Microbial Genetics: Structure, Function, and Regulation of Genetic Material

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Genetics: The Study of Inheritance

Key Terms and Concepts

Genetics is the study of inheritance and inheritable traits as expressed in an organism’s genetic material. The genome is the entire genetic complement of an organism, including its genes and nucleotide sequences. A gene is a specific nucleotide sequence that encodes for proteins or RNA molecules.

Structure of Nucleic Acids

Nucleotide Structure and Base Pairing

Nucleic acids, such as DNA and RNA, are polymers of nucleotides. Each nucleotide consists of a phosphate group, a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base. The bases pair specifically: adenine (A) with thymine (T) in DNA or uracil (U) in RNA, and guanine (G) with cytosine (C) in both DNA and RNA.

DNA is typically double-stranded and forms a double helix.
RNA is usually single-stranded.

Base pairing in DNA and RNA Double-stranded DNA structure and base pairing

Structure of Prokaryotic Genomes

Chromosomes and Plasmids

Prokaryotic cells are generally haploid, possessing a single, circular chromosome located in the nucleoid region. In addition to chromosomes, prokaryotes often contain plasmids—small, circular DNA molecules that replicate independently and can confer survival advantages, such as antibiotic resistance.

Fertility factors: Enable conjugation.
Resistance factors: Confer resistance to antibiotics.
Bacteriocin factors: Encode toxins against other bacteria.
Virulence plasmids: Carry genes for pathogenicity.

Prokaryotic chromosome and plasmid Diagram of bacterial chromosome and plasmid

Structure of Eukaryotic Genomes

Nuclear and Extranuclear DNA

Eukaryotic genomes are more complex, typically consisting of multiple linear chromosomes contained within a nucleus. Eukaryotic cells are often diploid. Extranuclear DNA is found in mitochondria and chloroplasts, resembling prokaryotic chromosomes and coding for a small fraction of cellular proteins.

Some fungi and protozoa also carry plasmids.

Eukaryotic chromosome structure and packaging Karyotype of eukaryotic chromosomes Mitochondrion structure Chloroplast structure

DNA Replication

Mechanism and Enzymes

DNA replication is essential for cell division and population growth. It is an anabolic process requiring energy and monomers (triphosphate deoxyribonucleotides). Replication is semiconservative: each new DNA molecule consists of one original and one daughter strand.

Replication begins at the origin of replication.
DNA polymerase synthesizes DNA in the 5′ to 3′ direction.
The leading strand is synthesized continuously; the lagging strand is synthesized discontinuously (Okazaki fragments).
Other enzymes: helicase, primase, ligase, gyrase, and topoisomerase.

Semiconservative DNA replication DNA replication fork and synthesis of leading and lagging strands

Gene Function: Genotype and Phenotype

Definitions and Relationships

The genotype is the set of genes in the genome, while the phenotype is the physical and functional expression of those genes. Not all genes are expressed at all times; gene regulation allows cells to adapt to changing environments.

Genotype and phenotype relationship

Central Dogma: Transcription and Translation

Flow of Genetic Information

The central dogma of genetics describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into proteins. This process involves several types of RNA and multiple steps.

Transcription: Synthesis of RNA from a DNA template.
Translation: Synthesis of polypeptides (proteins) from mRNA by ribosomes.

Central dogma of molecular biology Transcription and translation overview

Transcription: Steps and Types of RNA

Transcription occurs in three main steps:

Initiation: RNA polymerase binds to the promoter region with the help of sigma factors (in bacteria).
Elongation: RNA polymerase synthesizes the RNA strand by adding ribonucleotides complementary to the DNA template.
Termination: Transcription ends when RNA polymerase reaches a terminator sequence (self-termination or Rho-dependent termination).

Types of RNA: mRNA, tRNA, rRNA, regulatory RNA, ribozymes, and RNA primers.

Initiation of transcription Elongation of RNA transcript Termination of transcription Concurrent RNA transcription

Translation: The Genetic Code and Protein Synthesis

Translation is the process by which ribosomes use the genetic information in mRNA to synthesize polypeptides. The genetic code is a set of three-nucleotide codons that specify amino acids.

Participants: mRNA, tRNA, ribosomes (rRNA and proteins).
Stages: Initiation, elongation, and termination.
Initiation and elongation require energy in the form of GTP.

The genetic code table Ribosomal structures in prokaryotes and eukaryotes Ribosome and tRNA binding sites Initiation of translation in prokaryotes Elongation stage of translation

Regulation of Genetic Expression

Operons and Gene Regulation in Prokaryotes

Gene expression in bacteria is regulated to conserve energy. Operons are clusters of genes under the control of a single promoter and operator. There are two main types:

Inducible operons (e.g., lac operon): Activated by inducers when needed.
Repressible operons (e.g., trp operon): Transcribed continually until deactivated by repressors.

The lac operon: inducible operon

RNA as Regulators

Regulatory RNAs, such as microRNAs (miRNAs), small interfering RNAs (siRNAs), and riboswitches, can control translation by binding to mRNA and affecting its stability or translation efficiency.

Mutations and DNA Repair

Types and Effects of Mutations

A mutation is a change in the nucleotide base sequence of a genome. Mutations can be point mutations (substitutions, insertions, deletions) or frameshift mutations, which can have various effects on the resulting protein.

Silent mutation: No change in amino acid sequence.
Missense mutation: Change in one amino acid.
Nonsense mutation: Creates a stop codon, truncating the protein.
Frameshift mutation: Insertion or deletion shifts the reading frame, often resulting in nonfunctional proteins.

Types and effects of point mutations

Mutagens and DNA Repair Mechanisms

Mutagens, such as radiation and chemicals, increase the mutation rate. Cells have several DNA repair mechanisms:

Direct repair: Corrects specific base changes.
Single-strand repair: Repairs damage to one DNA strand.
Error-prone repair: Last-resort mechanism that may introduce mutations.

DNA repair mechanisms

Genetic Recombination and Horizontal Gene Transfer

Mechanisms of Gene Transfer in Prokaryotes

Genetic recombination involves the exchange of nucleotide sequences between DNA molecules. Horizontal gene transfer allows prokaryotes to acquire new genetic material from other cells, contributing to genetic diversity and the spread of traits such as antibiotic resistance. The main mechanisms are:

Transformation: Uptake of naked DNA from the environment.
Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria).
Conjugation: Direct transfer of DNA between cells via a pilus.

Bacterial conjugation via pilus Conjugation involving an Hfr cell

Antibiotic Resistance and Genetic Exchange

Clinical Relevance

Horizontal gene transfer, especially via plasmids, is a major mechanism by which bacteria acquire antibiotic resistance genes, leading to the emergence of 'superbugs' that are difficult to treat with standard antibiotics.