Genetics in Microbiology

Introduction to Genetics

Genetics is the study of genes, their functions, and how variations arise in genomes. In microbiology, understanding genetics is essential for exploring how microorganisms inherit traits, adapt, and evolve.

• Genotype: The genetic makeup of an organism; the set of genes it carries.

• Phenotype: The observable physical and physiological traits of an organism, determined by its genotype.

• Genome: The entire collection of genetic material in a cell or virus.

• Cells have DNA genomes, while viruses may have either DNA or RNA genomes.

Example: The genotype of Escherichia coli determines its ability to metabolize lactose, which is observed as a phenotype.

Organization of Genetic Material

Prokaryotic vs. Eukaryotic Genomes

The organization and size of genomes differ between prokaryotes and eukaryotes, reflecting their complexity and cellular structure.

• Prokaryotic cells: Usually have 1–3 circular chromosomes located in the nucleoid region; organized by histone-like proteins.

• Eukaryotic cells: Possess multiple linear chromosomes housed in the nucleus; DNA is wrapped around histone proteins.

• Plasmids: Small, circular DNA molecules found in many prokaryotes and some eukaryotes; often carry genes that confer advantages, such as antibiotic resistance.

Example: E. coli K12 has about 4,400 genes, while a human cell has about 24,000 genes.

Structure and Function of Nucleic Acids

DNA Structure

DNA (deoxyribonucleic acid) is a double-stranded molecule forming a double helix. It is composed of nucleotides, each with three components:

• Phosphate group

• Deoxyribose sugar

• Nitrogenous base: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)

Nitrogen bases pair specifically: A with T, and G with C. The strands are antiparallel, running 5’ to 3’ and 3’ to 5’.

• Phosphodiester bonds link nucleotides, giving DNA its directionality.

RNA Structure

RNA (ribonucleic acid) is typically single-stranded and contains ribose sugar. Its nitrogenous bases are Adenine (A), Guanine (G), Cytosine (C), and Uracil (U) (replacing Thymine).

• RNA can fold into complex structures and has 5’ to 3’ directionality.

• Types of RNA: Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomal RNA (rRNA)

The Central Dogma of Molecular Biology

Flow of Genetic Information

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• Transcription: DNA → RNA

• Translation: RNA → Protein

Some viruses and cells can perform reverse transcription (RNA → DNA) using the enzyme reverse transcriptase.

DNA Replication

Mechanism and Enzymes

DNA replication is the process by which a cell copies its genome before division. It is highly accurate and involves several key enzymes:

• Helicase: Unwinds the DNA helix.

• Primase: Synthesizes RNA primers.

• DNA polymerase: Synthesizes new DNA strands; proofreads and corrects errors.

• Ligase: Seals nicks in the sugar-phosphate backbone.

• Gyrase/Topoisomerase: Relieves supercoiling tension.

Replication starts at the origin of replication and proceeds bidirectionally, forming replication forks.

Leading and Lagging Strands

• Leading strand: Synthesized continuously in the direction of the replication fork.

• Lagging strand: Synthesized discontinuously in short segments called Okazaki fragments, later joined by ligase.

Replication is semiconservative: each new DNA molecule contains one parental and one newly synthesized strand.

Prokaryotic vs. Eukaryotic Replication

• Prokaryotes: Single origin of replication, faster process.

• Eukaryotes: Multiple origins of replication, more complex machinery, slower process.

Protein Synthesis (Gene Expression)

Transcription

Transcription is the synthesis of RNA from a DNA template, occurring in three steps:

1. Initiation: RNA polymerase binds to the promoter region.

2. Elongation: RNA polymerase synthesizes the RNA strand.

3. Termination: RNA polymerase reaches a termination sequence and releases the RNA transcript.

In eukaryotes, mRNA undergoes splicing to remove introns and join exons before translation.

Translation

Translation is the process by which ribosomes decode mRNA to build proteins. It involves:

1. Initiation: Ribosome assembles around the start codon of mRNA.

2. Elongation: tRNAs bring amino acids to the ribosome, matching codons with anticodons.

3. Termination: Ribosome encounters a stop codon and releases the completed polypeptide.

Multiple ribosomes can translate a single mRNA simultaneously (polysomes).

The Genetic Code

• Composed of 64 codons (triplets of nucleotides).

• 61 sense codons code for 20 standard amino acids; 3 are stop codons.

• The code is redundant: multiple codons can specify the same amino acid.

Post-Translational Modifications

• Proteins may be trimmed or modified by addition of organic (e.g., sugars, lipids) or inorganic (e.g., phosphate, metal ions) groups to become fully functional.

Regulation of Protein Synthesis

Gene Regulation Mechanisms

• Constitutive genes: Expressed continuously (housekeeping genes).

• Facultative genes: Expressed only in response to environmental changes.

• Regulation occurs at pre-transcriptional (e.g., operons, epigenetic changes) and post-transcriptional levels (e.g., mRNA splicing, stability).

Operons

Operons are clusters of genes under the control of a single promoter and regulatory elements. Two main types:

• Inducible operons: Off by default; activated in response to specific substrates (e.g., lac operon in E. coli).

• Repressible operons: On by default; repressed when end products are abundant (e.g., arg operon).

Epigenetic Regulation

• Involves chemical modifications (e.g., DNA methylation) that alter gene expression without changing the DNA sequence.

• Methylation typically silences genes by preventing transcription.

Mutations and Their Consequences

Types of Mutations

• Substitution: One nucleotide is replaced by another.

• Insertion: Addition of one or more nucleotides.

• Deletion: Removal of one or more nucleotides.

Mutation Effects

• Silent mutation: No change in amino acid sequence.

• Missense mutation: Changes one amino acid in the protein.

• Nonsense mutation: Converts a codon to a stop signal, truncating the protein.

• Frameshift mutation: Insertions or deletions not in multiples of three shift the reading frame, altering downstream amino acids.

• Reversion mutation: A second mutation restores the original sequence or function.

Sources of Mutations

• Spontaneous mutations: Occur naturally during DNA replication.

• Induced mutations: Caused by mutagens (chemical, physical, or biological agents).

• Carcinogens: Mutagens that increase cancer risk.

Mutation Detection and Repair

• Ames test: Detects mutagenic potential of compounds using Salmonella typhimurium strains.

• DNA proofreading: DNA polymerases correct errors during replication.

• Excision repair: Enzymes remove and replace damaged or mismatched DNA segments.

Plasmids and Horizontal Gene Transfer

Plasmids

• Small, circular DNA molecules that can replicate independently of chromosomal DNA.

• Often carry genes for antibiotic resistance (R plasmids).

• Used in biotechnology for gene cloning and protein production.

Conjugation

• Requires a fertility plasmid (F plasmid) encoding a pilus for DNA transfer between cells.

• High-frequency recombination (Hfr) strains can transfer chromosomal genes during conjugation.