A brief introduction to DNA

A brief introduction to: DNA (deoxyribonucleic acid)
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In 1928, British scientist Fredrick Griffith was trying to learn about how certain types of bacteria produce a serious lung disease known as pneumonia. He isolated two slightly different types of pneumonia bacteria from mice.

In the lab…
disease strain – grew into smooth colonies on the culture plates
harmless strain – produced colonies with rough edges
(easy to distinguish due to different appearances)
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This led to biologists realizing that genetic information could be transformed from one bacterium to another.
In 1944, a group of scientists led by Canadian biologist Oswald Avery at the Rockefeller Institute in New York decided to repeat Griffith’s work. They did this with the intention of finding what it is that causes the transformation.
they took the heat-killed bacteria and destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA. Transformation still occurred. When they then destroyed the DNA, transformation did not occur.
Through this experiment they discovered that the nucleic acid DNA stores and transmits genetic information. 
In 1952, two American scientists, Alfred Hershey and Martha Chase collaborated in studying viruses (nonliving particles smaller than a cell that can infect living organisms). One type of virus that infects bacteria is a “bacteriophage” (“bacteria-eater”). Bacteriophages are composed of DNA or RNA inside of a protein body. When a bacteriophage enters a bacteria, the virus attaches to the surface of the cell and injects its bacteria.
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[T2 bacteriophage (tan) invading an E. coli cell (green)].
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Radioactive markers helped the scientists determine which part of the virus entered the infected cell which led them to discover that genes were made of DNA, not protein.
Because proteins contain very little phosphorus and DNA contains no sulfur, the scientists grew viruses in cultures containing radioactive isotopes of phosphorus-32 and sulfur-35. The radioactive substances were used as markers.
* If sulfur-35 was found in the bacteria, then the viruses protein was injected into the bacteria.
* If phosphorus-32 was found in the bacteria, then it was the DNA that had been injected into the bacteria.
The Hershey-Chase experiment concluded that the genetic material of the bacteriophage was DNA!
After this, scientists wanted to know more!
1) carry information from one generation to the next.
2) they determine the heritable characteristics of organisms
3) all of a cell’s genetic information is replicated every time a cell divides (mitosis).
DNA (deoxyribonucleic acid)
DNA is a long molecule made of nucleotides.
Each nucleotide is made up of three basic components:
* a 5-carbon sugar called deoxyribose
* a phosphate group
* a nitrogenous (nitrogen containing) base >>>> there are 4 kinds of nitrogenous bases in DNA: adenine, thymine, guanine. and cytosine.
The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide.
Scientists concluded that the four different nucleotides could be strung together in many different ways, so it was possible they could carry coded genetic information.
***  Years earlier, an America biochemist named Erwin Chargaff discovered that the percentages of nucleotides are equal in any sample of DNA.
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  A British scientist named Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule. Aiming a powerful X-ray beam at concentrated DNA samples, she recorded the scattering pattern of the X-rays onto film.
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*** When an American Biologist named James Watson was shown a copy of Franklin’s X-ray pattern in 1953, he, along with British physicist Francis Crick, built a structural model that explained how DNA could carry information and how it could be copied. The published their results in April 1953.
Watson and Crick’s model of DNA was a double helix. (two strands are wrapped around each other)
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The Double Helix
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A double helix is shaped like a spiral staircase. Watson and Crick then discovered that hydrogen bonds could form between certain nitrogenous bases and provide enough force to hold the two strands together. Nitrogenous based can only form between adenine and thymine, and guanine and cytosine. This principle is called base pairing. This brought Chargaff’s rules full circle. For every adenine there had to be one thymine. For each cytosine molecule, there was one guanine molecule.
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