7.1 DNA Structure

Assessment Statements:• Describe the structure of DNA.

DNA – deoxyribonucleic acid• DNA nucleotide: deoxyribose (sugar),

phosphate group, nitrogenous base (A, T, G, C)• Double helix of antiparallel strands• Draw and label a DNA nucleotide:

PhosphateBase

Deoxyribose

Number the carbons.

RNA – ribonucleic acid• RNA nucleotide: ribose (sugar), phosphate

group, nitrogenous base (A, U, G, C)• Polymer is a single strand• Draw and label an RNA nucleotide:

Phosphate Base

Ribose

O

Number the carbons.

Watch Chargaff’s Ratio video clip: What is Chargaff’s Rule?• Base-pairing rules:• Adenine always pairs with

Thymine, Guanine always pairs with Cytosine

• *In RNA synthesis, Uracil will pair with Adenine

Purine bases have a double-ring structure; pyrimidine bases have a single-ring structure.

Label each base as a purine or pyrimidine.

How many H-bonds join A to T?How many H-bonds join G to C?

Purines Pyrimidines

If a DNA strand is made up of 20% Adenine nucleotides, what is the percentage of Cytosine nucleotides?

• 20% A, so 20% T because of base-pairing rules (=40%)

• 60% remains, so 30% C and 30% G

Watch Paired DNA Strands video clip:Describe the structure of DNA.• Double helix; two strands are complementary

(if you know sequence of 1 strand, can determine sequence of other)

Write the complementary sequence for the following DNA strand.

A T A T G C C A T T A T A C G G T A

DNA Structure• Sugar-phosphate backbone of each strand is joined with

phosphodiester bonds• Base pairs between strands are joined with hydrogen bonds• Label backbone, phosphodiester bonds, base pairs, and H-

bonds below:

DNA Structure• Sugar-phosphate backbone of each strand is joined with

phosphodiester bonds• Base pairs between strands are joined with hydrogen bonds• Label backbone, phosphodiester bonds, base pairs, and H-

bonds below:

DNA Structure• Sugar-phosphate backbone of each strand is joined with

phosphodiester bonds• Base pairs between strands are joined with hydrogen bonds• Label backbone, phosphodiester bonds, base pairs, and H-

bonds below:

DNA Structure• Sugar-phosphate backbone of each strand is joined with

phosphodiester bonds• Base pairs between strands are joined with hydrogen bonds• Label backbone, phosphodiester bonds, base pairs, and H-

bonds below:

DNA Structure• Sugar-phosphate backbone of each strand is joined with

phosphodiester bonds• Base pairs between strands are joined with hydrogen bonds• Label backbone, phosphodiester bonds, base pairs, and H-

bonds below:

Number 3’ and 5’ carbons.

Which bonds in DNA structure are stronger – phosphodiester or H-bonds?

• Phosphodiester – strong covalent bonds• H-bonds are weak

• Scientists to know: James Watson and Francis Crick solved DNA structure while working at University of Cambridge (England) in 1953

• Watson, Crick and Maurice Wilkins received Nobel Prize in 1962

• Rosalind Franklin’s unpublished DNA X-ray diffraction data provided crucial evidence for their solution

Watch Watson constructing base pairs clip:What “wonderful symmetry” is Watson referring to?• Base-pairing rules, one large and one small nitrogenous base

spanning center of double helix Read Watson and Crick’s original article in Nature. List several

pieces of evidence that helped them solve the structure of DNA.

The structure and function of DNA are clearly connected. What function did Watson and Crick’s structure immediately suggest? Explain.

• DNA replication could occur by separating double helix and using code to synthesize complementary strands