11. Nucleic Acids / Genes

Chapter 18

POHN

N

O

OH H

OO

NH2

O

HO

(OH)

Nucleic Acids – the Master MoleculesNucleotide:Phosphate + Sugar +

Heterocycle;nucleoside: No phosphate

DNA - DeoxyriboNucleic Acid RNA - RiboNucleic Acid

O

NN

OO

OP OOO

N

N N

NH

O

ONH2

N

NH

O

ON N

N

OPOO

O O

O

OP OOO

O

NH2

POO

O

NH2

A Sequence of DNA

Adenosine

Thymidine

Guanidine

Cytidine

N

NH

O

O

N

N

N

N

NHH

N

N

NH

O

H

N

N

HN

N

O

NH

Base-Pairing - the ‘Glue’ of the Double HelixThymine Adenine

Cytosine Guanine

The DNA of each human cell contains ~6 billion of these base-pairs

to ‘backbone’

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

transfer RNA: with anticodon and related amino acid

anticodon

valine

a.a. binding site

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