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Write a short essay explaining the evidence that dna is the genetic material

The genetic material The physical nature of the gene fascinated scientists for many years. A series of experiments beginning in the 1920s finally revealed that DNA was the genetic material.

Discovery of transformation A puzzling observation was made by Frederick Griffith in the course of experiments on the bacterium Streptococcus pneumoniae in 1928. This bacterium, which causes pneumonia in humans, is normally lethal in mice.

However, different strains of this bacterial species have evolved that differ in virulence in the ability to cause disease or death. In his experiments, Griffith used two strains that are distinguishable by the appearance of their colonies when grown in laboratory cultures.

In one straina normal virulent type, the cells are enclosed in a polysaccharide capsule, giving colonies a smooth appearance; hence, this strain is labeled S. Griffith killed some virulent cells by boiling them and injected the heat-killed cells into mice. The mice survived, showing that the carcasses of the cells do not cause death. However, mice injected with a mixture of heat-killed virulent cells and live nonvirulent cells did die.

Furthermore, live cells could be recovered from the dead mice; these cells gave smooth colonies and were virulent on subsequent injection.

  1. Humans have 22 pairs of autosomes and one pair of sex chromosomes.
  2. Then infected different bacteria for short time, vortexed in blender to separate phage coats from cells, and separated phage very small from cells larger by centrifugation.
  3. Correcting damage due to enviromental effects occurs side-by-side on one strand of DNA, screws up the ability of this bit of DNA to serve as template for replication or for correct reading of proteins. Therefore only DNA, not protein, was responsible for inheritance.
  4. The Okazaki fragments are united by a special ligase enzyme.
  5. Single-stranded DNA binding proteins.

Somehow, the cell debris of the boiled S cells had converted the live R cells into live S cells. The process is called transformation. Figure 8-1 The first demonstration of bacterial transformation.

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This same basic technique was then used to determine the nature of the transforming principle—the agent in the cell debris that is specifically responsible for transformation.

In 1944, Oswald Avery, C. McCarty separated the classes of molecules found in the debris of the dead S cells and tested them for transforming ability, one at a time. These tests showed that the polysaccharides themselves do not transform the rough cells.

Proof that DNA is Genetic Material

Therefore, the polysaccharide coat, although undoubtedly concerned with the pathogenic reaction, is only the phenotypic expression of virulence. In screening the different groups, Avery and his colleagues found that only one class of molecules, DNAinduced the transformation of R cells Figure 8-2.

They deduced that DNA is the agent that determines the polysaccharide character and hence the pathogenic character see pages 219—220 for a description of the mechanism of transformation. Demonstration that DNA is the transforming agent.

Chromosome

DNA is the only agent that produces smooth S colonies when added to live rough R cells. Hershey-Chase experiment The experiments conducted by Avery and his colleagues were definitive, but many scientists were very reluctant to accept DNA rather than proteins as the genetic material. The clincher was provided in 1952 by Alfred Hershey and Martha Chase with the use of the phage virus T2.

They reasoned that phage infection must entail the introduction injection into the bacterium of the specific information that dictates viral reproduction. The phage is relatively simple in molecular constitution.

Discovery of transformation

Hershey and Chase incorporated the radioisotope of phosphorus 32P into phage DNA and that of sulfur 35S into the proteins of a separate phage culture. They then used each phage culture independently to infect E. After sufficient time for injection to take place, they sheared the empty phage carcasses called ghosts off the bacterial cells by agitation in a kitchen blender. They used centrifugation to separate the bacterial cells from the phage ghosts and then measured the radioactivity in the two fractions.

The conclusion is inescapable: DNA is the hereditary material; the phage proteins are mere structural packaging that is discarded after delivering the viral DNA to the bacterial cell. The experiment uses two sets of T2 bacteriophages.

  1. How could such information be passed on from one generation to the next? DNA polymerase III cannot start a growing chain from scratch; needs a short primer a few nucleotides to add to.
  2. However, mice injected with a mixture of heat-killed virulent cells and live nonvirulent cells did die.
  3. These units condense into a chromatin fibre, which condenses further to form a chromosome. The structure of helicase has now been revealed.
  4. Cells often spend much more energy repairing DNA than synthesizing it. Correcting damage due to enviromental effects occurs side-by-side on one strand of DNA, screws up the ability of this bit of DNA to serve as template for replication or for correct reading of proteins.

In one set, the protein coat is labeled with radioactive sulfur 35Snot found in DNA. Why such reluctance to accept this conclusion?

DNA was thought to be a rather simple chemical.

How could such information be passed on from one generation to the next? Clearly, the genetic material must have both the ability to encode specific information and the capacity to duplicate that information precisely. What kind of structure could allow such complex functions in so simple a molecule? By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.