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THE GENETIC BASIS OF LIFE

THE GENETIC BASIS OF LIFE

(From http://generalhorticulture.tamu.edu)

 

Analogy

DEFINITIONS
DNA (deoxyribonucleic acid)- a double helix chain of sugar-phosphates (deoxyribo 
          sugar-phosphates) connected by nucleic acids (adenine, thymine, guanine, cytosine).
RNA (ribonucleic acid) - a single stranded chain of sugar-phosphates (ribo sugar-phosphates)
           containing nucleic acids (adenine, uracil, guanine, cytosine).
nucleic Acids - organic acids that form the base pairs of DNA and single-bases of  RNA.

  Base Pairing of Nucleic Acids between the double strands of  DNA
    A- T  (adenine-thymine)
    G - C (guanine-cytosine)
  Base Pairing of Nucleic Acids between DNA strands and RNA strands
    A - U  (adenine-uracil)
    G - C (guanine-cytosine)

gene - a length of DNA that codes for the production of a protein or protein subunit.
         - also codes for active RNAs (such as tRNA).
protein - a polymer or chain of amino acids.
enzyme - a protein that acts as a metabolic catalyst.

The Central Dogma of Molecular Biology

(From: http://www.accessexcellence.org)

Legend:
Transcription of DNA to RNA to protein: This dogma forms the backbone of molecular biology and is represented by four major stages.

1. The DNA replicates its information in a process that involves many enzymes: replication.

2. The DNA codes for the production of messenger RNA (mRNA) during transcription.

3. In eucaryotic cells, the mRNA is processed (essentially by splicing) and migrates from the nucleus to the cytoplasm.

4. Messenger RNA carries coded information to ribosomes. The ribosomes "read" this information and use it for protein synthesis. This process is called translation.

Proteins do not code for the production of protein, RNA or DNA.
They are involved in almost all biological activities, structural or enzymatic.

The Genetic Code

(From: http://www.accessexcellence.org)

 

DNA transfers information to mRNA in the form of a code defined by a sequence of nucleotides bases. During protein synthesis, ribosomes move along the mRNA molecule and "read" its sequence three nucleotides at a time (codon) from the 5' end to the 3' end. Each amino acid is specified by the mRNA's codon, and then pairs with a sequence of three complementary nucleotides carried by a particular tRNA (anticodon).

Since RNA is constructed from four types of nucleotides, there are 64 possible triplet sequences or codons (4x4x4). Three of these possible codons specify the termination of the polypeptide chain. They are called "stop codons". That leaves 61 codons to specify only 20 different amino acids. Therefore, most of the amino acids are represented by more than one codon. The genetic code is said to be degenerate.

 

 

Restriction Enzymes Cut DNA at Specific Sequences

to Create “Sticky Ends”

(From: http://www.accessexcellence.org)

The EcoRI restriction enzyme--the first restriction enzyme isolated from E. Coli bacteria--is able to recognize the base sequence 5' GAATTC 3'. Restriction enzymes cut each strand of DNA between the G and the A in this sequence. This leaves "sticky ends" or single stranded overhangs of DNA. Each single stranded overhang has the sequence 5" AATT 3'. These overhanging ends will bond to a fragment of DNA which has the complementary sequence of bases. See text of Background Paper for additional details.

Cloning DNA into a Plasmid to Produce Recombinant DNA

(From: http://www.accessexcellence.org)

Process by which a plasmid is used to import recombinant DNA into a host cell for cloning. The plasmid carrying genes for antibiotic resistance, and a DNA strand, which contains the gene of interest, are both cut with the same restriction endonuclease. They have complementary "sticky ends." The opened plasmid and the freed gene are mixed with DNA ligase, which reforms the two pieces as recombinant DNA.  This produces recombinant Deaths recombinant DNA stew transforms a bacterial culture, which is then exposed to antibiotics. All the cells except those which have been encoded by the plasmid DNA recombinant are killed, leaving a cell culture containing the desired recombinant DNA. DNA cloning allows a copy of any specific part of a DNA (or RNA) sequence to be selected among many others and produced in an unlimited amount. This technique is the first stage of most of the genetic engineering experiments: production of DNA libraries, PCR, DNA sequencing, et al.


