History and Structure of DNA

Deoxyribonucleic Acid A double-stranded polymer of nucleotides (each consisting of a deoxyribose sugar, a phosphate, and four nitrogenous bases) that carries the genetic information of an organism.

Bacteriophage Any bacteria infecting virus.

Deoxyribose Sugar Sugar molecule containing five carbons that has lost the –OH (hydroxyl group) on its 2’ (2 prime) carbon

Phosphate Group Group of four oxygen atoms surrounding a central phosphorus atom found in the backbone of DNA

Nitrogenous Bases An alkaline, cyclic molecule containing nitrogen.

Nucleotides Molecules that consist of a five carbon sugar (deoxyribose or ribose) with a nitrogenous base attached to their 1’ carbon and a phosphate group attached to their 5’ carbon.

Glycosyl bond A bond between a sugar and another organic molecule by way of an intervening nitrogen or oxygen atom.

Antiparallel Parallel but running in opposite directions; the 5’ end of one strand of DNA aligns with the 3’ end of the other strand in a double helix.

Complementary Base Pairing Pairing of the nitrogenous base of one strand of DNA with the nitrogenous base of another strand; adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

The History of DNA

Friedrich Miescher A Swiss biochemist who isolated nuclein from the nucleus of white blood cells in The nuclein was a non protein substance and was renamed DNA once the chemical composition of the substance was discovered.

Joachim Hammerling A Danish biologist who was studying green algae in the 1930’s trying to determine the location and source of hereditary material. When he removed the cap from the algae, the cells were able to regenerate new caps. When he removed the feet (where the nucleus was located) the cells were not able to regenerate. He hypothesized that the hereditary material must be contained in the nucleus.

Frederick Griffith Griffith was working in the 1920’s to try and develop a vaccine for pneumonia caused by the bacterium Strepococcus pneumoniae. He discovered the process of transformation through experiments that exposed mice to virulent and non-virulent forms of the pneumonia virus. Virulent pneumonia was able to transform non-virulent pneumonia into virulent pneumonia.

Oswald Avery, Maclyn McCarty and Colin MacLeod These three scientist were working together in New York in 1944 following Griffith’s work with pneumonia. They were intrigued by his discovery of a transforming agent. They were able to conclude that DNA was the transforming agent in Griffith’s experiment. Fun fact! Oswald Avery was born in Halifax, Nova Scotia!

Erwin Chargaff Chargaff revisited Levene’s conclusion of nucleotides in 1949 and discovered that they are not present in equal quantities. Each species had different composition of DNA, but all the same species have the same composition. In any DNA adenine is always equal to thymine and cytosine is always equal to guanine.

Alfred Hershey and Martha Chase Did Radioactive experiments with DNA and protein in 1952; they used phage bacteria in the experiments. They concluded that the DNA from the virus entered the bacteria cell and the virus protein stayed outside the bacteria. They concluded that the DNA transferred the genetic information; it plays a main role in the mechanisms of heredity

THE RACE IS ON!!!

Rosalind Franklin Franklin was with Maurice Wilkins in London using x-ray diffraction to analyse DNA and determine its structure. Franklin was able to produce a clear x-ray diffraction pattern that suggested that a molecule of DNA was helical in nature.

James Watson and Francis Crick Wilkins shared Franklin’s photo with James Watson and Francis Crick. Once they say the photo, they knew they could discover the molecular structure of DNA. They built their now famous model of DNA in 1953 and were awarded the Nobel prize, along with Wilkins in 1962.

2. What are the three main components of DNA? The three main components of DNA are a nitrogen base, a negatively charged phosphate group, and a deoxyribose sugar.

3. What are the four nitrogen bases of DNA nucleotides? DNA nucleotides may contain one of the following nitrogen bases; adenine, thymine, cytosine, and guanine.

4. Draw 4 DNA nucleotides using each of the four nitrogen bases.

5. How are the carbon atoms numbered in the 5 carbon deoxyribose sugar?. Deoxyribose sugar molecules contain 5 carbon atoms and they are numbered clockwise 1 to 5 beginning with the carbon atom to the immediate right of the oxygen atom. The first carbon atom is named 1' (1 prime) and so on.

6. What is the difference between a purine and a pyrimidine? The four nitrogen bases may be classified as either purines or pyrimidines. Adenine and guanines are the purines. They have a double ring structure. Thymine and cytosine are pyrimidines and have a single ring structure. A purine is always paired with a pyrimidine.

7. Use the concept of complementary base pairing to explain how if you know the sequence of one strand of DNA you can determine the opposite or complementary strand. Use an example in your explanation. The concept of complementary base pairing simply means that the purine adenine will always pair with the pyrimidine thymine and the purine guanine will always pair with the pyrimidine cytosine. This can allow you to figure out the opposite strand of DNA if you already know one strand. For example, if the known strand is AGTACTTGA, then by applying the concept of complementary base pairing you know that the opposite strand would be TCATGAACT.

8. What keeps the bases together in the middle of the DNA strand? The nitrogen bases in the middle of the DNA strand are bonded by weak hydrogen bonds. Adenine and thymine are held together by 2 hydrogen bonds. Cytosine and guanine are held together by 3 hydrogen bonds.

9. Fully explain how two DNA strands run anti-parallel. Use an example in your explanation. The two strands of DNA are arranged anti- parallel, that is one strand runs in the 5’ to 3’ direction while the other strand runs 3’ to 5’. The 5’ end of DNA is where the phosphate group is located and the 3’ end is where the hydroxyl group of the deoxyribose sugar is located