Bacterial Genetics Lecture 4 PDF

National University for Science Microbiology \ theory and Technology Second stage College of Pharmacy Lecture 4...

National University for Science Microbiology \ theory and Technology Second stage College of Pharmacy Lecture 4 Assist. Prof. Dr. Riyad E. Abed BACTERIAL GENETICS Genetics: Genetics is the study of genes including the structure of genetic materials, what information is stored in the genes, how the genes are expressed and how the genetic information is ‫وراﺛﺔ‬ ‫ﺗﺒﺎﯾﻦ‬ transferred. Genetics is also the study of heredity and variation. The arrangement of genes within organisms is its genotype and the physical characteristics an organism based on its genotype and the interaction with its environment, make up its phenotype. The order of DNA bases constitutes the bacterium's genotype. A particular organism may possess alternate forms of some genes. Such alternate forms of genes are referred to as alleles. The cell's genome is stored in chromosomes, which are chains of double stranded DNA. ‫ﺗﺴﻠﺴﻼت‬ Genes are sequences of nucleotides within DNA that code for functional proteins. The genetic material of bacteria and plasmids is DNA. The two essential ‫ﺣﻤﻞ‬ functions of genetic material are replication and expression. Most prokaryotic genes are carried on bacterial chromosome. while many bacteria contain additional genes on plasmids. 1- Bacterial chromosome: most bacteria have single, covalently closed, circular chromosomes. Not all bacteria have a single circular chromosome: some bacteria have multiple circular chromosomes, and many bacteria have linear chromosomes and linear plasmids. 2- Plasmids: Plasmids are extra-chromosomal elements found inside a bacterium. These are not essential for the survival of the bacterium but they confer certain extra advantages to the cell. Figure 1: Bacterial genome 1  ‫طﻔﺮة‬ ‫ﺻﺎغ‬ Mutation: The term “mutation” was coined by Hugo de Vries, which is derived from Latin word meaning “to change”. Mutation is defined as any change in base sequence of DNA. it occurs in two form: Transition (purine replaced by purine or pyrimidine replaced by pyrimidine) or Transversion (purine replaced by pyrimidine or vice versa) Mutations result from damage to DNA which ‫ﺷﺮاﺋﺢ‬ is not repaired, errors in the process of replication, or from the insertion or deletion of segments of DNA by mobile genetic elements. Mutations play a part in both normal and