The Central Dogma of genetics describes the flow of genetic information in this way: DNA is transcribed to RNA which is translated into protein. The exception to the rule is the mechanism utilized by retroviruses in which the specialized enzyme, reverse transcriptase, converts an RNA code into a replica DNA strand. Required components are shown in the box.

II. RECOMBINATION

DNA base complementarity assures that the genotype is correctly conveyed from parental to daughter strand. The integrity of this transfer determines genetic fidelity.

************************************************************ New DNA forms result from strand exchange and branch migration. A. Paired DNA duplex from a bacterial cell and from second source. B. RecA protein nicks the DNA in the paired DNA duplex. (Nicks made in the homologous strands) C. Each strand crosses over to pair with its complement in the other strand. D. Nicks are sealed by repair proteins. E. Crossover points can move by branch migration. F. The structure can generate planar molecules by rotation G. Nicks made in planar molecules; gaps are sealed to form either heteroduplex region-containing DNA or recombinant molecules.

III. GENETIC ELEMENTS

• T7 (makes its own polymerase that is more transcriptionally active than the host polymerase)
• T4 (well-studied and highly complicated)

• lambda (integrated prophage)
• P1 (extra-chromosomal prophage)

C. Transposons
1. simple - Insertional Sequence Elements (IS elements: example is Tn3) These elements are flanked by palindromic or reverted repeat sequences that make it easy to insert foreign DNA into the bacterial genome.

IV. GENETIC TRANSFER

B. Transduction-bacteriophage

1. Lysogeny - recombination and integration of viral DNA as a prophage in the host cell chromosome

2. Specialized - uptake of phage genes is limited to those adjacent to the integrated prophage on the bacterial chromosome, only those genes will be expressed by the host bacteria
3. Generalized - lytic infection results in packaging of bacterial DNA into some virions; many different viral and bacterial genes can be transmitted in this way to other bacteria in the surrounding environment
4. in vitro - phage vectors used in molecular transformation of bacteria

5. Lytic - infection with the bacteriophage virus results in lysis of the cell and release of both bacterial and viral genetic material

C. Transformation
1. natural - Streptococcus pneumoniae expresses DNA-binding proteins on the cell surface when in stationary phase growth conditions. This natural competent state allows uptake of "naked DNA"
2. in vitro (Ca ²⁺ heat shock, PEG, electroporation); artificially induced competence due to increased membrane permeability

V. MUTATION

• spontaneous -point (error)

- deletion
- insertional sequence element (IS)
• induced (mutagen)

- indirect (requires DNA replication)
- direct (affects transcription or translation)
- repair-induced (error prone)

• Transversion: Purine and Pyrimidine base exchange
• Transition: Purine-to-Purine -or- Pyrimidine-to-Pyrimidine base change
• CONSEQUENCES OF POINT MUTATIONS (ONE BASE PAIR)
  • Silent Mutation
  • Missense Mutation
  • Frameshift Mutation
  • Nonsense Mutation

C. CONSEQUENCES OF LARGE MUTATIONS
1. deletions - non-revertible
2. insertions (reversions are rare)
3. inversions (reversions are rare)

DELETION

INVERSION

D. SUPPRESSION
1. intragenic - a second mutation that reverses the first
2. extragenic - compensatory over-expression of another gene
- suppressor tRNA - - inhibits nonsense mutations (stop codon). These mutated tRNA's are charged with amino acids, but they recognize the stop codon, thereby circumventing the effects of nonsense mutations.

VI. REPAIR

C. Postreplication Repair or Recombinatorial Repair is the retrieval of missing information by genetic recombination when both DNA strands are damaged.

D. SOS Response - the induction of many genes (~15 total) after DNA damage or interruption of DNA replication.

E. Error-Prone Polymerase - induced as a "last resort" before bacterial cell death. It is used when no template DNA is available. gap retrieval (recombination)

VII. GENETIC ENGINEERING

VIII. GENETIC CONTROL OVER METABOLIC FUNCTION