Chapter 9

Genetically Modified Organisms

 

Genetic Engineering

§ Alteration of hereditary traits by molecular biological techniques

§ One or more genes may be modified

§ Genes may be moved from one organism to another

§ There is controversy over genetic engineering surrounding ethics, use in food sources, and humans

 

9.1 Protein Synthesis and Expression

§ In the early 1980s, genetic engineers at Monsanto® Company began producing recombinant bovine growth hormone (rBGH)

§ Made by genetically engineered bacteria

§ The bacteria were given DNA that carries instructions for making BGH

§ In cows, growth hormones increase body size and milk production

 

9.1 Protein Synthesis and Expression: From gene to protein

§ Protein synthesis – the process of using instructions carried on a gene to create proteins.

§ Several steps are involved and require both DNA and RNA.

§ Gene a sequence of DNA that encodes a protein

§ Protein – a large molecule composed of amino acids

 

§ DNA

§ Double-stranded

§ Each nucleotide composed of deoxyribose, phosphate, and nitrogenous base

§ 4 bases: adenine (A), thymine (T), guanine (G), cytosine (C)

 

§ RNA

§ Single-stranded

§ Nucleotides comprised of ribose, phosphate, and nitrogenous base

§ 4 bases: A, C, G, and Uracil

 

§ The flow of genetic information in a cell is DNA ® RNA ® protein

 

§ There are 2 steps in going from gene to protein

§ Transcription (DNA ® RNA)                                  

§ Translation (RNA ® Protein)

 

9.1 Protein Synthesis and Expression: Trancription

§ Transcription occurs in the nucleus.

§ RNA polymerase binds to the promoter region of the gene.

§ RNA polymerase zips down the length of gene, matching RNA nucleotides with complementary DNA nucleotides

 

§ The product of transcription is messenger RNA (mRNA).

 

9.1 Protein Synthesis and Expression: Translation

§ Translation occurs in the cytoplasm (outside the nucleus).

§ Translation requires: mRNA (made during transcription), amino acids, energy (ATP), and some helper molecules.

§ Ribosomes

§ Transfer RNA (tRNA)

 

§ Ribosomes

§ The ribosome is composed of rRNA and comprises a small and a large subunit.

 

§ Transfer RNA: tRNA carries amino acids and matches its anticodon with codons on mRNA

 

§ A protein is put together one amino acid at a time.

§ The ribosome attaches to the mRNA at the promoter region.

§ Ribosome facilitates the docking of tRNA anticodons to mRNA codons.

§ When two tRNAs are adjacent, a bond is formed between their amino acids.

 

9.1 Protein Synthesis and Expression: Genetic Code

§ The genetic code allows a specific codon to code for a specific amino acid.

§ A codon is comprised of three nucleotides = 64 possible combinations (43 combinations)

§ 61 codons code for amino acids (& codon redundancy)

§ 3 others are stop codons, which end protein synthesis

 

9.1 Protein Synthesis and Expression: Mutations

§ Changes in genetic sequence = mutations

§ Changes in genetic sequence might affect the order of amino acids in a protein.

§ Protein function is dependent on the precise order of amino acids

§ Possible outcomes of mutation:

1 - no change in protein (neutral mutation)

2 - non-functional protein

3 - different protein

 

§ Base-substitution mutation – simple substitution of one base for another

§ Frameshift mutation – addition or deletion of a base, which changes the reading frame

 

9.1 Protein Synthesis and Expression: An Overview of Gene Expression

§ Each cell in your body (except sperm and egg cells) has the same DNA.

§ But each cell only expresses a small percentage of genes.

§ Example: Nerve and muscle cells perform very different functions, thus they use different genes.

§ Turning a gene or a set of genes on or off = regulating gene expression

 

§ Nerves and cells have the same suite of genes, but express different genes.

 

9.2 Producing Recombinant Proteins: Cloning a Gene Using Bacteria

§ rBGH is a protein, and is coded by a specific gene.

§ Transfer of rBGH gene to bacteria allows for growth under ideal conditions.

§ Bacteria can serve as “factories” for production of rBGH.

