Lecture 10: Genetics of Microorganisms II PDF

Lecture #10: Genetics of microorganisms II Objectives By the end of this lecture, you should be familiar with: 1. Replication 2. Transcription 3. Translation 4. Mutation  Translation (protein synthesis) Occurs in the ribosome The ribosome -...

Lecture #10: Genetics of microorganisms II Objectives By the end of this lecture, you should be familiar with: 1. Replication 2. Transcription 3. Translation 4. Mutation  Translation (protein synthesis) Occurs in the ribosome The ribosome -Responsible for protein biosynthesis -Present in the cytoplasm Composed of RNA (rRNA) and r-proteins. 70S -Bacterial ribosome: 70S composed of: -Small subunit:30S -Large subunit: 50S 50S 30S (Eukaryotic (human) ribosome is 80S (60S+40S))  tRNA mRNA -The ribosome facilities the coupling between mRNA (codons) and the tRNA (anticodons) carrying the corresponding amino acid. Each Ribosome has three binding sites for tRNA: - (A-site) the Aminoacyl-tRNA site holds the tRNA carrying amino acid to be added to the peptide chain. - (P-site) the Peptidyl-tRNA site holds the tRNA carrying the growing polypeptide chain. - (E-site) the Exit site where the discharged tRNA leaves the ribosome.  Steps of translation 1- Initiation: - The small subunit (30S) recognizes the start codon (AUG) on the mRNA. - The large subunit (50S) joins to form 70S with the help of initiation factors (IFs). - AUG (start codon) corresponds to N-formyl methionine amino acid. - Initiator tRNA now occupies the P-site. 2-Elongation: - The incoming tRNA with a new amino acid occupies the A-site where the codon on the mRNA recognizes and hydrogen bonds with the anticodon on the tRNA. - The large subunit catalyzes peptide bond formation to join the polypeptide to the newly arrived aminoacid in the A-site. 3-Translocation: - Movement of the ribosome through the mRNA (to reach the next codon). - The empty tRNA occupies the E-site while the peptideyl tRNA occupies the P-site. - A-site is vacant again for another round. 4- Termination: -Requires termination nonsense (stop) codon (UAA, UAG, or UGA) appearing in the A-site. - Release factors attach to join a water molecule (no tRNA corresponds to the stop codons) and the ribosome is dissociated into its subunits and protein is released.  Mutation A permanent change in the DNA of the chromosome resulting in the alteration of information of that gene. -Hereditable change in the genotype. -Results from a change in the nucleotide sequence within a gene with the formation of the allele (another for of the gene). -Results in a change in the structure and hence functions of a protein.  Causes of mutations Spontaneous mutations: Changes in base sequences that occur as a result of natural phenomena e.g: Everyday radiation penetrating the atmosphere or errors during replication. Induced mutations when the microorganism exposed to Physical agents: (1)Heat, (2) Ultraviolet or (3) Ionizing radiation such as X-ray Chemical agents: (1) Nucleotide analogues (molecules which look like nucleotides) (2) DNA reactive chemicals such as nitrous acid (3) Alkylating agents (molecules which add methyl or ethyl groups)  Types of mutations A- Point mutations Silent mutation: is a change at the DNA level that does not result in any change of amino acid in the encoded protein (many codons could code for the same amino acid). (AAA or AAG still lysine). Mis-sense mutation: results in different amino acid being inserted in the protein. This type of mutation may result in a change in the phenotype. (AUU=Ile, AUG = Met) Non-sense mutation: change of sense codon to non-sense codon (stop codon) it results in synthesis of only a segment of the protein. (UGC= Cys, UGA=stop) B- Base pair deletion or insertion Due to break down of the sugar phosphate backbone of DNA and deletion or insertion of a DNA segment OR Deletion of only one base. Results in frame shift mutation. This usually results in new set of triplets or codons. Loss of A mRNA AUG CAG ACA UCU UAG Protein Met Gln Thr Ser Stop mRNA AUG CAG CAU CUU AG Protein Met Gln His Leu  Proof reading and repair mechanisms Proofreading: Replication error is usually corrected by DNA polymerase which has an exo-nuclease activity that recognizes and excises incorrect base pair and inserts an error free segment (proofreads and repairs). (it minimizes but does not prevent mutation). Summary: 1. Replication 2. Transcription 3. Translation 4. Mutation  Your Final Exam Lectures 1 To 10 40 points Types of Questions: T/F MCQ Match