8.2 Structure of DNA
Bell ringer:
• Have your homework on the side of your desk!
• 1-What was “transformed” in Griffith’s experiment?
• 2- How did Avery and his group identify the transforming principal?
• 3-What reason did scientist give for not accepting Avery’s conclusion that the genetic material was DNA?
• 4-How did Hersey and Chase confirm that DNA is the genetic material?

8.2 Structure of DNA
KEY CONCEPT
DNA structure is the same in all organisms.

8.2 Structure of DNA
DNA is composed of four types of nucleotides.
• DNA is made up of a long chain of nucleotides.
• Each nucleotide has three parts.
– a phosphate group
– a deoxyribose sugar
– a nitrogen-containing base phosphate group nitrogen-containing base deoxyribose (sugar)

8.2 Structure of DNA
• The nitrogen containing bases are the only difference in the four nucleotides.

8.2 Structure of DNA
Pyrimidines = single rings
• -thymine and cytosine

8.2 Structure of DNA
Purines= double ring
-Adenine and Guanine

8.2 Structure of DNA
Watson and Crick determined the three-dimensional structure of DNA by building models.
• They realized that DNA is a double helix that is made up of a sugarphosphate backbone on the outside with bases on the inside.

8.2 Structure of DNA
• Watson and Crick’s discovery built on the work of Rosalind
Franklin and Erwin Chargaff.
– Franklin’s x-ray images suggested that DNA was a double helix of even width.
– Chargaff’s rules stated that A=T and C=G.
– (# of A= # of T and # C = #G)

8.2 Structure of DNA
FRANKLIN:
• Produced x-ray photographs of DNA that indicated it was a helix.
• Her co-worker Wilkins showed her information to Watson and Crick (who were still studying proteins)
• Watson, Crick and Wilkins where awarded the Nobel prize in 1962 (no mention of Franklin’s contribution was made)
• Franklin died of cancer due to her exposure to the x-rays.

8.2 Structure of DNA
Nucleotides always pair in the same way.
• The base-pairing rules show how nucleotides always pair up in DNA.
– A pairs with T
– C pairs with G
• Because a pyrimidine
(single ring) pairs with a purine (double ring), the helix has a uniform width.
G
A
C
T

8.2 Structure of DNA
The two strands of DNA are complementary
(fit together and are opposite of one another)
They twist into a double helix

8.2 Structure of DNA
• The backbone is connected by covalent bonds.
• The bases are connected by hydrogen bonds.
hydrogen bond covalent bond

8.2 Structure of DNA
Individually the H bonds are weak, but all together they maintain the structure hydrogen bond covalent bond

8.2 Structure of DNA
Question:
• What sequence of bases would pair with the sequence
TGACTA?

8.2 Structure of DNA
Question
• Which base pairs do you think are help more tightly together? hydrogen bond covalent bond