abnormal biological processes including: evolution, cancer. ‫ﺗﻠﻘﺎﺋﯿﺎ‬ Mutations are heritable changes in genotype that can occur spontaneously or be induced by chemical or physical treatments. (Organisms selected as reference strains are called wild type, and their progeny with mutations are called mutants.) The process of mutation is called mutagenesis and the agent inducing mutations is called mutagen. Changes in the sequence of template DNA (mutations) can drastically affect the type of protein end product produced. For a particular bacterial strain under defined growth conditions, the mutation rate for any specific gene is constant and is expressed as the probability of mutation per cell division. Spontaneous mutation occurs naturally about one in every million to one in every billion divisions. Mutation rates of individual genes in bacteria range from 10 -2 to 10-10 per bacterium per division. Most spontaneous mutations occur during DNA replication. Mechanisms of mutation a. Substitution of a nucleotide: Base substitution, also called point mutation, involves the changing of single base in the DNA sequence. This mistake is copied during replication to produce a permanent change. If one purine [A or G] or pyrimidine [C or T] is replaced by the other, the substitution is called a transition. If a purine is replaced by a pyrimidine or vice- versa, the substitution is called a transversion. This is the most common mechanism of mutation. b. Deletion or addition of a nucleotide: deletion or addition of a nucleotide during DNA replication. When a transposon (jumping gene) inserts itself into a gene, it leads to disruption of gene and is called insertional mutation. 2 Results of mutation a. Missense mutation: Missense mutations are DNA mutations which lead to changes in the amino acid sequence (one wrong codon and one wrong amino acid) of the protein product. This could be caused by a single point mutation or a series of mutations..‫ ﯾﻤﻜﻦ أن ﯾﻜﻮن ھﺬا ﺑﺴﺒﺐ طﻔﺮة ﻧﻘﻄﺔ واﺣﺪة أو ﺳﻠﺴﻠﺔ ﻣﻦ اﻟﻄﻔﺮات‬.‫اﻟﻄﻔﺮات اﻟﺨﺎطﺌﺔ ھﻲ طﻔﺮات اﻟﺤﻤﺾ اﻟﻨﻮوي اﻟﺘﻲ ﺗﺆدي إﻟﻰ ﺗﻐﯿﯿﺮات ﻓﻲ ﺗﺴﻠﺴﻞ اﻷﺣﻤﺎض اﻷﻣﯿﻨﯿﺔ )ﻛﻮدون ﺧﺎطﺊ واﺣﺪ وﺣﻤﺾ أﻣﯿﻨﻲ ﺧﺎطﺊ واﺣﺪ( ﻟﻤﻨﺘﺞ اﻟﺒﺮوﺗﯿﻦ‬ b. Nonsense mutation: A mutation that leads to the formation of a stop codon is called a nonsense mutation. Since these codons cause the termination of protein synthesis, a nonsense mutation leads to incomplete protein products. ‫ ﻧﻈﺮا ﻷن ھﺬه اﻟﻜﻮدوﻧﺎت ﺗﺴﺒﺐ إﻧﮭﺎء‬.