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11. Nucleic Acids / Genes
Chapter 18
Nucleic Acids – the Master Molecules
Nucleotide:Phosphate + Sugar +
Heterocycle;nucleoside: No phosphate
NH2
O
HO
N
N
P
O
O
O
HO
OH H(OH)
DNA - DeoxyriboNucleic Acid RNA - RiboNucleic Acid
NH2
N
O
O
O
OP
O
N
N
Adenosine
O
NH
O
O
N
Thymidine
O
O
N
O
OP
O
N
O
O
O
OP
O
A Sequence of DNA
NH
N
Guanidine
NH2
NH2
N
O
O
O
OP
O
N
O
Cytidine
O
Base-Pairing - the ‘Glue’ of the Double Helix
Thymine
Adenine
H
O
NH
HN
N
N
N
Cytosine
H
N
N
Guanine
NH
O
N
O
N
to ‘backbone’
N
HN
N
N
O
HN
The DNA of each human cell contains ~6 billion of these base-pairs
DNA - Storehouse of the Genetic Code
The Double Helix
Chromosomes - chains of DNA contained in the
nucleus of every cell.
Arranged in 23 pairs (of each pair, 1 set comes
from sperm and 1 from egg).
Total length of these molecular threads in each cell
= ~2 meters !
When cells divide one strand from each ‘double
thread/helix ’ goes to each new cell thus carrying
the identical sequence/information.
The sequence of bases A, C, T & G contains the
information to direct the synthesis of all the proteins
in the body and is called the Genetic Code.
The sequence of bases, somewhere on a
chromosome, that is responsible for each protein
is called a gene.
If there are only 4 ‘bases/letters’ which must uniquely
code for 20 different amino acids then the
relationship cannot be 1:1 or 2:1 but must be 3:1,
ie. a triplet code.
Why is this so?
• we need to code for 20 different amino
acids using 4 different Bases (A,T,G,C)
• If only 2 bases used in the code, the number
of possibilities is 4x4=16
• Thus if we use a sequence of 3 bases , we
can code for 4x4x4=64 amino acids
RNA transforms genetic info into
action
• Messenger RNA (mRNA) –carries info
from DNA out of the nucleus into the
cytoplasm
• Transfer RNA (tRNA) –finds and transports
each amino acid to the protein synthesis site
Thymine(T) in DNA is replaced by
Uracil (U) in RNA
• Thymine
Uracil
But base pairing still OK
• A-U base pairs in RNA
Can you spot the error in this?
RNA contains U in place of T
• Thus, whenever U is present, must be a
ribose unit (not deoxy ribose)
Codons and anticodons!
• m-RNA sends its information to t-RNA via
complementary interactions between base
pairs
• Thus G in m-RNA codon becomes the
complementary base C in the t-RNA
anticodon (and vice versa)
• A in m-RNA codon becomes U in t-RNA
anticodon (and vice versa)
Lots of Possibilities!
• With 4 different bases in RNA arranged in codons
of 3 bases each, total number of codons
possible=4x4x4=64
• Some redundancy: ie GCA and GCC both code for
the amino acid lysine
• Signal to terminate the protein chain is given by
UAA, UAG or UGA
Building a new DNA Chain (Replication)
The GENETIC CODE - a Proposal
and a Nobel Prize
First published by James Watson* and Francis
Crick in 1953. Nobel prize awarded in 1962 to
Watson,Crick and M. Wilkens (based on X-ray
results by Rosemary Carter).
* head of the World Genome Project, ~1990 -2001.
Bedtime reading: The Double Helix (J. Watson)
The Genetic Code
AUG also = start
The Human Genome contains more than 100,000
genes each of which can be 1000 - 100,000 units
(base-pairs) long ......... but ..........
this is only ~3 - 5% of the total number of units
available!
Why? - maybe safety
How to find? - start/stop signals
anticodon
transfer RNA: with anticodon
and related amino acid
a.a.
binding site
valine
The human body can repair DNA/RNA by cutting,
splicing, inserting, but mutations can occur !
Mutation - any chemical or physical change that
alters the nucleic acid sequence in the DNA.
May be by chemical means, radiation, etc.
May be by substitution, insertion, deletion.
Every time a cell divides ~6 billion NA are matched
and ~ 2000 errors occur (most are repaired).
'Aging' is thought to be due to an increased
breakdown of RNA.
