11 Nucleic Acids Genes Chapter 18 Nucleic Acids

11. Nucleic Acids / Genes Chapter 18

Nucleic Acids – the Master Molecules Nucleotide: Phosphate + Sugar + Heterocycle; nucleoside: No phosphate DNA - Deoxyribo. Nucleic Acid RNA - Ribo. Nucleic Acid

Adenosine Thymidine Guanidine Cytidine A Sequence of DNA

Base-Pairing - the ‘Glue’ of the Double Helix Thymine Adenine Cytosine Guanine to ‘backbone’ 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 4 x 4=16 • Thus if we use a sequence of 3 bases , we can code for 4 x 4 x 4=64 amino acids

RNA transforms genetic info into action • Messenger RNA (m. RNA) –carries info from DNA out of the nucleus into the cytoplasm • Transfer RNA (t. RNA) –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=4 x 4 x 4=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 (1 st 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