED

POLYMERASE – PROOF READS EACH POLYMERASE – PROOF READS EACH NUCLEOTIDE AND REMOVES NUCLEOTIDE AND REMOVES INCORRECTLY PAIRED NUCLEOTIDES AND INCORRECTLY PAIRED NUCLEOTIDES AND ALLOWS SYNTHESIS TO CONTINUEALLOWS SYNTHESIS TO CONTINUE

EXCISION REPAIREXCISION REPAIR

ACCIDENTAL DNA CHANGES DUE TO ACCIDENTAL DNA CHANGES DUE TO EXPOSURE TO; CHEMICALS, RADIATION, EXPOSURE TO; CHEMICALS, RADIATION, UV LIGHT ETC.UV LIGHT ETC.

NUCLEASE – IS THE REPAIR ENZYMENUCLEASE – IS THE REPAIR ENZYME THE DAMAGED SEGMENT IS EXCISED AND THE GAP THE DAMAGED SEGMENT IS EXCISED AND THE GAP

IS FILLED BY DNA POLYMERASE AND SEALED BY IS FILLED BY DNA POLYMERASE AND SEALED BY LIGASELIGASE

Thymine DimersThymine Dimers

TELOMERESTELOMERES

DO NOT CONTAIN GENESDO NOT CONTAIN GENES MULTIPLE REPITITIONS OF A SHORT NUCLEOTIDE MULTIPLE REPITITIONS OF A SHORT NUCLEOTIDE

SEQUENCE SEQUENCE TTAGGG IN EUKARYOTESTTAGGG IN EUKARYOTES VARIES BETWEEN 100-1000VARIES BETWEEN 100-1000 PROTECTS THE EROSION OF DNAPROTECTS THE EROSION OF DNA

TELOMERASE (ENZYME) – CATALYZE THE TELOMERASE (ENZYME) – CATALYZE THE LENGTHENING OF THE TELOMERELENGTHENING OF THE TELOMERE

Only in Germ-line cellsOnly in Germ-line cells ALSO FOUND IN CANCER CELLSALSO FOUND IN CANCER CELLS

To start a new chain requires a To start a new chain requires a primerprimer, a short , a short segment of RNA, formed by Primase.segment of RNA, formed by Primase.


DNA polymerase can DNA polymerase can now add now add deoxyribonucleotides to deoxyribonucleotides to the 3’ end of the the 3’ end of the ribonucleotide chain.ribonucleotide chain.

Occurs at the Occurs at the Replication Fork.Replication Fork.


Fig. 16.14

Energy for synthesis…provided by Energy for synthesis…provided by the nucleotides themselves.the nucleotides themselves.

The raw nucleotides are nucleoside The raw nucleotides are nucleoside triphosphates.triphosphates. Each has a nitrogen base, deoxyribose, and a Each has a nitrogen base, deoxyribose, and a

triptriphosphate tail.hosphate tail.

As each nucleotide is added, the last two As each nucleotide is added, the last two phosphate groups are hydrolyzed to form phosphate groups are hydrolyzed to form pyrophosphate.pyrophosphate. The exergonic hydrolysis of pyrophosphate to two The exergonic hydrolysis of pyrophosphate to two

inorganic phosphate molecules drives the inorganic phosphate molecules drives the polymerization of the nucleotide to the new strand.polymerization of the nucleotide to the new strand.


Fig. 16.10

DNA polymerases can only add nucleotides to DNA polymerases can only add nucleotides to the free 3’ end of a growing DNA strand.the free 3’ end of a growing DNA strand.

A new DNA strand can only elongate in the 5’-A new DNA strand can only elongate in the 5’->3’ direction.>3’ direction.

This creates a problem at the replication fork This creates a problem at the replication fork because one parental strand is oriented 3’->5’ because one parental strand is oriented 3’->5’ into the fork, while the other antiparallel into the fork, while the other antiparallel parental strand is oriented 5’->3’ into the fork.parental strand is oriented 5’->3’ into the fork.

At the replication fork, one parental strand (3’-At the replication fork, one parental strand (3’-> 5’ into the fork), the > 5’ into the fork), the leading strand,leading strand, can be can be used by polymerases as a template for a used by polymerases as a template for a continuous complementary strand. continuous complementary strand.


The other daughter strand-- the The other daughter strand-- the lagging strandlagging strand, , is copied away from the fork in short segments is copied away from the fork in short segments (Okazaki fragments).(Okazaki fragments).

DNA ligaseDNA ligase fuses them fuses them

together.together.


Fig. 16.13

The leading strand requires the formation of The leading strand requires the formation of only a single primer as the replication fork only a single primer as the replication fork continues to separate.continues to separate.

The lagging strand requires formation of a new The lagging strand requires formation of a new primer as the replication fork progresses.primer as the replication fork progresses.

After the primer is formed, DNA polymerase After the primer is formed, DNA polymerase can add new nucleotides away from the fork can add new nucleotides away from the fork until it runs into the previous Okazaki until it runs into the previous Okazaki fragment. fragment.

The primers are converted to DNA before DNA The primers are converted to DNA before DNA ligase joins the fragments together.ligase joins the fragments together.


DNA molecules are constantly subject to DNA molecules are constantly subject to potentially harmful chemical and physical agents.potentially harmful chemical and physical agents. Reactive chemicals, radioactive emissions, X-rays, Reactive chemicals, radioactive emissions, X-rays,

and ultraviolet light can change nucleotides in ways and ultraviolet light can change nucleotides in ways that can affect encoded genetic information.that can affect encoded genetic information.

