Post on 14-Dec-2015
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