Ch' 9 : DNA : The Genetic Material

1
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.

2
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.

3
Averys Experiment

• After Griffiths 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.

4
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.

5
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

6
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

7
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

8
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.

9
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

10
DNA Structure

• The nucleotides of the DNA double helix are
  bonded together by hydrogen bonds.

11
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.

12
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.

13
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)

14
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.
• Its estimated that there is one error in 1
  billion nucleotides.

15
DNA Replication

• The end result of DNA replication is two
  identical new strands of DNA with the same
  sequence of nitrogen bases.

16
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.

17
DNA Replication Animations

• http//www.johnkyrk.com/DNAreplication.html
• http//www.ncc.gmu.edu/dna/repanim.htm
• http//bioteach.ubc.ca/TeachingResources/Molecular
  Biology/DNAReplication.swf
• http//science.nhmccd.edu/biol/bio1int.htmdna