DNA STRUCTURE OF DNA NOTES DNA DOUBLE HELIX
DNA
STRUCTURE OF DNA NOTES
DNA DOUBLE HELIX In the 1950’s, James Watson teamed up with Francis Crick to try and determine the structure of DNA. They proposed that DNA is made up of Two Chains that wrap around each other in the shape of a Double Helix, similar to a Spiral Staircase. Watson and Crick relied on other scientists’ work to develop their DNA model. X-ray diffraction photographs were produced by Rosalind Franklin and Maurice Wilkins. Died 2004 Still Alive: 89 Died 2004 Died 1958
DNA NUCLEOTIDES Nucleotides: repeating units of DNA made up of a 5 carbon sugar (deoxyribose), phosphate, and a nitrogen base Deoxyribose: five carbon sugar in a nucleotide Nitrogenous base: base containing nitrogen (adenine, thymine, cytosine, and guanine)
BONDS HOLD DNA TOGETHER The DNA double helix is similar to a spiral staircase. The alternating sugar and phosphates make up “hand rails”. The bases on one strand of DNA face- and form bonds called hydrogen bonds with the bases of the other strand. These base pairs form the “steps” of the stair case.
NITROGENOUS BASES There are 4 different kinds of nitrogenous bases, and are often represented by the first letter of their name: thymine (T), cytosine ( C ), adenine (A), and guanine (G) Purines: bases with a double ring of carbon (adenine and guanine) Pyrimidines: bases with a single ring of carbon (thymine and cytosine)
COMPLEMENTARY BASES In 1949 Erwin Chargaff observed that the percentage of adenine equals the percentage of Thymine and the percentage of cytosine equals the percentage of guanine Base pairing rules: in DNA the nitrogen bases pair up Complementary base pairs: A T and C G
COMPLEMENTARY BASES Because of these base pairing rules the order of bases on one chain of DNA is complementary to the order of the bases on the opposite DNA chain Complementary base pairing is important because hydrogen bonds hold the two strands of DNA together, and it helps explain how DNA replicates before a cell divides.
DNA MODELS DNA models are often simplified and look like straight ladders The Sugar -phosphate handrails are drawn as a straight line so the base pair steps between DNA strands are easier to see.
DNA DOUBLE HELIX: QUESTION CHECK 1. What are the names of 4 scientists who contributed to the discovery of the structure of DNA? James Watson, Francis Crick, Rosalind Franklin, Maurice Wilkins Died 2004 Still Alive: 91 Died 2004 Died 1958
DNA DOUBLE HELIX: QUESTION CHECK 2. What was the structure of DNA that Watson and Crick proposed and was later found to be correct? DNA is made of 2 chains that wrap around each other in the shape of a double helix (spiral staircase)
DNA NUCLEOTIDES: QUESTION CHECK 3. What are the 3 parts of a nucleotide? Phosphate, 5 carbon sugar, Base 4. A five carbon sugar in DNA is called Deoxyribose 5. A phosphate group consists of 1 phosphorous atom and 4 oxygen atoms 6. The nitrogenous bases contain Nitrogen and carbon atoms
BOND HOLD DNA: QUESTION CHECK 7. What type of bond holds together bases? Hydrogen Bonds
BOND HOLD DNA: QUESTION CHECK 8. How is DNA similar to a staircase? The phosphate and sugar side make the “handrails” while the bases and hydrogen bonds make the “Stair steps”
NITROGENOUS BASES: QUESTION CHECK 9. What are the 4 types of Bases? Adenine, thymine, cytosine, guanine 10. Which Bases are called purines? Adenine, guanine 11. Which bases are called pyrimidines? thymine, cytosine
COMPLEMENTARY BASES: QUESTION CHECK 12. What are the base pairs in DNA? Adenine &thymine then cytosine & guanine
COMPLEMENTARY BASES: QUESTION CHECK 13. For what two reasons are complementary base pairs important to DNA? 1. 2. Hydrogen bonds hold base pairs together. Helps DNA to replicate in cell division
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