Microbial Genetics: Structure and Organization of Microbial Genomes

Microbial Genetics

Fundamental Structural Features of DNA

DNA is the hereditary material in most organisms, and its structure determines how genetic information is copied and interpreted. Understanding these features is essential for studying microbial genetics.

• Polarity: DNA strands have a 5′ and 3′ end, indicating directionality.

• Antiparallel Strands: The two DNA strands run in opposite directions.

• Phosphodiester Bonds: Nucleotides are linked by phosphodiester bonds, forming the backbone of DNA.

• DNA Polymerase Directionality: DNA polymerases synthesize new DNA only in the 5′ → 3′ direction, adding nucleotides to the 3′ end.

• Semiconservative Replication: Each daughter DNA molecule contains one parental strand and one newly synthesized strand.

• Complementary Base Pairing: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

• Sequence Orientation: Knowing one strand's sequence allows determination of the complementary strand, but orientation must be considered.

Example: If a DNA strand is written 5′ → 3′, its complementary strand is written 3′ → 5′ directly underneath. To write the complementary strand in 5′ → 3′, reverse the sequence.

Base Composition and DNA Analysis

Base composition analysis helps determine the relative amounts of DNA bases in double-stranded DNA.

• Chargaff's Rules: In double-stranded DNA, %A = %T and %G = %C.

• Calculating AT and GC Content: %AT = 2 × %A; %GC = 2 × %G.

• Example: If Clostridium tetani DNA contains 14% G, it must contain 14% C, 36% A, and 36% T. GC content is 28%.

• Application: E. coli has an approximate 50% GC content.

DNA Polarity and PCR

Polymerase Chain Reaction (PCR) relies on DNA polarity and antiparallel structure for successful amplification.

• DNA Polymerase: Synthesizes DNA in the 5′ → 3′ direction.

• Primer Orientation: Primers must anneal to complementary sequences on opposite strands and be oriented so DNA synthesis proceeds toward the target region.

• High-Yield Rule: For conventional PCR, primers point toward each other with respect to the sequence being amplified.

Example: In PCR, selecting primers P1 and P4 allows amplification of the region between them due to their correct orientation.

Genome Organization Across Domains of Life

Genome structure varies among different domains of life, affecting genetic processes and inheritance.

• Chromosome Types: Organisms may have circular or linear chromosomes.

• Plasmids: Small, separate DNA molecules found in many bacteria.

• Organellar DNA: Eukaryotes have DNA in mitochondria and chloroplasts.

• Telomeres: Linear chromosomes in eukaryotes have telomeres at their ends.

• Bacterial Chromosomes: Commonly circular, but genome organization is diverse.

Domain	Chromosome Type	Plasmids	Organellar DNA
Bacteria	Circular (mostly)	Common	Absent
Archaea	Circular/Linear	Variable	Absent
Eukarya	Linear	Rare	Present (mitochondria, chloroplasts)

Additional info: Table inferred from lecture summary.

Genome Size and Gene Number

Genome size and gene number vary widely among organisms and are not directly proportional.

• Human Genome: ~3.2 Gb, <30,000 genes

• Yeast Genome (Saccharomyces cerevisiae): ~12.2 Mb, <6,000 genes

• E. coli Genome: ~4.7 Mb, ~4,000 genes

• Gene Density: Number of genes per Mb varies; larger genomes do not necessarily have more genes.

Gene Density in Bacterial vs. Human Genomes

Bacterial genomes have much higher gene density compared to human genomes due to differences in noncoding DNA and introns.

• Gene Density: Human: ~12–15 genes/Mb; Bacteria: ~500–1,000 genes/Mb

• Relative Gene Density: E. coli > yeast >> human

• Noncoding DNA: Eukaryotic genomes contain substantial noncoding DNA; human genes often have 7–8 introns.

Bacterial Chromosome Compaction

Bacterial chromosomes are highly compacted to fit within the small cell volume.

• Chromosome Length: E. coli chromosome is ~1 mm long when unpacked.

• Compaction Mechanisms: HU proteins (histone-like) help organize and compact DNA.

Contents of a Typical Bacterial Genome

Bacterial genomes contain more than just protein-coding genes; they include essential regulatory and functional elements.

• Protein-coding genes

• rRNA genes

• tRNA genes

• Origin of replication (ori): cis-acting DNA element

• Replication terminus: contains cis-acting elements such as dif

• Regulatory sequences: Essential for genome organization

Key Distinction: A genome is not simply a collection of protein-coding sequences; regulatory and other functional DNA sequences are essential.

Cis-acting DNA Elements vs. Trans-acting Factors

Gene regulation involves interactions between DNA sequences (cis-acting elements) and diffusible molecules (trans-acting factors).

• Cis-acting elements: DNA sequences serving as binding sites or regulatory regions.

• Trans-acting factors: Diffusible molecules, often proteins, that interact with cis elements to influence gene expression.

• Application: These concepts are central to transcriptional regulation and the flow from transcription to translation.

Coding vs. Template Strands

Understanding the distinction between coding and template strands is crucial for interpreting gene expression.

• Coding Strand: Also called sense or non-template strand; has the same sequence as RNA (except T for U).

• Template Strand: Also called noncoding, antisense, anticoding, or transcribed strand; used by RNA polymerase for RNA synthesis.

• Strand Designation: Coding and template strand designations are relative to individual genes, not absolute for the entire DNA molecule.

Example: For one gene, the top strand may be coding; for another gene, the bottom strand may be coding.

Open Reading Frames (ORFs) and Protein Sequence Determination

An open reading frame (ORF) is a sequence that can be translated into a protein without interruption by in-frame stop codons.

• Translation Direction: N-terminus to C-terminus; coding sequence read 5′ → 3′.

• ORF Identification: Used to determine the protein sequence encoded by a gene.

Genome Databases and Gene Information

Genome databases provide comprehensive information about genes, such as ldhA in E. coli.

• Gene location

• Gene size

• Coding sequence

• Predicted protein sequence

• Functional description

• Chromosomal arrangement

• Transcription orientation

Example: Using EcoCyc or similar databases, students can find DNA and protein sequences for specific genes.

Additional info: Academic context added for clarity and completeness, including table summarizing genome features across domains and expanded explanations of key concepts.