DNA bases often undergo spontaneous chemical DNA bases often undergo spontaneous chemical changes under normal cellular conditions.changes under normal cellular conditions.

Mismatched nucleotides that are missed by DNA Mismatched nucleotides that are missed by DNA polymerase or mutations that occur after DNA polymerase or mutations that occur after DNA synthesis is completed can often be repaired.synthesis is completed can often be repaired. Each cell continually monitors and repairs its genetic Each cell continually monitors and repairs its genetic

material, with over 130 repair enzymes identified in material, with over 130 repair enzymes identified in humans.humans.


DNA RepairDNA Repair

Mistakes during the initial pairing of template Mistakes during the initial pairing of template nucleotides --one error per 10,000 base pairs.nucleotides --one error per 10,000 base pairs.

DNA polymerase proofreads each new nucleotide DNA polymerase proofreads each new nucleotide against the template nucleotide as soon as it is against the template nucleotide as soon as it is added.added.

Missense:Missense:If there is an incorrect pairing, the If there is an incorrect pairing, the enzyme removes the wrong nucleotide and then enzyme removes the wrong nucleotide and then resumes synthesis.resumes synthesis.

The final error rate is only one per billion The final error rate is only one per billion nucleotides.nucleotides.

3. Enzymes proofread DNA during its 3. Enzymes proofread DNA during its replication and repair damage in existing replication and repair damage in existing

DNADNA


In In mismatch repairmismatch repair, special enzymes fix , special enzymes fix incorrectly paired nucleotides.incorrectly paired nucleotides. A hereditary defect in one of these enzymes is A hereditary defect in one of these enzymes is

associated with a form of colon cancer.associated with a form of colon cancer. In In nucleotide excision nucleotide excision

repairrepair, a , a nucleasenuclease cuts cuts out a segment of a out a segment of a damaged strand.damaged strand. The gap is filled in by The gap is filled in by

DNA polymerase and DNA polymerase and ligase.ligase.


Fig. 16.17

The importance of the proper functioning of The importance of the proper functioning of repair enzymes is clear from the inherited repair enzymes is clear from the inherited disorder xeroderma pigmentosum.disorder xeroderma pigmentosum. These individuals are hypersensitive to sunlight.These individuals are hypersensitive to sunlight. In particular, ultraviolet light can produce In particular, ultraviolet light can produce thymine thymine

dimersdimers between adjacent thymine nucleotides. between adjacent thymine nucleotides. This buckles the DNA double helix and This buckles the DNA double helix and

interferes with DNA replicationinterferes with DNA replication.. In individuals with this disorder, mutations in their In individuals with this disorder, mutations in their

skin cells are left uncorrected and cause skin cancer.skin cells are left uncorrected and cause skin cancer.


Limitations in the DNA polymerase create Limitations in the DNA polymerase create problems for the linear DNA of eukaryotic problems for the linear DNA of eukaryotic chromosomes.chromosomes.

The usual replication machinery provides no way The usual replication machinery provides no way to complete the 5’ ends of daughter DNA strands.to complete the 5’ ends of daughter DNA strands.

Repeated rounds of replication Repeated rounds of replication produce shorter and shorter DNA produce shorter and shorter DNA molecules.molecules.

4. The ends of DNA molecules are 4. The ends of DNA molecules are replicated by a special mechanismreplicated by a special mechanism



Fig. 16.18

The ends of eukaryotic chromosomal DNA The ends of eukaryotic chromosomal DNA molecules, the molecules, the telomerestelomeres, have special , have special nucleotide sequences.nucleotide sequences. In human telomeres, this sequence is typically In human telomeres, this sequence is typically

TTAGGG, repeated between 100 and 1,000 times.TTAGGG, repeated between 100 and 1,000 times. Telomeres protect genes from being eroded Telomeres protect genes from being eroded

through multiple rounds of DNA replication.through multiple rounds of DNA replication.


Fig. 16.19a

Eukaryotic cells have evolved a mechanism to Eukaryotic cells have evolved a mechanism to restore shortened telomeres.restore shortened telomeres.

TelomeraseTelomerase uses a short molecule of RNA as a uses a short molecule of RNA as a template to extend the 3’ end of the telomere.template to extend the 3’ end of the telomere. There is now room for There is now room for

primase and DNA primase and DNA polymerase to extend polymerase to extend the 5’ end.the 5’ end.

It does not repair the It does not repair the 3’-end “overhang,”3’-end “overhang,”but it does lengthenbut it does lengthenthe telomere.the telomere.


Fig. 16.19b

Telomerase is Telomerase is notnot present in most cells of present in most cells of multicellular organismsmulticellular organisms..

Therefore, the DNA of dividing somatic cells and Therefore, the DNA of dividing somatic cells and cultured cells does tend to become shorter.cultured cells does tend to become shorter.

Thus, Thus, telomere length may be a limiting factor telomere length may be a limiting factor in the life span of certain tissues and of the in the life span of certain tissues and of the organism.organism.

Telomerase is present in germ-line cellsTelomerase is present in germ-line cells, , ensuring that zygotes have long telomeres.ensuring that zygotes have long telomeres.

Active telomerase is also found in cancerous Active telomerase is also found in cancerous somatic cellssomatic cells.. This overcomes the progressive shortening that would This overcomes the progressive shortening that would

eventually lead to self-destruction of the cancer.eventually lead to self-destruction of the cancer.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

chapter 16 the molecular basis of inheritance copyright © 2002 pearson education, inc., publishing...

Documents

pearson education

benjamin cummings dna

benjamin cummingsfig

benjamin cummingsgriffith

s strain

pathogenic strain

genetic role of dna

benjamin cummingsthe