L1 - Central Dogma of Molecular Biology 2nd Year MED 2020-21(1) (1).pptx

Medical Genetics For 2nd year Medicine GB (739) 1/14/202 1 1 Lecture 2&3 CENTRAL DOGMA OF MOLECULAR BIOLOGY Review of the Basics 1/14/202 1 2 1/14/202 1 3 Conte nt •DNA replication. •DNA transcription. •Translation. 1/14/202 1 4 REPLICATI ON 1/14/2021 5 Replication (DNA synthesis...

Medical Genetics For 2nd year Medicine GB (739) 1/14/202 1 1 Lecture 2&3 CENTRAL DOGMA OF MOLECULAR BIOLOGY Review of the Basics 1/14/202 1 2 1/14/202 1 3 Conte nt •DNA replication. •DNA transcription. •Translation. 1/14/202 1 4 REPLICATI ON 1/14/2021 5 Replication (DNA synthesis) DNA to DNA 1/14/202 1 6 1/14/202 1 7 Characteristics of 1.Starts at replication fork (single in replication prokaryotes; multiple in eukaryotes). 2.Semiconservative. 3.Bidirectional. 4.Semi-discontinuous (leading strand ; lagging strand). 5.Both DNA strands act as template simultaneously. 6.Enzymes: helicase, topoisomerase, DNA – 8.polymerases, Synthesis ligase. 3’ direction (reading in 3’ proceed in 5’ 7.DNA polymerase requires a short RNA primer. 9 DNA Replication 1/14/202 1 1 0 1/14/202 1 1 1 Transcripti on 1/14/202 1 13 Gene structure 1/14/202 1 Gene structure 1/14/202 1 19 Transcripti on (RNA ▣ ▣ ▣ ▣ 1/14/202 1 synthesis) DNA to RNA Process of copying DNA to RNA. Differs from DNA synthesis in that only one strand of DNA, the template strand, is used to make mRNA Can involve multiple RNA polymerases. Divided into 3 stages: ◾ Initiation ◾ Elongation ◾ Termination 14 1/14/202 1 15 1/14/202 1 16 Synthesis of mRNA 1/14/202 1 2 2 Transcription: The final product 1/14/202 1 17 • Types of eukaryotic mRNA modifications 1) Capping: 2) Polyadenylated tail - Begins when mRNA is about 20-30 nucleotides long.(7 –methyl guanosine cap is added to 5’ end of mRNA). Importance: 1) Non-capped mRNA can not be recognized by ribosomes. 2)Protection against degradation. 3)Increase the stability of mRNA. 1/14/202 1 ( Poly-A tail) addition of 100 – 250 adenine nucleotides to the 3’ end of completed mRNA. Importance: 1) Help in passage of mRNA from nucleus into the cytoplasm. 2) Stabilizes the mRNA against degradation (increase life span of mRNA in cytoplasm) mRNA half lifetime: - about 10 hs. In Eukaryotic cell. - about 2 mins. In 25 3) Splicing: Intron I 5 ’ = Means removing the introns (non-coding regions) and splicing the exons (coding regions) together. •Removal of introns is performed by small nuclear ribonucleoprotein complex enzyme (snRNPC). P-PP- 1) capping G-P-PP- Intron II 3’ OH by addition 7methyl Guanosine to 5” end. Immat ure mRNA OH 2) Addition polyadenylate tail to 3’end. - G-P-PP- AAAAAA … 3) Splicing the Remove introns I & II Importance: formation a continuous protein coding message. G-P-PP-- exons to form continuous coding message Mature mRNA -- AAAAAA … Nu ear envelope cl 1/14/202 1 Mature mRNA becomes for transport & competent translation in G-P-PP-- -AAAAAA … 2 6 1/14/202 1 2 7 TRANSLATI ON 1/14/2021 2 8 Translation (Protein synthesis) RNA TO PROTEIN •Interpreting the information coded in the mRNA into proteins. •The nucleotides are read in triplets (set of three) called codons. •Each triplet code for a specific amino acid, and sometimes more than one codon exist for an amino acid. •mRNA are read by the translational machinery including ribosomes, tRNAs and rRNAs. •Like transcription, it also includes initiation, elongation and termination. 1/14/202 1 2 9 Characters of genetic code 1.Triplet code (three bases) 2.Universal for all organisms ex. : UUU = phenylalanine in all organisms. 