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Lecture

Lec 31- Transcription,.docx

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Department
Biology
Course
BIOL 1010
Professor
Igor Kovalchuk
Semester
Fall

Description
Transcription Lecture 31 Central dogma of Biology  Information from DNA is transmitted to DNA when it is replicated  Information from DNA is transmitted from DNA to RNA to proteins during phenotypic expression (transcription, translation) The dictionary of the genetic code  In the genetic code, nucleotide triplets specify amino acids.  Codon- a three nucleotide sequence in mRNA that specifies which amino acid will be added to a growing polypeptide or that signals termination Steps of Transcription  Transcription occurs in three stages o Initiation o Elongation o Termination Initiation  RNA polymerase binds directly or indirectly at regions of the DNA called promoters; transcription begins here o Promoters include the transcription start point and sequences before the start point as well. o The promoter determines which DNA strand will be used as a template o In prokaryotes, RNA polymerase binds directly to the promoter o In eukaryotes, other proteins (transcription factors) bind to the promoter first, followed by RNA polymerase Eukaryotic promoter  In eukaryotes, promoter is about 100 nucleotides long and contains: o 1. The initiation site o 2. Specific nucleotide sequences recognized by DNA- binding proteins called transcription factors that help bring RNA polymerase to the promoter  RNA polymerase II cannot recognize promoter unless specific transcription factors are bound first  3. TATA box: a short nucleotide sequence in the promoter that contains T and A, located about 25 nucleotides from the start point- o Specific transcription factors bind here o RNA polymerase recognizes the complex and binds The initiation of transcription at a Eukaryotic promoter  In eukaryotic cells, the enzyme that transcribes protein-coding genes into pre-mRNA is RNA polymerase II  This enzyme initiates RNA synthesis at promoters that commonly include TATA box, a nucleotide sequence typically something like TATAAAA  (A) Within the promoter, the TATA box is located about 25 nucleotides from the transcriptional start point  (B) RNA polymerase II cannot recognize the TATA box and other landmarks of the promoter on its own  Another protein, a transcription factor that recognizes the TATA box, binds to the DNA before the RNA polymerase can do so  (C) Additional transcription factors (purple) join the polymerase on the DNA  Protein-protein interaction are critical in the formation of the eukaryotic transcription initiation complex  The DNA double helix unwinds, and RNA synthesis begins at the start point on the template strand Elongation of the RNA strand  Once transcription begins the RNA polymerase moves along the DNA and performs two primary functions o In untwists and opens a short segment of DNA exposing about ten nucleotide bases. One of the exposed strands is template for base- pairing of nucleotides o It links incoming RNA nucleotides to the 3’ end of the elongating strand: thus RNA grows one nucleotide at a time in the 5’ to 3’ direction  During transcription mRNA grows about 30-60 nucleotides per second. As the mRNA strand elongates: o It peels away from its DNA template o The non-template strand of the DNA re-forms a DNA-DNA double helix by pairing with the template strand  Following in series, several molecules of RNA polymerase II can simultaneously transcribe the same gene o Cells thus can produce proteins in large amounts o The growing RNA strands hang free from each polymerase o The length of each strand varies and reflects how far the enzyme has travelled from the initiation site on template DNA Growth of a nucleic acid strand is always in the 5’ to 3’ direction  Unwinding (melting) of the DNA double helix  The enzyme which can unwind the double helix called helicase o Prokaryotic polymerases have the helicase activity, but eukaryotic polymerases do not o Unwinding of eukaryotic DNA is carried out by a specific transcription factor o Synthesis o RNA is based on the sequence of the DNA template strand o RNA polymerases use nucleoside triphosphate (NTPs) to contract an RNA strand Termination of transcription  Prokaryotes and eukaryotes use different signals to terminate transcription  Transcription proceeds until RNA polymerase transcribes a DNA sequence called a terminator. The transcribed terminator functions as the actually rumination signal o Additional proteins may cooperate with RNA polymerase in termination o The most common terminator in the sequence is AAUAAA  Eukaryotic genes contain a poly-A signal located downstream of the last exon  This signal is used to add a series of acetylate residues during RNA processing  Transcription often terminates at 0.5-2 kb downstream of the pol-A signal  The exact mechanism of termination is still unexplained Eukaryotic cells modify their RNA after transcription  RNA transcripts in eukaryotes are modified or processed, before laving the nucleus to yield functional mRNA. Eukaryotic RNA transcripts can be processed into two ways: o Covalent alteration of both the 3’and 5’ ends
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