Gene expression

Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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Gene expression: When a gene is expressed two processes occur:
1) transcription – DNA transcribed to produce mRNA.
2) translation – mRNA then translated to produce proteins.
Transcription Translation
DNA------------------------- mRNA-------------------------Protein
Transcription:-
• The process by which a molecule of DNA is copied into a complementary strand of RNA.
Translation:-
1. Codon code for a specific amino acid
2. 20 amino acids
3. 3 base code - 4 bases ( A,U,G,C )
4. 64 possible combinations ( 43)
5. Amino acids are coded for by more than one codon
Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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What are the genetic changes?
An alteration in a segment of DNA which can disturb gene s behavior and
sometimes leads to disease. It may be:
(1) Small genetic change, genetic drift (mutation)
(2) Large genetic change, genetic shift (recombination)
Mutation: changes in base sequence of DNA. Types of Mutations:
1. Spontaneous mutation:- occur in the absence of a mutagen, example:
Neisseria gonorrhoeae penicillin resistance original mutation was
spontaneous.
2. Induced mutation: - chemical or physical agents enhance mutation rate.
Mutagens
Tobacco products, nitrous acid, mold toxins, X-rays, Gamma rays, UV radiation,
some artificial sweeteners
Repair Mechanisms:-
1. Mismatch repair “proofreads” (DNA polymerase).
2. Excision repair (nucleases).
Recombination (Genetic Transfer in Bacteria):-
- Genetic transfer-results in genetic variation
- Genetic variation-needed for evolution
- Vertical genetic transfer
Genetic information passed from an organism to its offspring.
- Horizontal genetic transfer
Bacteria transfer genetic information form one organism to another in the
same generation, by three ways:
1. Transduction: donor DNA is carried in a phage coat and is
transferred into the recipient by the mechanism used for phage
infection DNA transferred from one bacterium to another by a virus.
2. Transformation: the direct uptake of donor DNA by the recipient
cell, may be natural or forced genes transferred from one bacterium to
another as “naked” DNA.
3. Conjugation: only one strand of DNA is transferred plasmids
transferred one bacterium to another via a pilus.
Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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Conjugation: - e.g. E. coli
Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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Transduction by a Bacteriophage: - e.g. Corynebacterium diphtheria
Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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Transformation: - e.g. Streptococcus peumoniae
Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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Recombinant DNA technology is a technology which allows DNA to be
produced via artificial means.
? Recombinant DNA technology works by taking DNA from two different
sources and combining that DNA into a single molecule. That alone,
however, will not do much.
? Recombinant DNA technology only becomes useful when that artificiallycreated
DNA is reproduced. This is known as DNA cloning.
Medical Microbiology, 24th Edition: Jawetz, Melnick, & Adelberg /Microbial Genetics
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Six basic steps are common to most recombinant DNA experiments:
1. Isolation and purification of DNA.
2. Cleavage of DNA at particular sequences by nucleases and restriction
endonucleases.
3. Ligation of DNA fragments.
4. Introduction of recombinant DNA into compatible host cells.
5. Replication and expression of recombinant DNA in host cells.
6. Identification of host cells that contain recombinant DNA of interest.
Applications of Recombinant DNA Technology:
1. Analysis of Gene Structure and Expression
2. Pharmaceutical Products
– Drugs
– Vaccines
3. Genetically modified organisms (GMO)
– Transgenic plants
– Transgenic animal
4. Application in medicine
? Human Gene Therapy
? Diagnosis of genetic disorders
? Forensic Evidence
? Human gene therapy seeks to repair the damage caused by a genetic
deficiency through introduction of a functional version of the defective gene.
To achieve this end, a cloned variant of the gene must be incorporated into
the organism in such a manner that it is expressed only at the proper time
and only in appropriate cell types. At this time, these conditions impose
serious technical and clinical difficulties.