I Remind ourselves with the Structure And the Functions Of The Genetic Material Nucleic acids structure There are two major classes Deoxyribonucleic acid DNA The most common macromolecule that encodes genetic information ID: 1009906
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1. Bacterial genetics I Dr. Ali Abdulwahid
2. I. Remind ourselves with the Structure And the Functions Of The Genetic Material
3. Nucleic acids structure There are two major classes:Deoxyribonucleic acid (DNA) : The most common macromolecule that encodes genetic informationRibonucleic acid (RNA) : plays an essential role in several genetic processes Nucleotides: Nucleotides are the structural units of nucleic acids. Nucleotides are named according to their nitrogenous base.
4. Parts of NucleotideEach nucleotide has three parts:A nitrogen base:.Pyrimidines: involving thymine (T), cytosine (C), and uracil (U), Purines : including adenine (A) and Guanine (G), Deoxyribose (DNA) or ribose (RNA) sugar A phosphate group
5. I. Deoxyribonucleic Acid (DNA) StructureDouble helix: consists of two long strands wrapped around each other Base pairing: A is always paired with T by two hydrogen bonds, G is always paired with C by three H bonds.Complementary : each strand act as a template for synthesis the another strand Antiparallel: One strand is oriented in the 5’ to 3’ direction and its complement is oriented in the 3’ to 5’ direction.
6. II. Ribonucleic Acid (RNA)Single-stranded molecule, shorter than DNA It has ribose sugar rather than deoxyribose.The base uracil (U) replaces thymine (T) in DNA.Three major kinds messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA).Small interfering RNA (siRNA) : A new class of RNAShort (20–25 nts), double-strandedAct as regulators for gene expression
7. II. Flow of genetic information
8. How the genetic information is stored in the DNA ?Genetic information is stored in DNA as a code. The unit of code is known as codon.Consists of a sequence of three bases such as ATG. Each codon codes for a single amino acid, but more than one codon may exist for a single amino acid.
9. How the genetic information is stored in the DNA ?It is first transferred (transcribed ) from one DNA strand to mRNA strand Then the mRNA base sequences serve as a template for amino acid sequences in the polypeptide (during translation process).
10. The gene Gene is a segment of DNA carrying a number of codons specifying for a particular polypeptide or RNA.Composed of two main regionspromoter region (recognised by RNA polymerase to start transcription processReading frame (or coding region), contain the codons that will be translated for a particular polypeptide or RNA A large number of genes constitute cell genome.Size of the gene and entire genome is expressed in the number of base pairs (bp) or kilobase pairs (kbp) (1 kbp = 1,000 base pairs).
11. Bacterial chromosome Haploid Single doublestranded molecule of DNA arranged in a circular form. Ranging in size ( in different species) from 580 kbp to more than 5220 kbp of DNA Not separated from the cytoplasm (no nucleus).The size of an E. coli cell chromosome 4,639,221 bp have a contour length of about 1.7 mm, some 850 times the length of the E. coli cellDNA from a lysed E. coli cell. In this electron micrograph several small, circular plasmid DNAs are indicated by white arrows. The black spots and white specks are artifacts of the preparation.
12. Bacterial chromosome The essential genes that are necessary for cell viability (house keeping genes) are carried on the chromosomeSome bacterial species (pathogenic bacteria) possess specific genes for pathogenic determinants. These genes are often clustered together in the DNA and are referred to as pathogenicity islands (PIs).These gene segments encode a collection of virulence factors that are important for pathogenesis, including, antibiotic resistance, adhesins, invasins, and exotoxins.
13. Non-chromosomal elements of genome Not all genes in a given cell are confined in the chromosome Many genes are located on plasmids and transposable elements These elements can replicate and encode information for production of cellular products They are not as stable as a chromosome (what does that mean ?)
