3A1 Continued

Phenotypes are determined through protein activities

PROTEINS DICTATE PHENOTYPEProteins dictate virtually every reaction in the cell directly responsible for observable characteristics

How do the proteins work to direct phenotype? Structural functions (for example, in the maintenance

of cell and/or tissue shape and rigidity) Transport of molecules Communication between cells. Substantial proportion of proteins are enzymes ,

catalyzing chemical reactions for the synthesis and transformation of virtually all biological molecules.

EXAMPLESVarying types and quantities of all biological molecules in the cells and tissues of an individual is what ultimately leads to phenotypic variation. Slight variation in the activity of an enzyme for

pigment synthesis in a plant may result in white flowers rather than red

Slight difference in a protein responsible for cell communication during the development of leaf tissue might result in variation of leaf shape

Understanding the path from genotype to phenotype is a major concern of modern molecular biology and one of the ultimate goals of the human genome project

GENETIC ENGINEERING

Gel Electrophoresis Separate DNA fragments by size Gel made of agarose or polyacrylamide Submersed in buffer that can carry current Subjected to an electrical field Negatively-charged DNA migrates towards

the positive pole Larger fragments move slower, smaller

move faster DNA is visualized using fluorescent dyes

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DNA Analysis

Polymerase chain reaction (PCR) Developed by Kary Mullis

Awarded Nobel Prize Allows the amplification of a small DNA fragment

using primers that flank the region Each PCR cycle involves three steps:

1. Denaturation (high temperature)2. Annealing of primers (low temperature)3. DNA synthesis (intermediate temperature)

Taq polymerase

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After 20 cycles, a single fragment produces over one million (220) copies!

Genetic Engineering Has generated excitement and controversy Expression vectors contain the sequences

necessary to express inserted DNA in a specific cell type

Transgenic animals contain genes that have been inserted without the use of conventional breeding

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In vitro mutagenesis Ability to create mutations at any site in a cloned

gene Has been used to produce knockout mice

A known gene is inactivated The effect of loss of this function is then

assessed on the entire organism An example of reverse genetics

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Medical Applications Medically important proteins can be produced

in bacteria Human insulin Interferon Atrial peptides Tissue plasminogen activator Human growth hormone Problem has been purification of desired proteins

from other bacterial proteins

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Genetically engineered mouse with human growth hormone

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Vaccines Subunit vaccines

Genes encoding a part of the protein coat are spliced into a fragment of the vaccinia (cowpox) genome

Injection of harmless recombinant virus leads to immunity

DNA vaccines Depend on the cellular immune response (not

antibodies)

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Gene therapy Adding a functional copy of a gene to correct a

hereditary disorder Severe combined immunodeficiency disease

(SCID) illustrates both the potential and the problems On the positive side, 15 children treated successfully

are still alive On the negative side, three other children treated have

developed leukemia (due to therapy)

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Agricultural Applications Ti (tumor-inducing) plasmid

Most used vector for plant genetic engineering Obtained from Agrobacterium tumefaciens, which

normally infects broadleaf plants Part of the Ti plasmid integrates into the plant

DNA and other genes can be attached to it However, bacterium does not infect cereals such

as corn, rice, and wheat

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Applications of PCR Allows the investigation of minute samples of DNA Forensics – drop of blood, cells at base of a hair Detection of genetic defects in embryos by

analyzing a single cell Analysis of mitochondrial DNA from early human

species

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