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Microbial Genetics UNIT 02: MICROBES AND HEALTH Life science Ramesh Kumar From the Virtual Microbiology Classroom on ScienceProfOnline.com Image: Prokaryotic Replication, U.S. National Library of Medicine Genetics • Genetics deals with the molecular structure and function of genes, gene behavior, patterns of inheritance from parent to offspring, and gene distribution, variation and change in populations. Nucleic Acid • Nucleic acids are biological molecules essential for life • Nucleic acids allow organisms to transfer genetic information from one generation to the next. • There are two types of nucleic acids: – Deoxyribonucleic acid, better known as DNA – Ribonucleic acid, or RNA. What is DNA ??? • DNA is a double stranded structure like a twisted ladder. It is embedded in the nucleus of eukaryotic cell but in prokaryotic it is lying in cytoplasm because of the absence of nucleus. • Discovered by Oswald Avery in 1944 with a team of scientists. • The nucleus of one human cell contains approximately 2 meter of DNA. DNA (cont….) • The basic structural units of DNA are nucleotides. – Nucleotides form a gene – Genes form DNA – DNA forms chromosome • Nucleotides are complex structures composed of three kinds of molecules as 1. Nitrogenous Base 2. Deoxyribose sugar 3. Phosphate group Nitrogen Base: • In DNA the nitrogen bases are 1. Adinine (A), 2. Thymine (T), 3. Guanine (G) 4. Cytocine (C) • The information in DNA is stored as a code made up of these four bases. Deoxyribose sugar Phosphate group • Deoxyribose sugar and Phosphate groups arrange alternatively and act as a backbone of DNA. • The deoxyribose sugar of one nucleotide is attached to the phosphate group of the next nucleotide. What is RNA??? • RNA is the second principal kind of nucleic acid. • It is a single stranded structure. • It has ribose sugar (C5 H10 O5) having one more oxygen atom than deoxyribose sugar (C5 H10 O4) in DNA • The nitrogen base in RNA contains Uracil instead of Thymine. RNA (cont….) • In RNA, thymine (T) is replaced by Uracil (U) and the rest of them are the same. • In RNA the nitrogen bases are 1. 2. 3. 4. Adinine Uracil Guanine Cytocine (A) (U) (G) (C) RNA Structure Types of RNA RNA is of three types as: – Messenger RNA (mRNA), – Transfer RNA (tRNA) – Ribosomal RNA (rRNA) Types of RNA (cont…) • mRNA: messenger RNA acts as a template for protein synthesis • tRNA: It brings amino acids to the ribosomal site where they are incorporated into proteins • rRNA: It forms ribosome and is its integral part. Difference in DNA & RNA DNA RNA Has Deoxyribose sugar Has Ribose sugar Double stranded Single stranded Nitrogen bases are A,G, T and C Are A, G, C and U Has one type Three types as mRNA, rRNA and tRNA Transfers hereditary characteristics Synthesizes protein Function of DNA • DNA) is the genetic code which ensures that daughter cells inherit the same characteristics as the parent cells • DNA is the code from which all protein is synthesized • DNA also contains all the genetic coding which is used to control functions • DNA is also used as a long term storage device to store the genetic instructions • genetic coding which is used to control behaviour and development Function of RNA o mRNA carries instructions on how to connect several "amino acids" o tRNA is the actual translator • Sequence of amino acids is put together Mutation Mutation • A change in the nucleotide sequence of a gene (< 1%), polymorphism (>1%) • May occur at the molecular or chromosomal level • The effect of mutations vary • Mutant refers to an unusual phenotype • Mutations are important to evolution Mutations • Change in the DNA • May occur spontaneously or by exposure to a radiation or chemicals • An agent that causes a mutation is a mutagen Spontaneous Mutation • De novo or new mutations • Not caused by exposure to known mutagen • Errors in DNA replication • DNA bases have slight chemical instability • Exist in alternating forms called tautomers • As replication fork encounters unstable tautomers, mispairing can occur Spontaneous Mutation Spontaneous Mutation Rate • Rate differs for different genes – Vary by size – Sequence dependence – Hot spots – Table 12.3 lists rates for several genes • On average, 1/100,000 each round of replication • Each individual has multiple new mutations • Most by are not in coding regions of genes