Chapter 12 Notes on DNA – Replication, Transcriptions and Translation
I.
A.
B.
C.
D.
E.
F.
Genetic Code
Fredrick Griffith (1928) Identified the process of transformation in bacteria
Avery, McCarty and MacLeod (1944) – isolated DNA and identified it as the molecule which
carried on transformation
1. DNA – Deoxyribonucleic acid- molecule which stores and transmits genetic
information
2. Bacteriaphages – simple organisms (viruses) composed of DNA core and protein
coat
Hershey and Chase (1952) used ratioactive isotopes to label DNA in bacteriaphages and
determined that DNA was responsible for transformation
Franklin and Wilkins (1950) – purified and photographed DNA using x-ray diffraction
Watson and Crick (1950) – developed a model for DNA structure which they called a double
helix
1. Phosphate group supported the backbone of the helix
2. Deoxiribose sugar – bonded with phosphate groups in the backbone
3. Nitrogenous Bases – Pairs of compounds that make cross-bridges that link two sides
of the double helix
a. Adenine to Thymine
b. Cytosine to Guanine
c. Hydrogen bonds hold the base pairs together
Chargaff (1950) – determined that these pairs always occur in equal numbers
II. DNA Replication
A. Before cells divide, they must duplicate their DNA in S-phase (Synthesis)
1. Complimentary base pairs- Each half of the DNA double helix can produce a copy of the
original molecule (semi-conservative replication)
- Phosphate group supports the backbone of the helix
- Deoxyribose sugar – bonded with phosphate groups in backbone
- Nitrogenous base pairs- create cross-bridges of double helix
a. Adenine – Thymine
b. Cytosine – Guanine
c. Hydrogen bonds hole the base pairs together
2. Replication – semi-conservative process by which DNA copies itself
a. Step 1- enzymes unzip the long nitrogen base sequence
b. Step 2 – enzymes replace unzipped base pairs with new bases
c. Step 3 enzymes proofread new sequence to make sure no mistakes exist
d. Example of original DNA
T-A
A-T
C-G
G-C
T-A
T-A
e. Complementary Strands of Replicated DNA
T-A
T-A
A-T
A-T
C-G
C-G
G-C
G-C
T-A
T-A
T-A
T-A
Old – New
New- Old (Semi-conservative)
III. RNA - ribonucleic Acid - messenger molecule between DNA and ribosome of cell which creates
proteins.
A. RNA differences from DNA
1. Sugar – ribose not deoxyribose
2. A single stranded molecule
3. Base pairs of RNA are
Adenine – Uracil
Guanine – Cytosine
B. Transcription of DNA to RNA – copying the genetic code to a messenger molecule
1. Transcription – process by which DNA copies onto a complimentary strand of RNA
2. Transcription must occur to transport information from DNA in nucleus out to
ribosomes in the cytoplasm.
3. mRNA (messenger) – relays information from DNA to ribosome
4. enzyme RNA polymerase separates and copies mRNA from DNA template
C. Protein Synthesis – Process of using RNA sequence to produce the proteins which control all
aspects of cell growth and development.
1. Codons – sequences of three (3) RNA base pairs which stand for an Amino Acid (AA) in a
protein molecule.
2. Refer to codons on p. 303
AUG – Methionine - Start codon
UAA – stop
UAG – stop
UGA – stop
D. Translation - decoding of information from mRNA to form a protein
1. Takes place in ribosome using….
a. Transfer RNA (tRNA) – a complex molecule containing three exposed base pairs
called anticodons which match with codons on mRNA
b. Ribosomal RNA (rRNA) executes protein synthesis on ribosome by binding proteins
together into chains.