Bacterial Genetics: Structure, Function, and Genetic Exchange

Chapter 8: Bacterial Genetics

Introduction to Bacterial Genetics

Bacterial genetics explores the structure, function, and transmission of genetic material in bacteria. Understanding these processes is fundamental to microbiology, as they underpin bacterial growth, adaptation, and evolution.

Location of DNA in Prokaryotic and Eukaryotic Organisms

• Prokaryotes: DNA is primarily located in a single, circular chromosome within the nucleoid region. Additional genetic material may be found in plasmids.

• Eukaryotes: DNA is contained within the nucleus (as linear chromosomes) and in organelles such as mitochondria and chloroplasts.

• Example: Escherichia coli has a single circular chromosome and may carry plasmids.

Definition and Function of a Gene

• Gene: A segment of DNA that encodes a functional product, usually a protein or RNA molecule.

• Function of DNA: Stores genetic information, directs synthesis of proteins, and transmits hereditary information.

The Central Dogma of Molecular Biology

• The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a protein.

• Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Structure of DNA

• Nucleotide Structure: Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, guanine).

• Complementary Base Pairing: Adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G) via hydrogen bonds.

• Double Helix: DNA is a double-stranded helix with antiparallel strands.

• Equation (Base Pairing): $\text{A} = \text{T}, \quad \text{C} = \text{G}$

DNA Replication

• Process: DNA replication is semiconservative; each new DNA molecule contains one original and one new strand.

• Key Enzymes:

  • Helicase: Unwinds the DNA helix.

  • DNA Polymerase: Synthesizes new DNA strands.

  • Primase: Synthesizes RNA primers.

  • Ligase: Joins Okazaki fragments on the lagging strand.

• Leading Strand: Synthesized continuously in the 5' to 3' direction.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

• Circular Chromosome Replication: Begins at the origin of replication and proceeds bidirectionally.

Comparison of DNA and RNA

• DNA: Double-stranded, contains deoxyribose, bases are A, T, C, G.

• RNA: Single-stranded, contains ribose, bases are A, U (uracil), C, G.

• Table: Comparison of DNA and RNA

Feature	DNA	RNA
Strands	Double	Single
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Location	Nucleus (eukaryotes), nucleoid (prokaryotes)	Cytoplasm

Kinds of RNA

• mRNA (Messenger RNA): Carries genetic code from DNA to ribosomes.

• tRNA (Transfer RNA): Brings amino acids to ribosomes during translation.

• rRNA (Ribosomal RNA): Forms the core of ribosome's structure and catalyzes protein synthesis.

Transcription

• Location: In prokaryotes, occurs in the cytoplasm; in eukaryotes, in the nucleus.

• RNA Polymerase: Enzyme that synthesizes RNA from a DNA template.

• Introns and Exons: Eukaryotic genes contain exons (coding regions) and introns (non-coding regions). Introns are removed by small nuclear ribonucleoproteins (snRNPs) during RNA processing.

Translation

• Triplet Code (Codon): Each set of three nucleotides (codon) codes for one amino acid.

• tRNA Features:

  • Anticodon: Sequence of three bases that pairs with the codon on mRNA.

  • Amino Acids: tRNA carries specific amino acids; essential amino acids must be obtained from the environment, nonessential can be synthesized by the cell.

• Ribosomes: Facilitate the assembly of amino acids into proteins.

Christmas Tree Structure in Prokaryotes

• Refers to the appearance of actively transcribed genes under electron microscopy, where multiple RNA polymerases transcribe a single gene simultaneously, resembling a Christmas tree.

Gene Expression and Regulation

• Constitutive Genes: Expressed continuously.

• Inducible Genes: Expressed only in the presence of an inducer.

• Repressible Genes: Expression can be inhibited by a repressor.

• Pre-transcriptional Control: Regulation of gene expression before transcription begins, often via regulatory proteins binding to DNA.

Plasmids and DNA Transfer

• Plasmid: Small, circular, double-stranded DNA molecule independent of the bacterial chromosome.

• Conjugative Plasmids: Carry genes for conjugation (transfer between bacteria).

• Resistance (R) Factors: Plasmids that carry antibiotic resistance genes.

Binary Fission in Bacteria

• Bacteria reproduce asexually by binary fission, where the cell duplicates its DNA and divides into two identical daughter cells.

Genetic Exchange in Bacteria

• Transformation: Uptake of naked DNA from the environment. Can be natural (occurs in some species) or artificial (induced in the lab).

• Conjugation: Direct transfer of DNA between bacteria via a pilus.

• Transduction: Transfer of DNA from one bacterium to another via bacteriophages (viruses that infect bacteria).