‫ﺗﺴﻤﻰ اﻟﻄﻔﺮة اﻟﺘﻲ ﺗﺆدي إﻟﻰ ﺗﻜﻮﯾﻦ ﻛﻮدون ﺗﻮﻗﻒ طﻔﺮة ﻻ ﻣﻌﻨﻰ ﻟﮭﺎ‬ ‫ ﻓﺈن اﻟﻄﻔﺮة اﻟﻌﺒﺜﯿﺔ ﺗﺆدي إﻟﻰ ﻣﻨﺘﺠﺎت ﺑﺮوﺗﯿﻨﯿﺔ ﻏﯿﺮ ﻣﻜﺘﻤﻠﺔ‬،‫ﺗﺨﻠﯿﻖ اﻟﺒﺮوﺗﯿﻦ‬ c. Silent mutation: Sometimes a single substitution mutation changes in the DNA base sequence results in a new codon still coding for the same amino acid. Since there is no change ‫ ﺗﺆدي ﺗﻐﯿﯿﺮات اﺳﺘﺒﺪال واﺣﺪة ﻓﻲ ﺗﺴﻠﺴﻞ ﻗﺎﻋﺪة اﻟﺤﻤﺾ اﻟﻨﻮوي إﻟﻰ ﺗﺮﻣﯿﺰ ﻛﻮدون‬،‫ﻓﻲ ﺑﻌﺾ اﻷﺣﯿﺎن‬ in the product, such mutations are called silent..‫ ﻓﺈن ھﺬه اﻟﻄﻔﺮات ﺗﺴﻤﻰ ﺻﺎﻣﺘﺔ‬،‫ ﻧﻈﺮا ﻟﻌﺪم وﺟﻮد ﺗﻐﯿﯿﺮ ﻓﻲ اﻟﻤﻨﺘﺞ‬.‫ﺟﺪﯾﺪ ﻟﻨﻔﺲ اﻟﺤﻤﺾ اﻷﻣﯿﻨﻲ‬ d. Frameshift mutation: Frameshift mutations involve the addition or deletion of base pairs causing a shift in the “reading frame” of the gene. This causes a reading frame shift and all of the codons and all of the amino acids after that mutation are usually wrong. Since the addition of amino acids to the protein chain is determined by the three base codons, when the overall sequence of the gene is altered, the amino acid sequence may be altered as well. ‫ﻗﺎﺗﻠﮫ‬ e. Lethal mutation: Sometimes some mutations affect vital functions and the bacterial cell become nonviable. Hence those mutations that can kill the cell are called lethal mutation..‫ وﺑﺎﻟﺘﺎﻟﻲ ﺗﺴﻤﻰ ﺗﻠﻚ اﻟﻄﻔﺮات اﻟﺘﻲ ﯾﻤﻜﻦ أن ﺗﻘﺘﻞ اﻟﺨﻠﯿﺔ طﻔﺮة ﻗﺎﺗﻠﺔ‬.‫ﻓﻲ ﺑﻌﺾ اﻷﺣﯿﺎن ﺗﺆﺛﺮ ﺑﻌﺾ اﻟﻄﻔﺮات ﻋﻠﻰ اﻟﻮظﺎﺋﻒ اﻟﺤﯿﻮﯾﺔ وﺗﺼﺒﺢ اﻟﺨﻠﯿﺔ اﻟﺒﻜﺘﯿﺮﯾﺔ ﻏﯿﺮ ﻗﺎﺑﻠﺔ ﻟﻠﺤﯿﺎة‬ ‫طﻔﺮة ﻣﺜﺒﻄﺔ‬ ‫اﻧﻌﻜﺎس‬ f. Suppressor mutation: It is a reversal of a mutant phenotype by another mutation at a position on the DNA distinct from that of original mutation. True reversion or back mutation results in reversion of a mutant to original form, which occurs as a result of mutation occurring at the ‫إﻧﮫ اﻧﻌﻜﺎس ﻟﻨﻤﻂ ظﺎھﺮي ﻣﺘﺤﻮر ﺑﻮاﺳﻄﺔ طﻔﺮة أﺧﺮى ﻓﻲ ﻣﻮﺿﻊ ﻋﻠﻰ اﻟﺤﻤﺾ اﻟﻨﻮوي ﻣﺘﻤﯿﺰ ﻋﻦ‬ same spot once again. ‫ واﻟﺬي‬،‫ ﯾﺆدي اﻻرﺗﺪاد اﻟﺤﻘﯿﻘﻲ أو اﻟﻄﻔﺮة اﻟﺨﻠﻔﯿﺔ إﻟﻰ ﻋﻮدة اﻟﻤﺘﺤﻮل إﻟﻰ اﻟﺸﻜﻞ اﻷﺻﻠﻲ‬.‫اﻟﻄﻔﺮة اﻷﺻﻠﯿﺔ‬ :‫طﻔﺮة ﻣﻤﯿﺘﺔ ﻣﺸﺮوطﺔ‬.‫ﯾﺤﺪث ﻧﺘﯿﺠﺔ ﺣﺪوث اﻟﻄﻔﺮة ﻓﻲ ﻧﻔﺲ اﻟﻤﻜﺎن ﻣﺮة أﺧﺮى‬ g. Conditional lethal mutation: Sometimes a mutation may affect an organism in such a way that the mutant can survive only in certain environmental condition. Example; a temperature sensitive mutant can survive at permissive temperature of 35oC but not at restrictive ‫ﻓﻲ ﺑﻌﺾ اﻷﺣﯿﺎن ﻗﺪ ﺗﺆﺛﺮ اﻟﻄﻔﺮة ﻋﻠﻰ اﻟﻜﺎﺋﻦ اﻟﺤﻲ ﺑﻄﺮﯾﻘﺔ ﻻ ﯾﻤﻜﻦ ﻟﻠﻤﺘﺤﻮل اﻟﺒﻘﺎء ﻋﻠﻰ ﻗﯿﺪ اﻟﺤﯿﺎة إﻻ ﻓﻲ ﺣﺎﻟﺔ‬ temperature of 39oC. ‫ ﻣﺜﺎل؛ ﯾﻤﻜﻦ ﻟﻠﻤﺘﺤﻮﻟﺔ اﻟﺤﺴﺎﺳﺔ ﻟﺪرﺟﺔ اﻟﺤﺮارة اﻟﺒﻘﺎء ﻋﻠﻰ ﻗﯿﺪ اﻟﺤﯿﺎة ﻋﻨﺪ درﺟﺔ ﺣﺮارة ﻣﺘﺴﺎھﻠﺔ ﺗﺒﻠﻎ‬.‫ﺑﯿﺌﯿﺔ ﻣﻌﯿﻨﺔ‬ ‫طﻔﺮة اﻻﻧﻌﻜﺎس‬.