Microbial Genetics: Study Guide and Structured Notes

I. Nucleic Acids

DNA Structure

DNA is the hereditary material in most organisms, composed of two antiparallel strands forming a double helix.

• Double helix, antiparallel strands (5' → 3' and 3' → 5')

• Nucleotides: Each nucleotide consists of a phosphate group, deoxyribose sugar, and a nitrogenous base.

• Bases:

  • Purines: A (adenine), G (guanine)

  • Pyrimidines: C (cytosine), T (thymine)

• Base pairing: A-T (2 H bonds), G-C (3 H bonds)

RNA Structure

RNA is typically single-stranded and plays various roles in gene expression and regulation.

• Single-stranded, ribose sugar

• Bases: A, U (uracil), G, C

II. Chromosomes & Plasmids

Prokaryotic Chromosomes

Prokaryotes have a single, circular, double-stranded DNA molecule located in the nucleoid region.

• Single, circular, double-stranded DNA

• Found in nucleoid, not membrane-bound

• Usually haploid (one copy of each gene)

Eukaryotic Chromosomes

Eukaryotes possess multiple, linear, double-stranded DNA molecules within a membrane-bound nucleus.

• Multiple, linear, double-stranded DNA

• Located in nucleus, histone proteins for packaging

• Usually diploid

Plasmids

Plasmids are small, circular, extrachromosomal DNA molecules found in prokaryotes.

• Replicate independently

• Carry non-essential genes (e.g., antibiotic resistance, virulence, fertility)

III. DNA Replication

Overview

DNA replication is the process by which a cell duplicates its DNA before cell division.

• Steps:

  1. Helicase unwinds DNA

  2. Primase synthesizes RNA primers

  3. DNA polymerase synthesizes new strand (5' → 3')

  4. Leading strand: continuous

  5. Lagging strand: discontinuous (Okazaki fragments) joined by DNA ligase

• Key Proteins:

  • Helicase (unwinds DNA)

  • Single-strand binding proteins (stabilize)

  • DNA polymerase III (main synthesis)

  • DNA polymerase I (removes RNA primers, replaces with DNA)

  • Ligase (seals nicks)

• Prokaryotes vs. Eukaryotes:

  • Prokaryotes: 1 origin, faster replication

  • Eukaryotes: multiple origins, slower replication, linear ends (telomeres + telomerase)

IV. Vertical vs. Horizontal Gene Transfer

Definitions

Gene transfer can occur vertically (parent to offspring) or horizontally (between organisms).

• Vertical: parent → offspring (binary fission, reproduction)

• Horizontal: transfer between organisms (same generation)

Griffith's Experiment (1928)

Demonstrated transformation in bacteria using Streptococcus pneumoniae.

• Live S (smooth, virulent) → mouse dies

• Live R (rough, nonvirulent) → mouse lives

• Heat-killed S → mouse lives

• Heat-killed S + live R → mouse dies → R transformed into S

Competent Cell

Cells able to take up naked DNA from environment.

V. RNA Types

mRNA, tRNA, Regulatory RNAs

RNA molecules play distinct roles in gene expression and regulation.

• mRNA: carries genetic info → protein

• tRNA: carries amino acids → ribosome

• Regulatory RNAs: control gene expression

VI. Transcription (DNA → RNA)

Steps

Transcription is the synthesis of RNA from a DNA template.

• Initiation: RNA polymerase binds promoter, unwinds DNA

• Elongation: RNA nucleotides added (5' → 3')

• Termination

Prokaryotes vs. Eukaryotes

• Prokaryotes: transcription & translation coupled, no mRNA processing

• Eukaryotes: mRNA processing (5' cap, 3' poly-A tail, splicing [introns removed, exons joined])

Replication vs. Transcription

• Replication: entire genome, DNA polymerase, primer needed

• Transcription: specific genes, RNA polymerase, no primer

VII. Translation (RNA → Protein)

Participants

Translation converts mRNA into a polypeptide using ribosomes and tRNAs.

• mRNA, tRNA, ribosomes (rRNA), amino acids, enzymes

Steps

• Initiation: small ribosome binds mRNA, start codon (AUG), tRNA (Met) enters P site

• Elongation: amino acids added, peptide bond formed, ribosome shifts A → P → E sites cycle

• Termination: stop codon (UAA, UAG, UGA), release factors free polypeptide

Ribosomal Sites

• A site: aminoacyl (new tRNA enters)

• P site: peptidyl (growing peptide held)

• E site: exit (tRNA leaves)

Genetic Code

• Start codon: AUG

VIII. Gene Expression & Regulation

Operons (Prokaryotes)

Operons regulate gene expression by controlling transcription of multiple genes.

• Function: regulate gene expression (on/off switch for multiple genes)

• Lac operon:

  • Components: promoter, operator, structural genes (lacZ, lacY, lacA), repressor

  • Off: repressor binds operator (no lactose present)

  • On: lactose (allolactose) binds repressor → repressor inactive → transcription occurs

IX. Mutations & DNA Repair

Types of Mutations

• Point mutations: silent, missense, nonsense

• Frameshift: insertion, deletion

Mutagens

• Chemicals (nitrous acid, base analogs)

• Radiation (UV → thymine dimers, ionizing)

DNA Repair

• Photoreactivation (light repair for thymine dimers)

• Nucleotide excision repair

• Mismatch repair

Identifying Mutants

• Positive selection: grow mutants directly (e.g., antibiotic resistance)

• Negative/indirect selection: replica plating to detect loss of function

• Ames test: screens chemicals for mutagenicity (reversion of histidine auxotrophs)

X. Horizontal Gene Transfer Methods

Transformation

Uptake of naked DNA (Griffith's experiment).

Transduction

Transfer via bacteriophage.

• Generalized: any gene transferred

• Specialized: specific genes near prophage transferred

Conjugation

Plasmid transfer via sex pilus.

• F+ cell: donor with F plasmid

• F- cell: recipient, becomes F+ after transfer

• Hfr cell: F plasmid integrated into chromosome; can transfer part of chromosomal DNA

XI. Transposons ("Jumping Genes")

Overview

Transposons are DNA segments that move from one location to another, sometimes carrying antibiotic resistance genes.

• Segments of DNA that move from one location to another

• May carry antibiotic resistance genes

• Insertion sequences vs. complex transposons

Additional info: These notes provide a comprehensive overview of microbial genetics, including molecular structures, gene transfer mechanisms, and mutation/repair processes, suitable for college-level microbiology students.