Microbial Genetics: Protein Synthesis, Gene Regulation, and Genetic Variation

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Microbial Genetics: Protein Synthesis, Gene Regulation, and Genetic Variation

Introduction to the Central Dogma

The central dogma of molecular biology describes the flow of genetic information within a biological system. It explains how genetic information stored in DNA is transcribed into RNA and then translated into proteins, which perform essential cellular functions.

DNA stores genetic information.
RNA acts as an intermediary, carrying instructions from DNA for protein synthesis.
Proteins are the functional molecules that result from gene expression.

Central dogma: DNA to RNA to Protein

RNA and Protein Synthesis

Types of RNA

RNA (ribonucleic acid) is a single-stranded molecule composed of nucleotides. It contains ribose sugar and uracil (U) instead of thymine (T).

mRNA (messenger RNA): Carries genetic code from DNA to ribosomes for translation.
tRNA (transfer RNA): Transports amino acids to the ribosome during protein synthesis.
rRNA (ribosomal RNA): Integral component of ribosomes, facilitating protein synthesis.

Transcription in Prokaryotes

Transcription is the process by which a complementary mRNA strand is synthesized from a DNA template. It occurs in three main stages:

Initiation: RNA polymerase binds to the promoter region of DNA, unwinding the double helix.
Elongation: RNA polymerase synthesizes the RNA strand by adding ribonucleotides complementary to the DNA template strand in the 5' to 3' direction.
Termination: Transcription ends when RNA polymerase reaches the terminator sequence, releasing the newly formed mRNA.

Stages of transcription: initiation, elongation, termination Detailed process of transcription with RNA polymerase

Defining a Gene

A gene is a segment of DNA that codes for a specific transcript, which may be translated into a protein or function as a non-coding RNA molecule. Genes are transcribed downstream from the promoter region.

The Genetic Code

The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of proteins. It is read in groups of three nucleotides called codons.

There are 64 possible codons, 61 of which code for amino acids (sense codons), and 3 are stop codons (nonsense codons).
The start codon (AUG) codes for methionine (or N-formylmethionine in bacteria).
The code is degenerate, meaning most amino acids are encoded by more than one codon.

The genetic code table

Translation

Translation is the process by which ribosomes synthesize proteins using the sequence encoded in mRNA. It involves:

Initiation: The ribosome assembles around the start codon on the mRNA.
Elongation: tRNA molecules bring amino acids to the ribosome, matching their anticodons to the codons on the mRNA. Peptide bonds form between amino acids.
Termination: When a stop codon is reached, the polypeptide is released, and the ribosome disassembles.

Translation process: tRNA, ribosome, and peptide bond formation

Simultaneous Transcription and Translation in Bacteria

In prokaryotes, transcription and translation can occur simultaneously because both processes take place in the cytoplasm. Ribosomes can begin translating mRNA before transcription is complete, forming structures called polysomes.

Simultaneous transcription and translation in bacteria

Transcription and RNA Processing in Eukaryotes

In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm. Eukaryotic genes contain exons (coding regions) and introns (non-coding regions). After transcription, introns are removed, and exons are spliced together to form mature mRNA.

Small nuclear ribonucleoproteins (snRNPs) are involved in splicing.
The start codon in eukaryotes codes for methionine.

RNA processing in eukaryotic cells: splicing of exons and introns

Regulation of Gene Expression in Bacteria

Constitutive and Regulated Genes

Constitutive genes: Expressed at a fixed rate; always "on".
Inducible genes: Expressed only when needed, turned "on" by inducers.
Repressible genes: Normally "on" but can be turned "off" by repressors.
Catabolite repression: Inhibits the use of alternative carbon sources when glucose is present.

The Operon Model

An operon is a cluster of genes under the control of a single promoter and operator, allowing coordinated regulation of gene expression.

Promoter: Site where RNA polymerase binds to initiate transcription.
Operator: DNA segment that controls access of RNA polymerase to the genes.
Regulatory gene: Encodes a repressor protein that can bind to the operator.