3. Redundant, redundancy of genetic code: more than one code specify one amino acid. (61 codon for 20 amino acid). ex. : Leucine has 6 different codons. 4.Reads as a series of nucleotide. - No comma between the codon. - The start codon determines the reading frame. 5) The codon could undergo mutation. 1/14/202 1 5 0 The Genetic Code 1/14/202 1 51 The Genetic Code 1/14/202 1 52 Translation Copy of genetic information in mRNA (codons) mRN A 1/14/202 1 Translate Polypeptide chain (protein) into Requirements Active tRNA Ribosom es 3 0 Functio 1. translate the information n: Properties: -Large subunit: contains 3 binding sites: P= For peptidyl-tRNA (hold peptide chain). A= For amino-acyltRNA, that delivers the next amino acid. -Small subunit E=contains Exit site mRNA binding site. - Separate entities are inactive. -become active when the 2 subunits are linked to mRNA -Polyribosomes (polysomes) during translation. are mRNA -linked clusters of ribosomal molecules . 1/14/202 1 in mRNA into protein. 2. hold mRNA, aminoacyl tRNA & polypeptide chain in a correct orientation during translation. 3. form the peptide bonds between amino acids. E- site A– P– site site L. Subunit mRNA S. subunit mRNA –binding site 31 Ribosome structure 1/14/202 1 32 tRNA “the Middle Man” • Is in a clover shaped structure • Brings the amino acids to the mRNA. • Has an anticodon loop to recognise the codons in the mRNA (by Watson-Crick base pairing). • Is responsible for the specificity 1/14/202 of the codon 1 33 1/14/202 1 34 tRNA Charging •Aminoacylation is the process of adding an aminoacyl group to a compound. •It produces tRNA molecules with their CCA 3' ends covalently linked to an amino acid •Each tRNA is aminoacylated (or charged) with a specific amino acid by an aminoacyl tRNA synthase. •There is normally a single aminoacyl tRNA synthetase for each amino acid, despite the fact that there can be more than one tRNA, and more than one anticodon, for an amino acid. 1/14/202 1 35 Process of Translation •Initiation •Elongation •Termination •Recognition of STOP codons •Usage of release factors 1/14/202 1 37 1/14/202 1 38 1/14/202 1 39 Translation Termination 1/14/202 1 4 0 • Steps of Protein Synthesis 1) Initiation - Begins when initiator complex (initiator tRNA & small ribosomal subunit) binds the start codon of mRNA. - That followed by formation of functioning ribosome by binding of large ribosomal subunit to initiator complex. 1/14/202 1 Anticodo n 3’ 5’ 5’ InitiatortRNA Methionine 5’ Small subun it 3’ Start codon 3’ mRN A Asite Psite 5’ Small subun it mRN A 4 1 2) 1) Begins by occupying A- site by Elongation 2nd tRNA with its amino acid, that carries anticodon complementary to the next mRNA –codon. 2) Peptide bond formation; Amino acid is detached from tRNA in Psite & joined by peptide bond to amino acid linked to tRNA in Asite. 3) Translocation, Ribosome moves one codon in direction 5’ – 3’, that leads to: release tRNA from Psite to the cytoplasm. - transfer tRNA & the peptide chain transfer from A- site to P-site. - A- site become open to by repeating steps 1, 2, 3 receive another suitable (elongation cycle), the tRNA. 1/14/202 1 ribosome peptide the chain reaches elongates thetill 1st step 5’ Small subun it 2nd step 5’ Small subuni t 3rd step GTP GDP+P+E 5’ mRN A 4 2 1/14/202 1 43 Growing polypeptide chain 1/14/202 1 44 3) Termination Releasing factor Polypeptide chain 5’ dissociation At stop codon: Release factor recognizes and binds the mRNA - stop codon. It terminates the protein synthesis by releasing: - Large & small ribosomal subunits 4 5 Translation= Polypeptide Synthesis 1/14/202 1 46 1/14/202 1 Chromosome e to 4 8 THANK YOU 1/14/202 1 53