Bacterial heredity and variation

Bacteria genetic material Bacteriophages Main contents Bacteria genetic material Bacteriophages Mechanisms of genotypic variation Medical application of bacteria variation

Bacterial genetics Genotypic variation DNA sequence change Heredity Variation Genotypic variation DNA sequence change Heritable, irreversible Phenotypic variation Occurs when the expression of gene is changed in response to its environment, reversible, inheritable

Bacteria genetic material Bacterial chromosome Plasmid Transposable element

Chromosome (nucleoid) A circular, double-stranded DNA

About 1000μm long Small genome Operon structure No intron Translation follow transcription almost at the same time Absence of non-encoding region No gene overlapped Haploid except rRNA

Plasmid extrachromosomal genetic elements Closed-circular, double-stranded DNA Characteristics Self-replication Encoding some bacterial properties F plasmid, R plasmid, Col plasmid, Vi plasmid

Not necessary for bacterial viability Transferability Transformation, conjugation, transduction Conjugative plasmid and nonconjugative plasmid Plasmid compatibility and incompatibility

Transposon,Tn A segment of DNA that is capable of moving to a new position within the same or another chromosome, plasmid, or cell and thereby transferring genetic properties. Twp types Insertion sequence (IS) Complex transposon (Tn)

Tnp, resistance, enterotoxin, etc IS and Tn transposase ABCDEF F’E’D’C’B’A’ ( IR ) (inverted repeats) IS Tnp, resistance, enterotoxin, etc Tn IR 1.细菌DNA分子中(核质,质粒)可自行移位的一个片段。 2.种类: ①插入序列(IS） 只有转位基因：→插入基因内部 →阻断基因的连续性 →导致突变。 ②转座子(Tn)---跳跃基因 转位基因+遗传信息：→转位→突变 Consequence: Drug resistance variation transmission

Bacteriophage (phage) Definition Viruses that infect bacteria,fungi,actinomycetes or spirochetes Obligate intracellular parasite Strict specificity Widely distributed

Bacteriophage (phage) Shape and structure tadpole, spherical, filamentous Chemical composition Nucleic acid(DNA or RNA) Protein Type of reproduction Self-Replication tail

Bacteriophage (phage) Life cycle Virulent phage—Lytic phage Lytic life cycle Temperate phage –Lysogenic phage Lysogenic life cycle

Bacteriophage (phage) Virulent phage Lytic life cycle Adsorption Penetration Biosynthesis Maturation Release

Bacteriophage (phage) Temperate phage Prophage Phage genome integrating with bacterial genomes. Lysogenic bacteria bacteria containing prophages Two life cycles Lysogenic and lytic Lytic cycle Lysogenic cycle

Bacteriophage (phage) Medical significance Identification and typing of bacteria Contribution to genetic transfer and recombination of bacteria

Mechanism of genetic variation Mutation Genetic transfer and recombination Transformation Transduction Conjugation Lysogenic conversion

Gene mutation Gene mutation any change to the sequence of bases in the DNA of an organism Wild type and mutant point mutations--- single base change nucluotide base insertion,deletion and subsitution—transition and transversion large piece of DNA mutation DNA inversion,duplication and deletion frameshift mutation

types of large DNA sequence mutation Wild type inversion duplication deletion substitution insertion types of large DNA sequence mutation A～G、X mean different DNA fragement

Gene mutation rules low frequency sponteneously---10-10-10-6 greatly increased rate induced by mutagen--- 10-6-10-4 randomly and non-directionally reversion of gene mutation forward mutation Wild type mutant Reverse mutation

影印试验(replica plating) (Lederberg 1952)

Gene transfer and recombination Unidirectional: donor to recipient Gene recombination New biological properties in recipient cells

Definition Transformation A process by which a bacterium directly takes up a free DNA fragment and then incorporate it into the recipient bacterial genome, resulting in the alteration of bacterial genetic constitution.

The discovery of transformation (1928 Griffith)

S colony R colony

Transformation Chromosomal DNA Plasmid DNA Bacteriophage DNA Free DNA transferred by transformation Chromosomal DNA Plasmid DNA Bacteriophage DNA conditions <10-20 genes High homology between donor and recipient Under the competent condition

Conjugation Definition A process by which there is a transfer of DNA from a living donor bacterium to a recipient bacterium by cell-to-cell contact. Often involves a sex pilus.

Conjugation DNA transferred by conjugation Plasmid(F plasmid or R plasmid) Chromosomal segment The donor bacterium F+ or male The recipient bacterium F- or female

Mechanism of F+ x F- Crosses Pair formation Conjugation bridge F+ F- DNA transfer Origin of transfer Rolling circle replication

R plasmid conjugation Components Resistance transfer factor(RTF) Resistance determinant (r-det) Medical significance Transmission of antibiotic resistance between bacteria

R plasmid Tn 9 Tn 21 Tn 10 Tn 8 RTF R determinant

R plasmid conjuagtion

Transduction Definition A process by which a DNA fragment is transferred from one bacterium to another by a bacteriophage. Two types Generalized transduction Specialized transduction

Generalized transduction

Generalized transduction Larger piece of DNA can be transferred by transduction compared to transformation More efficient than transformation Two consequences Complete transduction Abortive transduction

Specialized transduction gal bio

Specialized transduction

Lysogenic conversion Definition A process by which a temperate phage injects its own genome into a host bacterium to make the host a lysogeny. e.g., Diphtheria toxin (Corynebaterium diphtheriae)

Medical application of bacteria variation Application in diagnosis of bacterial infectious disease Preventing antibiotics resistance bacteria strain transmission Bacteria vaccine The application in discovering cancer-inducing substances Application in genetic engineering

V.cholera L-form