Lecture 10 Microbial genetics PDF

Preview

Summary

Genética Nannette...

Description

8

 Genetic information can be transferred from one bacterium to another, resulting in an altered genotype.  Three processes result in this transfer of genetic information 

Conjugation,



transformation,



transduction



- transfer of genetic information between members of the same species.



- transfer of genetic information between members of related but distinct bacterial species. – Usually confers survival advantages, that is, transfer of antibiotic resistance genes to another species – Major factor in speciation

Transduction

(or phages) are viruses that can infect a host bacterium by injecting their DNA.    

During reproduction, phages can be involved in genetic recombination called Bacteriophage T4 belongs to a group of related bacterial viruses called T-even viruses. The DNA is contained within an icosahedral protein coat which makes up the head of the virus. The head is connected to a tail, with binding sites recognizing the cell wall.

Figure 8-12

The life cycle of phage T4 is initiated when virus binds by adsorption to the bacterial host cell. 

The tail contracts, the central core penetrates the cell wall, and the DNA moves into the host cytoplasm. 

The bacterial DNA, RNA, and protein synthesis in the host cell is inhibited. 

Host

DNA degradation is initiated. Phage

DNA replication occurs, then components of the head, tail, and tail fibers are synthesized. When

approximately 200 new viruses have been constructed, the bacterial cell is ruptured.



The number of phages produced following the infection of bacteria can be determined by the ( ).



This technique entails performing serial dilutions of virally infected bacteria, which are then poured onto agar plates.



By counting the number of plaques (areas clear of bacteria) on the plates, the number of phages in the original culture can be determined.











occurs when: the phage DNA into the bacterial chromosome. it is replicated along with the chromosome. it is passed to daughter cells. The viral DNA that integrates into the bacterial chromosome is called a . Viruses that lyse the cell or behave as profhages are . Viruses that only lyse the cell are referred to as . A bacterium that harbors a prophage is said to be , capable of being lysed. An is a viral DNA that can be integrated into the host DNA or replicated in the bacterial cytoplasm.

Bacteriophages, which can themselves undergo genetic recombination, can be involved in a mode of bacterial genetic recombination called .



In , bacterial DNA instead of phage DNA is randomly packaged in a phage particle and transferred to a recipient host ( ).







Abortive Transduction: Bacterial DNA does not replicate but is transmitted to one progeny cell following each division. Complete Transduction: Bacterial DNA recombines with its homologous region of the bacterial chromosome and passes to all daughter cells. In , a small piece of bacterial DNA is packaged along with the phage DNA, and transfer is only strain-specific (not random).

Conjugation

 Bacteria undergo

, in which

– genetic information from one bacterium is transferred to another, it recombines with the second bacterium’s DNA.  When the F factor is present, the cell is able to form a sex pilus and serve as a donor of genetic information.  Physical contact is the first step in conjugation established by the F pilus (or sex pilus; pl. pili).

 During conjugation, a copy of the F factor is transferred from the F+ cell to the F recipient, converting the recipient to the F+ state.







F+ cells contain a fertility factor (F factor) that confers the ability to donate part of their chromosome during conjugation.

F factor is mobile and consists of a circular, double-stranded DNA molecule containing 19 genes.

One strand of the double helix moves into the recipient cell via the sex pilus, and the other one remains. Both re-form their double helix and become F+ (Figure 8-4). Figure 8-4











The genetic recombination of two auxotrophic strains producing prototrophs. Neither auxotroph grows on minimal medium, but prototrophs do, suggesting that genetic recombination has occurred Different strains of bacteria are involved in a unidirectional transfer of genetic material. Cells serving as donors of parts of their chromosomes are designated F+ cells (F for fertility). Recipient bacteria receive the donor DNA and recombine it with part of their own chromosome and are designated as F– cells.

 An ( ) strain behaves + as donor and is a special class of F cells.  An Hfr strain can donate genetic information to an F– cell, but the recipient does not become F+. – F+ F–  recipient becomes F+ (low rate of recombination). – Hfr F–  recipient remains F- (high rate of recombination).

Interrupted matings demonstrated that specific genes in an Hfr strain are transferred and recombined sooner than others (Figure 8-5).

An ordered linear transfer of genes is correlated with the length of time conjugation proceeded – The gene order and distance between genes could be predicted . – Basis for first genetic map in bacteria.

Gene transfer by Hfr strains led to the understanding that the chromosome is circular ( and ).

When F and F cells are mixed, conjugation occurs readily, and each F cell involved in conjugation with an F cell receives a copy of the F factor, no genetic recombination occurs. At an extremely low frequency in a population of F cells, the F factor integrates spontaneously from the cytoplasm to a random point in the bacterial chromosome converting the F cells to the Hfr state ( ).

During conjugation between an Hfr and an F– cell, the position of the F factor determines the initial point of transfer (Figure 8-8, Steps 2 and 3). Conjugation rarely allows the entire chromosome to pass across the conjugation tube. This procedure has established the location of approximately 1000 genes.



plays an important role in recombination involving either a single-stranded DNA molecule or the linear end of a double-stranded molecule that has unwound.

 Single-stranded is a common form of recombination in many bacterial species.  Double-stranded DNA entering a recipient cell undergoes onestrand degradation. – The remaining strand joins the host chromosome; RecA protein facilitates recombination.

 The is important for unwinding a double-stranded DNA molecule that serves as the source for genetic recombination. – RecBCD unwinds the helix facilitating recombination that involves RecA.

 This model of recombination based on the discoveries also applies to eukaryotes.





confer fertility and contain genes for sex pilus formation on which genetic recombination depends. consist of two components: the ( )and one or more ( ). – RTF encodes genetic information essential to transferring the plasmid between bacteria. – R-determinants confer resistance to antibiotics.



encode bacteria.

that can kill neighboring

Transformation

In , small pieces of extracellular DNA are taken up by a living bacterial cell and are integrated stably into the chromosome. Transformation involves: the entry of DNA into the host cell and recombination of the donor DNA with its homologous region into the recipient cell. The recombinant region contains one host strand (present originally) and one donor strand. These strands are from different sources, so this helical region is referred to as a . The two strands of DNA are not perfectly complementary in this region. Genes that are close enough to each other to be are .

    

are composed of a double-stranded closed circle of DNA Exist in multiple copies in the cytoplasm May contain one or more genes Use the same replication enzymes as host Are distributed to daughter cells Replicate independently of the bacterial chromosome...