GENETIC TRANSFER AND RECOMBINATION

Transcription

1 GENETIC TRANSFER AND RECOMBINATION

2 Genetic recombination! Genetic recombination is the rearrangement of genes to form new combinations. If two chromosomes break and are rejoined in such a way that some of the genes are reorganize between the two chromosomes, the process is called crossing over.! Crossing over occurs during meiosis in eukaryotes and during the processes of transformation, conjugation and transduction in prokaryotes

3 ! Genetic recombination contributes to population diversity: recombination more ability than mutations to provide beneficial change since it tends not to destroy gene function! Recombination: when one organism donates DNA to another organism. Transfer involves donor cell that gives portion of DNA to recipient cell, when donor DNA incorporated into recipient, recipient now called recombinant cell Usually involves small pieces of DNA (plasmids or chromosomal fragments)! The transferred donor DNA may then be integrated into the recipient's nucleoid by various mechanisms. In the case of Homologous recombination, homologous DNA sequences having nearly the same nucleotide sequences are exchanged by means of breakage and reunion of paired DNA segments. Bacterial genes are usually transferred to members of the same species but sometimes transfer to other species can also occur.! Genetic recombination may also involve acquisition of DNA for which there is no homologous DNA in the "recipient", as in most bacterial plasmid transfers! Generation of recombinant cells is very low frequency event (less than 1%): very few cells in population are capable of exchanging and incorporating DNA

4 Homologous Recombination Recombination can occur between homologous (similar) DNA sequences:! DNA with same genes! Special proteins In bacteria, transfer of genetic material is one-way, from donor to recipient

5 ! Genetic information can be transferred from organism to organism through Vertical transfer or Horizontal transfer! Vertical gene transfer: recombination during meiosis for sexual reproduction, genes passed from organism to offspring. Creates diversity in offspring but parent remains unchanged Generation 1 transfer to the next generation Generation 2 Generation 3

6 ! Horizontal gene transfer: genes passed to neighboring microbes of same generation. transfer within the same generation Donor cell Recipient cell

7 Bacterial Sexual Processes! Eukaryotes have the processes of meiosis to reduce diploids to haploidy, and fertilization to return the cells to the diploid state. Bacterial sexual processes are not so regular. However, they serve the same aim: to mix the genes from two different organisms together. The three bacterial sexual processes: 1. Transformation: naked DNA is taken up from the environment by bacterial cells. 2. Conjugation: direct transfer of DNA from one bacterial cell to another. 3. Transduction: use of a bacteriophage (bacterial virus) to transfer DNA between cells.

8 Transformation Genes are transferred from one bacterium to another as naked DNA! Transformation involves the uptake of free or naked DNA released by donor by a recipient. It was the first example of genetic exchange in bacteria to have been discovered.! Frederick Griffith (1928) - discovery of genetic material

9 ! Griffith s Experiment Two Strains of Streptococcus pneumoniae! The presence of a capsule around some strains of pneumococci gives the colonies a glistening, smooth (S) appearance while pneumococci lacking capsules have produce rough (R) colonies.

10 Strains of pneumococci with a capsule (type I) are virulent and can kill a mouse whereas strains lacking it (type II) are harmless. Griffith found that mice died when they were injected with a mixture of live non capsulated (R, type II) strains and heat killed capsulated (S, type I) strains. Neither of these two when injected alone could kill the mice, only the mixture of two proved fatal. Live S strains with capsule were isolated from the blood of the animal suggesting that some factor from the dead S cells converted the R strains into S type. The factor that transformed the other strain was found to be DNA by Avery, McLeod and McCarty in 1944.

11 ! The steps involved in transformation are: 1. A donor bacterium dies and is degraded. 2. A fragment of DNA (usually about 20 genes long) from the dead donor bacterium binds to DNA binding proteins on the cell wall of a competent, living recipient bacterium.

12 3. Nuclease enzymes then cut the bound DNA into fragments. One strand is destroyed and the other penetrates the recipient bacterium. 4. The Rec A protein promotes genetic exchange (recombination) between a fragment of the donor's DNA and the recipient's DNA

13 ! Recipient bacteria must be competent to take up DNA from dead cells and incorporate it into genome by recombination! Few strains of bacteria are naturally competent (generally Gram positive)! Bacteria can be made artificially competent! chemical agents to poke holes (calcium solutions) transformed cell than passes genetic recombination to progeny Competent: permeable to DNA: changes in cell wall that allow large molecule like DNA to get through! electric current