Microbial Genetics

?Microbial Genetics

?Molecular biology is the study of biology at a molecular level. It concerns with the interactions between the various systems of a cell, including the interrelationship of DNA, RNA and protein synthesis and learning how these interactions are regulated.
?Molecular genetics is the field of biology which studies the structure and function of genes at a molecular level.
Structure and Function of Genetic material:
DNA Structure:
In the 1950’s, Watson & Crick built the first model of DNA:-
• Two strands coiled called a double helix.
• Sides made of a pentose sugar Deoxyribose bonded to phosphate (PO4) groups by phosphodiester bonds.
• Center made of nitrogen bases bonded together by weak hydrogen bonds.















5’ end-means P comes off 5’ carbon of deoxyribose sugar
3’means P comes of 3’ end of deoxyribose sugar



DNA Replication
• DNA has to be copied before a cell divides
• DNA is copied during the S or synthesis phase of interphase
• New cells will need identical DNA strands
Semiconservative Model of Replication
• Idea presented by Watson & Crick
The two strands of the parental molecule separate, and each acts as a template for a new complementary strand
New DNA consists of 1 parental (original) and 1 new strand of DNA



RNA

RNA Functions
Three major RNAs:-
mRNA (messenger RNA): DNA transcript.
tRNA(transfer RNA): transfer amino acid during protein synthesis.
rRNA(ribosomal RNA): make up ribosomes.
Gene expression: When a gene is expressed two processes occur:
• 1) transcription – DNA transcribed to produce RNA
• 2) translation – RNA then translated to produce proteins
• Protein Synthesis
DNA--------- mRNA---------- Protein
Transcription Translation
Central Dogma of Molecular Genetics

Transcription:-
• The process by which a molecule of DNA is copied into a complementary strand of RNA.


Translation:-
Genetic Code
1. Codons 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







What are the genetic changes? An alteration in a segment of DNA, which can disturb a gene s behavior and sometimes leads to disease.
It may be:
(1) Small genetic change, genetic drifts (mutation)
(2) Large genetic change, genetic shift (recombination)
Mutation Repair Mechanisms:-
- Mismatch repair “proofreads” (DNA polymerase).
- Excision repair (nucleases).
Genetic Transfer in Bacteria (Recombination):
- Vertical gene transfer
Genetic information passed from an organism to its offspring.
- Horizontal gene transfer
Bacteria transfer genetic information form one organism to another in the same generation. Three ways:
1. Transduction
2. Transformation:
3. Conjugation:
Transduction by a Bacteriophage :- e.g. Corynebacterium diphtheria












Transformation: - e.g. Streptococcus peumoniae















Conjugation : - e.g. E. coli