Transcription & Translation notes

Transcription

1 Transcription & Translation notes

2 TRNSRIPTION

3 The entral Dogma DN RN proteins Protein Synthesis The DN inherited by an organism leads to specific traits by dictating the synthesis of proteins The process by which DN directs protein synthesis, gene expression includes two stages, called transcription and translation you

4 Transcription and Translation ells are governed by a cellular chain of command DN RN protein Transcription Is the synthesis of RN under the direction of DN Produces messenger RN (mrn) Translation Is the actual synthesis of a polypeptide, which occurs under the direction of mrn Occurs on ribosomes In a eukaryotic cell the nuclear envelope separates transcription from translation Extensive RN processing occurs in the nucleus TRNSRIPTION RN PROESSING mrn DN Nuclear envelope Pre-mRN Ribosome TRNSLTION Polypeptide (b) Eukaryotic cell. T compartment for tr transcript, called p ways before leavin

5 Transcription Transcription is the DN-directed synthesis of RN RN synthesis Is catalyzed by RN polymerase, which pries the DN strands apart and hooks together the RN nucleotides Follows the same base-pairing rules as DN, except that in RN, uracil substitutes for thymine

6 Five differences between RN and DN 1. Sugar in RN is ribose instead of deoxyribose 2. RN is single stranded and short 3. RN contains Uracil instead of Thymine 4. RN is disposable 5. RN can be outside the nucleus, DN can t If you are going from DN to RN what nitrogenous base would pair with denine? DN: G T T RN:

7 Types of RN Key players in Protein Synthesis - Messenger RN (mrn): carry instruction copies Ribosomal RN (rrn): makes up ribosome along with proteins Transfer RN (trn): brings amino acids to the ribosome

8 Big picture of protein synthesis

9 Synthesis of an RN Transcript RN polymerase synthesizes a single strand of RN against the DN template strand, adding nucleotides to the 3 end of the RN chain s RN polymerase moves along the DN it continues to untwist the double helix, exposing about 10 to 20 DN bases at a time for pairing with RN nucleotides Specific sequences in the DN signal termination of transcription When one of these is encountered by the polymerase, the RN transcript is released from the DN and the double helix can zip up again Elongation Newly made RN RN polymerase Non-template strand of DN T 3 end E G T G G T T Direction of transcription ( downstream ) RN nucleotides U Template strand of DN

10 Transcription Overview

11 Most eukaryotic mrns aren t ready to be translated into protein directly after being transcribed from DN. mrn requires processing. Transcription of RN processing occur in the nucleus. fter this, the messenger RN moves to the cytoplasm for translation. The cell adds a protective cap to one end. These both function to protect the RN from enzymes that would degrade Most of the genome consists of non-coding regions called introns Non-coding regions may have specific chromosomal functions or have regulatory purposes Introns also allow for alternative RN splicing Post RN Processing Thus, an RN copy of a gene is converted into messenger RN by doing 2 things: dd protective bases to the ends ut out the introns The original transcript from the DN is called pre-mrn. It contains transcripts of both introns and exons. The introns are removed by a process called splicing to produce messenger RN (mrn)

12 TRNSLTION

13 Translation Translation is the RN-directed synthesis of a polypeptide Translation involves mrn Ribosomes - Ribosomal RN trn Genetic coding - codons TRNSRIPTION TRNSLTION DN mrn Ribosome Polypeptide Polypeptide Ribosome mino acids trn with amino acid attached Gly trn U G G U U U G G nticodon 5 mrn odons 3

14 The Genetic ode Genetic information is encoded as a sequence of nonoverlapping base triplets, or codons DN molecule Gene 1 Gene 2 Gene 3 DN strand (template) 3 G G T 5 TRNSRIPTION mrn TRNSLTION 5 U G G U U U G G U odon 3 Protein Trp Phe Gly Ser mino acid