‫ درﺟﺔ ﻣﺌﻮﯾﺔ‬39 ‫ درﺟﺔ ﻣﺌﻮﯾﺔ وﻟﻜﻦ ﻟﯿﺲ ﻋﻨﺪ درﺟﺔ ﺣﺮارة ﻣﻘﯿﺪة ﺗﺒﻠﻎ‬35 h. Inversion mutation: If a segment of DNA is removed and reinserted in a reverse direction, it is called inversion mutation..‫ ﯾﻄﻠﻖ ﻋﻠﯿﮫ طﻔﺮة اﻧﻌﻜﺎس‬،‫إذا ﺗﻤﺖ إزاﻟﺔ ﺟﺰء ﻣﻦ اﻟﺤﻤﺾ اﻟﻨﻮوي وإﻋﺎدة إدﺧﺎﻟﮫ ﻓﻲ اﺗﺠﺎه ﻋﻜﺴﻲ‬ Based on extent of base pair changes, mutations can be of two types; microlesion and macrolesion. Microlesions are basically point mutations (affecting single base pairs) whereas macrolesions involve addition, deletion, inversion or duplication of several base pairs. ‫ اﻵﻓﺎت اﻟﺪﻗﯿﻘﺔ ھﻲ ﻓﻲ اﻷﺳﺎس طﻔﺮات ﻧﻘﻄﯿﺔ )ﺗﺆﺛﺮ ﻋﻠﻰ أزواج اﻟﻘﺎﻋﺪة اﻟﻤﻔﺮدة( ﻓﻲ ﺣﯿﻦ أن اﻟﻜﻠﯿﺔ ﺗﻨﻄﻮي ﻋﻠﻰ‬.‫ ﯾﻤﻜﻦ أن ﺗﻜﻮن اﻟﻄﻔﺮات ﻣﻦ ﻧﻮﻋﯿﻦ؛ اﻟﻤﯿﻜﺮوﻟﯿﺴﯿﻮن واﻟﻜﺮوﻟﯿﺴﯿﻮن‬،‫اﺳﺘﻨﺎدا إﻟﻰ ﻣﺪى ﺗﻐﯿﺮات اﻟﺰوج اﻷﺳﺎﺳﻲ‬.‫إﺿﺎﻓﺔ أو ﺣﺬف أو اﻧﻌﻜﺎس أو ﺗﻜﺮار اﻟﻌﺪﯾﺪ ﻣﻦ أزواج اﻟﻘﺎﻋﺪة‬ The mutations in DNA can occur spontaneously or can be caused by an external force or substance called a mutagen. Mutagens can be chemicals such as nitrous acid, which alters adenine to pair with cytosine instead of thymine. Other chemical mutagens include acridine dyes, nucleoside analogs that are similar in structure to nitrogenous bases, benzpyrene (from ‫إﺷﻌﺎع‬ smoke and soot) and aflatoxin. Radiation can also be a cause of DNA mutations. High energy light waves such as X-rays, gamma rays, and ultraviolet light have been shown to damage DNA. UV light is responsible for the formation of thymine dimers in which covalent links are ‫ﯾﻤﻜﻦ أن ﺗﺤﺪث اﻟﻄﻔﺮات ﻓﻲ اﻟﺤﻤﺾ اﻟﻨﻮوي ﺗﻠﻘﺎﺋﯿﺎ أو ﯾﻤﻜﻦ أن ﺗﺤﺪث ﺑﺴﺒﺐ ﻗﻮة ﺧﺎرﺟﯿﺔ‬ ‫ اﻟﺬي‬،‫ ﯾﻤﻜﻦ أن ﺗﻜﻮن اﻟﻤﻄﻔﺮات ﻣﻮاد ﻛﯿﻤﯿﺎﺋﯿﺔ ﻣﺜﻞ ﺣﻤﺾ اﻟﻨﯿﺘﺮوز‬.‫أو ﻣﺎدة ﺗﺴﻤﻰ اﻟﻄﻔﺮات‬ 3 ‫ ﺗﺸﻤﻞ اﻟﻤﻄﻔﺮات اﻟﻜﯿﻤﯿﺎﺋﯿﺔ اﻷﺧﺮى‬.‫ﯾﻐﯿﺮ اﻷدﯾﻨﯿﻦ ﻟﯿﻘﺘﺮن ﺑﺎﻟﺴﯿﺘﻮزﯾﻦ ﺑﺪﻻ ﻣﻦ اﻟﺜﯿﻤﯿﻦ‬ ،‫ وﻧﻈﺎﺋﺮ اﻟﻨﻮﻛﻠﯿﻮﺳﯿﺪ اﻟﻤﺘﺸﺎﺑﮭﺔ ﻓﻲ اﻟﺘﺮﻛﯿﺐ ﻣﻊ اﻟﻘﻮاﻋﺪ اﻟﻨﯿﺘﺮوﺟﯿﻨﯿﺔ‬،‫أﺻﺒﺎغ اﻷﻛﺮﯾﺪﯾﻦ‬ ‫ ﯾﻤﻜﻦ أن ﯾﻜﻮن اﻹﺷﻌﺎع أﯾﻀﺎ ﺳﺒﺒﺎ‬.‫واﻟﺒﻨﺰﺑﯿﺮﯾﻦ )ﻣﻦ اﻟﺪﺧﺎن واﻟﺴﺨﺎم( واﻷﻓﻼﺗﻮﻛﺴﯿﻦ‬ ‫ ﻟﻘﺪ ﺛﺒﺖ أن اﻟﻤﻮﺟﺎت اﻟﻀﻮﺋﯿﺔ ﻋﺎﻟﯿﺔ اﻟﻄﺎﻗﺔ ﻣﺜﻞ اﻷﺷﻌﺔ اﻟﺴﯿﻨﯿﺔ‬.‫ﻟﻄﻔﺮات اﻟﺤﻤﺾ اﻟﻨﻮوي‬ ‫ اﻷﺷﻌﺔ ﻓﻮق اﻟﺒﻨﻔﺴﺠﯿﺔ‬.‫وأﺷﻌﺔ ﻏﺎﻣﺎ واﻷﺷﻌﺔ ﻓﻮق اﻟﺒﻨﻔﺴﺠﯿﺔ ﺗﻀﺮ ﺑﺎﻟﺤﻤﺾ اﻟﻨﻮوي‬ ‫ﻣﺴﺆوﻟﺔ ﻋﻦ ﺗﻜﻮﯾﻦ دﯾﻤﺮات اﻟﺜﺎﯾﻤﯿﻦ اﻟﺘﻲ ﺗﻜﻮن ﻓﯿﮭﺎ اﻟﺮواﺑﻂ اﻟﺘﺴﺎھﻤﯿﺔ‬ established between the thymine molecules. These links change the physical shape of the DNA preventing transcription and replication. Significance of mutation: :‫أھﻤﯿﺔ اﻟﻄﻔﺮة‬.‫ﯾﻤﻜﻦ أن ﯾﺴﺎﻋﺪ اﻛﺘﺸﺎف طﻔﺮة ﻓﻲ اﻟﺠﯿﻦ ﻓﻲ ﺗﺤﺪﯾﺪ وظﯿﻔﺔ ھﺬا اﻟﺠﯿﻦ‬ ‫اﻛﺘﺸﺎف‬ Discovery of a mutation in a gene can help in identifying the function of that gene..