DNA: The Blueprint of Life

DNA & Scientists

Griffith and Transformation In 1928, British scientist Fredrick Griffith was trying to learn how certain types of bacteria caused pneumonia. He isolated two different strains of pneumonia bacteria from mice and grew them in his lab. Griffith made two observations: (1) The disease-causing strain of bacteria grew into smooth colonies on culture plates. (2) The harmless strain grew into colonies with rough edges.

Griffith's Experiments Griffith set up four individual experiments. Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another. Experiment 1: Mice were injected with the disease-causing strain of bacteria. The mice developed pneumonia and died.

Experiment 2: Mice were injected with the harmless strain of bacteria. These mice didn’t get sick. Harmless bacteria (rough colonies) Lives

Experiment 3: Griffith heated the disease-causing bacteria. He then injected the heat-killed bacteria into the mice. The mice survived. Heat-killed disease- causing bacteria (smooth colonies) Lives

Experiment 4: Griffith mixed his heat-killed, disease-causing bacteria with live, harmless bacteria and injected the mixture into the mice. The mice developed pneumonia and died. Live disease-causing bacteria (smooth colonies) Dies of pneumonia Heat-killed disease- causing bacteria (smooth colonies) Harmless bacteria (rough colonies)

Griffith concluded that the heat-killed bacteria passed their disease- causing ability to the harmless strain. Live disease-causing bacteria (smooth colonies) Heat-killed disease- causing bacteria (smooth colonies) Harmless bacteria (rough colonies) Dies of pneumonia

Transformation Griffith called this process transformation because one strain of bacteria (the harmless strain) had changed permanently into another (the disease-causing strain). Griffith hypothesized that a factor must contain information that could change harmless bacteria into disease-causing ones.

Avery and DNA Oswald Avery repeated Griffith’s work to determine which molecule was most important for transformation. Avery and his colleagues made an extract from the heat- killed bacteria that they treated with enzymes. The enzymes destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA. Transformation still occurred.

Avery and other scientists repeated the experiment using enzymes that would break down DNA. When DNA was destroyed, transformation did not occur. Therefore, they concluded that DNA was the transforming factor. Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next.

Chargaff's Rules Erwin Chargaff discovered that: The percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA. The percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA.

X-Ray Evidence This X-ray diffraction photograph of DNA was taken by Rosalind Franklin in the early 1950s. The X-shaped pattern in the center indicates that the structure of DNA is helical. Rosalind Franklin used X-ray diffraction to get information about the structure of DNA. She aimed an X-ray beam at concentrated DNA samples and recorded the scattering pattern of the X-rays on film.

The Double Helix Using clues from Franklin’s pattern, James Watson and Francis Crick built a model that explained how DNA carried information and could be copied. Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other.

What is DNA ?

Deoxyribonucleic acid, DNA Double-stranded molecule that carries information that determines an organism’s traits. Composed of a four-letter nucleotide/molecule alphabet referred to as A, T, C, and G. Order of the alphabet determines the characteristics of the living organism, much like the order of letters in our alphabet determines the words. Each cell in the human body contains >3 BILLION letters. The only difference between living organisms is the amount and order of the DNA alphabet.

DNA is like a fingerprint, in that it is unique to each individual and can be used to identify them, even though it is not visible to the naked eye.

Parts of DNA DNA is made up of nucleotides, building blocks of DNA. A nucleotide is a monomer of nucleic acids made up of: Deoxyribose – 5-carbon Sugar Phosphate Group Nitrogenous Base DNA molecule looks like a twisted ladder - shaped like a double helix.

The Double Helix In the “ladder,” bases – nitrogen-containing compounds are arranged in pairs. The sides of the ladder are composed of sugar molecule and phosphate group. Each of the building blocks of DNA is called a nucleotide.

The Bases There are four kinds of bases in DNA: Adenine (A) Guanine (G) Cytosine (C) Thymine (T) These bases can combine in billions of different ways, but they always pair this way: Hydrogen bonds form and hold these bases. This principle is called base pairing.

Where do you find DNA?

Prokaryotic Cells In prokaryotic cells, DNA is located in the cytoplasm. Most prokaryotes have a single DNA molecule containing nearly all of the cell’s genetic information.

Chromosome Supercoils Nucleosome DNA double helix Histones Coils Eukaryotic Chromosome Structure Eukaryotic DNA is located in the cell nucleus inside chromosomes.

DNA Replication?

DNA Replication When a cell divides, it makes new cells with exactly the same genetic information as the original cell. DNA cannot divide in two, instead the chromosomes must be copied. The process is called replication. Replication ensures that each new cell has the same genetic information as the original cell. Replication ensures that each resulting cell will have a complete set of DNA. Replication proceeds in both directions until each chromosome is completely copied.

DNA Replication (Cont.) During replication, the DNA molecule “unzips,” exposing each half of the base pairs. Free bases in the cytoplasm pairs up with the exposed bases on the each half of the DNA strand. The result is the formation of two strands of DNA identical to the original.

DNA FACT

FACT: You have about 9 million kilometers of DNA. That's enough to reach to the moon and back 13 times!

Strands of DNA are so tiny, you could fit about 5 million strands through the eye of a needle. FACT:

Each cell in your body contains 3 billion letters in the DNA: they would fill a stack of paperback books 200 ft high. FACT:

Each cell contains 9 feet of DNA. In an average meal, you eat approximately 55,000,000 cells or about 93,205 miles of DNA.