Bacterial Genetics

Bacterial Genetics

Bacterial Genome • Chromosome: • Plasmid:

Plasmids are extrachromosomal genetic elements capable of autonomous replication. An episome is a plasmid that can integrate into the bacterial chromosome. • IS or Tn • Bacteriophage

bacteriophage

• Bacteriophages ( phages ) are

obligate intracellular parasites that multiply inside bacteria by making use of some or all of the host biosynthetic machinery . They are viruses that infect bacteria.

Capsid DNA Contractile Sheath

Head

Tail

Tail Fibers Base Plate

Types of Bacteriophage • Lytic or Virulent Phages 毒性噬菌体 • Lysogenic or Temperate Phage 溶原性噬菌体或温和噬菌体

Lytic or Virulent Phages • Definition: Lytic or virulent phages are phages which can only multiply in bacteria and kill the cell by lysis at the end of the life cycle.

• Life Cycle – – – –

Adsorption 吸附 Penetration 穿入 Biological Synthesis 生物合成 Maturation and Release 成熟和释放

Adsorption 吸附

Penetration 穿入

Biological Synthesis 生物合 成

Maturation and Release 成熟和释放

Bacteriophage binding to and injecting their DNA into a bacterial cell

bacteriophage after contraction of sheath

normal bacteriophage

Lysogenic or Temperate Phage 溶原性噬菌体或温和噬菌体

Lysogenic bacterium

Induction

Animation summarizing the lysogenic life cycle of a temperate bacteriophage.

lysogenic conversion 溶原性转换 • When a cell becomes lysogenized,

occasionally extra genes carried by the phage get expressed in the cell. These genes can change the properties of the bacterial cell. This process is called lysogenic conversion.

Application of Bacteriophage • Used in treatment of bacterial

infections • Used for the identification of pathogenic bacteria (phage typing 噬 菌体型分型 ) • Used in molecular biology

Mechanism of bacterial variation

• Gene mutation • Gene transfer and recombination

Types of mutation • Base substitution • Deletion of one or more nucleotides • Insertion sequences

What can cause mutation? • Chemicals: nitrous acid; alkylating agents benzpyrene • Radiation: X-rays and Ultraviolet light • Viruses

Gene transfer and recombination MECHANISMS IN BACTERIA • Transformation • Conjugation • Transduction • Lysogenic

conversion • Protoplast fusion

(phage)

Transformation • Transformation is gene transfer

resulting from the uptake by a recipient cell of naked DNA from a donor cell. Certain bacteria can take up DNA from the environment and the DNA that is taken up can be incorporated into the recipient's chromosome.

Conjugation • Definition: Gene transfer from a donor to a recipient by direct physical contact between cells (during the mating ) • Mating types in bacteria – Donor • F factor (Fertility factor)

Donor

– F (sex) pilus

– Recipient • Lacks an F factor

Recipient

Physiological States of F Factor • Autonomous (F+) – Characteristics of F+ x Fcrosses • F- becomes F+ while F+ remains F+ • Low transfer of donor chromosomal genes

F+

Physiological States of F

Factor

• Integrated (Hfr) – Characteristics of Hfr x Fcrosses • F- rarely

becomes Hfr while Hfr remains Hfr • High transfer of certain donor chromosomal genes

F+

Hfr

Physiological States of F Factor

• Autonomous with donor genes (F’)

– Characteristics of F’ x F- crosses • F- becomes F’ while F’ remains F’ • High transfer of donor genes on F’ and low transfer of other donor chromosomal genes

Hfr

F’

Mechanism of F+ x F- Crosses • Pair formation – Conjugation bridge

• DNA transfer

F+

F-

F+

F-

F+

F+

F+

F+

– Origin of transfer – Rolling circle replication

Mechanism of Hfr x FCrosses • Pair formation – Conjugation bridge

• DNA transfer

Hfr

F-

Hfr

F-

– Origin of transfer – Rolling circle replication

• Homologous

recombination

Hfr

F-

Hfr

F-

Mechanism of F’ x F- Crosses • Pair formation – Conjugation bridge • DNA transfer

F’

F-

F’

F-

F’

F’

F’

F’

– Origin of transfer – Rolling circle replication

R Factors Resistance plasmids ( R factors) – These plasmids carry antibiotic resistance genes.

Structure of R Factors • RTF (Resistance

RTF

Transfer Factor) 0

Tn 21

• R determinant

Tn 8 Tn 9

– carries the transfer genes.

1 n T

– Resistance genes – Transposons

R determinant

Transduction: • Transduction

is defined as the transfer of genetic information between cells through the mediation of a virus (phage) particle. It therefore does not require cell to cell contact and is DNase resistant.

Transduction • Generalized transduction • Specialized transduction

Generalized Transduction • Generalized transduction is

transduction in which potentially any bacterial gene from the donor can be transferred to the recipient.

The mechanism of generalized transduction

Generalized transduction 1. It is relatively easy. 2. It is rather efficient (10-3 per recipient

with P22HT, 10-6 with P22 or P1), using the correct phage. 3. It moves only a small part of the chromosome which allows you to change part of a strain's genotype without affecting the rest of the chromosome. 4. The high frequency of transfer and the small region transferred allows finestructure mapping

Specialized transduction • Specialized transduction is transduction in which

only certain donor genes can be transferred to the recipient. • Different phages may transfer different genes but an individual phage can only transfer certain genes • Specialized transduction is mediated by lysogenic or temperate phage and the genes that get transferred will depend on where the prophage has inserted in the chromosome.

The mechanism of specialized transduction

Specialized transduction 1. Very efficient transfer of a small region--can be

useful for fine-structure mapping 2. Excellent source of DNA for the chromosomal region carried by the phage, since every phage carries the same DNA. 3. Can often be used to select for deletions of some of the chromosomal genes carried on the phage. 4. Merodiploids generated using specialized phage can be quite useful in complementation analyses.

Lysogenic conversion • The prophage DNA as a

gene recombined with chromosome of host cell.

Protoplast Fusion • Fusion of two protoplasts treated with lysozyme and penicillin.

Application of bacterial variation • Use in medical clinic: Diagnosis, Treatment, Prophylaxis. • Use in Genetic Engineering