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The Structure & Function of
Deoxyribose Nucleic Acid
Crash Course in
The Why’s & How’s
of
DNA
Fredrick Griffith 1928
Frederick Griffith
Oswald Avery 1944
Hershey & Chase 1952
Hershey & Chase
The Race
WATSON and CRICK
• Announced in 1953
• Used the results of other scientists to figure
out the structure of DNA
Erwin Chargaff
The Work of Biochemists
Watson & Crick Model
• Chemists found that DNA polymerized through the
formation of phosphodiester linkages
– This concluded a sugar-phosphate backbone
• By analyzing the total number of purines and
pyrimidines it was found that the number of A’s and
T’s were equal to the number of C’s and G’s
– This was called Chargaff’s rule after Erwin Chargaff
• X-ray diffraction showed a repeating scatter pattern
(.34 nm, 2.0nm, 3.4nm)
– This repeating pattern only makes sense if the molecule is
shaped as a double helix
Scatter Pattern X-ray Diffraction
Scatter Pattern X-ray Diffraction
• Watson & Crick began to analyze the size and
geometry of deoxyribose, phosphate groups, and
nitrogenous bases.
• Using things like bond angles, and measurements,
they were able to devise 2.0nm probably
represented the width of the helix, and .34 was likely
the distance between bases stacked in the spiral
• They arranged two strands of DNA running in
opposite directions (5`-3` and 3`-5`)
And the Winner IS…
DNA Size
• Width of the helix = 2.0nm
• Length of one full complete turn of helix =
3.4nm
• Distance between bases = .34nm
• Antiparallel Double Helix
– 3’  5’ & 5’ 3’
It is pointing at the
5’ end!
The Roof of the Sugar Molecule
C=O
Count Your Primes
Base Pairing
• Using the x-ray diffraction patterns and
measurements, it was found only to work if:
• Adenine always bonded with Thymine
• Guanine always bonded with Cytosine
• This phenomena is called Complimentary
Base Pairing
DNA REPLICATION
• Occurs during S-phase of the cell cycle
• DNA has a special “complimentary structure”
that acts as a template for reproduction
– This means, it allows for simple DNA copying
• The strand unzips, and the old strand acts for
a model to create a new “compliment”
• The strand copies in two directions:
– The Leading strand starts at the 3’ end and moves towards
the 5’ end
– The Lagging strand pieces together new nucleotides
starting at the replication fork and works toward the 5’
Prokaryotic Replication Proteins
• Helicase: Unwinds parental double helix at replication
fork
• Single Strand Binding Protein: Binds to and stabilizes
DNA strand after it has separated
• Topoisomerase: Relieves the “overwinding” strain that
can occur ahead of the replication fork (swivel motion)
• Primase: Synthesizes an RNA primer at the 5’ end of
replicating strand.
• DNA pol III: Using the template strand, covalently bonds
nucleotides to the 3’ end of the pre-existing RNA strand
or primer
• DNA pol 1: Removes RNA primers & replaces them with
DNA nucleotides
• DNA Ligase: Joins 3’ end of DNA that replaces primer to
the rest of strand (Joins Okazaki fragments of lagging
strand)
DNA Excision Repair
• Nucleotide excision repair (NER) is a particularly
important excision mechanism that removes DNA
damage induced by ultraviolet light (UV).
• Recognition of the damage leads to removal of a short
single-stranded DNA segment that contains the lesion
(By the Nuclease enzyme).
• The undamaged single-stranded DNA remains and DNA
polymerase uses it as a template to synthesize a short
complementary sequence.
• Final ligation to complete NER and form a double
stranded DNA is carried out by DNA ligase.
How Does a Cell Make Proteins?
• The RNA molecule comes out of the nuclear
envelope after it is transcribed from DNA (Its
like a photocopy)
– Transcription is the process of creating an RNA
strand from a template of DNA nucleotides
• The process of protein synthesis is called
translation
– Translation refers to the process of converting the
“3-nucleotide RNA codons” into amino acids and
then into amino acid chains
RNA & Protein Synthesis
• RNA has very specific blue prints that it uses
to build the various amino acids (proteins)
called Codons.
• These codons allow for the difficult job of the
synthesis of the many proteins to be grouped
into simple readable prints (codons) including
“stop and start” codons
The Genetic Code
• It is almost as if a cell is “pre-programmed”
with a guide for making life
• If there was such a “program” it would need
to be something contained in nearly EVERY
cell, so that each cell could individually work
at it
• We call this program “the Genetic Code”
• It is the control for life as we know it
Translation Initiation
Initiation
factors
bring
together
Completes
initiation
complex
Translation Elongation
Codon
recognition
accuracy
Peptide bond
formation
P site AA is attached to AA chain
GTP required for
Translocation:
movement
mRNA slides
through (P to E and
released)
Translation Termination
2 GTP
molecules
are
required
to break
translation
assembly
Protein
shaped
like
tRNA
R factor
hydrolyzes
bond
between
tRNA and
last AA
Sweet Moving Slide
Genetic Engineering Techniques
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Restriction Enzymes
DNA Recombination
DNA Insertion
DNA Sequencing
Transgenic Bacteria, Plants, and Animals
Cloning
Gel electrophoresis
DNA Polymerase Chain Reaction (PCR)
Genetic Engineering Techniques
• Restriction Enzymes:
– Genes can be cut at specific DNA sequences by proteins
known as Restriction Enzymes
• (We know over 3,000 different restriction enzymes)
– Each recognizes and cuts DNA at a particular sequence
(area of Bases)
• They are INCREDIBLY accurate, they will ONLY cut the
area that they recognize
• This amazing ability allows us to cut DNA into
fragments so that we can isolate it, separate it,
and/or analyze it.
