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Gene Expression : Transcription and Translation How Are Different Types of Cells Created and Maintained? By differential gene expression. The same genetic information is in all 100 trillion cells of any one person. Different cells use the same blueprint in different ways. How? In essence, the control of gene expression occurs by regulating the flow of information from DNA to protein. The “Central Dogma” of Molecular Genetics Transcription Translation DNA RNA Protein RNA processing Trait Review DNA Made up of 4 different nucleotides RNA Made up of 4 different nucleotides Proteins Made up of 20 different amino acids Gene Expression in Prokaryotes vs. Eukaryotes Transcription and translation may occur simultaneously Transcription and translation are separated in time and space The Genetic Code Problem: How do only 4 different nucleotides code for the 20 amino acids that make up proteins? Solution: Each group of 3 nucleotides codes for a different amino acid. These 3 nuclotide units are called codons. DNA RNA C G A U T A Amino Acid Valine Reading Frame The 3-nucleotide units (codons) must be read in the correct reading frame Start codons determine the reading frame The Genetic Code Dictionary There are multiple codons for each amino acid AUG is always the start codon UAA, UGA, and UAG are stop codons Transcription DNA is used to make a strand of RNA called the primary transcript (pre-mRNA) The pre-mRNA is further processed to create the finished mRNA mRNA exits the nucleus to be translated Transcription 3 main steps 1. Initiation 2. Elongation 3. Termination Transcription Initiation RNA polymerase binds to DNA at a region called the promoter RNA polymerase unwinds the DNA and adds nucleotides in the 5’ → 3’ direction Transcription Elongation RNA polymerase moves along the DNA strand, adding 60 nucleotides/sec DNA strands rejoin after polymerase passes by Transcription Termination Polymerase stops when it reaches a DNA sequence called the terminator The mRNA has been completely transcribed In eukaryotes, this is pre-mRNA and must be further processed mRNA Processing In eukaryotes, pre-mRNA must be further processed to mRNA before it leaves the nucleus Guanine is added to 5’ end, forming the 5’ cap 100’s of adenines are added to 3’ end, forming the poly-A tail Non-coding regions of RNA are spliced out Intron (non-coding sequences) are cut out by spliceosomes. Leaving only Exons (Coding sequences) making up the mRNA that leaves the nucleus. Alternative splicing patterns means one gene can make more than one protein mRNA Splicing Translation The process in which mRNA is used to make proteins Occurs in the cytoplasm using ribosomes Requires tRNA bound to amino acid 3 steps: initiation, elongation, termination tRNA A clover-shaped RNA molecule Structure of tRNA 3’ end of tRNA binds to specific amino acids Anti-codon on tRNA complements mRNA codon tRNA Synthesis RNA is made in the nucleus Amino acids float free in the cytoplasm Aminoacyl-tRNA synthase joins each amino acid to the appropriate tRNA Ribosomes 3 tRNA binding sites 2 subunits Composed of proteins and rRNA Translation: Initiation mRNA, tRNA and small ribosomal subunit bind with the P site at the start codon Large subunit binds using energy from GTP Translation: Elongation •mRNA is read 3 nucleotides at a time (Codons) •tRNA brings corresponding amino acid into the A site of the ribosome Ribosome catalyses dehydration synthesis reaction between amino acids in P site and A site Growing polypetpide now attached to tRNA in A site Ribosome moves forward one codon Free tRNA in P site exits out the back of ribosome tRNA (with polypeptide) moves into P site Translation: Termination Elongation continues until reaching a stop codon Release factor binds and hydrolyzes the bond between the last tRNA and its a.a., freeing the new protein Polyribosomes Many ribosomes may simultaneously translate from a single mRNA Gene Expression: Overview