Ch. 9 : DNA : The Genetic Material. Chromosomes are composed of two chromatids , which have segments called alleles , and are made up of chromatin . This chromatin is composed of strands of DNA. Alleles and genes are a sequence of DNA which code for a certain “trait”.
Ch. 9 : DNA : The Genetic Material • Chromosomes are composed of two chromatids, which have segments called alleles, and are made up of chromatin. This chromatin is composed of strands of DNA. • Alleles and genes are a sequence of DNA which code for a certain “trait”.
Identifying the Genetic Material • A series of experiments led to the discovery of DNA as the genetic material of life. • In 1928 Frederick Griffith performed an experiment on mice. While he was studying the bacteria which causes pneumonia, and a possible vaccine for this he found that the genetic material of a cell is inside the cell.
Avery’s Experiment • After Griffith’s experiment it was unclear if DNA or proteins where the genetic material of life. • In 1944 Oswald Avery and his co-workers were able to produce protein “killing” enzymes and DNA “killing” enzymes. When the DNA was destroyed no genetic material was able to be recovered.
Hershey-Chase Experiment • In 1952 Alfred Hershey and Martha Chase performed an experiment which “proved” that DNA is the genetic material of life. • They used sulfur and phosphorous to trace what was passed on from bacteria cells to new bacteria cells.
DNA Structure • After the Hershey-Chase experiment exposed DNA as the genetic material, the race was on to identify the shape of this material. • In 1953 James Watson and Francis Crick proposed that DNA is in the shape of a double helix. (spiral staircase) • This shape has been supported over the past 50 years, and accepted as the shape and structure of DNA
DNA Structure • DNA is now understood to have sugar phosphate backbones, with nitrogen bases bonded between the backbones. • The nitrogen bases are adenine(A), guanine (G), cytosine(C),and thymine (T) • Adenine and guanine are double rings and called the purines. • Cytosine and thymine are single rings and called pyrimidines
DNA Structure • DNA contains two backbones with base paired nitrogenous bases in between the sugar/phosphate backbones. • The nitrogenous bases are bonded to a complimentary base. • A-T, C-G
The Structure of DNA • DNA is deoxyribonucleic acid. • The molecule is in the shape of a double helix. (spiral staircase). • The molecule has a sugar (deoxyribose) and phosphate backbone, or sides of the helix, with nitrogen bases (2) inside of this outside structure.
DNA Structure • The DNA can be thought of as two parts or sides of a ladder. One of these parts – a sugar, phosphate and one nitrogen base is called a “nucleotide”
DNA Structure • The nucleotides of the DNA double helix are bonded together by hydrogen bonds.
Nitrogen Bases • There are four different nitrogen bases found on a DNA molecule. These are cytosine (C) , guanine (G), thymine (T) and adenine (A). • Adenine and guanine are double ring bases called the purines. • Cytosine and thymine are single ring bases known as the pyrimidines.
DNA Replication • During late interphase of the cell cycle the DNA must copy itself in preparation for mitosis. For DNA to be copied the double helix must “unzip” and an identical copy of nitrogen bases must be produced. • This process of making an identical strand of DNA is called DNA replication.
DNA Replication • DNA replication involves the following steps: • 1) an enzyme(DNA helicases) breaks the hydrogen bonds to unzip the DNA. • 2) “free” nucleotides float in and attach to their matching base pair with the aid of DNA polymerases(enzymes)
DNA Replication • In DNA replication errors can occur in the base pairs. • DNA polymerases proofread the code and can stop the process and backtrack to fix the error. • It’s estimated that there is one error in 1 billion nucleotides.
DNA Replication • The end result of DNA replication is two “identical” new strands of DNA with the same sequence of nitrogen bases.
DNA Replication Rate • DNA replication occurs not from one end to the other end. (This would take 33 days) • Replication occurs simultaneously at different areas, and put together as a whole. • These units are known as replication forks.
DNA Replication Animations • http://www.johnkyrk.com/DNAreplication.html http://www.ncc.gmu.edu/dna/repanim.htm http://bioteach.ubc.ca/TeachingResources/MolecularBiology/DNAReplication.swf http://science.nhmccd.edu/biol/bio1int.htm#dna