Genetic Engineering Techniques
• DNA RECOMBINANTS:
– DNA fragments (cut by restriction enzymes) may
be combined with bacterial DNA so that they can
later be inserted into a bacterial cell
– The small, circular DNA molecules in bacteria
(called plasmids) can be removed and cut with a
restriction enzyme.
– The cut ends are sticky to the foreign fragment,
and can allow for the formation of a recombinant
DNA molecule
Genetic Engineering Techniques
• DNA INSERTION:
– During the first 2 steps of genetic engineering, DNA
fragments containing the desired gene are obtained and
then inserted into DNA that has been removed from the
recipient cell (the cell where the DNA is going.)
• Forming recombinant DNA (New DNA)
– To insert the DNA into LIVING cells it is easiest to use
bacteria
• Bacteria in a solution of salt and the desired DNA will eventually
take up the DNA in its own DNA.
• These new bacteria are then cultured (grown) into a large colony.
• The technical term for a large number of cells grown from a single
cells Clone. So this is DNA cloning.
• You don’t HAVE to use bacteria but it is the easiest.
Genetic Engineering Techniques
• DNA sequencing:
– Sequencing DNA is when you read the nitrogenous bases
(ATCG) along the length of the DNA fragment.
– Only one strand of the double helix is used to sequence
the DNA, but they do need multiples of the used strand
(So you clone it)
– The DNA is divided to 4 groups that undergo different
chemical treatments that break the pieces and reveal the
Base sequence.
– The pieces are separated by electrophoresis.
Gel electrophoresis
• During gel electrophoresis, DNA is cut with a
restriction enzyme into small pieces
• Because DNA has a slight negative charge,
different charges are placed at either end of a
gel containing tray.
• When the DNA is placed into the tray it will
slowly move across the gel (towards the +)
• Because the pieces are different sizes they
move at different speeds.
DNA Fingerprinting
• The amazing complexity of the human
genome ensures that NO TWO INDIVIDUALS
are exactly the same.
• This biological “fact” (or more likely theory)
allows for a powerful new tool in criminal
investigations
• Now, finding bodily fluids and/or skin cells at
the scene of a crime or on a victim can link a
suspect to a crime with amazing reliability.
Electrophoresis
Electrophoresis
PCR
• Polymerase Chain Reaction (PCR) is a
technique used to copy DNA
– Original DNA strand (with desired gene) is heated
to separate strands
– AS it cools, primers are added to the solution
– DNA polymerase utilizes free nucleotides to copy
the template strands
– The procedure can be repeated as many times as
needed.
Genetic Engineering Techniques
• Transgenic
– It is now possible to insert genes from one
organism into another.
– Organisms that contain such foreign genes are
said to be Transgenic.
– Trans- across or moved genes
Transgenic Organisms
• Transgenic Bacteria:
– When a gene coding for a human protein (like a hormone
or enzyme) is inserted into bacteria, the new recombinant
cells may produce LARGE amounts of the protein.
– The human growth hormone, a hormone required for
growth and development, was incredibly rare before
genetic engineering.
– Now these transgenic bacteria (with the corresponding
foreign gene) are able to produce enough growth hormone
so that everyone who needs it has all they need.
– Other proteins, like insulin (used in the treatment of
diabetes) and interferon (used to block viral growth and
battle cancer) are also made by transgenic protein
Transgenic Organisms
• Transgenic Animals:
– DNA can be introduced into animal cells many
ways, including direct injection
– Growth hormones are used daily in many of the
cattle, fish, and poultry that we eat.
– We also use genetic engineering (DNA Fragment
Injection) to gain strains of AIDS that we use to
investigate the cure, further research into the
human immune system, and many other medical
researches.
Green Fluorescent Protein (GFP)
This transgenic animal is the result of
placing jellyfish genes in an albino rabbit,
and then bathing it in UV light
Transgenic Organisms
Transgenic Plants
• It is through the process of genetic
engineering that we have most of the plants
that we use for food.
• Not only can we selectively breed more
productive organisms, but we can use
transgenic organisms to make plants that are
resistant to diseases, insects, drought, winds,
etc.
CLONING
• In 1997 a Scottish scientist (Ian Wilmut)
cloned a sheep in which he named Dolly…
since this time we have cloned a multitude of
organisms (not including humans)
• To do this, the nucleus within an egg is
removed and replaced with the nucleus of an
adult cell.
• The cell is then placed into the reproductive
system a foster mother.
Making Dolly
Dolly’s life was cut short by cancer only a
few years following her creation…
Cloning
Human Genome
• Despite what many (older) books may say we
HAVE actually mapped the ENTIRE human
genome.
• This was something that was originally
considered impossible, but as of 2003, every
transcribing base in the human DNA strand is
mapped, and at least to a degree, understood
• We know the “loci” (location) of every protein
producing gene in the human body