MOLECULAR BIOLOGY OF THE GENE
I.
Experiments showed that DNA is the genetic material
a. Knowledge of DNA is relatively new
b. DNA was known as a substance 100 years ago, but not known
as a hereditary agent until much later
c. GRIFFITH’S EXPERIMENT
i. Identified DNA as genetic carrier
ii. Involved a type of bacteria
iii. Harmless form was turned into harmful form when
placed with harmful form of bacteria
d. Chromosomes were known to be comprised of
i. Protein  more versatile; 20 amino acids, therefore
thought to be genes
ii. DNA
e. HERSHEY and CHASE EXPERIMENT
i. Studied T2 bacteriophages (phages)
ii. Grew phages in radioactive elements
1. Sulfur for protein; phosphorous for DNA
iii. By tracing the radioactivity can determine what is the
hereditary carrier
iv. Results obtained, combined with earlier evidence
convinced scientific world that DNA was hereditary
material
II.
DNA and RNA are polymers of nucleotides
a. NUCLEOTIDES  monomer of nucleic acid; phosphate group
+ sugar + nitrogenous base
b. POLYNUCLEOTIDE  polymer of nucleotides
i. Because the bases can appear in any sequence, the
number of possible nucleotides is very large
c. SUGAR-PHOSPHATE BACKBONE  a repeating pattern of
sugar-phosphate-sugar-phosphate
d. DNA  Deoxyribonucleic Acid
i. Deoxyribo  deoxyribose; sugar found in the DNA
ii. Nucleic  found in the nucleus of the cell
iii. Acid  the phosphate group gives acidic properties
e. 4 types of bases found in DNA; 2 groups
i. PURINES (2 rings)
1. Adenine (A)
2. Guanine (G)
ii. PYRIMIDINES (1 ring)
1. Thymine (T)
2. Cytosine (C)
f. RNA  Ribonucleic Acid
i. Ribo  ribose; sugar found in RNA
1. Contains one more oxygen than deoxyribose
ii. Instead of T, RNA contains
1. URACIL (U)
III.
DNA is a double stranded helix
a. Structure denotes function, so learning the shape of DNA was
crucial to understanding its role in hereditary
b. DOUBLE HELIX  two strands of DNA coiled around each
other explained by Watson and Crick
i. Used data from Wilkins and Franklin
ii. Had X-Ray crystallography pictures of DNA
c. Watson and Crick explained double helix and that
i. A always joins with T
ii. C always join with G
iii. Allows from same distance between sugar backbones
which matched data
IV.
DNA replication depends on specific base pairing
a. Watson and Crick’s model suggested a possible copying
mechanism
b. The specific pairing of complementary bases is how DNA is
logically copied
c. While the general mechanism of DNA replication is
conceptually simple, the actual process is quite complex
V.
DNA replication: A closer look
a. DNA replication begins at specific sites called ORIGINS OF
REPLICATION
i. Proteins that start the process attach at these points
b. REPLICATION BUBBLES  where DNA splits and
replication occurs in both directions
c. There are many origins of replication, shortening the time for
the full replication process
d. The strands of DNA are oriented in opposite directions
i. This is important because the enzymes for replication
only work in a specific direction
e. DNA POLYMERASE  enzyme that adds nucleotides to
growing daughter strand
f. DNA LIGASE  ties pieces of DNA together into a single
strand
g. DNA replication ensures genetic information is copied and
passed along to somatic cells
h. Amazingly accurate; only 1 mistake every billion nucleotides
placed
VI.
The DNA genotype is expressed as proteins, which provide the
molecular basis for phenotypic traits
a.
b.
c.
d.
TRANSCRIPTION  DNA is converted to RNA
TRANSLATION  RNA is converted into a protein
Proteins control the phenotype of an organism
BEADLE and TATUM
i. One gene : One polypeptide
ii. Showed that a mutant mold deficient in only one gene
could not grow normally on medium that normally
supported growth
VII. Genetic information written in codons is translated into amino acid
sequences
a. DNA’s language is written as a linear sequence of nucleotide
bases on a polynucleotide
b. Specific sequences of bases, each with a beginning and end,
make up genes
c. DNA is transcribed to RNA; still the language of nucleic acids
d. Transcribed RNA sequence is complementary to the DNA
e. Transcribed RNA is translated into the polypeptide language
f. CODON  three base word that specifies a specific amino acid
g. 3 bases allows for 20 amino acids (43 vs 42 vs 4)
VIII. The genetic code is the Rosetta stone of life
a. AUG  dual function; signals start of polypeptide and codes
for MET
b. 3 STOP CODONS  end polypeptide chain/translation
c. Wobble hypothesis  overlap at 3rd base; sometimes amino
acid is coded by different codons
IX.
Transcription produces genetic messages in the form of RNA
a. Transcription occurs in the nucleus
b. Similar to replication; the 2 strands of DNA must split but only
one strand serves as template
c. RNA POLYMERASE  enzyme that places and links
nucleotides for RNA being transcribed
d. PROMOTER  a specific binding site for RNA polymerase to
start transcribing the gene
e. Transcription has 3 stages
i. INITIATION
ii. ELONGATION
iii. TERMINATION
f. TERMINATOR  sequence that signals the RNA polymerase
to detach
X.
