DNAInformation and Heredity,

Cellular Basis of LifeChapter 12(M)

Genes are made of DNA

The following scientists proved that

DNA is the genetic material Fredrick Griffith (1928) Oswald Avery ( 1944) Hershey and Chase (1952)

Fredrick Griffith (1928) The discovery of the genetic role of

DNA began with research by Frederick Griffith.

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

pathogenic.

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

He recovered the pathogenic strain from the dead mouse’s blood.

Some harmless bacteria had been “transformed” into the deadly strain

For the next 14 years scientists tried to identify the transforming substance.

Transforming Substance? Could be:

DNAProteins

Because scientists already knew chromosomes consist of these substances. So the debate started.

Oswald Avery ( 1944) Treated Griffiths mixture with

Protein digesting enzymes remove all proteins

DNA digesting enzymes remove all DNA

Avery contd. Is Protein the transforming factor?

treated Griffith’s mixture of heat treated deadly strain and live harmless strains with protein-destroying enzymes grew the strains

The bacterial colonies were still transformed

Concluded that protein could not be the transforming factor

Avery contd. Is DNA the transforming factor?

treated the mixture with DNA-destroying enzymes grew the strains

The bacterial colonies failed to transform

Concluded that DNA is the genetic material of the cell

Scientists were still skeptical proteins made of 20 AAs, DNA only 4 bases

Hershey and Chase (1952) Used viruses to prove that DNA is the genetic material.Viruses consist of a DNA (sometimes

RNA) enclosed by a protective coat of protein.

To replicate, a virus infects a host cell and takes over the cell’s metabolic machinery.

Viruses that specifically attack bacteria are called bacteriophages or just phages.

Conclusion

Phage DNA entered the bacterial cell, proteins did not

DNA carries the genetic information.

The Role of DNA

Storing Information The genetic material stores information needed by every living cell

Copying Information before a cell divides this info must be copied

Transmitting Information Each daughter cell must receive a complete copy of all the information

Structure of DNACh 12.2

Building Blocks Of DNANucleotides A ring-shaped sugar

called deoxyribose A phosphate group (a

phosphorus atom surrounded by four oxygen atoms)

A nitrogenous base ("nitrogen-containing") : a single or double ring of carbon and nitrogen atoms with functional groups

Nitrogenous Bases

The four nucleotides in DNA differ only in their nitrogenous bases

Bases:Thymine (T) single ringCytosine (C) single ringAdenine (A) double ringGuanine (G) double ring

Bases

Structure of DNA Early 1950s R. Franklin

DNA helix with2 strands with the phosphate linked to the sugar, diameter is 2nm, each turn has 10 bases

Watson & Crick (1953)Using Franklins work built a model of DNA

Each strand is complementary to the other

A pairs with T, G with C

Base Pairing Chargaff’s Rule

Adenine forms a base pair with Thymine

Guanine forms a base pair with Cytosine

Amounts are about the same

The Double Helix

Figure 16.5 The double helix

DNA Strands Two strands double helix Compared to a ladder Sides Sugar phosphate

backbone Rungs pairs of nitrogenous

bases Order of bases genetic code Difference in order gives

individuality to each living organism

Base pairing is the key that allows DNA to be copied

A pairs with T, G with C

Replication of DNA

Ch 12.3

ReplicationComplete set of genetic

instructions passes from one generation to the next

The DNA molecule must be copiedBase pairing allows DNA to be copied

DNA 2 strands, one is used as a template

Replication Process by which DNA is copied

Mechanism of Replication

Takes place in the nucleus DNA untwists & both strands are

replicated almost simultaneously 50 bases /sec are added The parent DNA strands serve as

a template for making a new strand

DNA polymerase unzips the 2 strands

Each strand is used as a template

Free nucleotides present in the nucleus, pair with the exposed bases

A with T, G with C As bases pair DNA Ligase an

enzyme links the phosphate of each nucleotide to the sugar of the previous one

Pairing & bonding continue till 2 new strands are formed.

Enzyme works from 5’3’ Synthesis is anti parallel 5’3’

and 3’5’ As bases pair DNA Ligase an

enzyme links the phosphate of each nucleotide to the sugar of the previous one

Pairing & bonding continue till 2 new strands are formed.

DNA Replication is Semi conservative