1. TRANSFERIMIENTO LATERAL DE GENES

4. Fig. 8.7 transmission electron micrograph of F pilli of E. coli
Unlike the shorter attachment pili (fimbriae), this long type of pilus is used for transfer of genes in conjugation and is often called a sex pilus. F+ F-
Fig. 8.7 transmission electron micrograph of F pilli of E. coli

5. F' cells
Formation of an F' cell from an Hfr cell, and transfer of a bacterial chromosome segment to a recipient cell.

6. F+ F-
An F+ x F- mating- The F+ cell transfers one strand of DNA from its plasmid to the F- cell via the conjugation bridge. As this occurs, the complementary strands of F plasmid DNA are synthesized.
Fig An F+ x F- mating.

8. F', formed by the excision of the F factor plasmid from the Hfr strain contains some host genes in addition to the gene for F plasmid.
The F' transfers its genetic material like an F+ strain
Fig The formation and transfer of F’ plasmids

10. Viral reproduction: the lytic cycle
Generalized schematic for viral reproduction in a host bacterium, through the lytic cycle.
In the lytic cycle, the virus (phage) multiplies in the host cell and the progeny viruses are released by lysis of cell.

11. Viral reproduction: the lysogenic cycle
Generalized schematic for viral reproduction in a host bacterium, through the lysogenic cycle.
In the lysogenic cycle, viral DNA is integrated into the host genome and replicates as the chromosome replicates, producing lysogenic progeny cells

14. Generalized transduction: Lytic phage

15. Specialized transduction: Lysogenic phage

17. Transposable elements in prokaryotes
Insertion sequence (IS) elements
Transposons (Tn)
Bacteriophage Mu

18. Insertion elements and transposons
Insertion sequences (IS) are short DNA sequences, about 700 to 5000 bp which can move from one location in a DNA sequence to another. They have short bp inverted repeats on their ends. They encode a transposase which catalyses site-specific recombination.
Simple transposons are mobile genetic elements in which a one or more genes are flanked by two insertion sequences.

19. Composite transposons
Structures of some bacterial transposable elements.
A composite transposon contains antibiotic genes flanked by two insertion sequences as direct or inverted repeats Shown here is the Tn5 transposon, with inverted repeats.
The Tn3 transposon.

20. Insertion sequence (IS) elements
Simplest type of transposable element found in bacterial chromosomes and plasmids
Encode only genes for mobilization and insertion
Range in size from 768 bp to 5 kb
IS1 first identified in E. coli’s glactose operon is 768 bp long and is present with 4-19 copies in the E. coli chromosome
Ends of all known IS elements show inverted terminal repeats (ITRs)

22. Integration of IS element in chromosomal DNA

23. Three different mechanisms for transposition
Conservative transposition
Replicative transposition
Retrotransposition

24. Three different mechanisms for transposition
Conservative transposition: The element itself moves from the donor site into the target site
Replicative transposition: The element moves a copy of itself to a new site via a DNA intermediate
Retrotransposition: The element makes an RNA copy of itself which is reversed-transcribed into a DNA copy which is then inserted (cDNA)

25. Conservative transposition

26. Replicative transposition

27. Retrotransposition

28. common feature of mobile elements
Generation of short direct repeats flanking the newly inserted element
This results for a staggered cut being made in the DNA strands at the site of insertion

30. Transposons (Tn)
Similar to IS elements but are more complex structurally and carry additional genes
2 types of transposons:
Composite transposons
Noncomposite transposons

31. Composite transposons

32. IS10R is an autonomous element, while IS10L is non-autonomous

33. Composite Transposons
Tetracycline resistance is carried by a transposable element
The transposon is a composite transposon, composed of IS-elements flanking an included sequence, in this case containing an antibiotic resistance gene
IS10R is an autonomous element
while IS10L is non-autonomous
Composite transposons probably evolved from IS elements by the chance location of a pair in close proximity to one another. Inactivation of one element by mutation would not harm ability to transpose and would assure continued transposition of the entire transposon

34. Noncomposite transposons

35. Noncomposite transposons (Tn)
Carry genes (e.g., a gene for antibiotic resistance)
Ends are non-IS element repeated sequences
Tn3 is 5 kb with 38-bp ITRs and includes 3 genes; bla (-lactamase), tnpA (transposase), and tnpB (resolvase, which functions in recombination)

36. Examples of DNA-intermediate mobile elements
Insertion Sequences (IS) elements in bacteria
P elements in Drosophila
AC/DS (dissociation) elements in maize
AC is a full-length autonomous copy
DS is a truncated copy of AC that is non-autonomous, requiring AC in order to transpose
At least seven major classes of DNA transposons in the human genome (3% of total genome)

37. Structure and transposition of a transposable element