Using restriction enzymes for Mapping or Finger Printing

When DNA from the same source is digested with a particular restriction enzyme it will always give a set of the same sized fragments. For example if lambda bacteriophage DNA is cut with EcoR1 we know that it will give six fragments of the sizes:  21.23, 7.42, 5.8, 5.65, 4.87, 3.53 kbp. This is because, mutations apart, the phage sequence will always be the same, and so EcoR1 cutting sites will always be present in the same places. The fragments can be separated and their sizes determined by agarose gel electrophoresis.

We can use the positions of restriction enzyme sites as convenient markers along DNA sequences. The map obtained can be used for DNA identification and to plan DNA manipulations.

Finger Printing

Gel Showing Banding from use of Different Restriction Enzymes

 

Genetically Modified Organisms (GMO) or Transgenic Crops

(From: http://www.colostate.edu/programs/lifesciences/TransgenicCrops/index.html)

Authors:  Pat Byrne, Sarah Ward, Judy Harrington, Lacy Fuller (Web Master)

 

Crops and acreage

 

Transgenic crop production area by country (source: James, 2000b)

Country

Area planted in 2000
(millions of acres)

Crops grown

USA

74.8

soybean, corn, cotton, canola

Argentina

24.7

soybean, corn, cotton

Canada

7.4

soybean, corn, canola

China

1.2

cotton

South Africa

0.5

corn, cotton

Australia

0.4

cotton

Mexico

minor

cotton

Bulgaria

minor

corn

Romania

minor

soybean, potato

Spain

minor

corn

Germany

minor

corn

France

minor

corn

Uruguay

minor

soybean

Widely Used GMOs

Worldwide production area of transgenic crops – Traits

(source: Science 286:1663, 1999).

Trait

Area planted in 1999 (millions of acres)

Herbicide tolerance

69.4

Bt insect resistance

22.0

Bt + herbicide tolerance

7.2

Virus resistance

0.3

Herbicide Tolerance  Herbicide tolerant crops resolve many of those problems because they include transgenes providing tolerance to the herbicides Roundup® (chemical name: glyphosate) or Liberty® (glufosinate). These herbicides are broad-spectrum, meaning that they kill nearly all kinds of plants except those that have the tolerance gene. Thus, a farmer can apply a single herbicide to his fields of herbicide tolerant crops, and he can use Roundup and Liberty effectively at most crop growth stages as needed.

Weed-infested soybean plot (left) and Roundup Ready® soybeans after Roundup treatment. Source: Monsanto

 

Bt Insect-Resistant Crops

"Bt" is short for Bacillus thuringiensis, a soil bacterium whose spores contain a crystalline (Cry) protein. In the insect gut, the protein breaks down to release a toxin, known as a delta-endotoxin. This toxin binds to and creates pores in the intestinal lining, resulting in ion imbalance, paralysis of the digestive system, and after a few days, insect death.

European corn borer (left) and cotton bollworm (right) are two pests controlled by Bt corn and cotton, respectively.
Source: USDA.

Bt insect-resistant crops currently on the market include

·         Corn: primarily for control of European corn borer, but also corn earworm and Southwestern corn borer. Cotton: for control of tobacco budworm and cotton bollworm

·         Potato: for control of Colorado potato beetle. Bt potato has been discontinued as a commercial product.

 

Papaya ringspot virus

Papaya is a tropical fruit rich in Vitamins A and C, but susceptible to a number of serious pests and diseases. The transgenic variety UH Rainbow, resistant to the papaya ringspot virus, is currently in production in Hawaii.

Papaya is an important source of vitamins in tropical areas. Source: USDA

 

 

Risks And Concerns

(http://www.colostate.edu/programs/lifesciences/TransgenicCrops/risks.html)

The introduction of transgenic crops and foods into the existing food production system has generated a number of questions about possible negative consequences. People with concerns about this technology have reacted in many ways, from participating in letter-writing campaigns to demonstrating in the streets to vandalizing institutions where transgenic research is being conducted. What are the main concerns? What scientific support is there for these concerns?