Griffith – Experimented on mice and observed some harmless strains of bacteria could change into harmful strains. He called this transformation.
Avery – Discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation to the next.
More DNA History
Hershey-Chase – Concluded that the genetic material in bacteria was DNA not proteins
Watson & Crick – created the double helix model for DNA.
Watson & Crick
The Importance of DNA
Structure of DNA
DNA is a long molecule made up of units called nucleotides.
Each nucleotide is made up of three parts: a 5-carbon sugar called dioxyribose, a phosphate group, and a nitorgenous base (Nitrogen Containing).
The backbone of DNA is formed by sugar and phosphate groups of the nucleotide.
The nitrogenous base stick out from the sides and can be joined together in any order, meaning that any sequence of bases is possible.
There are four kinds of nitrogenous bases.
They are divided into two classes: purines and pyrmidines
Purines – Adenine and Guanine
Pyrmidines – Cytosine and Thymine
Chargaff discovered how the nitrogenous bases bond together.
He discovered that Adenine always bonds with Thymine and that Cytosine always bonds with Guanine.
The Genetic Code
Prokaryotes & DNA
In prokaryotes, DNA molecules are located in the cytoplasm of the cell.
Most prokaryotic DNA is a single circular molecule that contains nearly all the cell’s genetic information.
Eukaryotes & DNA
Many eukaryotes have 1000 times as much DNA as prokaryotes.
DNA is located in the nucleus in the form of chromosomes.
Chromosomes are DNA wound tightly around proteins called histones.
E. Coli have about 4,639,221 base pairs. It is about 1.6mm in length. This sounds small until you realize the bacteria is only 1.6 µm in diameter.
Thus DNA must be wrapped tightly to fit into cells. Imagine fitting 900 yards (300m) of rope into a backpack.
During DNA replication, the DNA molecule separates into two strands, then produces two new complimentary strands following the rules of base pairing (Chargaff Rules). Each strand of double helix of DNA serves as a template, or model, for the new strand.
How It Occurs
DNA replication is carried out by a series of enzymes.
The enzymes unzip the DNA molecule creating two strands that serve as templates.
Complimentary bases are added to the strands, for example a strand of DNA with the bases ATTCGAG would have a complimentary strand of TAAGCTC.
Each new DNA molecule has one new stand and one strand from the original molecule.
The enzyme DNA polymerase, the principal enzyme, “proofreads” the new DNA strands, helping to maximize the odds that each molecule is a perfect copy of the original.