Transcription and Translation. Chapter 10. Objectives of Transcription. Understand the process of transcription Recognize the role of RNA Polymerase Recognize the significance of the Promoter and Termination regions of every gene
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Transcription and Translation Chapter 10
Objectives of Transcription • Understand the process of transcription • Recognize the role of RNA Polymerase • Recognize the significance of the Promoter and Termination regions of every gene • Explain how transcribed RNA is modified prior to exiting the nucleus. Understand the significance of this process
Objectives of Translation • Understand the process of translation • Recognize the role of mRNA, rRNA, and tRNA • Understand how protein may be modified prior to use
Transcription • Consists of three stages • Initiation: attachment of RNA Polymerase to the promotor region on DNA • Elongation: building of the mRNA from the 3’ end of the nucleotide polymer • Termination: release of RNA polymerase and mRNA following transcription of the terminator region of the DNA
Initiation • Genes on the DNA begin with a promoter region consisting of a sequence of A & T (TATA box) and the first nucleotide involved in the peptide sequence • Transcription factors (proteins that assist the binding of RNA polymerase to the promoter) are found in association with the promoter region • Transcription initiation complex: transcription factors & RNA polymerase bound to the promoter region of the DNA –We are ready to make RNA!
Once initiation is complete the 2 strands of the DNA unwind RNA polymerase builds a mRNA strand complimentary to the DNA transcription unit (60 bases/sec) Once the RNA Polymerase passes the DNA strands reform their double helix Elongation
Termination • When the RNA Polymerase transcribes the terminator region of the DNA, the Polymerase releases the mRNA • The transcribed termination sequence on the mRNA is AAUAAA
Modification of mRNA • Transcribed mRNA (pre-mRNA) must be modified before leaving the nucleus • modifications include: • addition of 5’cap • Prevents “unraveling” • addition of poly A tail • Prevents “unraveling” • Helps ribosome attach • Assists in the export of mRNA from nucleus
Further Modifications* • Transcribed RNA is “too long” and is shortened in the nucleus • Exons are segments of the pre-mRNA that contain information that will be reflected in the polypeptide • Introns are segments of the pre-mRNA that separate (intervene) exons * Discussed in: “A closer look” on page 176
Why bother with introns? • Intron removal may regulate gene activity and the passage of mRNA into the cytoplasm • Genes may play roles in multiple proteins, introns may enable a gene to be diverse in function • May increase recombination (new combinations of genetic material) such that multiple genes may be required to make certain polypeptide sequences through a “cut and paste” kind of mechanism
Translation • Flow of genetic information from mRNA to protein • Change in cellular language from nucleotide to protein • tRNA acts as the interpreter • There is a different tRNA for “each kind of codon” except the stop codons
How does Transfer RNA work? • tRNA has a 3-D structure that includes two important regions • 3’ end attaches to a specific AA • anticodon (2nd loop) that compliments the codon on mRNA There are fewer than 64 tRNA molecules needed because of the ability of certain tRNA molecules to bind to more than one codon. This condition is known as Wobble Enzymes attach specific Amino Acids to specific tRNA molecules
Ribosome Anatomy • Ribosomes: structures composed of rRNA (2 subunits) and protein • Responsible for aligning tRNA anticodons with mRNA codons • Stage for synthesis of peptides • Each ribosome has: • a binding site for mRNA • two binding sites for molecules of tRNA • First site: holds tRNA with growing peptide chain • Second site: holds tRNA containing the next AA to be added to the polypeptide
The Process • Initiation: Binding of the small and large ribosomal subunits to mRNA and the first tRNA • Initiation factors and energy (GTP) involved in the formation of initiation complex • Small subunit attaches to mRNA near the 5’cap • tRNA carrying the AA Methionine attaches to the start codon • Large subunit attaches
Process continued Elongation: lengthening of polypeptide • Codonrecognition: Site 2 codon forms bonds with the anticodon of tRNA • Peptide bond formation: large subunit rRNA catalyzes the formation of a peptide bond between the AA of tRNA’s at 1 & 2 sites. AAs at 1 site transferred to 2 site • Translocation: rRNA shifts position by one codon with respect to mRNA. The H bonded mRNA-tRNA unit is moved from the 1 site to the 2 site. Any tRNA occupying 1 site is released. Translocation requires energy. mRNA is read from 5’ to 3’.
Processcontinued • Termination: stoppage of synthesis and peptide release • elongation continues until the a stop codon is read by the ribosome • Release factor protein binds to the stop codon • Release factor adds H2O to the protein causing it to be released from tRNA • Translation assembly falls apart
Modification of the Polypeptide • Most synthesized proteins must undergo further modification before use including • attachment of sugar, lipids, functional groups • leading end of protein may have some AA removed (recall that all protein sequencing starts with AA methionine but not all finished protein do) • Polypeptide chain may be divided up into smaller units • Protein may require several polypeptide chains