(468) 2117 Chapter 8 Part A - Microbial Genetics

Overview of Microbial Genetics

• Traits influenced by genetics and heredity include:

  • Cell shape: coccus or bacillus (rod)

  • Structural features: cell wall composition (gram-positive or gram-negative)

  • Metabolism: type of energy utilized

  • Motility: presence of flagella

  • Interactions with other cells: production of antibiotics

• This chapter covers microbial genetics to understand:

  • Emergence of new diseases

  • Relatedness among organisms

  • Genetic expression and regulation

Understanding Genetics

• Genetics: Study of genes, their information, expression, and replication.

• Chromosomes: Structures containing DNA and genes.

• Gene: Specific segment of DNA coding for one protein.

• Genome: All genetic information in a single cell.

• Genetic Code: Rules for converting nucleotide sequences into proteins.

• Central Dogma: DNA is transcribed to RNA, which is translated into protein.

Genetic Makeup: Genotype and Phenotype

• Genotype: Genetic makeup of an organism.

• Phenotype: Physical expression of a trait.

• Example:

  • Blue eyes = phenotype (recessive: bb)

  • Brown eyes = phenotype (dominant: BB or Bb)

• Punnett Squares: Tool for predicting offspring traits.

Bacterial Genetics

• Bacteria typically have a single circular chromosome made of DNA.

• DNA Location: In prokaryotes, DNA is in the nucleoid (no nucleus).

Structure of DNA

• DNA consists of millions of base pairs; only 2% functionally impactful.

• Non-coding DNA: Filler DNA (junk), useful for DNA profiling.

Genetic Information Flow

• Vertical Gene Transfer: Transfer of genetic info from parent to offspring.

• Horizontal Gene Transfer: Exchange of genetic info between nearby unrelated cells.

DNA Structure and Replication

• Double Helix: DNA's structure, twisted ladder.

• Backbone: Sugar-phosphate groups.

• Bases (A, T, C, G) held by weak hydrogen bonds.

  • Hydrogen Bonds: Easy to break for replication.

• Anti-parallel Strands: Strands of DNA run in opposite directions (5' to 3', 3' to 5').

• DNA replication is semi-conservative: Each new strand contains one original strand.

• DNA Polymerase: Adds nucleotides to new strands; follows helicase.

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized discontinuously in fragments (Okazaki fragments).

• Ligase: Joins Okazaki fragments together.

Energy in DNA Replication

• Nucleotides provide energy for replication through hydrolysis of phosphate groups.

• Bacteria: Bi-directional replication due to circular chromosomes.

Accuracy of DNA Replication

• High accuracy ensured by DNA polymerase (proofreading ability).

• Occasional mutations can occur, affecting genetic code.

RNA: A Key Role in Protein Synthesis

• RNA (Ribonucleic Acid): Composed of ribose sugar.

• Nucleotides: Composed of ribose, phosphate, and bases (A, U, C, G).

• Types of RNA:

  • rRNA: Component of ribosomes.

  • tRNA: Transfers amino acids to growing protein chain.

  • mRNA: Messenger carrying genetic information from DNA.

Central Dogma of Biology

• Transcription: Coping DNA into mRNA.

  • Occurs in the nucleus for eukaryotes; simultaneous with translation in prokaryotes.

• Translation: mRNA translated into proteins by ribosomes.

• Codons: Sets of three nucleotides coding for amino acids.

• Start Codon: AUG (methionine); signals where translation begins.

• Stop Codons: Indicate end of translation.

Codon Properties

• 61 sense codons code for 20 amino acids.

• Degeneracy: Similar codons for the same amino acid protect against mutations.

Translation Process

• Ribosome reads mRNA and assembles amino acids.

• tRNA has anticodon that matches mRNA codon.

• Amino acids joined by peptide bonds to form proteins.

• Ribosome has three sites: A (arrival), P (primary), E (exiting).

Eukaryotic vs Prokaryotic Transcription & Translation

• In eukaryotes, transcription occurs in the nucleus and mRNA must be processed.

• Non-coding introns are removed and coding exons are spliced together with snRNPs (snurps).

• In prokaryotes, no nucleus allows simultaneous transcription and translation.