12.2 The Structure of DNA

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1. The building blocks of DNA are

nucleotides

2. Nucleotides in DNA are made of three basic components: a sugar called: , a
, and a nitrogenous .

deoxyribose; phosphate; base

3. DNA contains four kinds of nitrogenous bases:

adenine , thymine ,
guanine , and cytosine .

4. In DNA, can be joined in any order.

nucleotides

5. The nucleotides in DNA are joined by bonds.

covalent

6. Complete the table to describe each scientist's contribution to solving the structure of DNA.
Scientist
Contribution

Rosalind Franklin
Took X-ray diffraction pictures that revealed the double-helix structure of DNA.
Built a model of the DNA molecule that explained both the Crick structure and the properties of DNA.
7. Complete the table by estimating the percentages of each based on Chargaff's rules.
3 33 33 17 17 The Double-Helix Model
For Questions 8-13, on the lines provided, label the parts of the DNA molecule that correspond to the numbers in the diagram.
James Watson and Francis

LOOK AT 12.2 CHART IN NOTES

A 9.
nucleotide
10.nitrogenous base (adenine)
hydrogen bonds
deoxyribose sugar
8. 11.
G
C A
G
G C 13. A T
T
C
phosphate group 12.
base pair (guanine and cytosine)

LOOK AT 12.2 CHART IN NOTES

14. The drawing below shows half of a DNA molecule. Fill in the appropriate letters for the other half. Explain why you drew your sketch the way you did.
CG
AT
GC
GC
CG
CG
TA AT
CG The bases link with hydrogen bonds in pairs across the "rungs" of the
ladder: A with T and G with C.

LOOK AT 12.2 CHART IN NOTES

15. Complete this table to show how the structure of the DNA molecule allows it to perform each essential function.
Key
A = Adenine C = Cytosine G = Guanine T = Thymine
Function
Structure of the Molecule
Store information
Each strand of the double helix carries a sequence of bases, arranged something like letters in a four-letter alphabet.
Copy information
The base pairs can be copied when hydrogen bonds break and the strands pull apart.
Transmit information
When DNA is copied, the sequence of base pairs is copied, so genetic information can pass unchanged from one generation to the next.

LOOK AT 12.2 CHART IN NOTES

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