UNIT 1:
GENETICS:
NUCLEIC ACID
DNA
(Campbell & Reece:
Chapters, 5, 16)

LOCATION OF DNA IN
A CELL
• Chromatin is a complex of DNA and
protein, and is found in the nucleus of
eukaryotic cells.
• Histones are proteins that are
responsible for the first level of DNA
packing in chromatin
• The chromatin network in the nucleus of
a cell will coil up tightly during cell
division and form individual
chromosomes.

• Chromosomes are always duplicated
during this process (2 sets of identical
genetic information to ensure each cell
receives identical genetic info to the
parent cell during cell division).
• A duplicated chromosome consists of 2
chromatids attached to each other by a
centromere.
• Each chromatid consists of several genes.
• Genes consists of a long DNA strand.
• A string of DNA coiled around a few
histones is called a nucleosome.

LOCATION OF DNA IN
A CELL
Locus: Position of gene on chromosome

CHROMOSOME DUPLICATION
AND DISTRIBUTION

2. DNA STRUCTURE
• DNA molecules are polymers called
polynucleotides.
• Each polynucleotide is made of
monomers called nucleotides.
• Each nucleotide consists of :
• a nitrogenous base (Adenine,
Thymine, Cytosine or Guanine)
• a pentose sugar (DNA =
Deoxyribose sugar),
• and a phosphate group.

• Nucleotide monomers are linked
together to build a polynucleotide.
• Adjacent nucleotides are joined by
covalent bonds that form between the –
OH group on the 3’ carbon of one
nucleotide and the phosphate on the 5’
carbon on the next nucleotide.
• These links create a backbone of sugar-
phosphate units with nitrogenous bases
as appendages.
• The sequence of bases along a DNA
polymer is unique for each gene.

A polynucleotide and a single nucleotide

The different nitrogenous bases in DNA
Single ring structure
Double ring structure

• A DNA molecule has two polynucleo-
tides spiralling around an imaginary
axis, forming a double helix.
• In the DNA double helix, the two
backbones run in opposite 5’ → 3’
directions from each other, an
arrangement referred to as antiparallel.
• One DNA molecule includes many genes.
• The nitrogenous bases in DNA pair up
and form hydrogen bonds: adenine (A)
always with thymine (T), and guanine
(G) always with cytosine (C).

A DNA double helix structure

3. DISCOVERY OF THE
DNA STRUCTURE
• Early in the 20th century, the identifi-
cation of the molecules of inheritance
loomed as a major challenge to biologists.

The Search for the Genetic
Material: Scientific Inquiry
• T. H. Morgan’s group showed that genes
are located on chromosomes, the 2
components of chromosomes are DNA &
protein which became the candidates for
the genetic material.
• Key factor in determining the genetic
material was choosing appropriate
experimental organisms - bacteria & the
viruses that infect them were chosen.

• Discovery of the genetic role of DNA began
with research by Frederick Griffith in 1928.
• Griffith worked with 2 strains of a
bacterium, 1 pathogenic (S cells) & 1
harmless (R cells)
• Heat-killed pathogenic strain were mixed
with living cells of harmless strain and the
result = some living cells became
pathogenic.
• This phenomenon was called
transformation, now defined as a change
in genotype & phenotype due to
assimilation of foreign DNA.

• More evidence for DNA as the genetic
material came from studies of viruses that
infect bacteria.
• Such viruses, called bacteriophages (or
phages), are widely used in molecular
genetics research.
Bacterial cell
Phage head
Tail sheath
Tail fiber
DNA

• 1952: A. Hershey & M. Chase
experiments showing that DNA is
the genetic material of T2 phage.
• To determine the source of genetic
material in the phage, they designed
an experiment showing that only 1 /
2 components of T2 (DNA or
protein) enters an E. coli cell during
infection
• They concluded that the injected
DNA of the phage provides the
genetic information.

• After most biologists became
convinced that DNA was the genetic
material, the challenge was to
determine how its structure accounts
for its role…
• M Wilkins & R Franklin used X-ray
crystallography to study molecular
structure.
• Franklin produced a picture of the DNA
molecule using this technique.

A picture of the DNA molecule using
crystallography by Franklin.

