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►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
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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.
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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.
2.
3.
4.
5.
Codons code for a specific amino acid
20 amino acids
3 base code - 4 bases ( A,U,G,C )
64 possible combinations ( 43)
Amino acids are coded for by more than one codon
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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
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Transformation: - e.g. Streptococcus peumoniae
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Conjugation : - e.g. E. coli
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