Chapter 8 Microbial Genetics

Microbial Genetics Chapter 8

I Structure and function of genetic material
A. DNA is composed of repeating nucleotides
1. Base pairs (the hydrogen bonds from which DNA is connected)
a. Adenine & Thymine
b. Cystosine & Guanine
2. Deoxyribose sugar & Phosphate group
a. The “backbone” or basis of DNA is deoxyribose.
b. Phosphate is held together by hydrogen bonds between AT & CG.
II DNA replication (Reproduction)
(see next page for steps)
A. Bacteria replication begins at the origin of replication.
B. Circular molecule
C. DNA polymerase
III RNA and Protein Synthesis (Regulation)
A. Transcription: a strand of mRNA is synthesized from the genetic info of DNA.
1. Uracil replaces Thymine. (AT becomes AU)
2. promoter site: the region where RNA polymerase binds to DNA and transcription begins.
3. terminator site: where RNA polymerase and newly formed mRNA are released from the DNA, signaling the end point for transcription of the gene.
B. Translation: translates the language of nucleic acids into the language of proteins.
1. codons: (the language of mRNA) groups of 3 nucleotides that “code” for a particular amino acid.
a. 64 possible codons
b. 20 amino acids (therefore amino acids have more than one codon)
c. degeneracy of the code: 20 amino acids/64 codons
d. sense codons: code for amino acids
e. nonsense codons: signal the end of the synthesis of protein.
i. UAA, UGA, UAG
f. Met/start codon
i. AUG
2. Ribosomes: the sites of translation that move along mRNA.
a. Transfer (t)RNA: transports amino acids to the ribosome.
b. anticodon: a complimentary tRNA molecule made for a specific amino acid. (3 nucleotides by which a tRNA recognizes an mRNA codon.) AUG for mRNA meets UAC for tRNA.
IV Mutations: a change in the base sequence (genetic material) of DNA. Carcinogenic and can be genetic.
A. Types
1. base substitution
2. nonsense mutation
3. Spontaneous: occurs without known intervention of mutation causing agents.
B. Mutagens: chemicals and radiation that bring about mutations.
1. Chemical mutagens
2. Radiation
a. ionizing
b. nonionizing
i. UV light
3. Viruses
V Genetic transfer and recombination
A. Recombination, general overview:
1. Donor cell gives a portion of its total DNA to a different recipient cell.
2. Recipient cell (aka recombinant) DNA from donor is added to its own DNA.
3. Vertical gene transfer: DNA passed from an organism to its offspring.
4. horizontal gene transfer: DNA passed laterally to other microbes of the same generation.
B. Recombination: the rearrangement of genes to form new combinations. Types:
1. Transformation
a. DNA released when donor cell dies.
b. DNA taken up by adjacent cells.
c. competence: the ability to take up foreign DNA.
i. ex: antibiotic resistance
d. Gram + that can take up outside DNA (exogenous)
i. Streptococcus pneumoniae
ii. Staphylococcus aureus
iii. Bacillus subtilis
e. Gram – (neg) that can take up exogenous DNA
i. Neisseria meningitis
ii. Neisseria gonorrhea
iii. Haemophilus influenzae
iv. E. coli
2. Conjugation: transfer of a plasmid (F- Factor) DNA from one bacterium to another. (page 243)
a. Plasmid transfer in gram – bacteria occurs only between strains of the same species or closely related species.
b. Need cell to cell contact. With a sex pilus. (This is how we get a double coil plasmid.)
c. Usually only occurs in gram – bacteria. The exceptions are gram +:
i. Bacillus subtilis
ii. some Streptococcus
iii. Enterococcus faecalis
3. Transduction: A virus carries DNA from one bacteria to another.
(pg 244)
a. Generalized, not specific.
b. Occurs between unrelated species.
c. Can often lead to changes in microbial pathogenicity.
i. Botulism toxin of Clostridium botulinum
ii. Capsule of Streptococcus pneumoniae
iii. E. coli 0157.H7 toxin. (pathogenic)

VI Plasmids and transposons (only went over plasmids)
A. Plasmids: circular pieces of DNA that replicate independently from the cell’s chromosome.
1. Plasmids usually carry only genes that are not essential for growth of the cell.
2. Types
a. dissimilation plasmids: code for enzymes to utilize unusual sugars and hydrocarbons. (Pseudomonas)
b. bacteriocins: toxic proteins that kill other bacteria.
i. plasmids cause the synthesis of bacteriocins
c. resistance (R)factors: carry genes for antibiotic resistance and other antimicrobial factors.
i. encode antibiotic resistance in gram negative bacteria.
d. other plasmids contribute to the pathogenicity of microbes.
i. S. mutans, the cause of cavities.