Molecular Biology Lv2 Nucleic Acids Lesson 1 PDF

Molecular Biology Lv2 By Prof. Sami Mohamed Badr University in Cairo, School of Biotechnology Lecture #2  STEPWISE FORMATION OF PROTEINS ON RIBOSOMES A. INITIATION 1. The AUG Start Codon Is Recognized by Methi...

Molecular Biology Lv2 By Prof. Sami Mohamed Badr University in Cairo, School of Biotechnology Lecture #2  STEPWISE FORMATION OF PROTEINS ON RIBOSOMES A. INITIATION 1. The AUG Start Codon Is Recognized by Methionyl-tRNAiMet  STEPWISE FORMATION OF PROTEINS ON RIBOSOMES A. INITIATION 2. Bacterial Initiation of Protein Synthesis Begins Near a Shine-Dalgarno Sequence in mRNA AGGAGG Initiation factors (IFs) + (30S) ribosomal subunit preinitiation complex + fMet-tRNAiMet + mRNA 30S initiation complex  STEPWISE FORMATION OF PROTEINS ON RIBOSOMES A. INITIATION 2. Bacterial Initiation of Protein Synthesis Begins Near a Shine-Dalgarno Sequence in mRNA 3’ end of 16S rRNA interacts with an 8-nucleotide sequence at 5’-end of mRNA called the Shine-Dalgarno sequence ( SD) after its discoverers This sequence, located near the AUG start codon, base-pairs to a sequence at or very near 3’ end of the 16S rRNA, thereby binding the mRNA and small ribosomal subunit to each other Shine-Dalgarno AUG (start) codon mRNA 5’-end mRNA 3’-end 16S rRNA 3’-end 30S ribosomal subunit STEPWISE FORMATION OF PROTEINS ON RIBOSOMES A. INITIATION 3. Eukaryotic Initiation of Protein Synthesis Occurs at the 5’ End and Internal Sites in mRNA Eukaryotic Initiation factors (eIFs) m7G Cap Analog is a modified ribonucleotide (initiation sequence)  STEPWISE FORMATION OF PROTEINS ON RIBOSOMES B. ELONGATION 1. During Chain Elongation Each Incoming Aminoacyl-tRNA Moves through Three Ribosomal Sites Elongation factors (EFs) Peptidyltransferase Center fMet-tRNAiMet Amino acid P-site A-site mRNA 5’-end AUG  STEPWISE FORMATION OF PROTEINS ON RIBOSOMES B. ELONGATION 1. During Chain Elongation Each Incoming Aminoacyl-tRNA Moves through Three Ribosomal Sites Peptidyltransferase Center P-site Amino acid E-site A-site mRNA 5’-end  STEPWISE FORMATION OF PROTEINS ON RIBOSOMES C. TERMINATION 1. Protein Synthesis Is Terminated by Release Factors When a Stop Codon Is Reached Termination (release) factors: RF1 and RF2 (and eukaryotic eRF1)  STEPWISE FORMATION OF PROTEINS ON RIBOSOMES C. TERMINATION 2. Simultaneous Translation by Multiple Ribosomes and Their Rapid Recycling Increase the Efficiency of Protein Synthesis  MOLECULAR STRUCTURE OF GENES AND CHROMOSOMES MOLECULAR DEFINITION OF A GENE Classical Definition Gene: “a unit of DNA that contains the information to specify synthesis of a single polypeptide chain.” Molecular Definition Gene: “the entire nucleic acid sequence that is necessary for the synthesis of a functional polypeptide.” Which means: ► the coding region ►the DNA sequences required for synthesis of a particular RNA transcript: Promoter Enhancers Terminator poly(A) site GENES: 3 Types: Protein Coding mRNA, rRNE genes and tRNA genes  MOLECULAR STRUCTURE OF GENES AND CHROMOSOMES MOLECULAR DEFINITION OF A GENE 1. Bacterial Operons Produce Polycistronic mRNAs A cluster of genes comprising a single transcription unit is referred to as an operon. Since a cistron is defined as a genetic unit that encodes a single polypeptide, trp mRNA, which encodes several polypeptides, is said to be polycistronic. MOLECULAR STRUCTURE OF GENES AND CHROMOSOMES MOLECULAR DEFINITION OF A GENE 2. Most eukaryotic mRNA are monocistronic and contain introns 3. Eukaryotic genomes have Simple and multiple complex transcription units MOLECULAR STRUCTURE OF GENES AND CHROMOSOMES MOLECULAR DEFINITION OF A GENE 3. Simple and complex transcription units are found in eukaryotic genomes  CHROMOSOMAL ORGANIZATION OF GENES AND NONCODING DNA 1. Genomes of Higher Eukaryotes Contain Much Nonfunctional DNA 80 kb region Human β-Globin gene..!...‫ سؤال‬RBCs  CHROMOSOMAL ORGANIZATION OF GENES AND NONCODING DNA 2. Cellular DNA Content Does Not Correlate with Phylogeny Protein-coding genes The huge differences in the size of genomes are due mainly to the (nonfunctional DNA) Solitary genes Duplicated and diverged genes (functional gene Much of this apparently nonfunctional DNA is families and nonfunctional pseudogenes) composed of repetitious DNA sequences, some of which are never transcribed and most all Tandemly repeated genes encoding rRNA, 5S rRNA, of which are likely dispensable. tRNA, and histones Repetitious DNA The different classes of eukaryotic DNA sequences discussed in the following sections are Simple-sequence DNA summarized here: Moderately repeated DNA (mobile DNA elements) Transposons Viral retrotransposons Long interspersed elements (LINES; nonviral retrotransposons) Short interspersed elements (SINES; nonviral retrotransposons) Unclassified spacer DNA CHROMOSOMAL ORGANIZATION OF GENES AND NONCODING DNA 3. Protein-Coding Genes May Be Solitary or Belong to a Gene Family solitary genes Example: The 15-kb DNA sequence encoding chicken lysozyme constitutes a simple transcription unit (i.e., a single gene) containing four exons and three introns  CHROMOSOMAL ORGANIZATION OF GENES AND NONCODING DNA 3. Protein-Coding Genes May Be Solitary or Belong to a Gene Family gene family - a set of duplicated genes that encode proteins with similar but nonidentical amino acid sequences The different b-globin genes probably arose by duplication of an ancestral gene, most likely as the result of an “unequal crossover” during recombination in a germ-cell (egg or sperm) precursor CHROMOSOMAL ORGANIZATION OF GENES AND NONCODING DNA 4. Tandemly Repeated Genes Encode rRNAs, tRNAs, and Histones rRNA genes tRNA genes Break...!!