Chapter 9: Gene Transfer, Mutations, and Genome Evolution. Chapter Overview. ● The mosaic nature of genomes ● Gene transfer: Transformation; conjugation; and transduction ● Genetic recombination ● Mutations: Types and causes ● Mechanisms of DNA repair ● Mobile genetic elements
Transformation is the process of importing free DNA into bacterial cells.
- the cells need to be competent.
Many cells are capable of natural transformation and naturally competent.
-others require artificial manipulations.
- Perturbing the membrane by chemical (CaCl2) or electrical (electroporation) methods
Natural Transformation occus Bacillus sp., Haemophilussp., Neisseriasp., Acinetobactersp., Streptococcus sp., Pseudomonas sp.
Gram-positive bacteria transform DNA using atransformasomecomplex.
Gram-negative bacteria transform DNA without the use of competence factors (CF).
some Gram negative organisms are always competent or they become competent when starved.
also, they do not use transformasomes.
most Gram-negative species is sequence-specific.
Thus limiting gene exchange between genera
Conjugation is the transfer of DNA from one bacterium to another, following cell-to-cell contact by pilus on the donor cell.
- The pilus attaches to the receptor on the recipient cell
- Two cell fuse and single-stranded DNA passes from donor to recipient cell.
presence of special
A well-studied example in E. coli is the fertility factor (F factor). Also called fertility plasmid
Conjugation begins with contact between the donor cell, called the F+ cell, and a recipient F– cell.
Female cells become male cells and be able to transfer the plasmid
Relaxosome: many genes necessary for DNA transfer (halicase, endonuclease, etc.
The F-factor plasmid can integrate into the chromosome.
- The cell is now designated Hfr, or high-frequency recombination strain.
Hfr + F- Hfr + F-
an Hfr and F-, the
recipient gets some
of the Hfr genes
plus some of the donor’s
genes. The recipient
since not all Hfr
genes are transferes.
The entire chromosome take about 100 min to transfer as opposed only 5 min for free plasmid
- Aberrant excision results in an F′ factor or F′ plasmid,which carries chromosomal genes.
Transfer of Genes into Eukaryotes
- Agrobacterium tumefaciens, which causes crown gall disease
- Contains a tumor-inducing plasmid (Ti) that can be transferred via conjugation to plant cells
Transduction is the process in which bacteriophages carry host DNA from one cell to another.
There are two basic types:
- Generalized transduction: Can transfer any gene from a donor to a recipient cell
- Specialized transduction: Can transfer only a few closely linked genes between cells
Generalized Transduction domains.
Any gene from a donor chromosome is packaged into a bacteriophage and transferred to a new cell upon infection.
Steps of generalized transduction domains.
P1 phage of E.Coli. and P22 phage of Samonella are examples of generalized transduction.
Specialized Transduction domains.
Bacteria have developed a kind of “safe sex” approach to gene exchange.
This protection system, called restriction and modification, involves:
- Enzymatic cleavage (restriction) of alien DNA, by restriction endonucleases
- Protective methylation (modification) of host DNA
DNA Restriction and Modification
Figure 9.9 gene exchange.
Two different DNA molecules in a cell can recombine by one of several mechanisms:
- Generalized recombination requires that the two recombining molecules have a considerable stretch of homologous DNA sequences (>50 bp).
- Site-specific recombination requires very little sequence homology between the recombining DNA molecules.
- But it does require a short sequence recognized by the recombination enzyme
Recombination gene exchange.
RecA gene exchange. proteins or Synaptases play critical role in recombination
-double stranded DNA becomes single-stranded DNA by creating a nick
-single-stranded binding proteins bind to the ssDNA
-RecA finds homology and mediated strand invasion
A gene exchange.mutation is a heritable change in the DNA.
Mutations can come in several different forms:
Types of Mutations
- Point mutation: Change in a single base
- Insertion (addition) and deletion (subtraction) of one or more bases
-Inversion: DNA is flipped in orientation
- Reversion: DNA mutates back to original sequence
- Silent mutation:Does not change the amino acid sequence
DNA template TTT point mutation T TC
DNA coding AAA AAG
m-RNA UUU UUC
Amino acid Phenylalanine Phenylalanine
Though DNA strand has changed, the protein sequence is the same
Missense mutation classes::Changes the amino acid sequence to another
Nonsense mutation classes::Changes the amino acid sequence to a stop codon
Frame-shift mutation classes:: Changes the open-reading frame of the gene
Spontaneous mutations are rare because of the efficiency of DNA proofreading and repair pathways.
However, they can arise for many reasons:
1)Tautomeric shifts in DNA bases that alter base-pairing properties [ GT or A C]
2) Oxidative deamination of bases
Mutations Arise in Diverse Ways
3) Formation of apurinic sites DNA proofreading and repair pathways.[loss of purine]
Mutations can be caused by DNA proofreading and repair pathways.mutagens:
- Base analogs
- Base modifiers
- X-rays and gamma rays: Break the DNA
- Ultraviolet rays: Form pyrimidine dimers
Mutations Arise in Diverse Ways
Mutagenic agents and their effects. DNA proofreading and repair pathways.
UV radiation can induce dimerization DNA proofreading and repair pathways.