Introduction

1. Microbial Genetics and Variation 2019
MICROBIAL GENETICS ANDVARIATION
History of Genes
1.The genetics were deduced by Gregor Mendel in 1865 on the basis of breeding
experiments with peas.
He assumed that each trait is determined by a pair of inherited ‘factors’ which are now called
gene.
2. Mendel’s work was rediscovered in 1900 by Hugo de Vries, Correns and Tschermak.
3. Wilhelm Johannes coined the term ‘GENE’ in 1909.
4. William Bateson in 1905 coined the term genetics.
5.In 1972, Walter Fiers and his team determined the sequence of a gene in bacteriophage
MS2 coat protein.
6.Richard J Roberts and Phillip Sharp found the gene can be split into segments making it
possible that a single gene might be coding for several proteins.
A comparison of Genes
Gene
 The gene is the Functional unit of Heredity.
 Each gene is a segment of DNA that give rise to a protein product or RNA.
 A gene may exist in alternative forms called alleles.
 Chromosome in fact carries genes.
 Each chromosome consists of a linear array of genes.
 A specific DNA sequence that contains geneticinformation
 Information required to make a specific type of protein
 that information is stored in the sequence on the “sense” strand
 we say that a gene encodes a protein
 thus a DNA molecule can contain many genes
 The gene sequence is always written 5‟.
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2. Microbial Genetics and Variation 2019
Genetic Code
• Proteins are composed of 20 different amino acids
• A sequence of 3 nucleotides is used to code each amino acid.
• Each triplet of nucleotides is called a codon.
• Start codon AUG codes for amino acid methionine.
• 3 stop codons
• There are 64 codons in the genetic code 43
=64
• Several different codons can code for the same aa, but no codon ever has more than
one amino acid counterpart.
• Codons are always written in the form of the RNA transcript from the original DNA
molecule.
Base triplet that codes for an amino acid
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Translation
 Three parts: Initiation-start codon (AUG)
 Elongation-ribosome moves along mRNA
 Termination: stop codon reached/polypeptide released and new protein forms
 rRNA=subunits that form the 70 S ribosomes (protein synthesis occurs here)
 tRNA=transfers amino acids to ribosomes for protein synthesis)
Gene mutation
o Changes in base sequence of DNA/lethal and inheritable Can be:
 Harmful
 Lethal
 Helpful
 Silent
o Information coded in the DNA sequence is used to make proteins
o A change in the genetic information is called a mutation. The outcome
depends on the nature of the “change:.
o 3 types of DNA sequence mutations.
o Mutation can also happen due to:
 changes in long DNAsequences
 changes in the structure of genes/ chromosomes
 changes in the number of genes/ chromosomes
e.g. Mutations leading to cystic fibrosis
Regulation of Gene expression
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4. Microbial Genetics and Variation 2019
 It is the process by which information from a gene is used in the synthesis of a
functional gene product.
 Regulation of gene expression is of two types:
1. Positive regulation: When the expression of genetic is quantitatively increased by the
presence of specific regulatory element is known as positive regulation
2. Negative regulation: when the expression of genetic information diminished by the
presence of specific regulatory element
 Gene regulation is essential for viruses, prokaryotes and eukaryotes as it increases the
versatility and adaptability of an organism by allowing the cell to express protein
when needed.
 The first discovered example of a gene regulation system was the lac operon,
discovered by Jacques Monod , in which protein involved in lactose metabolism are
expressed by E.coli only in the presence of lactose and absence of glucose.
Principles of Gene Regulation:
1) RNA polymerase binds to DNA at promoters.
2) Transcription initiation is regulated by proteins that bind to or near promoters.
 Repression of a repressible gene:( i.e., negative regulation) repressors (vs. activators)
bind to operators of DNA.
 Repressor is regulated by an effector, usually a small molecule or a protein, that binds
and causes a conformational change.
 Activator binds to DNA sites called enhancer to enhance the RNApolymerase
activity. (i.e., positive regulation) Induction of a inducible gene, e.g, heat-shock genes.
Genetic Transfer in Bacteria
A. Genetic transfer-results in genetic variation
B.Genetic variation-needed for evolution
A and B changes in three ways:
Transformation: genes transferred from one bacterium to another as “naked” DNA
Conjugation: plasmids transferred 1 bacterium to another via a pilus
Transduction: DNA transferred from 1 bacterium to another by a virus
Gene Protein synthesis
• Genes are a sequence of nucleotides in DNA that code for a particular protein.
• Proteins drive cellular processes, determine physical characteristics, and manifest
genetic disorders by their absence or presence.
• Information is copied from DNA into mRNA, this is transcription.
• mRNA leaves the nucleus and enters the cytoplasm of the cell:
Ribosomes use the mRNA as a blueprint to synthesize proteins composed of aa, this is
translation.
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Polymerase chain reactions
 PCR, polymerase chain reaction, is an in-vitro technique for amplification of a region
of DNA whose sequence is known or which lies between two regions of known
sequence.
 PCR a technique used in molecular biology to amplify a single copy or a few copies
of a segment of DNA across several orders of magnitude, generating thousands to
millions of copies of a particular DNAsequence.
Applications
 Genome mapping and gene function determination
 Biodiversity studies (e.g. evolution studies)
 Diagnostics (prenatal testing of genetic diseases, early detection of cancer, viral
infections)
 Detection of drug resistance genes
 Forensic (DNAfingerprinting)
Genetic engineering
- Changing the DNA in living organisms is to create something new. This organism is
called genetically Modified Organism (GMO).
Example: i) Bacteria that produce human insulin
- Genetically Modified organism are called transgenic organism; since genes are
transferred from one organism to another.
- Some genetic engineering techniques are as follows:
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1.Artificial selection: breeders choose which organism to mate to produce offspring with
desired traits.
A. Selective breeding: when animals with desired characteristics are mated to produce
offspring with those desired traits. Eg: Cow, Egg
B. Hybridization: two individuals with unlike characteristics are crossed to produce the
best in both organisms.Eg: Lion+ Tiger, Grapes+Apple
C. Inbreeding: breeding of organism that genetically similar to maintain desired
traits.
2. Cloning: creating an organism that is an exact genetic copy of another.
3. Gene splicing: DNA is cut out of one organism and put into another organism
4. Gel electrophoresis: a technique used to compare DNA from two or more organisms.
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