Eukaryotic RNA is processed before leaving the nucleus
a. mRNA  messenger RNA; carries the message from DNA to
ribosome (translation machinery)
b. Before leaving the nucleus, the mRNA is processed
c. One type of processing is the addition of a cap and tail
i. Protects mRNA from cellular enzymes
ii. Helps ribosome recognize the mRNA
d. Second type of processing is removal of non-coding regions
e. INTRONS  regions of DNA that don’t code for anything
f. EXONS  regions that are expressed
g. RNA SPLICING  removal of introns to produce final mRNA
product
XI.
Transfer RNA molecules serve as interpreters during translation
a. tRNA  transfer RNA; a molecular interpreter that
converts/translates the nucleic acid language to the amino acid
language (polypeptide)
b. Amino acids are readily available in the cell from digested food
or recycled proteins
c. ANTICODON  complementary triplet to the codon on the
mRNA
d. The tRNA picks up the appropriate amino acid and brings it to
the appropriate codon of the mRNA sequence to arrange the
amino acids in the appropriate order
e. Too complex for tRNA to accomplish on own, there are a
bunch of enzymes that accomplish this translation step along
with tRNA
XII. Ribosomes build polypeptides
a. Ribosomes are composed of proteins and rRNA
b. Ribosome coordinates mRNA, tRNA, amino acids to allow
syntheis
c. P site  holds growing polypeptide
d. A site  holds next amino acid to be added
XIII. An initiation codon marks the start of an mRNA message
a. Translation can be divided into 3 phases
i. INITIATION
ii. ELONGATION
iii. TERMINATION
b. Initiation brings together the mRNA, the first amino acid
attached to tRNA, and ribosome subunits
i. 2 steps to initiation
1. mRNA binds to small subunit of ribosome; tRNA
with the start codon binds (MET = AUG)
ii. Large ribosomal subunit attaches, creating a functional
ribosome initiator tRNA fits into P site
XIV. Elongation adds amino acids to the polypeptide chain until a stop
codon terminates translation
a. 3 steps of elongation
i. Codon recognition
ii. Peptide bond formation
iii. Translocation
b. Elongation continues until a STOP CODON enters the A site
XV. Review: The flow of genetic information in the cell is DNA 
RNA  protein
a. TRANSCRIPTION  occurs in the nucleus
b. TRANSLATION  occurs in the cytoplasm
XVI. Mutations can change the meaning of genes
a. MUTATION  any change in the nucleotide sequence of DNA
b. Mutations can be classified as
i. SUBSTITUTIONS
ii. INSERTIONS
iii. DELETIONS
c. A base substitution may change an amino acid in a polypeptide,
changing the protein
d. Insertions and deletions are more severe
i. They alter the “READING FRAME” and impact all the
amino acids “downstream” from the change
e. MUTAGENESIS  the creation of mutations
i. Can occur in a number of ways
1. SPONTANEOUS MUTATIONS
a. Result of error in DNA replication or
recombination
b. Also may arise from unknown origin
2. MUTAGEN
a. Physical (radiation) or chemical agent
f. While mutations are usually harmful, they are also extremely
useful
i. In nature, promotes evolution
ii. In the lab, provides a basis for comparison and a tool of
genetic research
XVII. Viral DNA may become part of the host chromosome
a. In a sense, viruses are nothing more than packaged genes
b. Two reproductive cycles
i. LYTIC CYCLE
1. Always leads to lysis; breaking open of the host
cell
ii. LYSOGENIC CYCLE
1. Viral replication occurs without the death of the
host cell
2. PROPHAGE
a. Viral DNA inserted into bacterial
chromosome
XVIII. Many viruses cause disease in animals
a. Viruses use host cells machinery to reproduce
b. RNA viruses reproduce in host cytoplasm
c. DNA viruses reproduce in nucleus
d. Viruses that attack quickly repairable tissue are usually
recoverable while non-repairable tissues (eg. Nerves / polio) are
much more severe
e. Use vaccines to counter viruses
f. Antibiotics don’t work; difficult to find a drug that will kill a
virus and not the host cell
XIX. Plant viruses are serious agricultural pests
a. Plants protect themselves by having an epidermal layer
b. Damaging this layer leaves plant to susceptible
XX. Emerging viruses threaten human health
a. Some viruses
i. AIDS
ii. FLU
iii. EBOLA
iv. HANTAVIRUS
b. How do viruses arise?
i. Not known; hypothesis that fragments of nucleic acids
somehow manage to move between cells and evolve into
viruses
XXI. The AIDS virus makes DNA on an RNA template
a. AIDS (HIV) is a retrovirus
i. Creates DNA using an RNA template
ii. Requires the enzyme REVERSE TRANSCRIPTASE
b. This viral DNA can insert into host DNA making it a
“PROVIRUS”
c. AIDS = Acquired Immune Deficiency Syndrome
d. HIV = Human Immunodeficiency Virus
i. So named because of the cells it attacks
XXII. Virus research and molecular genetics are intertwined
a. Understanding viruses helps us to understand our own heredity
also helps us to combat deadly viruses that plague humans