* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download c - Holterman
Gene regulatory network wikipedia , lookup
Non-coding DNA wikipedia , lookup
Vectors in gene therapy wikipedia , lookup
Two-hybrid screening wikipedia , lookup
Promoter (genetics) wikipedia , lookup
Real-time polymerase chain reaction wikipedia , lookup
RNA interference wikipedia , lookup
Peptide synthesis wikipedia , lookup
Metalloprotein wikipedia , lookup
Protein structure prediction wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
RNA silencing wikipedia , lookup
Eukaryotic transcription wikipedia , lookup
Silencer (genetics) wikipedia , lookup
Transcriptional regulation wikipedia , lookup
RNA polymerase II holoenzyme wikipedia , lookup
Point mutation wikipedia , lookup
Proteolysis wikipedia , lookup
Deoxyribozyme wikipedia , lookup
Polyadenylation wikipedia , lookup
Amino acid synthesis wikipedia , lookup
Nucleic acid analogue wikipedia , lookup
Biochemistry wikipedia , lookup
Gene expression wikipedia , lookup
Messenger RNA wikipedia , lookup
Genetic code wikipedia , lookup
Transfer RNA wikipedia , lookup
The Molecular Basis of Inheritance G C A T T A 1 nm C G C A C 3.4 nm G T G T A T A A T T A G A Figure 16.7a, c C 0.34 nm T (a) Key features of DNA structure (c) Space-filling model Transcription • Transcription is the DNAdirected synthesis of RNA • RNA synthesis – Is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides – Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine Home Work Questions TATA BOX -It is a type of promoter sequence, which specifies to other molecules where transcription begins. Home Work Questions - INTRONS: intervening non-coding sequence in a eukaryotic gene. - EXONS: the coding region of a eukaryotic gene. Each gene is composed of one or more exons. *Translation* • Translation is the RNA-directed synthesis of a polypeptide • Translation involves – – – – TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide mRNA Ribosomes - Ribosomal RNA Transfer RNA Genetic coding – codons Amino acids Polypeptide tRNA with amino acid Ribosome attached Gly tRNA Anticodon A A A U G G U U U G G C Codons 5 mRNA 3 *The Genetic Code* • Genetic information is encoded as a sequence of nonoverlapping base triplets, or codons Gene 2 DNA molecule Gene 1 Gene 3 DNA strand (template) 3 A C C A A A C C G A G T 5 TRANSCRIPTION mRNA 5 U G G U U U G G C U C A Codon TRANSLATION Protein Trp Amino acid Phe Gly Ser 3 *The Genetic Code* Codons: • Basic unit of genetic code • set of 3 nucleotides in mRNA that codes for an amino acid placement on polypeptides. • 4 bases and 3 positions in each codon, there are 4 x 4 x 4 = 64 possible codons • 64 codons but only 20 amino acids, therefore most have more than 1 codon • 3 of the 64 codons are used as STOP signals which mark the end of the protein • One codon is used as a START signal: start of every protein *The Genetic Code* Second mRNA base U C A UAU UUU UCU Tyr Phe UAC UUC UCC U UUA UCA Ser UAA Stop UAG Stop UUG Leu UCG CUU CUC C CUA CUG CCU CCC Leu CCA CCG Pro AUU AUC A AUA AUG ACU ACC ACA ACG Thr GUU G GUC GUA GUG lle Met or start GCU GCC Val GCA GCG Ala G U UGU Cys UGC C UGA Stop A UGG Trp G U CAU CGU His CAC CGC C Arg CAA CGA A Gln CAG CGG G U AAU AGU Asn AAC AGC Ser C A AAA AGA Lys Arg G AAG AGG U GAU GGU C GAC Asp GGC Gly GAA GGA A Glu GAG GGG G Third mRNA base (3 end) First mRNA base (5 end) • A codon in messenger RNA is either translated into an amino acid or serves as a translational start/stop signal *Transfer RNA* • • • • Consists of a single RNA strand, only 80 nucleotides long Each carries a specific amino acid on one end and has an anticodon on the other end Enzymes pair up the proper tRNA molecules with their corresponding amino acids. tRNA brings the amino acids to the ribosomes, 3 A C C A 5 C G The “anticodon” is the 3 RNA bases that G C C G matches the 3 bases of the codon on the U G U A mRNA molecule A U A U U C UA C A C AG * G * U G U G C C * * * U C * * G AG C (a) Two-dimensional structure. The four base-paired regions and three G C U A loops are characteristic of all tRNAs, as is the base sequence of the * G amino acid attachment site at the 3 end. The anticodon triplet is A A* C unique to each tRNA type. (The asterisks mark bases that have been U * chemically modified, a characteristic of tRNA.) A G A Amino acid attachment site Anticodon C U C G A G A G * * G A G G Hydrogen bonds *Transfer RNA* • 3 dimensional tRNA molecule is roughly “L” shaped 5 3 Amino acid attachment site Hydrogen bonds A AG 3 Anticodon (b) Three-dimensional structure 5 Anticodon (c) Symbol used in the book *Ribosomes* • • Ribosomes facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis The 2 ribosomal subunits are constructed of proteins and RNA molecules named ribosomal RNA or rRNA DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Growing polypeptide Exit tunnel tRNA molecules Large subunit E P A Small subunit 5 mRNA 3 (a) Computer model of functioning ribosome. This is a model of a bacterial ribosome, showing