Searching for Genetic Searching for Genetic MaterialMaterial

Science as a processScience as a process Until 1940’s no one new what the genetic Until 1940’s no one new what the genetic

material wasmaterial was

• Proteins and DNA were candidatesProteins and DNA were candidates Most scientists thought proteins were Most scientists thought proteins were

the genetic material because of their the genetic material because of their diversity and sizediversity and size

At this time little was known about At this time little was known about nucleic acidsnucleic acids

Searching for Genetic materialSearching for Genetic material Discovery of genetic role of DNA was in Discovery of genetic role of DNA was in

1928 by Frederick Griffith1928 by Frederick Griffith Studied the bacterium streptococcus Studied the bacterium streptococcus

pneumoniae ( pneumonia)pneumoniae ( pneumonia) Used two strains ( pathogenic and Used two strains ( pathogenic and

harmless) and discovered the process of harmless) and discovered the process of transformationtransformation

Oswald Avery took Griffiths data and found Oswald Avery took Griffiths data and found that the only substance capable of that the only substance capable of transformation was DNA- met with transformation was DNA- met with skepticismskepticism

DNA as the Genetic MaterialDNA as the Genetic Material Evidence for viral DNAEvidence for viral DNA

Viruses are non-living and are made of little more Viruses are non-living and are made of little more than DNA surrounded by a protein coatthan DNA surrounded by a protein coat

Hershey and ChaseHershey and Chase Used bacteriophages Used bacteriophages (phages)- viruses that infect (phages)- viruses that infect

bacteriabacteria Discovered -DNA, not protein, is the hereditary Discovered -DNA, not protein, is the hereditary

material material How?How?• Implanted radioactive elements ( S and P) into Implanted radioactive elements ( S and P) into

viruses and then let them inject their heredity viruses and then let them inject their heredity material into the bacteria and later separated what material into the bacteria and later separated what they did vs. what they did not inject.they did vs. what they did not inject.

DNA StructureDNA Structure ChargaffChargaff

1947- reported that the composition of DNA 1947- reported that the composition of DNA varies from species to speciesvaries from species to species

Found the in the DNA of each species amount of Found the in the DNA of each species amount of the bases are not equal but present in a ratiothe bases are not equal but present in a ratio

Found A=T and always C=GFound A=T and always C=G

Discovery of the Structure of Discovery of the Structure of DNADNA

Watson & CrickWatson & Crick with help from Wilkins and with help from Wilkins and Franklin ( 1950’s)Franklin ( 1950’s) W and C used crystallography photos of W and C used crystallography photos of

DNA, and Chargaff’s data to determine the DNA, and Chargaff’s data to determine the structure of DNAstructure of DNA

ConcludedConcluded• DNA is a Double Helix of DNA is a Double Helix of nucleotidesnucleotides• Thought of DNA as a ladder or a winding Thought of DNA as a ladder or a winding

stair case made of two strands of stair case made of two strands of nucleotidesnucleotides

DNA Bonding and StructureDNA Bonding and Structure Double helix is made of nucleotides that are eitherDouble helix is made of nucleotides that are either

PurinesPurines

• Adenine and Guanine- double ring structureAdenine and Guanine- double ring structure PyrimidinesPyrimidines

• Thymine and Cytosine- single ring structureThymine and Cytosine- single ring structure Purines always pair with pyrimidines due to structurePurines always pair with pyrimidines due to structure

A always bonds with T forming 3 H-bonds that hold A always bonds with T forming 3 H-bonds that hold them togetherthem together

C always bonds with G forming 2 H-bonds that hold C always bonds with G forming 2 H-bonds that hold them togetherthem together

Model explains Chargaffs dataModel explains Chargaffs data

Figure 16.5 The double helixFigure 16.5 The double helix

How does DNA replicate itself?How does DNA replicate itself? 3 theories3 theories

Conservative- Parental molecule remains Conservative- Parental molecule remains intact and makes an all new second copyintact and makes an all new second copy

Semi conservative- ( Watson and Crick) Semi conservative- ( Watson and Crick) two strands of parental molecule separate two strands of parental molecule separate and each is used as a template for a new and each is used as a template for a new strandstrand

Dispersive- each strand of both daughter Dispersive- each strand of both daughter molecules contains mixture of old and molecules contains mixture of old and newly made partsnewly made parts

DNA ReplicationDNA Replication

DNA Replication: a closer DNA Replication: a closer looklook

Teams of enzymes and other proteins carry out Teams of enzymes and other proteins carry out DNA replicationDNA replication Where to start?Where to start?• Origin of replication-Origin of replication- where replication begins where replication begins• Depending on what type of cell can have 1 or Depending on what type of cell can have 1 or

many of these ( Pro- 1, Euk- many)many of these ( Pro- 1, Euk- many)• Region is recognized by a specific set of Region is recognized by a specific set of

nucleotidesnucleotides At this site DNA will be split open and a At this site DNA will be split open and a

replication bubble will formreplication bubble will form DNA replication will move in both directionsDNA replication will move in both directions