Intercalation into DNA
• Planar molecules such as Polynuclear
Aromatic Hydrocarbons (PNAH’s) can
slide into the “grooves” in the DNA double
helix, potentially causing mutations, and
cancer induction
Most mutations are detrimental. If at a crucial
position the defective protein will lack biological
activity, the cell/organism dies and the DNA will not
be reproduced.
Non-lethal mutations often lead to metabolic
abnormalities or hereditary/genetic diseases,
eg. sickle-cell anemia, hemophilia or
PKU (phenylketonuria) - cannot convert Phe to Tyr
(precursor of neurotransmitters); can cause severe
mental retardation.(~1 in 12,000). But can be
cured/controlled if detected within 3 weeks of birth.
Cloning
• First remove the nucleus of an egg cell
• Replace it in the cell by a nucleus from
body cell of a mature adult, thus producing
a cell which has a full complement of
chromosomes
• Induce cell division and implant into the
reproductive system of a surrogate mother
CLONING
Cloned mammals
• Dolly
(1st cloned mammal) UK
What about meat and milk from
cloned animals?
• Is it safe to eat/drink?
• What do you think?
The US FDA thinks so!
• Jan 15, 2008 announcement
• But cloned animals are very expensive at
present, thus not likely to be a major
issue……yet!
Human Cloning??
• Technically possible
Cloning and Stem Cells
• Cloning of human embryonic cells is being
considered for production of stem cells to
treat many diseases
• Also controversial –When does life start??
Genetically Modified Food
• Selective breeding-started with Gregor
Mendel’s work on peas in 1800’s
• Cross pollination or fertilization will change
genetic makeup of “new organism”
• Ie. Cattle cross breeding
Santa Gertrudis cattle
• Cross breed Brahman (poor quality meat)
but high resistance to heat and humidity
with English shorthorn (good meat but low
resistance to heat and humidity) : outcome
was a new breed (Santa Gertrudis) with
good quality meat and good resistance to
heat and humidity
Cross breeding of Cattle
• English shorthorn (LHS)+ Brahma =
• Santa Gertrudis (RHS)
Disadvantages to selective breeding
• Relatively slow and imprecise (also got
cattle with poor meat and poor resistance to
heat/humidity!)
• Trial and error !
Genetic Modification by DNA
manipulation
• Recombinant DNA technology
• Isolate the segment of DNA that encodes
for a protein conferring desirable traits
• Extract the DNA segment using DNA
restriction enzymes
• Copy the DNA segment using PCR
(polymerase chain reaction)
Kary Mullis (1993 Chem Nobel Prize)
• PCR!
Raw materials for PCR
• Add oligonucleotide “Primers” which
hybridize to the complementary DNA
strands in the region of interest
• Then DNA polymerase enzyme extends
each DNA strand
DNA amplification by PCR
• Tiny amounts of DNA can be made into
enough to permit analysis of the sequence
• Up to 1 million x more DNA in an hour
• Valuable in forensics
• A few nanograms is sufficient
• GMO impossible without it!
DNA manipulation cont’d
• Then splice copies of this DNA into the
cells of the organism lacking the desired
trait
• New “modified DNA” then causes the
organism to build the protein of interest
• Many successful and useful applications of
this and some “exploitations”
Tomato plants growing in salty soil
• Traditionally tomatoes would not tolerate
salty soil
• Recombinant DNA technology inserted a
gene for salt tolerance, thus permitting the
use of soil for agriculture that was otherwise
a wasteland
Other useful modifications
• Soybeans and canola have been genetically
modified to increase their content of “heart
healthy” monounsaturated fatty acids
• GMO animals: 98% of GMO animals are
mice:used for research
The Harvard Mouse
• The oncomouse (develops cancer)
• US supreme court; it cannot be patented!
Exploitation of the technology
• “Roundup ready” corn (Monsanto)
• Corn has been genetically modified to be
resistant to the herbicide “Roundup”
• Thus crops can be sprayed with Roundup
and only the weeds are harmed
• Monsanto has monopoly on seed
Signs of the times in Agriculture
What do you think?
• Relative benefits vs. harm of GMO foods
On balance…….
• Concerns: seeds and pollen from GMO
crops disrupt others through cross
pollination
• Effects on animals/insects who consume
them or use them for their habitat
• Effects on humans! Unknown allergies
• Products of GMO should be labelled
Labelling GE foods in Europe
Canadian “Policy”
• 2004: Feds adopted a “voluntary labelling”:
result: zero labelled products!
• 2007: NDP private member’s bill calling for
mandatory labelling of all GMO products in
Canada
• USA: 17 states currently considering it