‫ ﺧﺎﺻﺔ ﻹﻧﺘﺎج اﻟﻠﻘﺎﺣﺎت‬،‫ﯾﻤﻜﻦ إﺣﺪاث اﻟﻄﻔﺮات ﻓﻲ اﻟﻤﻨﻄﻘﺔ اﻟﻤﺮﻏﻮﺑﺔ ﻹﻧﺸﺎء ﻣﺘﺤﻮﻟﺔ ﻣﻨﺎﺳﺒﺔ‬ Mutations can be induced at a desired region to create a suitable mutant, especially to produce vaccines. ‫ظﮭﻮر‬.‫ﯾﻤﻜﻦ أن ﺗﺆدي اﻟﻄﻔﺮات اﻟﻌﻔﻮﯾﺔ إﻟﻰ ظﮭﻮر ﻣﻘﺎوﻣﺔ ﻟﻠﻤﻀﺎدات اﻟﺤﯿﻮﯾﺔ ﻓﻲ اﻟﺒﻜﺘﯿﺮﯾﺎ‬ Spontaneous mutations can result in emergence of antibiotic resistance in bacteria. Mutations can result in change in phenotype such as appearance of novel surface antigen, alternation in physiological properties, change in colony morphology, nutritional requirements, biochemical reactions, growth characteristics, virulence and host range. ،‫ﯾﻤﻜﻦ أن ﺗﺆدي اﻟﻄﻔﺮات إﻟﻰ ﺗﻐﯿﯿﺮ ﻓﻲ اﻟﻨﻤﻂ اﻟﻈﺎھﺮي ﻣﺜﻞ ظﮭﻮر ﻣﺴﺘﻀﺪ ﺳﻄﺤﻲ ﺟﺪﯾﺪ‬ Transfer of Genetic Material ‫ واﻟﻤﺘﻄﻠﺒﺎت‬،‫ واﻟﺘﻐﯿﯿﺮ ﻓﻲ ﻣﻮرﻓﻮﻟﻮﺟﯿﺎ اﻟﻤﺴﺘﻌﻤﺮة‬،‫واﻟﺘﻨﺎوب ﻓﻲ اﻟﺨﺼﺎﺋﺺ اﻟﻔﺴﯿﻮﻟﻮﺟﯿﺔ‬.‫ وﻧﻄﺎق اﻟﻤﻀﯿﻒ‬،‫ واﻟﻔﻮﻋﺔ‬،‫ وﺧﺼﺎﺋﺺ اﻟﻨﻤﻮ‬،‫ واﻟﺘﻔﺎﻋﻼت اﻟﻜﯿﻤﯿﺎﺋﯿﺔ اﻟﺤﯿﻮﯾﺔ‬،‫اﻟﻐﺬاﺋﯿﺔ‬ Sometimes when two pieces of DNA come into contact with each other, sections of each DNA strand will be exchanged. This is usually done through a process called crossing over in which the DNA breaks and is attached on the other DNA strand leading to the transfer of genes and possibly the formation of new genes. ‫إﻋﺎدة اﻟﺘﺮﻛﯿﺐ‬ Genetic recombination is the transfer of DNA from one organism to another. The transferred ‫ﻣُﺘَﺒَﺮﱢع‬ donor DNA may then be integrated into the recipient's nucleoid by various mechanisms. In the case of homologous recombination, homologous DNA sequences having ‫ﺟﻤﻊ ﺷﻤﻞ‬ nearly the same nucleotide sequences are exchanged by means of breakage and reunion of paired DNA segments. Genetic information can be transferred from organism to organism through vertical transfer (from a parent to offspring) or through horizontal transfer methods such as conjugation, transformation or transduction. Bacterial genes are usually transferred to members of the same species but occasionally transfer to other species can also occur. ‫أﻋﻀﺎء‬ Transformation: 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. This was first demonstrated in an experiment conducted by Griffith in 1928. Transformation is gene transfer resulting from the uptake by a recipient cell of naked DNA from a donor cell. Certain bacteria (e.g. Bacillus, Haemophilus, Neisseria, Pneumococcus) can take up DNA from the environment and the DNA that is taken up can be incorporated into the recipient's chromosome. Some bacteria are able to take up DNA naturally. However, these bacteria only take up DNA a particular