Induced Mutations • Caused by mutagens, many are also carcinogens and cause cancer Examples: • Alkylating agents: remove a base • Acridine dyes: add or remove base • Xrays: break chromosomes delete a few nucleotides • UV radiation: creates thymidine dimers • Site-directed mutagenesis: Exposure to Mutagens • Workplace • Industrial accidents – Chernobyl • Medical treatments • Weapons • Natural sources Point Mutations A change of a single nucleotide • Transition purine replaces purine A to G or G to A or pyrimidine replaces pyrimidine C to T or T to C • Transversion purine replaces pyrimidine or pyrimidine replaces purine A or G to T or C T or C to A or G Missense Mutation • A point mutation that changes the codon • Causes a substitution of an amino acid • Missense mutations may affect protein function severely, mildly, or not at all. Example: • Hemoglobin mutation • Glutamic acid to valine causes sickle cell anemia Nonsense Mutation • A point mutation changing a codon for an amino acid into a stop codon • Creates truncated proteins that are often nonfunctional • Some have dominant effects due to interference with normal functions Example: • A factor XI deficiency is a nonsense mutation changing glutamic acid to a “stop” • Short protein cannot function in clotting Insertions or Deletions • The genetic code is read in triplet nucleotides • Addition or subtraction of nucleotides not in multiples of three leads to a change in the reading frame • Causes a frameshift and alters amino acids after mutation • Addition or subtraction of nucleotides in multiples of three leads to addition or subtraction of entire amino acids Gene Transfer Mechanisms – Conjugation (cont.) Transformation and Transduction • Conjugative Transposons • These genetic elements share attributes with conjugal plasmids and transposons • A transposition event between two cells • Requires all the functions found in conjugation plus those of transposition Transformation • Tranformation is process of taking up naked DNA in a stably inherited form. • Two major types of transformation 1. Natural transformation (only a subset of microbes do this) • - usually linear DNA • 2. Artificially-induced (most, but not all microbes can be induced to take up DNA • - usually plasmid DNA • A cell that is proficient to take up DNA is described as competent Transduction • Genetic exchange mediated by bacterial viruses (bacteriophage) • Two basic types of bacterial viruses • Lytic viruses – infect cells, multiply rapidly, lyse cells • Lysogenic viruses – infect cells, can integrate into genome and go dormant (a prophage) • - at some point, can excise, multiply and lyse cells • Bacteriophage have a range of morphologies from simple filaments to large complex structures • May contain either RNA or DNA associated with a protein coat • Almost all bacteria have phage associated with them Attach to specific receptors on the surface of their host bacteria Transfer their nucleic acid into the host cell Lysogeny of bacteriophage Integrate into host genome - Enter a semidormant state (eg. Lambda phage) METABOLISM OF PROTEINS DIGESTION AND ABSORPTION OF PROTEIN NITROGEN BALANCE Amount of nitrogen taken in food = Amount of nitrogen excreted/day • POSITIVE NITROGEN BALANCE Amount of nitrogen taken in food > Amount of nitrogen excreted/day • NEGATIVE NITROGEN BALANCE Amount of nitrogen taken in food < Amount of nitrogen excreted/day TRANSAMINATION & DEAMINATION TRANSAMINATION: Transamination is a reaction in which one or more amino acids are converted into other amino acid. The reaction is catalyzed by transaminases or aminotransferases. DEAMINATION: A catabolic reaction in which amino group is removed forming an keto acid & ammonia. The reaction is catalyzed by amino acid oxidase. CH3-CH-COOH amino acid oxidase CH3-C-COOH + NH3 NH2 Alanine O Pyruvic acid (keto acid) • The transamination reaction results in the exchange of an amine group on one acid with a ketone group on another acid. DECARBOXYLATION Decarboxylation is a reaction in which carboxyl group of an amino acid is removed & converts into an amine group. H R-C-COOH NH2 + CO2 NH2 amino acid dehydrolase R-CH2- pyridoxal phosphate Example: Decarboxylation of histidine to form histamine UREA CYCLE CENTRAL DOGMA Crick's central dogma: Information flow is from DNA to RNA via the process of transcription, and thence to protein via translation.