University:
California State University, Northridge
Course:
BIOL 360 | Genetics
Academic year:

2007
Views:

165

Pages:

19
Author:

banenetta0l

DNA: The Genetic Material Chapter 10 DNA as the Genetic Material • DNA was first extracted from nuclei in 1870 • named ‘nuclein’ after their source. • Chemical analysis – determined that DNA was a weak acid rich in phosphorous. • Its name provides a lot of information about DNA: – – – – deoxyribose nucleic acid: it contains a sugar moiety (deoxyribose), it is weakly acidic, and is found in the nucleus. • Because of its: – nuclear localization – subsequent identification as a component of chromosomes – it was implicated as a carrier of genetic information. 2 Are genes composed of DNA or protein? • Chromosomes are also known to contain protein – so early on it was a challenge to demonstrate that DNA was indeed the molecule that contained the genetic information. • DNA – Only four different subunits make up DNA – Chromosomes contain less DNA than protein by weight • Protein – 20 different subunits – greater potential variety of combinations – Chromosomes contain more protein than DNA by weight • Classical experimental data confirmed DNA as the genetic material. 3 Bacterial transformation implicates DNA as the substance of genes • 1928 – Frederick Griffith – experiments with smooth (S), virulent strain Streptococcus pneumoniae, and rough (R), nonvirulent strain – Bacterial transformation demonstrates transfer of genetic material • 1944 – Oswald Avery, Colin MacLeod, and MacIyn McCarty – – determined that DNA is the transformation material 4 Griffith experiment: transformation of bacteria Fig. 6.3 b 5 Griffith’s Experiment • Griffith observed that live S bacteria could kill mice injected with them. • When he heat killed the S variants and mixed them with live R variants, and then injected the mixture in the mice, they died. • Griffith was able to isolate the bacteria from the dead mice, and found them to be of the S variety. • Thus the bacteria had been Transformed from the rough to the smooth version. • The ability of a substance to change the genetic characteristics of an organism is known as transformation. • Scientists set out to isolate this ‘transforming principle’ since they were convinced it was the carrier of the genetic information. 6 Avery, MacLeod, McCarty Experiment: Identity of the Transforming Principle 7 Hershey and Chase experiment • 1952 – Alfred Hershey and Martha Chase provide convincing evidence that DNA is genetic material • Waring blender experiment using T2 bacteriophage and bacteria • Radioactive labels 32P for DNA and 35S for protein 8 Hersey-Chase Experiment 9 Hershey and Chase experiment • Performed in 1952, using bacteriophage, a type of virus that have a very simple structure: – an outer core and an inner component. • The phage are made up of equal parts of protein and DNA. • It was known that the phage infect by anchoring the outer shell to the cell surface and then deposit the inner components to the cell, infecting it. • Scientists were interested in finding out whether it was the protein component or the DNA component that got deposited inside the infected cell. • By incorporating radiolabel either in the protein or the DNA of the infecting phage, – they determined that the DNA was indeed introduced into the infected bacteria, causing proliferation of new phage. 10 The Watson-Crick Model: DNA is a double helix • 1951 – James Watson learns about x-ray diffraction pattern projected by DNA • Knowledge of the chemical structure of nucleotides – deoxyribose sugar, phosphate, and nitrogenous base • Erwin Chargaff’s experiments demonstrate that – ratio of A and T are 1:1 – ratio of G and C are 1:1 • 1953 – James Watson and Francis crick propose their double helix model of DNA structure 11 X-ray diffraction patterns produced by DNA fibers – Rosalind Franklin and Maurice Wilkins 12 Fig. 6.6 13 14 DNA is polar: 5’ to 3’ Beta N-glycosidic bond connects the sugar to the nitrogen base. A phosphodiester bond connects one nucleotide to the next. The sugar phosphate backbone is identical in every DNA molecule. In any DNA purine = pyrimidine 15 Watson-Crick Model • Double helical structure. • The two strands are antiparallel • It is a right handed helix, this structure is called the B DNA. • Complementary base pairing: – three hydrogen bonds between C and G; – two hydrogen bonds between A and T. • The arrangement of the nitrogen bases determines the genetic message. • At each position, there are 4 possibilities, – therefore for a 100 base pair long molecule of DNA, – there are 4100 variations possible. 16 •Complementary base pairing: – three hydrogen bonds between C and G; –two hydrogen bonds between A and T. 17 Chargaff’s Rules • Ratios of nucleotides • A=T and G=C • A+T does not have to equal G+C 18 19

DNA: The Genetic Material

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