Bacterial genetics lec. No.5 Lectures of D.r Oruba khalid

Bacterial genetics Genetics define & analyze heredity Bacterial genetics Genetics define & analyze heredity. The unit of heredity is the gene which is a segment of DNA that carries information for specific biochemical or physiological properties. Each characteristic of bacteria is controlled by a specific gene. DNA:- genetic information which is stored as a sequence of bases in deoxyribonucleic acid (DNA).

DNA Structure Double stranded Nucleotide Base Pairs Nitrogen Bases Sugar Phosphate Base Pairs Hydrogen Bonds A-T C-G Alpha helix 5’ – phosphate group 3’ – hydroxyl group

The complementary bases enable one strand to provide the information for copying or expression of information in other strand. Each base determines the genetic information of DNA. DNA is a circular single closed loop found in the cytoplasm of bacteria. This DNA carries the gene.

Genes present along the circle of DNA Genes present along the circle of DNA. Number & location of gene on DNA differ from one bacterium to another. Some bacteria have several hundreds or thousands of genes. GENOME:- is a complete set of genes (more than one gene that carry the information & characters of microorganisms). Genome also can be identified as the totality of genetic information in the organism.

Chromosome function (protein synthesis):- In protein synthesis there is: DNA------- transcription mRNA translation Protein Each amino acid has at least a triplet bases for each codon. • There is an anticodon which is complementary to the codon for example codon UGG has anticodon ACC. • Codon: is a sequence of three bases that determine the structure of one amino acid. • The types of RNA in bacteria & any cell are : (1) mRNA (2)tRNA (3)rRNA.

Bacterial genetics Experiments by Nature of the genetic material Griffith (1928); working on S- forms and R-forms of the strep. Pneumonia Avery (1944); mixing DNA extract of the S-forms with and without Dnase, then mixing it with R- forms Hershy and Chase (1952), used radioactive isotops on bacteriophages (S25-for head protein ) and (P32- for nucleic acids)

Gene transfer: Recepient Recombination ; is the reassortment of nucleotide sequences within the DNA molecule Recombination may occur between Donor Recepient   Chromosomal DNA Plasmid DNA Viral DNA

Sometimes rearrangement occurs within the DNA molecule itself without an external DNA Any recombination or spontaneous rearrangement leads to what is called genotypic change, this may or may not leads to phenotypic change The fate of the transferred DNA depends on: Its capability to be taken by the host cells Its stability within the host chromosome

There are three mechanisms for gene transfer in bacteria Conjugation Transformation Transduction

Plasmids

Conjugation Plasmids extrachromosomal circular DNA, M.W. 106 -108 Daltons Not very necessary to the cell, but add selective advantage over other organisms May be lost from the cell under environmental stress, such as: changes in temperature, presence of certain dyes, or nutrient depletion. Plasmid replication occur during cell division and during conjugation. Presence of certain plasmids possesses the properity of conjugation, therefore called conjugative plasmids. Non-conjugative plasmids may get transferred at the same times with the conjugative plasmids in a process called plamid mobilization Certain plasmids are stably inherited called ( compatible plasmids), others are lost after few divisions called( non compatible plasmids)

Conjugation Sex

Plasmid F-factor F = fertility ~25 genes Sex Pilus F+ = male F- = female ~25 genes Sex Pilus Replicates in synchrony with host DNA Rolling method DNA replicates from parent If integrated with host DNA becomes Hfr

R-plasmids (resistance plasmids) responsible for resistance to drug Classes of plasmids Sex factor plasmids: the cell that possess this plasmid is called F+, male or the donor cell, while the one that do not possess it is called; F-, or the recipient cell. R-plasmids (resistance plasmids) responsible for resistance to drug Col- plasmids: responsible for production of bacteriocins. Heavy metal ion resistant plasmids: responsible for resistance to heavy metal ions that the bacteria may get exposed in the envirnment Plasmids of catabolic activity: responsible for degredation of highly complex compounds, such as: hydrocarbons Virulence plasmids: reponsible for production of certain virulence factors such as: toxin , hemolysin, adhesive factors,… etc Genetic Engineering

During the process of conjugation, the sex pilus, which is formed due to stimulation by the conjugative plasmid, the sex pilus is attached to the reciepent cell on a special receptor, where the tip of the pilus leads to lyses of a tiny area on the recipent cell wall. Attachment will stimulate the plasmid to replicate where a single strand is and the other remains in the donor cell where another complementary strand is synthesized. The receipent cell will have the same extra character possessed by the donor cell. The pilus is cut off of unknown reason.

References: 1- Jawetz, Melnick, & Adelberg’s.( 2013). Medical Microbiology (Twenty-Sixth Edition). 2- Kenneth Todar. (2008).Todar’s Online Textbook of Bacteriology ,University of Wisconsin.