Microbial Genetics Lecture 1

Microbial Genetics - Lecture 1

• Genetics is the branch of biology studying genes, genetic variation, and heredity in living organisms.
• Classical genetics focuses on visible results of reproduction based on Mendel's experiments.
• Molecular genetics studies the structure and function of nucleic acids at the molecular level, including chromosome structure, replication, and gene expression.
• Nucleic acids (DNA and RNA) are the carriers of genetic material in all living organisms.
• Nucleic acids have the ability to duplicate themselves and transfer traits to progeny through transcription to mRNA and translation into proteins.

DNA Structure

• DNA consists of nucleotides with a backbone of alternating phosphate and sugar (2-deoxyribose) and nitrogenous bases.
• Purines include adenine (A) and guanine (G).
• Pyrimidines include cytosine (C) and thymine (T).
• A pairs with T (2 hydrogen bonds).
• G pairs with C (3 hydrogen bonds).
• Nucleotides are joined by phosphodiester bonds.
• DNA is a double helix with two antiparallel chains of nucleotides.
• In prokaryotes, DNA is circular and super-coiled, with proteins similar to histones.
• In eukaryotes, DNA is organized with histone proteins and coiled into nucleosomes.

DNA Replication

• DNA replication is semiconservative, producing two copies each containing one original and one new strand.
• Replication starts at an origin of replication, a specific segment of DNA.
• DNA replication occurs at replication forks, areas of strand separation where new DNA synthesis happens.
• A replicon consists of an origin of replication and the DNA replicated from that origin.
• Bacteria have a single replicon, while eukaryotes have multiple replicons for efficient replication of large molecules.

Different modes of polymerization in DNA replication

• Polymerization of nucleotides occurs in the 5' to 3' direction.
• Original strand - called the "leading strand" is replicated in a continuous 5' to 3' process.
• Lagging strand is replicated discontinuously in small fragments (Okazaki fragments).
• DNA polymerase III synthesizes the leading strand continuously, while primase synthesizes RNA primers for discontinuous lagging strand. The primers are replaced with DNA by polymerase I, and gaps between Okazaki fragments are closed by DNA ligase.

Types of RNA

• RNA is a single-stranded molecule that coils back on itself.
• RNA uses the sugar ribose instead of deoxyribose.
• Uracil (U) replaces thymine (T) in RNA.
• Three main types of RNA:
  • Ribosomal RNA (rRNA): Components of ribosomes and involved in protein synthesis.
  • Transfer RNA (tRNA): Carries amino acids to ribosomes during protein synthesis. Each tRNA has an anticodon that binds to a codon on mRNA.
  • Messenger RNA (mRNA): Carries the genetic code for protein synthesis from DNA to ribosomes. mRNA is formed in transcription from DNA, using RNA polymerase.

DNA Replication Summary and Further factors

• DNA replication is carried out by DNA polymerases.
• Cells in all organisms contain multiple, highly specialized DNA polymerases. Specific polymerases are needed for prokaryotes, and others for eukaryotes.
• The rate of DNA synthesis varies between prokaryotes (750-1000 bp/second) and eukaryotes (50-100 bp/second).
• DNA polymerase III is the primary enzyme for DNA synthesis
• To initiate replication, DNA polymerase requires a primer, which is a short RNA strand.
• Polymerase I has exonuclease activity to remove mismatched nucleotide to repair damage or errors.
• Proofreading removes incorrect nucleotides immediately after they are added, enhancing the fidelity in the process. The proofreading function improves the accuracy of DNA replication, reducing errors to one in every 10⁹ to 10¹⁰ bp.
• Proteins involved in DNA replication:
  • DNA Polymerase III
  • Primase
  • Helicase
  • Topoisomerase
  • Single-stranded Binding proteins (SSBs)
  • DNA Ligase
  • And more

This quiz covers the fundamentals of microbial genetics, focusing on the basic principles of classical and molecular genetics. It discusses the structure of DNA, including the genetic material's nucleotides and pairing rules. Dive deep into how these concepts relate to heredity and genetic variation.