Transcription and Translation, Slides of Environmental science

Download Transcription and Translation and more Slides Environmental science in PDF only on Docsity! TRANSCRIPTI ON OVERVIEW OF TRANSCRIPTION  Transcription is the first step in gene expression, in which information from a gene is used to construct a functional product such as a protein.  The goal of transcription is to make a RNA copy of a gene's DNA sequence.  For a protein-coding gene, the RNA copy, or transcript, carries the information needed to build a polypeptide (protein or protein subunit).  Eukaryotic transcripts need to go through some processing steps before translation into proteins. Me 2! ANA © serena? & Template DNA gx ACC AT CAG TIC ¢ RNA Polymerase STAGES OF TRANSCIPTION initiation, elongation, and termination 1. Initiation RNA polymerase binds to a sequence of DNA called the promoter, found near the beginning of a gene. Each gene (or group of co-transcribed genes, in bacteria) has its own promoter. Once bound, RNA polymerase separates the DNA strands, providing the single-stranded template needed for transcription. A SO io § Se © AS and 3’ mean? The two ends of a strand of DNA or RNA strand are different from each other. That is, a DNA or RNA strand has directionality. • At the 5’ end of the chain, the phosphate group of the first nucleotide in the chain sticks out. The phosphate group is attached to the 5' carbon of the sugar ring, which is why this is called the 5' end. • At the other end, called the 3’ end, the hydroxyl of the last nucleotide added to the chain is exposed. The hydroxyl group is attached to the 3' carbon of the sugar ring, which is why this is called the 3' end. Many processes, such as DNA replication and transcription, can only take place in one particular direction relative the the directionality of a DNA or RNA strand. RNA polymerase Coding, strand In DNA Temploce strand RNA Codima Strand RNA Alule Alule > Template. strand TACTAGAGCATT Eukaryotic RNA modifications  In bacteria, RNA transcripts can act as messenger RNAs (mRNAs) right away. In eukaryotes, the transcript of a protein-coding gene is called a pre-mRNA and must go through extra processing before it can direct translation. Eukaryotic pre-mRNAs must have their ends modified, by addition of a 5' cap (at the beginning) and 3' poly-A tail (at the end). Many eukaryotic pre-mRNAs undergo splicing. In this process, parts of the pre-mRNA (called introns) are chopped out, and the remaining pieces (called exons) are stuck back together. End modifications increase the stability of the mRNA, while splicing gives the mRNA its correct sequence. (If the introns are not removed, they'll be translated along with the exons, producing a "gibberish" polypeptide.) TRANSLATIO N The genetic code During translation, a cell “reads” the information in a messenger RNA (mRNA) and uses it to build a protein. Actually, to be a little more technical, an mRNA doesn’t always encode—provide instructions for—a whole protein. Instead, what we can confidently say is that it always encodes a polypeptide, or chain of amino acids. First letter Second letter UUU UCU) UAU UGU U ie UCC | co, wey uec fos e UA} eu UCA UAA Stop|UGA Stop] A UUG UCG } UAG Stop|UGG Trp |G CUU } CCU) CAU } His | CSU U GUC Vcy | CCC lee ca CGC lag |e CUA CCA AA) in Goa [07 1A CUG. ECG | ee EGG G AUU 7 ACU) AAU AGU U AUC file | ACC |, aac fAS" |nee Se | ¢ AUA | ACA AAA }uys AGA } arg | A AUG’ Met | ACG | AAG AGG G GUU } GCU} ex} Asp | GGU U GUC |), | GCC | a, |GACI/? | Gac ey |¢ GUA GCA AA) ty GGA A Gus J GcG | Bae 6Ge G Third letter o In an mRNA, the instructions for building a polypeptide are RNA nucleotides (As, Us, Cs, and Gs) read in groups of three. These groups of three are called codons. o There are 6161 codons for amino acids, and each of them is "read" to specify a certain amino acid out of the 2020 commonly found in proteins. One codon, AUG, specifies the amino acid methionine and also acts as a start codon to signal the start of protein construction. o There are three more codons that do not specify amino acids. These stop codons, UAA, UAG, and UGA, tell the cell when a polypeptide is complete. All together, this collection of codon- amino acid relationships is called the genetic code, because it lets cells “decode” an mRNA into a chain of amino acids _z Amino acid Large ribosomal subunit tRNA Anticodon 5’ mRNA ...AU G Ld UGGUUC... 3’ Codon =a Small ribosomal subunit Ribosomes Ribosomes are the structures where polypeptides (proteins) are built. They are made up of protein and RNA (ribosomal RNA, or rRNA). Each ribosome has two subunits, a large one and a small one, which come together around an mRNA—kind of like the two halves of a hamburger bun coming together around the patty The ribosome provides a set of handy slots where tRNAs can find their matching codons on the mRNA template and deliver their amino acids. These slots are called the A, P, and E sites. Not only that, but the ribosome also acts as an enzyme, catalyzing the chemical reaction that links amino acids together to make a chain. Steps of translatio n initiation (starting off), elongation (adding on to the protein chain), and termination (finishing up) Extending the chain: Elongation Elongation is the stage where the amino acid chain gets longer. In elongation, the mRNA is read one codon at a time, and the amino acid matching each codon is added to a growing protein chain. Each time a new codon is exposed: A matching tRNA binds to the codon The existing amino acid chain (polypeptide) is linked onto the amino acid of the tRNA via a chemical reaction The mRNA is shifted one codon over in the ribosome, exposing a new codon for reading 5 tRNA binds to matching codon mRNA is shifted forward, exposing a new codon 5 <—— The polypeptide is linked to the new amino acid 3" Finishing up: Termination Termination is the stage in which the finished polypeptide chain is released. It begins when a stop codon (UAG, UAA, or UGA) enters the ribosome, triggering a series of events that separate the chain from its tRNA and allow it to drift out of the ribosome. After termination, the polypeptide may still need to fold into the right 3D shape, undergo processing (such as the removal of amino acids), get shipped to the  right place in the cell, or combine with other polypeptides before it can do its job as a functional protein.