DNA: the Molecular DNA: the Molecular Basis of InheritanceBasis of Inheritance

Chapter 11Chapter 11

DNA: the genetic materialDNA: the genetic material

In the 1860’s Mendel verified that In the 1860’s Mendel verified that traits were inheritable components traits were inheritable components passed from parent to offspring. passed from parent to offspring.

These traits seemed to obey certain These traits seemed to obey certain laws. laws. What were the laws that What were the laws that inheritable traits obeyed?inheritable traits obeyed?

By the 1920’s it had been established By the 1920’s it had been established that chromosomes were the basis of that chromosomes were the basis of inheritance. Scientists understood it inheritance. Scientists understood it was something in chromosomes that was something in chromosomes that caused the multitudinous variety of caused the multitudinous variety of organism present on earth. organism present on earth. What What are the two components of are the two components of chromosomes? What is the chromosomes? What is the structure of chromosomes?structure of chromosomes?

DNA: the genetic materialDNA: the genetic material

The structure The structure of of

chromosomechromosomes:s:

The Griffith Experiment:The Griffith Experiment:

Showed that DNA was the material Showed that DNA was the material that transferred inheritable traitsthat transferred inheritable traits

Was a “lucky” findWas a “lucky” find Griffith’s experiment with Griffith’s experiment with

Streptococcus Streptococcus pneumoniaepneumoniae His experiment was confirmed by His experiment was confirmed by

Oswald Avery in 1944Oswald Avery in 1944 What was the significance of What was the significance of

these two experiments?these two experiments?

DNA or Protein?DNA or Protein?

Why would protein be a good Why would protein be a good candidate for passing on of hereditary candidate for passing on of hereditary material?material?

Why was DNA not thought to be a Why was DNA not thought to be a good candidate?good candidate?

How was DNA shown to be the real How was DNA shown to be the real “power” behind inheritance once and “power” behind inheritance once and for all?for all?

Hershey and Chase experimentHershey and Chase experiment

Hershey-Hershey-Chase Chase

experiment:experiment:

Watson and Crick and Chargaff Watson and Crick and Chargaff and Franklin and Wilkins and…and Franklin and Wilkins and…

Using the information gathered by Using the information gathered by Erwin Chargaff, Maurice Wilkins and Erwin Chargaff, Maurice Wilkins and Rosalind Franklin, Francis Crick and Rosalind Franklin, Francis Crick and James Watson were able to deduce James Watson were able to deduce the structure of DNA: a double helix the structure of DNA: a double helix surrounding paired nitrogen basessurrounding paired nitrogen bases

Back ground and explanation of DNA Back ground and explanation of DNA structurestructure

The nature of DNA The nature of DNA replicationreplication

““It has not escaped our notice It has not escaped our notice that the specific pairing we that the specific pairing we have postulated immediately have postulated immediately suggests a possible copying suggests a possible copying mechanism for the genetic mechanism for the genetic material.”material.” - - J.D. Watson & F.H.C. J.D. Watson & F.H.C. Crick in “Molecular Structure of Nucleic Crick in “Molecular Structure of Nucleic Acids: A Structure for Deoxynucleic Acids: A Structure for Deoxynucleic acids”acids”

How does DNA replicate?How does DNA replicate?

The two DNA strands by being The two DNA strands by being complementary to one anothercomplementary to one another serve serve as a template for new strands as a template for new strands ((semiconservative model)semiconservative model)

Other models of replication had to be Other models of replication had to be rejected (rejected (conservative, and conservative, and dispersive)dispersive)

MeselsonMeselson and Stahl’s experiment and Stahl’s experiment

The enzymes of replicationThe enzymes of replication

HelicaseHelicase PrimasePrimase DNA polymerase IDNA polymerase I DNA polynerase IIIDNA polynerase III DNA ligaseDNA ligase Single strand binding proteinSingle strand binding protein DNA topoisomeraseDNA topoisomerase Telomerase Telomerase

Telomeres Telomeres

Repetitive strands at the end of Repetitive strands at the end of chromosomes chromosomes

Human sequence is TTAGGG Human sequence is TTAGGG repeated about 2500 timesrepeated about 2500 times

Per replication each human loses Per replication each human loses about 50-200 base pairsabout 50-200 base pairs

Cells die after about 20-30 divisionsCells die after about 20-30 divisions Telomerase regenerates telomeresTelomerase regenerates telomeres

DNA proof reading and DNA proof reading and repairrepair

1 base pair in 101 base pair in 1066 is mismatched is mismatched Result would be 1000 mutations per Result would be 1000 mutations per

cell divisioncell division Mechanisms are in place to proofread Mechanisms are in place to proofread

and repair DNAand repair DNA

DNA proofreadingDNA proofreading

DNA polymerase “reads” DNA strand DNA polymerase “reads” DNA strand and finds mismatched bases.and finds mismatched bases.

The mismatched base is excised and The mismatched base is excised and replacedreplaced

Lowers the error rate to one in 10Lowers the error rate to one in 101010

How does the DNA polymerase How does the DNA polymerase “know” which strand is faulty?“know” which strand is faulty?