its overall shape. The eukaryotic ribosome is roughly similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules and proteins. Building a Polypeptide Amino end Growing polypeptide Next amino acid to be added to polypeptide chain tRNA 3 mRNA 5 Codons (c) Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon basepairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged tRNA leaves via the E site. *Building a Polypeptide* • We can divide translation into three stages – Initiation – Elongation – Termination • The AUG start codon is recognized by methionyl-tRNA or Met • Once the start codon has been identified, the ribosome incorporates amino acids into a polypeptide chain • RNA is decoded by tRNA (transfer RNA) molecules, which each transport specific amino acids to the growing chain • Translation ends when a stop codon (UAA, UAG, UGA) is reached Initiation of Translation • The initiation stage of translation brings together mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosome Large ribosomal subunit P site 3 U A C 5 5 A U G 3 Initiator tRNA GTP GDP E A mRNA 5 Start codon mRNA binding site 1 5 3 Small ribosomal subunit A small ribosomal subunit binds to a molecule of mRNA. In a prokaryotic cell, the mRNA binding site on this subunit recognizes a specific nucleotide sequence on the mRNA just upstream of the start codon. An initiator tRNA, with the anticodon UAC, base-pairs with the start codon, AUG. This tRNA carries the amino acid methionine (Met). 3 Translation initiation complex 2 The arrival of a large ribosomal subunit completes the initiation complex. Proteins called initiation factors (not shown) are required to bring all the translation components together. GTP provides the energy for the assembly. The initiator tRNA is in the P site; the A site is available to the tRNA bearing the next amino acid. Elongation of the Polypeptide Chain • In the elongation stage, amino acids are added one by one to the preceding amino acid TRANSCRIPTION 1 Codon recognition. The anticodon of an incoming aminoacyl tRNA base-pairs with the complementary mRNA codon in the A site. Hydrolysis of GTP increases the accuracy and efficiency of this step. Amino end of polypeptide DNA mRNA Ribosome TRANSLATION Polypeptide E mRNA Ribosome ready for next aminoacyl tRNA 5 3 P A site site 2 GTP 2 GDP E E P P A GDP 3 Translocation. The ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs, bringing the next codon to be translated into the A site. GTP E P A A 2 Peptide bond formation. An rRNA molecule of the large subunit catalyzes the formation of a peptide bond between the new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. This step attaches the polypeptide to the tRNA in the A site. Termination of Translation • The final stage is termination when the ribosome reaches a stop codon in the mRNA Release factor Free polypeptide 5 3 3 5 5 3 Stop codon (UAG, UAA, or UGA) 1 When a ribosome reaches a stop 2 The release factor hydrolyzes 3 The two ribosomal subunits codon on mRNA, the A site of the the bond between the tRNA in and the other components of ribosome accepts a protein called the P site and the last amino the assembly dissociate. a release factor instead of tRNA. acid of the polypeptide chain. The polypeptide is thus freed from the ribosome. *Termination of Translation* • The final step in translation is termination. • ribosome reaches a STOP codon, there is no corresponding transfer RNA. • A small protein called a “release factor” attaches to the stop codon. • The release factor causes the whole complex to fall apart: messenger RNA, the two ribosome subunits, the new polypeptide. • The messenger RNA can be translated many times, to produce many protein copies. A summary of transcription and translation in a eukaryotic cell DNA TRANSCRIPTION 1RNA is transcribed from a DNA template. 3 RNA transcript 5 RNA polymerase Exon RNA PROCESSING 2 In eukaryotes, the RNA transcript (premRNA) is spliced and modified to produce mRNA, which moves from the nucleus to the cytoplasm. RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase NUCLEUS Amino acid FORMATION OF INITIATION COMPLEX CYTOPLASM AMINO ACID ACTIVATION tRNA 3 After leaving the 4 Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. nucleus, mRNA attaches to the ribosome. mRNA Growing polypeptide Activated amino acid Ribosomal subunits 5 TRANSLATION 5 E A A A A U G G U U U A U G Figure 17.26 Codon Ribosome Anticodon A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome one codon at a time. (When completed, the polypeptide is released from the ribosome.) Post-translation • The new polypeptide is now floating loose in the cytoplasm if translated by a free ribosme. • It might also be inserted into a membrane, if translated by a ribosome bound to the endoplasmic reticulum. • ***Polypeptides fold spontaneously into their 4 levels of structure (quaternary) , 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. All of these have special purposes for protein function and GENE EXPRESSION