Bacterial Heredity & Genetics Overview

Bacterial Heredity & Variation

• Bacteria have diverse genetic material: chromosomes, plasmids, transposable elements, integrons, and phage genomes.
• Bacterial phages are an overview of mechanisms of transfer and recombination of bacterial genes: transformation, transduction, conjugation, and lysogenic conversion.
• Gene mutation is a mechanism of bacterial genetic variation.
• Bacterial genetic variation plays a significant role in drug resistance, pathogenesis, virulence, diagnosis, and vaccination.
• Manipulation of cloned DNA is also influenced by bacterial genetic variation.

Bacterial Genetics

• Genetic mechanisms are central to bacterial diversity and disease-producing power.
• New antibiotic resistance and emerging pathogens are frequently reported.
• Bacteria that were previously successfully treated with antimicrobials now show resistance.
• Previously controlled diseases are reappearing.
• New diseases are emerging and spreading.
• Genetic mechanisms such as mutation, recombination, transformation, transduction, conjugation, and transposition are involved in these phenomena.

Bacterial Genome

• DNA molecules that replicate as discrete units are called replicons in bacteria.
• Some bacterial strains only have a chromosome as their replicon.
• Other strains have additional replicons such as plasmids and bacteriophages.
• Bacterial genomes vary in size from roughly 4 x 10⁹ to 8.6 x 10¹⁰ daltons.
• Mycoplasma genitalium has the smallest microbial genome, at 580,070 base pairs, encoding 525 genes.
• Scytonema hofmanni has a large genome of 12,073,012 base pairs, with nearly 12,356 putative protein-coding genes.
• Most bacteria have a haploid genome consisting of a single circular, double-stranded DNA molecule.
• Some species have more than one chromosome (e.g., Deinococcus radiodurans).
• Certain strains of Bacillus subtilis have linear chromosomes.
• Borrelia (Lyme disease) and Streptomyces have linear chromosomes.
• Agrobacterium tumefaciens has one linear and one circular chromosome.
• Vibrio cholerae has two circular chromosomes.

How Bacterial Genomes Differ From Higher Forms of Life

• Bacterial genes are composed of exons and introns.
• Exons code for proteins.
• Introns do not code for proteins, but may regulate gene expression.

Binary Fission and Generation Time

• Most bacteria reproduce asexually by binary fission.
• Binary fission is where a cell divides into two cells.
• Generation time is the time required for a bacterial cell to divide or for a population of cells to double.
• Generation times can be as short as 15 minutes (e.g., E. coli) or as long as several days (e.g., Mycobacteria).

The Flow of Genetic Information

• Genetic information in a cell is used to produce proteins, which are essential for cell function.
• DNA is transferred from one generation of cells to the next resulting in new gene combinations.

Mutation

• Mutations are heritable changes in the structure of genes.
• The normal, usually active type of a gene is a wild-type allele.
• The mutated (usually inactive) form is a mutant allele.
• Spontaneous development of mutations is a major factor in bacterial evolution.
• Mutations in bacteria occur at a low frequency (e.g. 10^-6).
• The consequences of bacterial mutations are immediately obvious within the mutated cell because bacteria are typically haploid.

Kinds of Mutations

• Mutations can be spontaneous or induced.
• Some mutations are replacements or substitutions (point mutations).
• Missense mutations occur when changes in a gene cause a different amino acid to be coded for.
• Insertion mutations involve the addition of many base pairs.
• Nonsense mutations occur when a codon specifying an amino acid changes to a codon that does not specify any amino acid.
• Deletion mutations involve the removal of a contiguous segment of numerous base pairs.
• Micro-deletions and micro-insertions result in frame shift mutations, which lead to changes in the reading frame of the mRNA, and the subsequent mRNA transcript amino acid sequence.
• Duplications produce a redundant segment of DNA.
• Transversions involve changing a purine to a pyrimidine (or vice-versa).
• Suppressor mutations reverse the effect of a mutant phenotype.
• Conditional lethal mutants are sensitive to temperature changes (e.g., temperature-sensitive mutants).

Development of Antibiotic Resistance

• Antibiotics may lead to the evolution of resistant bacterial strains.

Types of Plasmids

• Plasmids are small, circular, extrachromosomal DNA molecules.
• Non-integrating plasmids replicate independently.
• Integrated plasmids (episomes) can insert into the bacterial chromosome and become a permanent part of the genome.
• One method of grouping plasmids is by their ability to transfer.
• Conjugative plasmids carry tra genes that facilitate conjugation.
• Nonconjugative plasmids cannot initiate conjugation alone, but can be transferred with conjugative plasmids.

Plasmids by Compatibility

• Certain plasmids may co-exist in a single cell, but related plasmids are often incompatible
• This is due to regulation of vital plasmid functions.
• Plasmids can be assigned to compatibility groups based on this incompatibility.

Classification of Plasmids by Function

• Plasmids can be classified by their functions.
• Fertility plasmids (F-plasmids) contain tra genes, enabling conjugation.
• Resistance plasmids (R-plasmids) confer resistance to antibiotics or poisons. These were formerly known as R-factors.
• Col plasmids produce bacteriocines which are proteins that kill bacteria.
• Degradative plasmids have the ability to digest unusual substances.
• Virulence plasmids cause bacterial cells to become pathogenic.

Use of Plasmids in Molecular Biology

• Plasmids are used to produce large amounts of proteins in research.
• Researchers can use plasmids to manipulate DNA.
• This method allows for the creation of large quantities of target proteins including antibiotics and insulin.