14. I. Plasmids double-stranded circular DNA moleculesseparated from the bacterial chromosome.Replicating independently of the chromosome Their number per bacterial cell varies from a few to thousandsTheir sizes range from a few thousand base pairs to more than 100 kbpMay be incorporated into the chromosome
15. I. Plasmids Many plasmids carry genes that code for certain phenotypic characteristics of the host cell. The following plasmid types are medically relevant:Virulence plasmids. Carry determinants of bacterial virulence, e.g., enterotoxin genes or hemolysin genes.Resistance (R) plasmids. Carry genetic information bearing on resistance to antibacterial agents (antibiotics). R plasmids may carry several R genes at once Some plasmids have also been described that carry both virulence and resistance gene
16. II. Transposable elements Pieces of DNA that move from one genetic elements to another in the same or a different DNA moleculeDo not exist as separate entities within the bacterial cell, must be incorporated to a plasmid or chromosome Its insertion often disrupt of a gene and inactivates it.The spread of antibiotic-resistance elements among disease-causing bacterial populations is mediated in part by transposition
17. II. Transposable elements Each type of transposons carry the information that encodes for functions using for its transmissibility Including transposase enzyme that mediate the excision of the transposon and insertion it in the target DNA sequences It also flanked by short inverted repeats (IR) at there terminals
18. Classes of transposonsBacteria have two classes of transposons Insertion sequences (simple transposons): contain only the sequences required for transposition and the genes for proteins (transposases) that promote the process.Complex transposons : contains genes for movement as well as genes that encode for other feature such as antibiotic resistance.
19. Mechanism of transposition: Two transposases recognize and bind to IR sequences, join together and promote DNA double-strand cleavage. The DNA-transposase complex then inserts its DNA cargo at specific DNA motifs elsewhere in the genome, creating short TSDs upon integration.Their transfer can cause deletions, duplications, and inversions of DNA segments Its insertion often disrupt of a gene and inactivates it.
20. The central dogma of molecular biology The flow of information from DNA to RNA to protein is often referred to as the central dogma of molecular biology It refers to three major processes include: ReplicationTranscription Translation
21. DNA replication Semiconservative:each strand serves as a template for synthesizing new DNA strandsBidirectional:begins at one point (ori locus) and moves in both directionsTerminated at termination site (ter) in the opposite to ori site.
22. TranscriptionThe DNA acts as a template for the transcription of RNA by RNA polymeraseThe bases in mRNA will be complementary to one ( template) strand of DNAAdapted from : https://courses.lumenlearning.com/microbiology/chapter/rna-transcription/Adapted from: http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-21/21_09.jpg
23. Translation The process requires three major components mRNA, ribosomes, and tRNAs in addition to various accessory proteins.The mRNA carries the coded information for making specific proteins from DNARibosomes: the sites of translation, their structure facilitates the joining of one amino acid to another.tRNA: contains (anticodon) at one end and amino acid at the other end. Different amino acid sequence form different protein.
24. Regulation of Gene ExpressionBacteria demonstrate an impressive capacity for adapting to their environment.A number of regulatory bacterial mechanisms are known, for example posttranslational regulation, translational regulation, transcription termination.The most important is regulation of the initiation of transcription by means of activation or repression Ex. The lac peron
25. Lactose fermentation requires three enzymes coded by structural genes Lac Z, Lac Y and Lac A of Lac operon respectivelyThe operon is composed of:The operator, which is a sequence of bases that controls the expression (transcription) of the structural genes The promoter, where the RNA polymerase binds and transcribes the structural genes.The lac operon The structure of lac operonA distant Regulatory gene (not part of the operon) codes for a repressor protein , which is a protein molecule that can combine with either operator region on the chromosome or with the inducer (lactose).
26. For transcription to occur , RNA polymerase has to attach to DNA at a promoter region and transcribe the DNA. when lactose (inducer) is not present in the medium, repressor molecule is bound to the operator, preventing the passage of RNA polymerase from promoter to the structural genes. The repressor molecule has an affinity for lactose, in the presence of which it leaves the operator region free enabling the transcription to take place. When lactose present is completely metabolized, the repressor again attaches to the operator, switching off transcription.
27. Thank you for your attention
28. References Jawetz, M. & Adelberg’s. 2019. Medical Microbiology , Twenty-Eight Edition. The McGraw-Hill Companies, Inc. USANelson, D. L., & Cox, M. M. (2017). Lehninger principles of biochemistry (7th ed.). W.H. Freeman.Kumar, S. 2012. Textbook of microbiology. Jaypee Brother Medical Publishers (P) Ltd. New Delhi, India. Kayser, F.H., Bienz, K.A., Eckrt, J. and R. M. Zinkernagel. 2005. Medical Microbiology. Georg Thieme Verlag. Stuttgart, Germany.Passarge, E. 2001. Color atlas of genetics. Georg Thieme Verlag. Stuttgart, Germany.http//www.lamission.edu/lifesciences/chapter 8: Microbial genetics