1. DNA Structure and Replication 8.2 and 8.3
Chapter 8
DNA Structure and Replication
8.2 and 8.3

2. What is DNA?
Deoxyribonucleic acid (DNA) is a long molecule made up of units called nucleotides.
Nucleotides are made up of:
5-carbon sugar (deoxyribose)
Phosphate group
Nitrogenous base (adenine, guanine, cytosine, or thymine)
All the genetic information for an organism is carried on its DNA.

3. Nucleotides Form DNA
Nucleotide Structure

4. History of DNA
DNA winds around itself, forming a twisted ladder (double helix)
Chargaff discovered the rules of base pairs in 1950
Rosalind Franklin took a picture of DNA in 1952 using X-ray diffraction
Watson and Crick coined the term “Double Helix” in 1953

5. Groups of Bases Purines (two ring) Pyrimidines (one ring) Adenine
Guanine
Pyrimidines (one ring)
Cytosine
Thymine

6. Structure of DNA Cont’d
Chargaff’s Rules:
The four bases (adenine, guanine, thymine, cytosine) pair up in a certain way
A = T
C = G
The bases are held together by hydrogen bonds (hydrogen bonds are weak bonds)

7. A C T G
Chargaff’s Rules G A T C

8. Storage of DNA Prokaryotes – DNA is found in the cytoplasm
Most have single circular DNA molecule
Eukaryotes – Much more DNA than in prokaryotes (up to 1000x as much!)
Condensed into chromosomes
Chromosome number varies wildly from one species to the next. (humans have 46)

10. DNA Replication
Before a cell divides, it must copy its DNA in a process called replication.
During DNA replication:
DNA unwinds using an enzyme called topoisomerase
DNA then unzips into two strands using helicase
DNA polymerase makes complimentary strands using base pairing.
Each strand of the original DNA molecule is used as a template for the new DNA.

11. The area where DNA unwinds is called a replication fork.