8 4 Transcription KEY CONCEPT Transcription converts a

8. 4 Transcription KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule.

8. 4 Transcription RNA carries DNA’s instructions. • The central dogma states that information flows in one direction from DNA to RNA to proteins.

8. 4 Transcription makes three types of RNA. • Transcription copies DNA to make a strand of RNA. • OBJECT: to make an accurate copy of a small section of a genome (GENE) • Transcription results in 3 types of RNA: – Messenger RNA (m. RNA) carries the message that will be translated to form a protein. – Ribosomal RNA (r. RNA) forms part of ribosomes where proteins are made. – Transfer RNA (t. RNA) brings amino acids from the cytoplasm to a ribosome.

8. 4 Transcription INITIATION 2 Strands of DNA 3’ 5’ 5’ 3’ Q: Which strand of DNA provides the template for the m. RNA? Ø 3’-5’ strand sense or template strand results in a 5’ 3’ m. RNA copy

8. 4 Transcription

8. 4 Transcription INITIATION • Promoter sequences ID’s the correct strand location. – The “TATA” box – Binding site of RNA Polymerase • 2 promoter sequences signal a gene – Different sequences! • Gene starts at the end of promoter #2 • A transcription factor is added to the end of the promoter

8. 4 Transcription Elongation • Transcription is catalyzed by RNA polymerase. – RNA polymerase and other proteins form a transcription complex. – The transcription complex recognizes the start of a gene and unwinds a segment of it. start site transcription complex nucleotides

8. 4 Transcription Elongation – Nucleotides pair with one strand of the DNA. (U replaces T) – RNA polymerase bonds the nucleotides together in 5’ to 3’ direction – The DNA helix winds again as the gene is transcribed. – No Okazaki’s … No proofreading DNA RNA polymerase moves along the DNA

8. 4 Transcription Termination • RNA Polymerase hits a terminator sequence • Similar concept as promoter (2 seq’s in order) • m. RNA separates from RNA polymerase which detaches from DNA m. RNA

8. 4 Transcription

8. 4 Transcription In prokaryotes (bacteria) … translation happens right after In eukaryotes ‘processing’ of m. RNA is needed before the translation in a polypeptide chain ØIntrons must be removed

8. 4 Transcription m. RNA Splicing

8. 4 Transcription Processing Ø“ 5’ Cap” & “Poly-A” tail added to m. RNA • Addition of 5’ cap At beginning of pre-m. RNA transcript 7 guanine bases added • Addition of poly(A) tail Added to 3’ end by poly-A polymerase Protects m. RNA from attack by RNA-digesting enzymes in cytosol

8. 4 Transcription Intron Splicing Ø Spliceosomes (sn. RP’s) remove introns from m. RNA transcript

8. 4 Transcription The transcription process is similar to replication. • Transcription and replication both involve complex enzymes and complementary base pairing. • The two processes have different end results. – Replication copies all the DNA; transcription copies one gene growing RNA strands a gene. – Replication makes one copy; DNA transcription can make many copies.