Chapter 12: DNA

DNA has 3 functions: Storing information Copying information Transmitting information

12.1 The Role 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)

12.2 The Structure of DNA

Edwin Chargaff (1949)Chargaff’s Rule

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

Solving the Structure of DNA

Adenine

Thymine

Guanine

Cytosine

35%

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.

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

The Double Helix Model

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

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

Practice Base Pairing

Why does DNA replicate?(Hint: Think back to Mitosis) DNA is replicated in the S phase of the cell

cycle, before the cell divides.

12.3 DNA Replication

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.

Copying the Code

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.