Chapter 13
packet: DNA
and Protein
Synthesis Part I
•Genes – the instructions for inherited
traits
•Genes are made of DNA, short for
Deoxyribonucleic acid
Discovery of the structure of DNA
•DNA is in the shape of a double helix –
discovered by Franklin & Wilkins through X-ray
diffraction of DNA (a)
•1953 - Watson & Crick used above information
to construct 1st model of DNA (b)
Structure of DNA
• DNA is a made of nucleotides
• Nucleotides are composed of:
• Phosphate
• 5-carbon sugar (deoxyribose)
•1 of 4 nitrogencontaining bases adenine (A),
thymine (T),
guanine (G), or
cytosine (C)
•Hydrogen bonds – holds
2 strands of nucleotides
together
•Shape: double helix
(Two strands twist around
each other)
•Base pairing:
•A & T
•C & G
(complementary base
pairs)
DNA Replication
• Purpose: makes an exact copy of DNA
before the cell divides; ensures that each
new cell gets a complete copy of the DNA
DNA
DNA
Replication
DNA
DNA
Cell Division
DNA
• Steps:
1. Unwinding and separating DNA strands
2. Adding complementary bases
• If parent strand has A  T is added
• If parent strand has T  a is added
• If parent strand has C  G is added
• If parent strand has G  C is added
3. Formation of two identical DNA molecules
Overview of DNA replication
Ladder configuration and DNA
replication
• Each old strand (parent strand) of
nucleotides serves as a template for each
new strand (daughter strand).
• Proofreading and repair limits error rate
to less than 1 per billion nucleotides.
• Website showing animation:
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dnarna2.swf
Video clip from class:
http://www.youtube.com/watch?v=4jtmOZaIvS0
Enzymes involved in replication:
•DNA Helicase – breaks the hydrogen
bonds between base pairs during
replication
•DNA polymerase – adds new nucleotides
to parent strand of DNA