15 First mrn base (5 end) Third mrn base (3 end) The Genetic ode odons: 3 base code for the production of a specific amino acid, sequence of three of the four different nucleotides Since there are 4 bases and 3 positions in each codon, 64 possible codons, but only 20 amino acids, therefore most have more than 1 codon 3 of the 64 codons are used as STOP signals; they are found at the end of every gene and mark the end of the protein U G Second mrn base U G UUU UU UU UGU Phe Tyr ys UU U U UG UU U Ser U Stop UG Stop UUG Leu UG UG Stop UGG Trp UU U U UG UU U U UG GUU GU GU GUG Leu lle Met or start Val U G U G GU G G GG Pro Thr la U G U G GU G G GG His Gln sn Lys sp Glu GU G G GG GU G G GG GGU GG GG GGG rg Ser rg Gly U G U G U G U G One codon is used as a STRT signal: it is at the start of every protein codon in messenger RN is either translated into an amino acid or serves as a translational start/stop signal

16 Transfer RN onsists of a single RN strand that is only about 80 nucleotides long Each carries a specific amino acid on one end and has an anticodon on the other end special group of enzymes pairs up the proper trn molecules with their corresponding amino acids. trn brings the amino acids to the ribosomes, The anticodon is the 3 RN bases that The matches anticodon the 3 is bases the 3 of RN the codon bases on that the matches mrn molecule the 3 bases of the codon on the mrn molecule (a) Two-dimensional structure. The four base-paired regions and three loops are characteristic of all trns, as is the base sequence of the amino acid attachment site at the 3 end. The anticodon triplet is unique to each trn type. (The asterisks mark bases that have been chemically modified, a characteristic of trn.) mino acid attachment site * G U 3 G UU G * * G U G U * U * G GG U * * 5 G G G U U U G * G U nticodon G * U * * G G G G G* Hydrogen bonds

17 mrn 5 Building a Polypeptide mino end odons Growing polypeptide 3 Next amino acid to be added to polypeptide chain trn (c) Schematic model with mrn and trn. trn fits into a binding site when its anticodon basepairs with an mrn codon. The P site holds the trn attached to the growing polypeptide. The site holds the trn carrying the next amino acid to be added to the polypeptide chain. Discharged trn leaves via the E site. The UG start codon is recognized by methionyl-trn or Met Once the start codon has been identified, the ribosome incorporates amino acids into a polypeptide chain RN is decoded by trn (transfer RN) molecules, which each transport specific amino acids to the growing chain Translation ends when a stop codon (U, UG, UG) is reached

18 summary of transcription and translation in a eukaryotic cell TRNSRIPTION 1 RN is transcribed from a DN template. 3 DN 5 RN transcript RN polymerase RN PROESSING Exon 2 In eukaryotes, the RN transcript (premrn) is spliced and modified to produce mrn, which moves from the nucleus to the cytoplasm. NULEUS RN transcript (pre-mrn) Intron minoacyl-trn synthetase FORMTION OF INITITION OMPLEX mino acid MINO ID TIVTION YTOPLSM trn 3 fter leaving the 4 Each amino acid nucleus, mrn attaches attaches to its proper trn to the ribosome. with the help of a specific enzyme and TP. mrn Growing polypeptide ctivated amino acid Ribosomal subunits 5 E U G GU U U U G Ribosome odon nticodon TRNSLTION 5 succession of trns add their amino acids to the polypeptide chain as the mrn is moved through the ribosome one codon at a time. (When completed, the polypeptide is released from the ribosome.) Figure 17.26

19 Post-translation The new polypeptide is now floating loose in the cytoplasm if translated by a free ribosome. It might also be inserted into a membrane, if translated by a ribosome bound to the endoplasmic reticulum. Polypeptides fold spontaneously into their active configuration, and they spontaneously join with other polypeptides to form the final proteins. Sometimes other molecules are also attached to the polypeptides: sugars, lipids, phosphates, etc. ll of these have special purposes for protein function.

20 Just Like onstruction DN is the master plan mrn is the everyday blueprint rrn is the builder trn is the gopher mino cids are the wood Proteins are the building 1. What amino acids would be coded by the following DN sequence? GGGTT