Molecular Genetics
BACKGROUND: 1860 - Gregor Mendel determined patterns of inheritance
1868 Friedrich Miescher discovered material inside the cell nucleus
(chromosomes) is half protein and half something else
- other half later discovered to be DNA (deoxyribonucleic acid)
1902 - Walter Sutton= genetic material is found on chromosomes
Conclusion:
1. Chromosomes are made up of DNA & Protein
2. Which one makes up the GENES?
Experiments to Determine DNA or Protein
1. Frederick Griffith (1928): was attempting to develop a vaccine against
pneumonia. He never succeeded but did make some important discoveries
concerning DNA .

Took 2 strains of bacteria Streptococcus pnuemoniae; inject them into mice in
4 different experiments:
#1) Bacteria Enclosed in a smooth mucous coat (smooth = S strain) = kill
mice
#2) Bacteria with Coat absent (rough = R strain)= mice live
#3) Heated strain S bacteria = made harmless, mice lived
#4) Mixed heated S strain with R strain = MICE DIED!!
Conclusion: Transformation had taken place. Transformation = process by
which bacterial cells incorporate DNA from dead bacterial cells (transfer of
genetic information). The question remains: Is DNA or protein portion of the
chromosome responsible for transformation?
2. Oswald Avery, Colin Macleod, Maclyn McCarty (1944):
Strong evidence for DNA as the transforming principle.
 Used Enzymes: (repeated Griffith experiment)
Use a Protein destroying enzyme = transformation still occurs
Use a DNA
“
“
= NO transformation!
3. Martha Chase & Alfred Hershey (1952) : Proved DNA is the hereditary
material
Used a Bacteriophage = a virus that infects a bacteria cell; made of a DNA core
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& protein coat; attached radioactive labels ( P to DNA; S to Protein) in two
different batches
 Viruses given time to attach to bacteria and inject their genetic material
 Separated the mixture using a high speed centrifuge, this removes any
viral material remaining on the outside
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
S radioactivity found only in liquid
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
P radioactivity found only in bacteria
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Molecular Genetics
ALL NEW VIRUSES produced in future generations contained only radioactive
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P
CONCLUSION: DNA and NOT protein must be the genetic material
The Structure of a DNA Molecule
Nucleotides = subunits of DNA; made up of 3 components:
1. 5 - Carbon Sugar molecule (deoxyribose)
2. Phosphate group
3. Nitrogen Base (4)
Purines
- Adenine & Guanine
- Double ring structure
Pyrimidines
- Cytosine & Thymine
- Single ring structure
Insert figure 7-1 comparison of Dna and Rna
Determining the Structure of a DNA Molecule
Erwin Chargaff (1950) : discovered in cells that equal amounts of A & T and G &
C always exist.
Chargaff’s Rule: A=T ; C=G (Purine always bonded to a pyrimidine)
Rosalind Franklin (1954) : used X-ray diffraction to determine that DNA is a long,
thin molecule. She interpreted the shape of a DNA molecule to be in the shape
of a helix (single coil)
James Watson & Francis Crick (1962) : determined the structure of a DNA
molecule to be in the shape of a Double Helix (twisted ladder)
 DNA molecule is made of COMPLEMENTARY strands:
one strand :
ATTGCAT
Complement :
TAACGTA

Twisted ladder structure:
Sugar - Phosphate backbone = outside rails of the ladder, held together by
strong covalent bonds
Nitrogen Base Pairs = make up the inside rungs (steps) held together by
weak hydrogen bonds
Replication: process by which genetic information gets copied such as during
Interphase of the cell cycle
 Involves separating “unzipping” the DNA molecule into 2 strands
 Each strand serves as a template for making a new complementary strand
 The process is SEMI CONSERVATIVE = each new molecule consists of one
new and one old strand of DNA
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Molecular Genetics