Mobile Genetic Elements (Jumping Genes)

• Transposons are DNA segments that can move from one position to another on a DNA molecule.
• The process is known as transposition.
• Transposons are not self-replicating, requiring integration into other replicons for stable maintenance in bacterial genomes.
• The transposition relies on the ability of transposons to synthesize recombination enzymes called transposases.
• Transposons were first discovered in corn, where they varied the expression of colors in the kernels.

Mechanisms of Transposition

• Direct transposition is a "cut and paste" mechanism.
• In this type of transposition, the transposon is physically moved to a new site.
• Replicative transposition is a "copy and paste" mechanism. In this type of transposition, a copy of the transposon is made and inserted at a new site.

Mechanisms of Bacterial Gene Transfer

• Transformation: DNA is taken up from the environment by a recipient bacterium and incorporated into its genome.
• Conjugation: DNA is transferred from a donor bacterium to a recipient bacterium via direct contact. Recipient cells become genetically altered.
• Transduction: Bacteriophages transfer DNA from one bacterium to another.

Regulation of Gene Transfer

• Bacterial species can differ in their ability to transfer DNA, though the mechanisms are common in various bacterial species.
• Transformation is regulated by bacterial chromosome genes.
• Transduction is controlled by bacteriophage genes.
• Conjugation is controlled by plasmid genes.

Transformation - Griffith's Experiment

• Griffith's experiment demonstrated transformation in bacteria.
• In this type of gene transfer, the genetic information is transferred via extracellular DNA.
• Dead bacteria transfer their genetic information to living cells.

Transformation - Process

• Transformation involves four steps in bacteria (1) a donor bacterium dies (2) a fragment of DNA is released (3) The DNA fragment is bound by recipient DNA binding proteins to the cell wall (4) The fragment of DNA is integrated into the genome.

Transformation - Competence

• The ability to take up DNA from the environment is referred to as competence.

Conjugation

• Conjugation is the transfer of genetic material through cell-to-cell contact.
• Conjugation is regulated entirely by the genes encoded on the plasmid.

Conjugation - F+ Conjugation

• F+ conjugation involves a transfer for the F-plasmid (or sex pilus).
• F+ conjugation involves the transfer of genetic material through cell-to-cell contact.

Conjugation - Hfr Conjugation

• In Hfr conjugation, part of the donor's whole chromosome is transferred into the recipient.
• The process involves a sex pilus.
• The conversion of an F+ cell to an Hfr cell is reversible.

Conjugation - Steps (F+ Conjugation)

• F+ males have a plasmid carrying genes for a sex pilus
• The sex pilus attaches to F- females
• The pilus retracts forming a bridge between cells
• A plasmid strand enters the recipient bacterium.
• Both bacteria now have a complete F+ plasmid

Conjugation - Steps (Hfr Conjugation)

• F+ males have a plasmid incorporated into their chromosome. This forms an Hfr male.
• The sex pilus attaches to F- females.
• The pilus retracts forming a bridge.
• Part of the donor's chromosome moves into the recipient.
• The donor bacterium makes a complementary strand.
• The recipient bacterium receives the complementary strand.
• Donor DNA undergoes genetic recombination with the recipient.

Composition of RTF

• RTFs (resistance transfer factors) are plasmids that carry genes for antibiotic resistance.
• RTFs consist of two components: a transfer component which facilitates conjugational transfer and resistance determinants for various drugs.
• The number of drug resistance determinants ranges from 8 to 8+

Transduction

• Transduction is the transfer of genetic material from one bacterium to another by bacteriophage (a virus that infects bacteria).
• Transduction was discovered by Joshua Lederberg and Norton Zinder in 1952.
• The U-tube experiment demonstrated Transduction
• Generalized transduction: Random bacterial DNA is packaged into a bacteriophage.
• Specialized transduction: Specific bacterial genes are packaged into a bacteriophage.

Steps in Generalized Transduction

• A lytic bacteriophage adsorbs to a susceptible bacterium.
• The bacteriophage genome enters the bacterium.
• A bacteriophage head packages bacterial DNA by mistake instead of phage DNA.
• The bacteriophages are released from the bacterium.
• The bacteriophage carries the bacterial DNA.

Specialized Transduction

• The process where a temperate bacteriophage carries a specific portion of bacterial DNA (e.g., the gal gene) during a lysogenic cycle.
• This enables the recipient cell to gain new genetic characteristics (metabolize galactose, for example).

Genetic Mechanisms of Drug Resistance

• Bacteria can acquire drug resistance through various mechanisms, including mutations and gene transfer (transformation, transduction, and conjugation).
• Some bacteria can alter the permeability of their cellular membranes to prevent drugs from entering or by producing enzymes that inactivate particular drugs.
• Others can use alternative metabolic pathways.

Genetic Manipulations of DNA

• DNA cloning and other techniques can manipulate DNA and create new products or organisms.
• Recombinant plasmids are created by inserting a specific gene fragment into a plasmid vector.
• This technique has reshaped medicine and drug development.

Other Key Points

• Bacterial genomes, which carry genetic information, are crucial to understand the way they function and evolve.
• Bacteria reproduce by binary fission, resulting in the rapid generation of new cells, which enables the rapid spread of mutations through a population.
• Several different genetic mechanisms are central to bacterial gene transfer (e.g., transformation and conjugation).

Explore the complex world of bacterial heredity and genetic variation. This quiz covers topics such as genetic mechanisms, antibiotic resistance, and the role of phages and mutations in bacterial diversity. Gain insights into how these factors influence disease and treatment outcomes.