Molecular Biology Revision Summary
Discovery of DNA
Bacteriophage
● Virus which infects and replicates within bacteria
● Prietin (and lipid) envelope surrounding a nucleic acid core
The Griffith Transformation Experiment (1928)
Injected rough or smooth strain of bacteria into
mice to determine whether they would survive
● Rough strain meant mice lived
● Smooth strain meant mice died (due to
protective coat preventing attack from mouse
immune system)
● Killing smooth strain meant mice lived
● Mixing strains then killing smooth strain
meant mice died
Lead to Theory of Transformation Principle: a
molecule must exist which transferred from the smooth
the rough bacteria, transforming them into the smooth
strain which killed the mice
The Avery-McCartney Experiment
(1944)
Aim was to isolate the transforming factor from the
Griffith transformation experiment
1. Broke down the smooth bacteria and isolated
the major molecules (RNA, proteins, DNA,
polysaccharides, lipids) using enzymes
2. When DNA was destroyed using DNAse the
mouse lived
Showed inheritable factors came from chromosomes however these contain DNA and
proteins (work ignored as most scientists thought DNA not complex enough to carry
hereditary information)
,Hershey and Chase Experiment
(1952)
1. T2 bacteriophage used as only contains
proteins and DNA so could be used to see
which transferred genetic material
2. Split T2 phage into two batches labelling
proteins of one with Sulphur 35 and DNA of
the other with Phosphorus 32 as radioactive
markers
3. They combined the batches with E. coli
bacteria and centrifuged so the heavier
bacteria would form a pellet at the bottom of
the tube
4. Found radioactive phosphorus in the pellet so concluded that DNA is what
transfers the genetic material
DNA and Gene Structure
Structure of DNA
● DNA is a polynucleotide
● Each nucleotide has three parts (2’ deoxyribose
sugar, phosphate group, base)
● Bases are either purine with two rings (adenine
and guanine) or pyrimidine with one ring
(cytosine and thymine)
● Four nucleotides are:
2’ - deoxyadenosine 5’ - triphosphate / dATP
2’ - deoxyguanosine 5’ - triphosphate / dGTP
2’ - deoxycytidine 5’ - triphosphate / dCTP
2’ - deoxythymidine 5’ - triphosphate / dTTP
● Bases attach to the sugar via a β-N-glycosidic bond
● Nucleotides joined together via a phosphodiester bond (two phosphates are
removed to form DNA polymer)
● DNA code is read in the 5’ -P terminus to 3’ -OH terminus direction
,Discovery of DNA structure
DNA double helix was discovered by James
Watson and Francis Crick in 1953 based on data
from Rosalind Franklin and others
Features of the DNA double helix are:
● Two strands are antiparallel (read in
opposite directions)
● It has major and minor grooves (allowing
DNA to be accessed and read)
● DNA helix can exist in different forms
● Two strands held together by hydrogen
bonds creating complementary base-pair
interactions
● A pairs with T via 2 H bonds (weaker) and
C pairs with G via 3 H bonds (stronger)
● Percentage of G and C nucleotides is called
GC content (humans have GC content of
40.3%)
Different forms of DNA
B-DNA A-DNA Z-DNA
Most common and Rare and forms only in Found naturally in certain
predominant in DNA dehydrating conditions regions of B-DNA
Right-handed helix Right-handed helix Left-handed
10 base pairs per turn 11 base pairs per turn 12 base pairs per turn
0.34 nm between base pairs 0.29 nm between base pairs 0.37 nm between base pairs
Diameter is 2.37 nm Diameter is 2.55 nm Diameter is 1.84 nm
Major groove wide, deep Major groove narrow, deep Major groove is flat
Minor groove narrow, Minor groover wide, Minor groove narrow, deep
shallow shallow
, Genes
Describing lengths of DNA
1 bp = 1 base pair
1 kb = 1,000 base pairs
1 Mb = 1,000,000 base pairs
Description of genes and their structure
Gene: segment of a DNA molecule in which the DNA sequence contains biological
information. Discontinuous and divided into:
- Introns: information not needed to make proteins. Mean length 3365 bp. 90% total
gene length
- Exons: contains information needed to make proteins. Mean length 145 bp. 10%
total gene length
Genes undergo gene expression via transcription to form RNA (mRNA, rRNA, tRNA,
snRNA, snoRNA, scRNA, miRNA, siRNA) and then translation to form proteins (structural,
motor, catalytic, transport, storage, protective, regulatory)
Description of gene families and how they arise
Simple multigene families: all genes the same as gene product needed in large amounts
