PHD Lecture Notes Gene Tech I

7/28/2019 PHD Lecture Notes Gene Tech I

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Microbial genetics

and Genetic elements in Microbes

LECTURE 1

Course teacher:

Dr. G.GOPALASWAMY

Professor


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Contents

Genetic elements in microbes

Genomes, DNA, RNA basic concepts

Transfer of genetic information in prokaryotes and

eukaryotes - DNA replication supercoiling in bacteriaTranscription and translation

Structure and importance of bacterial and eukaryoticplasmids -Role of plasmids in bacteria

Transposable elements present in bacteria insertionsequences, transposons, special viruses


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Kinds of genetic elements


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Genomes vary in size smallest virus 4-5 genesE. coli single chromosome containing 4,288

genes; 1 mm; 1,000X longer than cellHuman cell 46 chromosomes containing 31,000genes; 6 feet; 180,000X longer than cell

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Basic Concepts Nucleic AcidsComposed of chains of nucleotides

Nucleic acid molecules are usually composed of 4 different nucleotides A nucleic acid molecule may contain several thousandsor millions of nucleotides

Each nucleic acid molecule has its own order, orsequence, of nucleotides The correct sequence of nucleotides is essential for thenucleic acids function


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Basic Concepts Nucleotide structure A nucleotide consists of:

Nitrogenous basePentose sugarPhosphate group

Nitrogenous bases:Purines: adenine & guanine

Pyrimidines: cytosine, thymine (in DNA), & uracil (in RNA)Pentose sugars:Ribose (found in RNA)Deoxyribose (found in DNA)


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Basic Concepts DNA: Deoxyribonucleic acidPentose sugar: 2 -deoxyribose

Nitrogenous bases: Adenine and guanine (purines)Cytosine and thymine (pyrimidines)Structure is typically a double-stranded helix

Nucleotide sequences of the strands are complementary to each other, A pairing with T and C pairing with G


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DNA structure 2 strands twisted into a helixsugar -phosphate backbone

nitrogenous bases form steps in ladder

constancy of base pairingA binds to T with 2 hydrogen bonds

G binds to C with 3 hydrogen bonds

antiparallel strands 3to 5 and 5to 3

each strand provides a template for the exactcopying of a new strand

order of bases constitutes the DNA code

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Basic Concepts RNA: Ribonucleic acidPentose sugar: RiboseNitrogenous bases: Adenine and guanine (purines)Cytosine and uracil (pyrimidines)Structure is typically single-stranded; however, theremay be internal complementary regions within an RNA

strand that can form double- stranded hairpin loops (Cto G; A to U) An RNA strand can also form a double-strandedstructure with a DNA strand; in this case, the U on theRNA will base-pair with the A on the DNA.


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Basic Concepts The nucleotide sequence of a nucleic acid moleculeencodes the amino acid sequence of a protein.Genome: The entire nucleotide sequence of an

organism; transmitted to offspring during reproductionDeoxyribonucleic acid (DNA): DNA molecules serve asthe genome for the proteins of all cellular organisms,both eukaryotic and prokaryotic. DNA also serves as thegenome for certain viral groups.Ribonucleic acid (RNA): RNA molecules serve as anintermediate in gene expression in eukaryotic andproyaryotic organisms, as well as some viruses. RNA serves as the genome for certain viral groups.


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Basic Concepts DNA Replication: The sequence of a nucleotides in aDNA molecule serves as a template to copy itself, so twoidentical copies of the DNA helix are formed.Transcription: The sequence of nucleotides in a DNA molecule serves as a template for the synthesis of anRNA molecule; typically, only a small segment of theDNA is copied. This is the first step in gene expression.

Translation: The sequence of nucleotides in an RNA molecule serves to direct the assembly of amino acidsinto a protein chain on a ribosome. This is the secondstep in gene expression.


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Central dogma of life


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Basic Concepts GeneContemporary understanding:

A segment on a DNA moleculeUsually at a specific location (locus) on a chromosome or plasmidCharacterized by its nucleotide sequence

Genes play three notable roles:To encode the nucleotide sequences of mRNA, which in turnencodes the amino acid sequences of proteinsTo encode the nucleotide sequences of tRNA or rRNA To regulate the expression of other genes

Mutation:Change in the nucleotide sequence of a gene, usually resulting froman error during DNA replcation


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Genotype - genes encoding all the potentialcharacteristics of an individual

Phenotype -actual expressed genes of an individual(its collection of proteins)


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Basic Concepts PhenotypeThe appearance or discernible characteristics of a trait in anindividualMutation in a gene responsible for a phenotype may cause achange in the phenotype.Typically, more that one gene is responsible for a phenotype.

GenotypeThe genetic makeup of an individual with reference to one ormore specific genes A genotype is designated by using symbols to represent themutations of the gene


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Basic Concepts Microbial Genotypes & PhenotypesMicrobial phenotypes are usually designated by anonitalicized 3-letter abbreviation that in some way refers

to the appearance or effect of the phenotype. Variation inthe phenotype may be designated by superscripts such as+ or for auxotrophic mutations, R or S forantibiotic resistance mutations, etc.Microbial genes are usually designated by an italicized, 3-letter abbreviation (that often refers to the phenotypiceffect by which the gene was discovered) plus a letter thatdistinguishes one gene in a family from other genes thatproduce the same phenotype.


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Basic Concepts Microbial Genotypes & Phenotypes (cont.)For example:

A lysine prototroph of E. coli, capable of making its ownlysine, is designated lys +. An auxotrophic mutant, incapable of making its own lysine, is designated lys .There are several different genes responsible for lysineproduction in E. coli. These genes encode the different

enzymes in the metabolic pathway for lysine synthesis. Thegenes are designated lysA, lysB, lysC , and so on.Mutation in any of the genes responsible for lysine productionmay block the lysine pathway and produce the Lys phenotype.


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DNA-protein relationship 1. Each triplet of nucleotides (codon) specifies aparticular amino acid.

2. A proteins primary structure determines its shape& function.

3. Proteins determine phenotype. Living things are what their proteins make them.

4. DNA is mainly a blueprint that tells the cell whichkinds of proteins to make and how to make them.


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DNA-protein relationship


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prokaryotic and eukaryotic chromosom


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gene transfer in prokaryote and eukaryote

Polycistronic mRNAMonocistronic mRNA


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Gene expression Transcription DNA is used to synthesize RNA RNA polymerase is the enzyme responsible

Translation making a protein using the informationprovided by messenger RNA occurs on ribosomes

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Importance of super coiling in DNA replication


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Stages of super coiling


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Plasmids


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Simplified model of mosaic structure of circular plasmi


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Phenotype conferred by plasmids


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Phenotype conferred by plasmids


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Biggest plasmids of agricultural importance


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Structure of transposons


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THE END

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PHD Lecture Notes Gene Tech I