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

N

N

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

NH

H

N

N

NH

O

H

N

N

HN

N

O

NH

Base-Pairing - the ‘Glue’ of the Double Helix

Thymine 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