• Franklin’s X-ray crystallographic
images of DNA enabled Watson to
deduce:
• that DNA was helical
• the width of the helix
• the spacing of the nitrogenous
bases
• Width suggested that the DNA
molecule was made up of 2 strands,
forming a double helix

Representations of DNA molecule

• Watson and Crick: built models of a
double helix to conform to the X-rays &
chemistry of DNA.
• Franklin concluded there were 2
antiparallel sugar-P backbones, with
the N bases paired in the molecule’s
interior.
• But: How did bases pair? A-A?/A-T?/
A-C?/A-G?......

• But: How did bases pair?
• They worked it out by using the
following image:
Purine + purine: too wide
Pyrimidine + pyrimidine:
too narrow
Purine + pyrimidine: width
consistent with X-ray data

• W & C noted that the pairing of the
Nitrogen bases was specific, dictated
by the base structures.
• Adenine (A) paired only with thymine
(T), & guanine (G) paired only with
cytosine (C)
• The W-C model explains Chargaff’s
rules which states that; in any
organism the amount of A = T, & the
amount of G = C

4. THE ROLE OF DNA
• DNA is vital for all living beings – even
plants.
• It is important for:
• inheritance,
• coding for proteins and
• the genetic instruction guide for
life and its processes.
 DNA holds the instructions for an
organism's or each cell’s development
and reproduction and ultimately death.
 DNA can replicate itself.

NON-CODING DNA
 Multicellular eukaryotes have many
introns(non-coding DNA) within genes and
noncoding DNA between genes.
 The bulk of most eukaryotic genomes
consists of noncoding DNA
sequences, often described in the past as
“junk DNA”
 Much evidence indicates that noncoding
DNA plays important roles in the cell.
 Sequencing of the human genome reveals
that 98.5% does not code for
proteins, rRNAs, or tRNAs.

 About 24% of the human genome
codes for introns and gene-related
regulatory sequences.
 Intergenic DNA is noncoding DNA
found between genes:
 Pseudogenes are former genes that
have accumulated mutations and are
nonfunctional
 Repetitive DNA is present in
multiple copies in the genome

5. DNA REPLICATION
• Replication begins at special sites
called origins of replication, where
the 2 DNA strands separate, opening
up a replication “bubble” (eukaryotic
chromosome may have many origins
of replication.
• The enzyme helicase unwinds the
parental double helix.
• Single stranded binding protein
stabilizes the unwound template
strands.

• A replication fork forms.
• The enzyme: Topoisomerase breaks,
swivels and re-joins the parental DNA
ahead of the replication for, to
prevent over winding.
• The unwind complimentary strands
now act as individual template for 2
new strands.
• RNA nucleotides are added to each
DNA template by the enzyme RNA
primase to form RNA primers on both
templates

• The enzyme DNA polymerase III add
free DNA nucleotides to the RNA
primer 3’ carbon.
• The free nucleotides bond with H-
bonds to their complimentary bases
on the DNA templates.
• Along one template strand of DNA,
the DNA polymerase synthesizes a
leading strand continuously, moving
toward the replication fork.

• To elongate the other new strand,
called the lagging strand, DNA
polymerase must work in the
direction away from the replication
fork.
• The lagging strand is synthesized as a
series of segments called Okazaki
fragments.
• Each fragment has an RNA primer and
added DNA strand.
• All the fragments are then joined with
the help of the enzyme DNA ligase.

• DNA polymerase I then removes RNA
primers and replaces it with DNA
nucleotides.

6. PROOFREADING AND
REPAIRING DNA
• DNA polymerases proofread newly made
DNA, replacing any incorrect nucleotides
• In mismatch repair of DNA, repair
enzymes correct errors in base pairing.
• DNA damaged by chemicals, radioactive
emissions, X-rays, UV light, & certain
molecules (in cigarette smoke for example)
• In nucleotide excision repair, a nuclease
cuts out & replaces damaged stretches of
DNA.

7.BIOTECHNOLOGY
AND GENOMICS
DNA CLONING
BIOTECH PRODUCTS

DNA CLONING
–Cloning is the reproduction of genetically
identical copies of DNA, cells or
organisms through some asexual means.
–DNA cloning can be done to produce
many identical copies of the same gene –
for the purpose of gene cloning.
–When cloned genes are used to modify a
human, the process is called gene
therapy.

