Bacterial Genetics PDF

Bacterial genetics Samanthika Jagoda, BVSc (Perad), MVM (Massey), PhD (Tokyo)  Learning Objectives On completion of the course, the student should be able to: Describe the molecular structure of DNA, including base pairing and 5’→ 3’orientation Describe the process of DNA r...

Bacterial genetics Samanthika Jagoda, BVSc (Perad), MVM (Massey), PhD (Tokyo)  Learning Objectives On completion of the course, the student should be able to: Describe the molecular structure of DNA, including base pairing and 5’→ 3’orientation Describe the process of DNA replication in a bacterial cell Explain the relationship between genotype and phenotype Describe the different pathways through which the inter- bacterial gene transfer is materialized and how genetic variation can influence microbial structures and their functions  Genetics: Manipulation of DNA to study cellular and organismal functions  Genetic information in bacteria is encoded in DNA All of the DNA found in a bacterial cell is collectively referred to as bacterial genome  Bacterial genome 1. Chromosome 2. Plasmids 3. Bacteriophage DNA Bacteriophage DNA 1. Bacterial chromosome Small, circular, double stranded DNA Located in the cytoplasm - floating freely Self replicating Size may vary -less than 5Mb Most bacteria are HAPLOID – single chromosome per cell (one copy of each gene) Exceptions: e.g Vibrio cholerae – 2 chromosomes *HW  Chromosome is supercoiled and tightly packaged in the bacterial nucleoid Total length – 1mm (1000 times larger than the entire cell) Core genome Accessory genome Genes located on the Genes located on the chromosome plasmids and bacteriophages The subset of genes Genes that are variably that are present in all present among individual genomes genomes Offer a selective advantage to the bacterial cell/not Essential for normal essential for viability survival of the e.g encodes for resistance to bacterium antibiotics, proteins inhibitory to other bacteria, virulence factors – toxins, capsule, attachment proteins 2. Plasmids Extrachromosomal genetic elements Closed, circular, double stranded DNA Located in the cytoplasm, much smaller than the chromosome (vary from 5 to 500 kb in size) Can replicate independently of the chromosome Mobile - can move between bacteria of the same or different species ( a mobile genetic element)  Plasmids encode traits that are not essential for bacterial viability Control pathogenic properties resistance to antibiotics production of toxins synthesis of cell surface structures required for adherence or colonization Virulence factors encoded by plasmids tetanus toxin of Clostridium tetani -Tetanus Capsule of Bacillus anthracis - Anthrax 3. Bacteriophages (bacterial viruses/phages) extrachromosomal genetic material Phage genomes vary in size (2 to 200 kilobases per strand) Consist of double-stranded DNA or single- stranded DNA Encodes for exotoxins e.g Phage CTX encodes cholera toxin in Vibrio cholarae Bacteriophages Bacteria can acquire an accessory genome through the horizontal transfer of genetic elements Watson-Crick double helical structure of DNA Complete DNA molecule consists of two long chains wrapped around each other in a double helix The helix has two grooves between the two strands  Base Pairing A’s pair only with T’s and G’s pair only with C’s Each A-and-T pair and each G-and-C pair in DNA is called a complementary base pair  The 5′ and 3′ Ends has a phosphate the last nucleotide lacks attached to its 5′ a phosphate at its 3′ carbon that does not carbon. Because it has connect it to another only a hydroxyl group nucleotide  Antiparallel Construction In a DNA molecule, two chains run in opposite orientations  Replication of bacterial DNA Bacteria reproduce by binary fission – produce two genetically similar daughter cells Genetic material must replicate accurately so that progeny inherit all of the specific genetic determinants (the genotype) of the parental organism Two old strands are separated and used as templates to synthesize new complementary strands  Replication of the chromosome in bacteria; begins at a specific location – origin of chromosomal replication (oriC) goes on at a rate of 1000 nucleotides per second Replication begins in at one point and moves in both directions from there The structure where the two strands are separated and the new synthesis is occurring is referred to as the replication fork  Replication of bacterial DNA DNA unwind – two parent strands separate (DNA helicase) Each parent DNA strand acts as a template for the synthesis of new complementary DNA strand (DNA polymerase) Ends of new strands are joined to form circular chromosome ( DNA ligase) What is a gene? Genes are a small part of larger DNA molecules It is the basic unit of heredity that determines a particular characteristic or trait Most genes are codes describing how to make different types of protein (encode proteins)  Bacterial genomes differ in size (0.6 to 8.0 Mb) Usually contain DNA to encode 1000-4000 different genes Gene density is relatively high in bacteria (1 gene per 1.1 kilobase of DNA) E.coli 4.6 X 106 base pairs 4126 bacterial proteins  Gene expression Transcription of DNA into messenger RNA and translation of mRNA into amino acids that form protein  We can describe any particular DNA molecule using the first letters of these bases eg. ATGCGCTGGTAG This string of letters is called a DNA sequence Each 3-nucleotide sequence, called a codon, carries information for a specific amino acid  The genetic code Like DNA, proteins can be thought of as strings of letters Instead of four bases (A, T, G, C), proteins have 20 amino acids. Every three bases in a gene (codon) encodes one amino acid e.g ATG encodes the amino acid methionine  The genetic code There are 64 different three letter words that can be made with a four letter alphabet (There are 64 different codons in DNA) The assignment of each of the possible codons to amino acids (correspondence between codons and amino acids) is called the genetic code The genetic code determines how the nucleotides in mRNA specify the amino acids in a polypeptide  Genotype Phenotype (inherited genetic information) (all the observable properties of an organism) Actual sequence of DNA of the organism Some genetic information is expressed under defined environmental conditions (nutrient availability) e.g Bacillus anthracis capsule only in vivo Both genotype and environment can influence phenotypic expression  Why bacteria are suitable for genetic experiments? Bacteria are haploid – easy to produce mutant phenotypes Very short generation time Multiply asexually (by cell division) -all the progeny are genetically identical to their parent and to each other Can be propagated in a small space – agar plate Possible to obtain a measurable number of discrete colonies by serial dilutions Possible to select rare mutants by providing selective conditions Can be stored in a dormant state for long time Ability to exchange DNA between strains of a bacterium  Bacterial colonies are clones of the original bacterium Each colony is composed of millions of bacteria, all derived from the original bacterium