Bacterial genetics molecularbiology.pptx

Cell genetics

1. Microbial genetics

2. Nucleoid of prokaryotes Most prokaryotes contain a single circular chromosome A few have linear chromosomes Some bacteria contain multiple chromosomes e.g. Vibrio DNA is double stranded and nucleoproteins present. Most of the genes are in the chromosome e.g. genes for growth, cell structure. Size varies from species to species Translation of mRNA to protein starts before transcription is finished since the nucleoid is in the cytoplasm.

3. Linear chromosomes of bacteria In  Streptomyces ,  Agrobacterium , and  Borrelia Free ends of dsDNA are susceptible to degradation by intracellular nucleases or fusion with other chromosomes. This is solved by having telomeres. Linear chromosomes of bacteria have hairpin or invertron telomeres different from vertebrate or eukaryotic telomeres What is the function of telomeres?

4. Linear chromosome (cont.) Hairpin loops are of single-stranded DNA at the ends of the linear chromosomes e.g. in Borrelia Invertron telomeres (terminal inverted repeats) have proteins covalently bound to the 5’ end of linear chromosome e.g. in Streptomyces

5. Genomes of a few bacteria

6. Bacterial DNA lacks introns Bacterial chromosomes lack introns and therefore, there is no mRNA splicing before translation in bacteria.

7. mRNA in bacteria Can be polycistronic because of operons hence many sites of initiation and termination codons Transcription and translation in the cytoplasm and concurrently 5’ end has shine Dalgarno sequence-recognition site for ribosomes hence lacks cap structure Poly A tail: Present or absent????? Very little or no post-transcriptional modification Shorter life span

8. Operons of bacteria In bacteria, several genes producing several proteins to serve the same purpose may be serially arranged. All the genes share one promoter and one repressor, hence regulated together Operons are for saving energy and rapidly adapt ing to changes in the environment Operons also reported in some eukaryotes and viruses

9. Plasmids in prokaryotes Bacteria often contain small circular dsDNA molecules called plasmids, that code for drug resistance and other forms of resistance, virulence, mating Some bacteria have linear plasmids e.g …… Size varies Plasmids have their own origin of replication and replicate independent of the chromosome

10. Plasmids Not essential for life of bacteria Copy Number: up to 40/cell contain 50-100 genes Curing: process of eliminating plasmid from bacteria Spontaneous (plasmids unstable in nature) induced Acridine Radiation Thymine starvation High temp

11. Types of plasmids classified based on function i . Resistance plasmids ii. Fertility plasmids iii. Virulence plasmids iv. Col plasmids v. Degradative plasmids A plasmid can code for more than one property

12. Transposons and insertional sequences Constitute the jumping genes that move from one DNA location to another. Transposons carry other genes in addition to genes for transposition. IS carry genes for transposition only. Jumping genes may code for drug resistance e.g. transposons or cause mutations, involve in integration of plasmid DNA into chromosome or regulate gene expression e.g. IS.

13. Genome of fungi Have several chromosomes in the nucleus of each cell Chromosomes composed of DNA and histone proteins. Fungal cells are haploid or diploid depending on the stage of life cycle. When diploid cells are formed, meiosis takes place to produce haploid spores. Gene transcription occurs in the nucleus but translation occurs in the cytoplasm. Chromosomes are linear and protected by telomeres with unique sequence repeats

14. Plasmids in fungi Plasmids are reported in fungi. Most plasmids are mitochondrial (in filamentous fungi), some in the nucleus (in S. cerevisiae ) or cytoplasm. Circular plasmids are common only in Neurospora spp., but linear plasmids have been found in many fungi. The role of these plasmids is not well understood.

15. Introns present in fungi Introns are short and therefore mRNA splicing takes place.

16. Transposons and IS in fungi Transposons and IS are found in eukaryotes Like those of other eukaryotes, each fungal transposable element is either of class I or of class II. Class I elements transpose by a RNA intermediate and employ reverse transcriptases but class II elements transpose directly at the DNA level. Eukaryotic transposable elements, including those from fungi carry no selectable genes.

17. Read about Replication and binary fission Growth curve Changes to the genome of a cell through point mutations and Genetic recombination or Gene transfer (Transduction, transformation and conjugation) Types of transduction, types of bacteriophages (virulent/lytic and temperate), lytic and lysogenic cycles

18. Telomere sequences

19. In Eukaryotes