DNA ReplicationDNA Replication Steps in the processSteps in the process

Replication begins at Replication begins at Origin of replicationOrigin of replication Replication forkReplication fork forms forms DNA helicasesDNA helicases bind and unwind the DNA bind and unwind the DNA Single-strand binding proteinsSingle-strand binding proteins stabilize stabilize

moleculemolecule DNA polymerase IIIDNA polymerase III will begin replication will begin replication

the molecule by pulling nucleotides and the molecule by pulling nucleotides and attaching them in a complimentary attaching them in a complimentary sequencesequence

Problems with DNA ReplicationProblems with DNA Replication Replication will move in either direction Replication will move in either direction

from the origin of replicationfrom the origin of replication However, the DNA molecule in anti-However, the DNA molecule in anti-

parallel- sugar phosphate backbones parallel- sugar phosphate backbones move in opposite directionsmove in opposite directions

DNA has a 3’ end and a 5’ endDNA has a 3’ end and a 5’ end DNA polymerase will only attach DNA polymerase will only attach

nucleotides to the 3’ endnucleotides to the 3’ end

• Thus DNA molecule can only elongate Thus DNA molecule can only elongate in the 5’ – 3’ directionin the 5’ – 3’ direction

Solving the problems of DNA Solving the problems of DNA replicationreplication

Due to anti-parallel structure of DNA one Due to anti-parallel structure of DNA one DNA polymerase will move copy in one DNA polymerase will move copy in one direction and the other in the oppositedirection and the other in the opposite Creates a leading and lagging strandCreates a leading and lagging strand

• Leading strand just moves forwardLeading strand just moves forward

• Lagging strand make segments Lagging strand make segments ( Okazaki fragments) and then another ( Okazaki fragments) and then another protein ( DNA ligase) attaches them protein ( DNA ligase) attaches them together to form the final strandtogether to form the final strand

Almost like backstitching Almost like backstitching

Priming DNA synthesisPriming DNA synthesis DNA polymerase can only add nucleotides to an DNA polymerase can only add nucleotides to an

existing polynucleotide that is already pared with existing polynucleotide that is already pared with the complementary strandthe complementary strand DNA polymerase cannot actually initiate DNA polymerase cannot actually initiate

synthesissynthesis Therefore DNA polymerase must start at a primer Therefore DNA polymerase must start at a primer

regionregion Short stretch of RNA( 10 nucleotides)Short stretch of RNA( 10 nucleotides) Primase ( enzyme) is what binds the RNA Primase ( enzyme) is what binds the RNA DNA polymerase III starts here and later another DNA polymerase III starts here and later another

DNA polymerase I goes back and replaces the DNA polymerase I goes back and replaces the RNA nucleotides to complete the DNA strandRNA nucleotides to complete the DNA strand

Figure 16.14 Priming DNA synthesis with RNAFigure 16.14 Priming DNA synthesis with RNA

Figure 16.15 The main proteins of DNA replication and their functionsFigure 16.15 The main proteins of DNA replication and their functions

DNA RepairDNA Repair Mismatch repair of nucleotidesMismatch repair of nucleotides

Repaired by DNA polymeraseRepaired by DNA polymerase Proofreads itself and errors only amount to Proofreads itself and errors only amount to

about 1 in 1 billion nucleotidesabout 1 in 1 billion nucleotides Maintenance repairMaintenance repair

DNA is subjected to many things and damageDNA is subjected to many things and damage This can result in mutations or changes in the This can result in mutations or changes in the

codecode• Repaired by Repaired by nucleasenuclease- cut out damaged - cut out damaged

section of DNA and replace with goodsection of DNA and replace with good• Called Excision repairCalled Excision repair

Telomere ends- non-coding region of DNATelomere ends- non-coding region of DNA Telomerase replaces lost nucleotidesTelomerase replaces lost nucleotides

TELOMERESTELOMERES

Impossible on lagging strand to copy the end of Impossible on lagging strand to copy the end of 5’ strand5’ strand

This leaves a gap that would shorten DNA every This leaves a gap that would shorten DNA every time it replicatestime it replicates

Telomeres (TTAGGG) repeated many times Telomeres (TTAGGG) repeated many times protects genes by postponing erosion of genes protects genes by postponing erosion of genes from this shortening effectfrom this shortening effect

Telomerase – lengthens telomeresTelomerase – lengthens telomeres Contains an RNA sequence that is the Contains an RNA sequence that is the

template for a telomeretemplate for a telomere Present in germ cells (for future gametes)Present in germ cells (for future gametes) Increased activity in cancer cells (allows for Increased activity in cancer cells (allows for

more cell division)more cell division)

Figure 16.19a Telomeres and telomerase: Telomeres of mouse chromosomesFigure 16.19a Telomeres and telomerase: Telomeres of mouse chromosomes