the sequence of bases gets preserved
1.
2.
3.
4.
5.
6.
7.
8.
9.
Steps in the process of Replication
Enzyme Helicase unwinds the DNA helix (1A)
A Y-shaped Replication Fork results (1B)
Single stranded DNA binding proteins prevent the strands from
recombining (1C)
Topoisomerase removes any twists or knots that form (1D)
RNA Primase initiates DNA replication at special nucleotide sequences
called origins of replication using RNA Primers
DNA Polymerase attaches to the RNA primers and begins elongation =
adding DNA nucleotides to the complement strand DNA polymerase
moves in the 3’  5’ direction along each template (3)
The Leading Complementary Strand ( 5’ 3’ ) is assembled
continuously (4)
The Lagging Complementary Strand ( 3’ 5’ ) is assembled in short
Okazaki fragments which are joined by DNA Ligase (5A, 5B)
RNA primers get replaced by DNA nucleotides
Insert figure 7-2 Dna replication pic
Mutations: any sequence of nucleotides that does not match the original DNA
molecule from which it was made
Mutagen = anything that causes a mutation to occur (UV light, radiation, drugs,
chemicals etc.)
 DNA can “proof read” itself
 DNA polymerase often does this
 Excision repair enzymes can fix mistakes
Types of Mutations
Original DNA MESSAGE:
THE DOG RAN AND THE FOX DID TOO
Dna is read by the cell 3 base letters (CODON) at a time, this is called a
Reading Frame
1. Point (substitution) = an incorrect nucleotide
THE HOG RAN AND THE FOX DID TOO
2. Deletion = missing nucleotide
THE DOG RAN AND THE FOX DID TO
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Molecular Genetics
3. Insertion = additional nucleotide is added
 Frameshift mutation = reading frame is every 3 bases (Codon)
THE DOG RAA NAN DTH EFO XDI DTO O
4. Duplication = section of nucleotides gets repeated
THE DOG THE DOG THE DOG RAN AND THE FOX DID TOO
5. Inversion = sequence of nucleotides gets turned around
THE GOD RAN AND THE FOX DID TOO
6. Translocation = sequence of nucleotides gets moved to another
chromosome
THE DOG RAN AND THE CAT HAS FUN ALL DAY
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Protein Synthesis
DNA in chromosomes contains genetic instructions
Those instructions regulate development, growth, and metabolic activities.
They also determine cell type and characteristics
DNA controls the cell by using codes of Polypeptides (Proteins)
Polypeptides (Proteins) = enzymes that regulate chemical reactions or
structural components
GENE (genotype) = genetic information for a particular trait
From a molecular viewpoint = traits are the end product of metabolic processes
regulated by enzymes!
The GENE is the DNA segment that codes for a particular polypeptide (protein) =
One-gene-one-polypeptide hypothesis
Protein Synthesis = process by which enzymes and other proteins are
manufactured from the information contained in DNA
Consists of three steps:
1. Transcription = transfer of information from a strand of DNA to a strand of
RNA
2. RNA Processing = modifies the RNA molecule with deletions and additions
3. Translation = processed RNA used to assemble amino acids into a
polypeptide
3 types of RNA are involved in the process:
1. Messenger RNA (mRNA) = carries protein building instructions out of the
nucleus
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Molecular Genetics
2. Transfer RNA (tRNA) = carries amino acids to ribosomes
3. Ribosomal RNA (rRNA) = building blocks of ribosomes which coordinate the
activities of mRNA and tRNA
RNA
 Is single stranded
 Bases = A, G, C and U (Uracil) replaces T
 Sugar = Ribose
Codon = a triplet group of 3 adjacent nucleotides in mRNA; codes for one
specific amino acid
Anticodon = a triplet group of 3 adjacent nucleotides in tRNA; complementary to
mRNA
Transcription:
1. Initiation = RNA polymerase attaches to promoter regions on DNA and
begins to unzip the DNA into 2 strands. Promoter region contains the
sequence T-A-T-A (called the TATA box)
2. Elongation = RNA nucleotides are assembled using one side of the DNA
molecule as a template (5’ 3’)
3. Termination = RNA polymerase reaches a special sequence of nucleotides
that serve as a stop point; Usually AAAAAAA
RNA Processing:
Alterations take place before the mRNA leaves the nucleus
 A 5’ Cap is added to the 5’ end of the molecule
 5’ Cap = GTP (guanosine triphosphate)
 This provides stability to the mRNA
 Provides a point of attachment for the ribosome (small unit)
 Poly-A Tail added to the 3’ end
 A sequence of 150 to 200 adenine nucleotides
 The tail provides stability
 Controls the movement of the mRNA across the nuclear membrane
 Some mRNA segments get removed
 Exons = sequences that express a code for a protein
 Introns = intervening sequences that are noncoding
 SnRNPs (small nuclear ribonucleoproteins) = delete out the introns
and splice the exons
Insert fig 7-4
Translation:
1. Initiation = small ribosomal subunit attaches to a special region near the 5’
end of the mRNA
2. A tRNA with the anticodon UAC attaches to the mRNA start codon AUG
3. Large ribosomal subunit now attaches to the mRNA
4. Elongation = tRNA’s deliver their amino acids to the growing polypeptide
5. Ribosome moves over to the next codon and repeats the process
6. Polypeptide chain elongates one amino acid at a time
7. Termination = occurs when ribosome encounters a stop codon
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Molecular Genetics
The completed protein can now be used by the cell as a structural unit or
as an enzyme!!
Insert figure 7-5
Insert figure 7-3
DNA Organization:
 DNA packaged with proteins forms a matrix called Chromatin
 During cell division DNA = compact Chromosomes
 Transposons = segments of DNA able to move to new locations on the same
chromosome or to a different chromosome altogether
 Transposons have the effect of a mutation
Control of Gene Expression
 Every cell in a human contains the exact same sequences of DNA
 Cells obviously have different functions however
 Gene expression is regulated by the activation then of only certain genes
Example: gene regulation in E. coli (well understood)
OPERONS = sequence of DNA that direct particular biosynthetic pathways.
There are 4 major parts to an Operon:
1. A regulatory gene produces a repressor protein that prevents gene
expression by blocking the action of RNA polymerase
2. Promoter region of DNA attaches to RNA polymerase to begin transcription
3. Operator region blocks the action of RNA polymerase
4. Structural Genes contain DNA that codes for several related enzymes that
direct the production of a product
Lac Operon = in E. coli controls the breakdown of Lactose
 Lactose is required to turn on the operon that codes for the enzymes
that break down lactose.
 If lactose is not present the enzymes are not made.
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