Microbial+Genetics+1

Microbial Genetics Topics

• DNA Structure

• DNA Replication

• Protein Synthesis

  • Transcription

  • Translation

• Mutations

  • Types of Mutations

    • Base Substitution (point mutations)

    • Frameshift Mutations

• Mutagens

  • Chemical

  • Radiation

• Identifying Mutagens (the Ames Test)

• Genetic Recombination and Transfer

  • Transformation

  • Conjugation

  • Conjugation Hfr

  • Transduction

DNA Structure

• Definition: DNA (deoxyribonucleic acid) is the genetic carrier and is structured as a double helix.

• Composed of two strands twisted together, oriented in opposite directions.

• Basic unit: Nucleotide (Deoxynucleotide)

  • Three subunits:

    • Deoxyribose (sugar)

    • Phosphate group

    • One nitrogenous base

• Nitrogenous Bases:

  • Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

  • All nucleotides share the same sugar and phosphate but vary by their base.

DNA Complementary Base Pairing

• Base Pairing Rules:

  • A pairs with T (A=T)

  • C pairs with G (C=G)

• Example:

  If one strand is 5’ A T T C G A G C T 3’The complementary strand is 3’ T A A G C T C G A 5’

DNA Orientation

• Strand Orientation:

  • One strand starts with 5’ and ends with 3’.

  • The complementary strand runs in the opposite direction.

Eukaryotic vs. Prokaryotic DNA

• Eukaryotic DNA:

  • Linear structure

  • Multiple chromosomes

• Prokaryotic DNA:

  • Circular structure

  • Generally one chromosome

  • Some bacteria have plasmids (small circular DNA)

DNA Replication

• Process:

  • Two strands of DNA separate upon breaking hydrogen bonds.

• Example:Original strands:5’ T A C G A C 3’3’ A T G C T G 5’

• Each strand serves as a template to make a complementary strand.

• Outcome: Two identical DNA molecules are formed, each containing one old and one new strand (semiconservative replication).

Genetic Information Flow

• Flow:

  • DNA → RNA → Protein

• Transcription: DNA is transcribed into RNA

• Translation: RNA is translated into a protein

• Location:

  • Eukaryotes: Transcription in nucleus; Translation in cytoplasm

  • Prokaryotes: Both occur in the cytoplasm simultaneously

Differences Between DNA and RNA

Types of RNA

1. mRNA (messenger RNA): Carries genetic information

2. rRNA (ribosomal RNA): Component of ribosomes; aids in protein synthesis

3. tRNA (transfer RNA): Transfers amino acids to ribosomes

Protein Synthesis

• Steps:

  • Transcription

  • Translation

Transcription

• Occurs on one strand of DNA.

• Example:Original DNA:5’ A T G G G C A T C C T A G G C A T G T A A 3’Complementary mRNA:5’ A U G G G C A U C C U A G G C A U G U A A 3’

• Codons: Blocks of three bases in mRNA that encode amino acids

  • 64 codons total

  • Start codon: AUG (encodes methionine)

  • Stop codons: Signal termination of protein synthesis

Translation

• tRNA Structure:

  • Function: Brings amino acids to ribosomes

  • Anticodon is complementary to mRNA codon

  • Example:

    • Codon: AUG → Anticodon: UAC

    • Codon: GGC → Anticodon: CCG

Mutations

• Definition: Change in base sequence of DNA causing altered amino acid sequence in proteins

• Types of Mutations:

  • Base Substitution (point mutations):

    • Missense: One amino acid replaced by another (e.g., sickle cell anemia)

    • Nonsense: Codon changes to a stop codon

  • Frameshift Mutations:

    • Caused by insertion or deletion of bases; disrupts reading frame

Mutagens

• Definition: Agents causing mutations

• Types:

  • Chemical mutagens

  • Radiation (ionizing and nonionizing)

Identifying Mutagens: The Ames Test

• Uses Salmonella typhimurium to detect mutagenic potential

• His- strain (unable to synthesize histidine) can revert to His+

• Higher colony counts on histidine deficient medium indicate mutagen presence

Genetic Recombination and Transfer

• Definition: Exchange of genetic material between organisms

• Types:

  • Transformation: Uptake of naked DNA

  • Conjugation: Direct transfer via pilus

  • Conjugation Hfr: Involves the transfer of part of genomic DNA

  • Transduction: Gene transfer via viruses

Homologous Recombination

• Exchange of similar DNA sequences between two DNA molecules

• Example: Swap between two markers (e.g., cap colors) producing recombinant DNA