Transcription

Recall: What is the Central Dogma of molecular genetics?

LE 17-7 Elongation Non-template strand of DNA RNA polymerase RNA nucleotides 3 end Newly made RNA Template strand of DNA Direction of transcription (“downstream”) Promoter 5 3 RNA polymerase Start point DNA Transcription unit Unwound DNA RNA tran- script Template strand of DNA Initiation Elongation Termination Rewound DNA RNA transcript 5 Completed RNA transcript Transcription is catalyzed by RNA polymerase DNA strands are separated RNA nucleotides are linked by RNA polymerase mRNA grows in the 5’  3’ direction Only 1 DNA strand serves as template U pairs with A

LE 17-7a-2 Promoter Transcription unit DNA Initiation Start point RNA polymerase Unwound DNA RNA tran- script Template strand of DNA Promoter approx. 100 nucleotides long upstream from gene have a lot of A’s and T’s in them—RNA polymerase recognizes these sequences DNA is unwound and separates into 2 strands RNA polymerases bind to DNA at promoter region

LE 17-7a-1 Promoter Transcription unit RNA polymerase Start point DNA nucleotide sequence on the template strand of DNA that is transcribed into a single RNA molecule by RNA polymerase includes initiation and termination sequences

LE 17-8 Promoter TATA box Start point Transcription factors Several transcription factors Additional transcription factors RNA polymerase II Transcription factors RNA transcript Transcription initiation complex Eukaryotic promoters Template DNA strand  ranscription factor: DNA binding proteins, bound specific DNA nucleotide sequence that help RNA polymerase recognize and bind the promoter  box short nucleotide sequence rich in Ts and As ~ 25 nucleotides upstream from the initiation site RNA polymerase II Recognizes complex between TATA transcription factor and the DNA binding site When RNA polymerase binds to promoter, the enzyme separates the two DNA strands

LE 17-7a-3 Promoter 5 3 Transcription unit 3 5 DNA Start point RNA polymerase Initiation Unwound DNA RNA tran- script Template strand of DNA Elongation Rewound DNA RNA transcript -mRNA is synthesized using one strand of the DNA -Template strand is 3’ -  5’ on the DNA strand -Therefore mRNA is made in the direction of 5’  3’ -RNA sequence is complementary to the template strand and identical to the coding strand

LE 17-7a-4 Promoter 3 5 Transcription unit DNA Initiation RNA polymerase Start point Template strand of DNA RNA tran- script Unwound DNA Elongation Rewound DNA RNA transcript Termination Completed RNA transcript -End of gene is reached when RNA polymerase reaches the terminator sequence--common sequence in eukaryotes is AAUAAA -mRNA comes off the DNA strand (disassociates)

LE 17-7b Elongation Non-template strand of DNA RNA polymerase RNA nucleotides 3 end Newly made RNA Template strand of DNA Direction of transcription (“downstream”)

LE Protein-coding segment 5 Start codon Stop codon Poly-A tail Polyadenylation signal 5 3 Cap UTR RNA processing—Capping and Tailing (pre-mRNA – transcript that will be processed to functional mRNA) Both the 5’ and 3’ ends are covalently modified 5’ cap - GTP is added – for protection - easy recognition for small ribosomal subunit - cap prevents mRNA from digestion 3’ end –poly-A polymerase adds a poly-A tail ( nucleotides) - may inhibit degradation of mRNA in cytoplasm

LE ExonIntronExonIntronExon 3 Pre-mRNA Coding segment Introns cut out and exons spliced together Cap Poly-A tail 5 3 UTR Split Genes and RNA splicing Introns – noncoding regions (initially transcribed but not translated) Excised from transcript before mRNA leaves the nucleus Exons – coding regions--linked together by RNA splicing

LE Exon 1 5 IntronExon 2 Other proteins Protein snRNA snRNPs RNA transcript (pre-mRNA) Spliceosome 5 components Cut-out intron mRNA Exon 1Exon 2 5 Small nuclear ribonucleoproteins (snRNPs) (complex of proteins and small nuclear RNAs found only in the nucleus) play role in RNA splicing Spliceosome – large molecular complex that catalyzes RNA splicing reactions

LE Gene Transcription RNA processing Translation Domain 2 Domain 3 Domain 1 Polypeptide Exon 1IntronExon 2IntronExon 3 DNA Function/Importance of Introns may control gene activity may help regulate the export of mRNA may allow a single gene to direct the synthesis of different proteins (if the same RNA transcript is processed differently i.e. One transcript remove all introns, in another leave introns and translate them

LE TRANSCRIPTION RNA PROCESSING RNA transcript 5 Exon NUCLEUS FORMATION OF INITIATION COMPLEX CYTOPLASM 3 DNA RNA polymerase RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase Amino acid tRNA AMINO ACID ACTIVATION 3 mRNA A P E Ribosomal subunits 5 Growing polypeptide E A Activated amino acid Anticodon TRANSLATION Codon Ribosome