Microbial Genetics and Biotechnology: Study Notes

Microbial Genetics and Biotechnology

Student Learning Objectives

• Define genetics and list different components of the genetic material.

• Describe protein synthesis, transcription, and translation, and learn about differences between eukaryotes and prokaryotes.

• Explain and compare protein synthesis in eukaryotes and prokaryotes.

• Define recombinant DNA technology and provide examples of its therapeutic and agricultural applications.

• Discuss safety issues and ethics of using DNA technology.

Genetics: The Foundation of Heredity

Definition and Components

• Genetics: The science of heredity, including the study of genes, how they carry information, replicate, and determine characteristics.

• Genome: The total genetic information in a cell, including chromosomes and plasmids (in prokaryotes).

• Genomics: The molecular characterization and sequencing of genomes.

• Chromosomes: Structures containing DNA that physically carry hereditary information.

• Genes: Segments of DNA that code for functional products (proteins or RNAs).

Experimental Evidence for DNA as Genetic Material

• Hershey and Chase experiments with bacteriophages demonstrated that DNA, not protein, is the genetic material.

• Radioactive tracers showed only DNA enters the bacterium and directs viral replication.

Applications of Genetics

• Gene editing: Adding, deleting, or inserting DNA in chromosomes using bacterial enzymes (e.g., CRISPR).

• Gene silencing: Inhibiting gene expression.

• Gene therapy: Treating diseases by replacing abnormal genes.

• Genetic analysis of mutants reveals gene function, order of gene products, and protein interactions.

Advantages of Using Microbes in Biotechnology

• Microbes are inexpensive, abundant, and often work at normal temperature and pressure.

• They may not produce toxic waste like chemical processes.

• DNA technology with microbes is used for clinical, environmental, and agricultural products.

• Applications include tracking infectious disease outbreaks and forensic microbiology.

Mutations and Their Effects

How Mutations Affect Expression and Function

• Mutations in DNA can alter mRNA, leading to altered proteins and functions.

• Mutations can be spontaneous or induced by mutagens (e.g., chemicals, radiation).

Biotechnology and Genetic Engineering

• Biotechnology: Use of microorganisms, cells, or cell components to make products.

• Genetic engineering: Manipulation of DNA to create recombinant DNA.

• Tools include selection (artificial selection of microbes) and mutation (induced by mutagens or site-directed mutagenesis).

The Importance of Heredity

• Microbial traits are inherited and include shape, metabolism, motility, and interactions.

• Genes are transmitted vertically (to offspring) and horizontally (to other bacteria).

• Plasmids: Extra-chromosomal DNA transferred between cells, often carrying antibiotic resistance genes.

• Conjugation pili: Structures for DNA transfer between bacteria.

Pili and Horizontal Gene Transfer

• Pili are longer than fimbriae, fewer per cell, and function in motility and DNA transfer.

• Horizontal gene transfer spreads antibiotic resistance, especially in the intestine.

Recombination and Immune Evasion

• Bacteria can recombine DNA to produce new flagellar proteins, evading immune responses (e.g., Salmonella).

• Recombination can be vertical (parent to offspring) or horizontal (between cells of the same generation).

Clones and Vectors in Recombinant DNA Synthesis

• Vectors: DNA molecules (plasmids, viruses) used to transfer genes between cells.

• Clones: Cells or organisms carrying recombinant genes.

• Large quantities of gene products can be harvested from clones.

DNA and Chromosomes

• Bacteria have a single circular chromosome, supercoiled and attached to the membrane.

• Example: E. coli chromosome is about 4.6 million base pairs and 1 mm long.

Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell	Eukaryotic Organelle
DNA	One circular; some two circular	Linear	Circular
Histones	In archaea	Present	Present
First Amino Acid in Protein Synthesis	Formylmethionine (bacteria); Methionine (archaea)	Methionine	Formylmethionine
Ribosomes	70S	80S	70S
Growth	Binary fission	Mitosis	Binary fission

The Flow of Genetic Information

• Expression: DNA is transcribed into mRNA and translated into protein.

• Recombination: Genes can be exchanged horizontally between cells.

• Replication: DNA is copied for cell division (vertical gene transfer).

DNA Replication

• One parental DNA molecule is converted to two identical offspring molecules.

• Complementary base pairing: Adenine (A) pairs with Thymine (T), Guanine (G) with Cytosine (C).

• Topoisomerases (gyrases): Relax supercoiling.

• Helicases: Unwind DNA strands.

• DNA polymerase: Synthesizes new DNA, proofreads, and removes mismatches.

Important Enzymes in DNA Replication, Expression, and Repair

Enzyme	Function
DNA Gyrase	Releases supercoiling ahead of the replication fork
DNA Ligase	Makes covalent bonds to join DNA strands
DNA Polymerase	Synthesizes DNA, proofreads, and facilitates repair
Endonucleases	Cut DNA backbone in a strand of DNA
Exonucleases	Cut DNA from an exposed end
Helicase	Unwinds double-stranded DNA
Primase	Makes RNA primers from a DNA template
RNA Polymerase	Copies RNA from a DNA template
Topoisomerase or Gyrase	Relaxes supercoiling ahead of the replication fork

Operons and Gene Regulation

• Inducible operon: Genes are switched "off" until an inducer turns them "on" (e.g., lac operon).

• Repressible operon: Genes are switched "on" until a co-repressor turns them "off" (e.g., trp operon).

Mutations and Mutagens

• Alleles: Different forms of a gene.

• Mutations: Changes in DNA sequence, can be endogenous or exogenous.

• Mutagens: Environmental agents causing mutations (e.g., ethylmethane sulfonate).

Recessive and Dominant Mutant Alleles

• Diploid: Two copies of each chromosome (e.g., humans).

• Haploid: One copy of each chromosome (e.g., bacteria, yeast).

• Polyploid: More than two copies (some plants, cancer cells).

Alterations in Phenotype and Function

• Diseases can result from dysfunctional proteins due to gene mutations.

• Mutations can increase disease susceptibility.

• Causes: endogenous (within the cell) or exogenous (environmental).

Phenotype

• An organism's phenotype is its collection of proteins.

• Microbial proteins are often enzymatic (catalyze reactions) or structural (form complexes).

Toxic Proteins: Endotoxins and Exotoxins

• Endotoxins: Lipid A portion of lipopolysaccharide in gram-negative bacteria, released after cell death.

• Exotoxins: Proteins produced and secreted by bacteria (mostly gram-positive).

• Both can cause severe immune responses.

Gene Location and Toxins

Toxin	Organism	Gene Location	Host Cell Receptor	Biological Effects
Bordetella toxin	Bordetella pertussis	Chromosomal	Unknown	Increases cAMP in host cells
Anthrax toxin	Bacillus anthracis	Plasmid	Tumor endothelial marker	Edema, cell death
Botulinum toxin	Clostridium botulinum	Phage	Polysialogangliosides	Decreases neurotransmitter release

Antibiotic Resistance

• Mutations in bacterial genomes can confer resistance (e.g., Staphylococcus aureus and MRSA).

• Resistance genes often spread via plasmids and horizontal gene transfer.

Biofilms

• Produced by altered gene expression in bacteria when populations are large (quorum sensing).

• Example: Dental plaque by Streptococcus mutans.

Summary

• Genetic information is stored in DNA, expressed via transcription and translation, and can be altered by mutation or recombination.

• Biotechnology uses microbial genetics for medical, agricultural, and industrial applications.

• Understanding gene transfer, regulation, and mutation is essential for controlling microbial traits and combating antibiotic resistance.