Discovery of DNA
Bacteriophage
● Virus which infects and replicates within bacteria
● Prietin (and lipid) envelope surrounding a nucleic acid core
The Griffith Transformation Experiment (1928)
Injected rough or smooth strain of bacteria into
mice to determine whether they would survive
● Rough strain meant mice lived
● Smooth strain meant mice died (due to
protective coat preventing attack from mouse
immune system)
● Killing smooth strain meant mice lived
● Mixing strains then killing smooth strain
meant mice died
Lead to Theory of Transformation Principle: a
molecule must exist which transferred from the smooth
the rough bacteria, transforming them into the smooth
strain which killed the mice
The Avery-McCartney Experiment
(1944)
Aim was to isolate the transforming factor from the
Griffith transformation experiment
1. Broke down the smooth bacteria and isolated
the major molecules (RNA, proteins, DNA,
polysaccharides, lipids) using enzymes
2. When DNA was destroyed using DNAse the
mouse lived
Showed inheritable factors came from chromosomes however these contain DNA and
proteins (work ignored as most scientists thought DNA not complex enough to carry
hereditary information)
,Hershey and Chase Experiment
(1952)
1. T2 bacteriophage used as only contains
proteins and DNA so could be used to see
which transferred genetic material
2. Split T2 phage into two batches labelling
proteins of one with Sulphur 35 and DNA of
the other with Phosphorus 32 as radioactive
markers
3. They combined the batches with E. coli
bacteria and centrifuged so the heavier
bacteria would form a pellet at the bottom of
the tube
4. Found radioactive phosphorus in the pellet so concluded that DNA is what
transfers the genetic material
DNA and Gene Structure
Structure of DNA
● DNA is a polynucleotide
● Each nucleotide has three parts (2’ deoxyribose
sugar, phosphate group, base)
● Bases are either purine with two rings (adenine
and guanine) or pyrimidine with one ring
(cytosine and thymine)
● Four nucleotides are:
2’ - deoxyadenosine 5’ - triphosphate / dATP
2’ - deoxyguanosine 5’ - triphosphate / dGTP
2’ - deoxycytidine 5’ - triphosphate / dCTP
2’ - deoxythymidine 5’ - triphosphate / dTTP
● Bases attach to the sugar via a β-N-glycosidic bond
● Nucleotides joined together via a phosphodiester bond (two phosphates are
removed to form DNA polymer)
● DNA code is read in the 5’ -P terminus to 3’ -OH terminus direction
,Discovery of DNA structure
DNA double helix was discovered by James
Watson and Francis Crick in 1953 based on data
from Rosalind Franklin and others
Features of the DNA double helix are:
● Two strands are antiparallel (read in
opposite directions)
● It has major and minor grooves (allowing
DNA to be accessed and read)
● DNA helix can exist in different forms
● Two strands held together by hydrogen
bonds creating complementary base-pair
interactions
● A pairs with T via 2 H bonds (weaker) and
C pairs with G via 3 H bonds (stronger)
● Percentage of G and C nucleotides is called
GC content (humans have GC content of
40.3%)
Different forms of DNA
B-DNA A-DNA Z-DNA
Most common and Rare and forms only in Found naturally in certain
predominant in DNA dehydrating conditions regions of B-DNA
Right-handed helix Right-handed helix Left-handed
10 base pairs per turn 11 base pairs per turn 12 base pairs per turn
0.34 nm between base pairs 0.29 nm between base pairs 0.37 nm between base pairs
Diameter is 2.37 nm Diameter is 2.55 nm Diameter is 1.84 nm
Major groove wide, deep Major groove narrow, deep Major groove is flat
Minor groove narrow, Minor groover wide, Minor groove narrow, deep
shallow shallow
, Genes
Describing lengths of DNA
1 bp = 1 base pair
1 kb = 1,000 base pairs
1 Mb = 1,000,000 base pairs
Description of genes and their structure
Gene: segment of a DNA molecule in which the DNA sequence contains biological
information. Discontinuous and divided into:
- Introns: information not needed to make proteins. Mean length 3365 bp. 90% total
gene length
- Exons: contains information needed to make proteins. Mean length 145 bp. 10%
total gene length
Genes undergo gene expression via transcription to form RNA (mRNA, rRNA, tRNA,
snRNA, snoRNA, scRNA, miRNA, siRNA) and then translation to form proteins (structural,
motor, catalytic, transport, storage, protective, regulatory)
Description of gene families and how they arise
Simple multigene families: all genes the same as gene product needed in large amounts
