Chapter 12: DNA

12.1 The Role of DNA

• DNA has 3 functions:– Storing information– Copying information– Transmitting information

12.2 The Structure of DNA

• DNA – deoxyribonucleic acid• Nucleotides – monomers that

make up a DNA molecule– 3 parts:

• 5 Carbon sugar (deoxyribose)• Phosphate group• Nitrogenous base

– Adenine (A)– Guanine (G)– Cytosine (C)– Thymine (T)

Solving the Structure of DNA

Edwin Chargaff (1949)Chargaff’s Rule

[A]=[T][C]=[G]

Adenine Thymine Guanine Cytosine35%

45%

• Rosiland Franklin used X-ray diffraction to visualize DNA structure (1952).

• Watson and Crick were the first to model DNA as a double helix in 1953.

The Double Helix Model

• All the previous discoveries show us the structure of DNA and how DNA can function as a carrier of genetic information.

• Characteristics of the double helix model:– Antiparallel strands– Hydrogen Bonding– Base Pairing

Antiparallel StrandsRun in opposite directionsBase PairingBases are held together by hydrogen bonds.Base PairingA bonds with T, and C bonds with G

Complimentary base pairs

hydrogen bond covalent bond

Practice Base Pairing

• If the sequence of bases on one strand of a DNA molecule is TCGAACTGA, the sequence on the other (complimentary) strand is:

12.3 DNA Replication• Why does DNA replicate?

(Hint: Think back to Mitosis)– DNA is replicated in the S phase of

the cell cycle, before the cell divides.

Copying the Code

• Step 1– The DNA double

helix unwinds • DNA helicase

(enzyme).– breaks the H

bonds between the bases.

• The areas where the double helix separates are called replication forks.

• Step 2– DNA polymerase

(enzyme) moves along each DNA strand adding complimentary bases according to the base pairing rules.

– DNA polymerase also proofreads the DNA molecule to reduce errors.

• Step 3– The process

continues until all of the DNA has been copied.

– DNA polymerase detaches.

• This process produces two DNA molecules each composed of one new and one original strand.

• Both DNA molecules produced are identical to each other.

• DNA replication takes place at many points on a eukaryotic chromosome.– Allows replication to happen more

quickly.