Structure of the operon: promoter, operator, structural genes

Inducible Operons (e.g., lac Operon)

In inducible operons, genes are not transcribed unless an inducer is present. The lac operon in E. coli is a classic example:

In the absence of lactose, the repressor binds to the operator, blocking transcription.
When lactose (allolactose) is present, it binds to the repressor, inactivating it and allowing transcription of genes needed for lactose metabolism.

lac operon: structure and regulation lac operon: repressor active, operon off lac operon: repressor inactive, operon on

Repressible Operons (e.g., trp Operon)

In repressible operons, genes are transcribed until they are turned off by a corepressor. The trp operon is a well-known example:

When tryptophan is scarce, the repressor is inactive, and genes for tryptophan synthesis are transcribed.
When tryptophan is abundant, it acts as a corepressor, activating the repressor, which binds to the operator and blocks transcription.

trp operon: structure and regulation trp operon: repressor inactive, operon on trp operon: repressor active, operon off

Positive Regulation and Catabolite Repression

Catabolite repression ensures that bacteria preferentially use glucose over other sugars. When glucose is scarce, cyclic AMP (cAMP) accumulates and binds to the catabolite activator protein (CAP), which then activates transcription of the lac operon.

Positive regulation of the lac operon by cAMP and CAP

Epigenetic and Post-Transcriptional Control

Epigenetic control: Methylation of DNA can turn genes off, and these modifications can be inherited but are reversible.
Post-transcriptional control: Mechanisms such as riboswitches and microRNAs (miRNAs) can regulate gene expression after transcription by affecting mRNA stability or translation.

miRNA-mediated post-transcriptional gene regulation

Genetic Variation: Mutation and Recombination

Mutation

A mutation is a permanent change in the DNA sequence. Mutations can be:

Silent (neutral): No effect on protein function.
Missense: Change in one amino acid.
Nonsense: Introduction of a stop codon, truncating the protein.
Frameshift: Insertion or deletion of nucleotides shifts the reading frame, altering downstream amino acids.

Base substitution mutation Missense mutation Nonsense mutation Frameshift mutation

Mutagens and Mutation Rate

Mutagens: Physical or chemical agents that increase mutation rates (e.g., radiation, chemicals).
Spontaneous mutations: Occur naturally at a low rate.
Mutation rate: Probability of mutation per gene per cell division; mutagens can increase this rate by 10–1000 times.

Identifying Mutants

Direct (positive) selection: Identifies mutants by their ability to grow under selective conditions.
Indirect (negative) selection: Identifies mutants that cannot grow or perform a function (e.g., auxotrophs) using replica plating.

Genetic Transfer and Recombination in Bacteria

Genetic Recombination

Genetic recombination is the exchange of genes between DNA molecules, creating new gene combinations and contributing to genetic diversity.

Crossing over: Exchange of DNA segments between homologous chromosomes.

Gene Transfer Mechanisms

Vertical gene transfer: Genes passed from parent to offspring.
Horizontal gene transfer: Genes transferred between cells of the same generation via transformation, transduction, or conjugation.

Plasmids and Transposons

Plasmids: Small, self-replicating DNA molecules that can carry genes for antibiotic resistance, toxin production, or metabolic functions.
Transposons: DNA segments that can move within and between DNA molecules, sometimes carrying antibiotic resistance genes.

Transformation

Transformation involves the uptake of naked DNA fragments from the environment by a bacterial cell, which may then incorporate the DNA into its genome by recombination.

Conjugation

Conjugation is the transfer of plasmids or chromosomal DNA from one bacterium to another via direct cell-to-cell contact, often mediated by sex pili in Gram-negative bacteria.

Transduction

Transduction is the transfer of bacterial DNA from one cell to another by a bacteriophage (bacterial virus). It can be generalized (random DNA) or specialized (specific genes).

Summary Table: Types of Mutations

Type of Mutation	Description	Effect on Protein
Silent	Base change does not alter amino acid	No effect
Missense	Base change results in different amino acid	May alter function
Nonsense	Base change creates stop codon	Truncated protein
Frameshift	Insertion/deletion shifts reading frame	Usually nonfunctional protein

Conclusion

Understanding microbial genetics, including the mechanisms of protein synthesis, gene regulation, and genetic variation, is fundamental to microbiology. These processes underlie microbial adaptation, evolution, and the development of traits such as antibiotic resistance.