Microbial Genetics

• Nucleic Acids:

  • Heredity material found in cells
  • Large molecules that are acidic in nature
  • Associated with the nuclear material of cells
• Two types of Nucleic Acids:

  • Deoxyribonucleic Acid (DNA)
  • Ribonucleic Acid (RNA)
• Deoxyribonucleic Acid (DNA):

  • Responsible for all cellular activity
  • Directs the production of proteins
  • Double stranded and helical
  • Maintained by weak hydrogen bonds
  • Very stable and can survive high temperatures, high salt concentrations, and acid environments
• Ribonucleic Acid (RNA):

  • Consists of a long chain of nucleotide units
  • Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate
  • Usually single-stranded
• Three types of RNA:

  • mRNA (messenger)
  • tRNA (transfer)
  • rRNA (ribosomal)
• mRNA (messenger):

  • Complementary to one strand of DNA
  • Functions to carry genetic material from the chromosome to the ribosome (Transcription)
• tRNA (transfer):

  • Responsible for transferring information from mRNA to rRNA (Translation)
• rRNA (ribosomal):

  • Associated with the ribosome
  • Accepts information from tRNA and correlates the information to synthesize proteins (Protein Synthesis)
• Nucleic Acids are constructed from a string of small molecules called Nucleotides:

  • Nucleotides consist of a 5-carbon sugar (pentose), one or more phosphate groups, and a base containing nitrogenous rings
• Base Types:

  • Purines contain 2 nitrogenous rings (Adenine and Guanine)
  • Pyrimidines contain 1 nitrogenous ring (Cytosine and Thymine in DNA, Uracil replaces Thymine in RNA)
• Rules for Base Pairings:

  • Adenine pairs with Thymine in DNA (A-T)
  • Uracil replaces Thymine in RNA
  • Guanine pairs with Cytosine (G-C) and forms stronger bonds
• Central Dogma Theory:

  • Represents the flow of genetic information in a living cell: DNA—>RNA-->protein
  • Major processes involved are replication, transcription, and translation
• Reverse Transcriptase:

  • Involves copying RNA information into DNA using reverse transcriptase
  • Adds another pathway to the central dogma of molecular biology
• DNA Replication in Bacteria:

  • Bacteria contain 1 chromosome and many contain plasmids
  • Enzymes known as polymerases transport nucleotides to duplicate DNA during replication
  • Replication is semi-conservative
• RNA Synthesis in Bacteria:

  • Involves the assembly of nucleotides by RNA polymerase
  • RNA polymerase binds to DNA at a promoter site near the gene to be transcribed
• Protein Synthesis in Bacteria:

  • Carried out in the cytoplasm
  • Involves DNA duplication by mRNA (Transcription) and transfer of information by tRNA to rRNA (Translation)
• Codon:

  • Group of three nucleotides in DNA that acts as a code for placing an amino acid in a protein molecule
• Plasmids:

  • Small, circular, double-stranded DNA molecules distinct from chromosomal DNA
  • Naturally exist in bacterial cells and some eukaryotes
  • Provide genetic advantages like antibiotic resistance
  • Can be transferred through conjugation
• Fertility F-plasmids:

  • Contain transfer genes for gene transfer through conjugation
  • Episomes that can be inserted into chromosomal DNA
  • Bacteria with F-plasmid are F positive (F+), without are F negative (F–)
• Resistance Plasmids:

  • Contain genes for defense against environmental factors
  • Can transfer themselves through conjugation, leading to antibiotic resistance
• Virulence Plasmids:

  • Turn bacteria into pathogens, causing disease
  • Easily spread and replicated among affected individuals
• Escherichia coli (E. coli) has several virulence plasmids
• E. coli is found naturally in the human gut and in other animals
• Certain strains of E. coli can cause severe diarrhea and vomiting
• Salmonella enterica is another bacterium that contains virulence plasmids
• Degradative plasmids help the host bacterium to digest compounds not commonly found in nature
• Contain genes for special enzymes that break down specific compounds
• Degradative plasmids are conjugative
• Col plasmids contain genes that make bacteriocins (colicins) which kill other bacteria and defend the host bacterium
• Bacteriocins are found in many types of bacteria including E. coli
• Transposons are small pieces of DNA found in chromosomes and plasmids
• Transposons can move from one location to another in a cell's genome
• Transposons are also known as "jumping genes"
• DNA transposons move using a cut-and-paste mechanism
• Retrotransposons move in a copy-and-paste fashion via an RNA intermediate
• Retrotransposons increase their copy number more rapidly than DNA transposons
• Mutation can involve exchanging base pairs, inserting or deleting base pairs, rearranging sections in the DNA molecule, or exchanging DNA regions with another molecule (recombination)
• Some mutations are harmful, some beneficial, some neutral
• UV light and nitrous acid are examples of mutagens that cause mutations
• UV light causes adjacent thymines to react and bond with each other, leading to thymine dimers