time in their growth cycle (log phase) when they produce a specific protein called a competence factor. Uptake of DNA by Gram positive and Gram negative bacteria differs. In Gram positive bacteria the DNA is taken up as a single stranded molecule and the complementary strand is made in the recipient. In contrast, Gram negative bacteria take up double stranded DNA. 4  ‫ ﯾﺴﺘﺨﺪم‬،‫ ﺑﺎﻹﺿﺎﻓﺔ إﻟﻰ ذﻟﻚ‬.‫ﯾﺤﺪث اﻟﺘﺤﻮل ﻓﻲ اﻟﻄﺒﯿﻌﺔ وﯾﻤﻜﻦ أن ﯾﺆدي إﻟﻰ زﯾﺎدة اﻟﻔﻮﻋﺔ‬.‫اﻟﺘﺤﻮل ﻋﻠﻰ ﻧﻄﺎق واﺳﻊ ﻓﻲ ﺗﻘﻨﯿﺔ اﻟﺤﻤﺾ اﻟﻨﻮوي اﻟﻤﺆﺗﻠﻒ‬ Significance: Transformation occurs in nature and it can lead to increased virulence. In addition, transformation is widely used in recombinant DNA technology. Transduction: ‫ﺗﻄﻔﻞ‬ Bacteriophage are viruses that parasitize bacteria and use their machinery for their own replication. During the process of replication inside the host bacteria the bacterial chromosome or plasmid is erroneously packaged into the bacteriophage capsid. Thus newer progeny of phages may contain fragments of host chromosome along with their own DNA or entirely host chromosome. When such phage infects another bacterium, the bacterial chromosome in the phage also gets transferred to the new bacterium. This fragment may undergo recombination with the host chromosome and confer new property to the bacterium. Life cycle of bacteriophage may either by lytic or lysogenic. In the former, the parasitized bacterial cell is killed with the release of mature phages while in the latter the phage DNA gets incorporated into the bacterial chromosome as prophage. Conjugation: ‫اﻗﺘﺮان‬ In 1946 Joshua Lederberg and Tatum discovered that some bacteria can transfer genetic information to other bacteria through a process known as conjugation. Bacterial conjugation is the transfer of DNA from a living donor bacterium to a recipient bacterium. Plasmids are small autonomously replicating circular pieces of double-stranded circular DNA. Conjugation involves the transfer of plasmids from donor bacterium to recipient bacterium. Plasmid transfer in Gram-negative bacteria occurs only between strains of the same species or closely related species. ‫ﻟﺰج‬ In Gram-positive bacteria sticky surface molecules are produced which bring the two bacteria into contact. Plasmid-mediated conjugation occurs in Bacillus subtilis, Streptococcus lactis, and Enterococcus faecalis but is not found as commonly in the Gram-positive bacteria as compared to the Gram-negative bacteria. 5 6