 

§ Restriction enzymes – Used by bacteria as a form of defense. Restriction enzymes cut DNA at specific sequences. They are important in biotechnology because they allow scientists to make precise cuts in DNA.

§ Plasmid – Small, circular piece of bacterial DNA that exists separate from the bacterial chromosome. Plasmids are important because they can act as a ferry to carry a gene into a cell.

 

§ Step 1. Remove the gene from the cow chromosome

 

§ Step 2. Insert the BGH gene into the bacterial plasmid

 

§ Recombinant – Indicates material that has been genetically engineered. A gene that has been removed from its original genome and combined with another.

§ After step 2, the GBH is now referred to as r (recombinant) GBH.

 

§ Step 3. Insert the recombinant
plasmid into a bacterial cell

 

§ The same principles apply to other proteins.

§ Clotting proteins for hemophiliacs are produced using similar methods.

§ Insulin for diabetics is also produced in this way.

 

§ About 1/3 of cows in the US are injected with rBGH. rBGH increases milk volume from cows by about 20%.

§ Controversy over safety to humans: USDA and Monsanto argue that milk from rBGH-treated cows is indistinguishable from non-treated. Activists disagree.

§ Welfare of cows? Europe and Canada banned rBGH over concerns of effects of rBGH on the health of cows.

 

9.3 Genetically Modified Foods

§  All agricultural products are the result of genetic modification through selective breeding. Artificial selection does not move genes from one organism to another, but does drastically change the characteristics of a population.

 

9.3 Genetically Modified Foods: Why Genetically Modify Crop Plants?

§ Increase shelf life, yield, or nutritional value

§ Golden rice has been genetically engineered to produce beta-carotene, which increases the rice’s nutritional yield.

 

9.3 Genetically Modified Foods: Modifying Plants with the Ti Plasmid and Gene Gun

§ Unlike rBGH, crop plants are directly modified. In order to do this, the target gene must be inserted into the plant cell. Two methods to do this:

§ Ti plasmid

§ Gene gun

 

§ Transgenic organism – the result of the incorporation of a gene from one organism to the genome of another. Also referred to as a Genetically Modified Organism (GMO).

 

9.3 Genetically Modified Foods: Effect of GM Crops and the Environment

§ Benefits: Crops can be engineered for resistance to pests, thus farmers can spray fewer chemicals.

§ Concerns: Pests can become resistant to chemicals. GM crops may actually lead to increased use of pesticides and herbicides. GM crop plants may transfer genes to wild relatives.

 

9.4 Genetically Modified Humans: Stem Cells

§ Stem cells – undifferentiated cells, capable of growing in to many different kinds of cells and tissues.

 

§ Stems cells might be used to treat degenerative diseases such as Alzheimer’s or Parkinson’s, multiple sclerosis, or liver, lung, or heart disease.

§ Stem cells could also be used to grow specific tissues to treat burns, heart attack damage, or replacement cartilage in joints.

 

9.4 Genetically Modified Humans: Human Genome Project

§ Human Genome Project – international effort to map the sequence of the entire human genome (~20,000 – 25,000 genes).

§ For comparative purposes, genomes of other model organisms (E. coli, yeast, fruit flies, mice) were also mapped.

§ It was sequenced using the technique of chromosome walking.

 

9.4 Genetically Modified Humans: Gene Therapy

§ Gene therapy – replacement of defective genes with functional genes

§ Two approaches:

§ Germ line gene therapy

§ Somatic cell gene therapy

 

§ Germ line gene therapy – embryonic treatment

§ Embryo supplied with a functional version of the defective gene.

§ Embryo + cells produced by cell division have a functional version of gene.

 

§ Somatic cell gene therapy – fix or replace the defective protein only in specific cells.

§ Used as a treatment of SCID (severe combined immunodeficiency)

 

§ All somatic cells have limited lifetimes.

§ Therapy is not permanent and requires several treatments per year.

 

9.4 Genetically Modified Humans: Cloning Humans

§ Human cloning occurs naturally whenever identical twins are produced.

§ Cloning of offspring from adults has already been done with cattle, goats, mice, cats, pigs, and sheep.

§ Cloning is achieved through the process of nuclear transfer.