–Otherwise, the organisms are called
transgenic organisms – these
organisms today are used to produce
products desired by humans.
–A. Recombinant DNA (rDNA) and B.
polymerase chain reaction (PCR) are
two procedures that scientists can use
to clone DNA

CLONING OF AN ORGANISM

A. CLONING A HUMAN GENE
HOW IS INSULIN MADE BY DNA CLONING?
(A. RECOMBINANT DNA)
• A large quantity of insulin are being produced
by recombinant DNA technology.
• This process is as follows:
1. DNA that codes for the production of
insulin is removed from the chromosome of a
human pancreatic cell.
2. Restriction enzymes cut the gene from the
chromosome (isolating the gene for insulin)
I

Insulin gene – cut
out with restriction
enzyme
Isolated insulin gene

3. A plasmid (acting as a vector/carrier of
new gene) is removed from the bacterium
and cut open with a restriction enzyme to
form sticky ends
Plasmid removed from
bacterium
Cut by restriction
enzyme
plasmid
Nucleus
BACTERIUM CELL
Sticky ends

• 4. Ligase (enzyme) is added to join the
insulin gene to the plasmid of the
bacterium cell - forming recombinant
DNA.
• 5. The recombinant DNA can then be
reinserted into the bacterium, the
bacterium will then produce more
insulin, therefore cloning the gene.
Insulin gene placed in plasmid
by enzyme Ligase ( attached
to sticky ends)

• 6. When the bacterium reproduces it makes
the insulin inserted into the plasmid.
• 7. The bacteria are kept in huge tenks with
optimum pH, temperature and nutrient
values, where they multiply rapidly, producing
enormous amounts of insulin, this is then
purified and sold.
Recombinant DNA placed into bacterium cell

B. POLYMERASE CHAIN REACTION
• PCR – Used in genetic profiling.
• To solve crimes – criminals usually leave DNA
evidence at the scene of the crime in the form
of saliva, blood, skin, semen and hair. These all
contain DNA. If only a little bit of DNA is found
or the DNA is old, we can make copies of the
available DNA by means of PCR.
• From the DNA produced through PCR, DNA
fingerprint can be generated.

PCR method
• 1. Sample containing DNA is heated in a
test tube to separate DNA into single
strands.
• 2. Free nucleotides are added to the test
tube with DNA polymerase (enzyme), to
allow DNA replication.
• 3. DNA is cooled to allow free nucleotides
to form a complementary strand along side
each single strand.
• 4. In this way the DNA is doubled giving
sufficient amount of DNA to work with.

DNA SCREENING AND FINGERPRINT
TECHNIQUE
• 1. Sample of DNA is cut into fragments by means of
restriction enzymes.
• 2. Negative charged electrode at one end of a
rectangular flat piece of gel and a positive
electrode is placed at the other end.
• 3. The DNA is placed at the negative end of the gel
and starts to move to the positive end. Smaller
fragments move faster than the larger ones.
Separation occurs on the basis of size. This process
is called gel electrophoresis.

• 4. DNA is then pressed flat against the gel and
transferred to filter paper.
• 5. Radioactive probes bind to special DNA
fragments.
• 6. X-rays are taken of the filter paper. The DNA
probes show up as dark bands on the film. The
pattern of these bands is the DNA fingerprint.

DNA fingerprinting

GEL ELECTROPHORESIS

DNA FINGERPRINT

BIOTECHNOLOGY PRODUCTS
• Today transgenic bacteria, plants and animals
are called genetically modified organisms
(GMO’s).
• The products that GMO’s produce are called
biotechnology products.

GENETICALLY MODIFIED BACTERIA
• Recombinant DNA is used to make transgenic
bacteria.
• They are used to make insulin, clotting factor
VIII, human growth hormone and hepatitis B
vaccine.
• Transgenic bacteria is used to protect the
roots of plants from insect attack, by
producing insect toxins.

GENETICALLY MODIFIED PLANTS
• Example = pomato
• Genetically modified to produce potato's
below the ground and tomato's above the
ground.
• Foreign genes transferred to cotton, corn, and
potato strains have made these plants
resistant to pests because their cells now
produce an insect toxin.
• Read p. 253 for more examples

GENETICALLY MODIFIED ANIMALS