Transcription • Vocabulary of transcription:• transcription - synthesis of RNA under the direction of DNA • messenger RNA (mRNA) - carries genetic message from DNA to the

ribosome for the synthesis of protein • translation - synthesis of protein under the direction of mRNA • ribosome - site of protein synthesis (translation) • primary transcript - RNA that comes directly from the transcription of

DNA • codon - a triplet code of DNA (64 possible) that codes for an amino acid • reading frame - indicates the proper starting & ending points for the

transcription of DNA • amino acid (AA) - building block of proteins based on a 3-letter code of

DNA

Genetic Code• 64 codons for 20 amino acids

– Called a redundant code as different codons create the same amino acid• AUG start … Methionine• UAA, UAG, & UGA Stop

– Each amino acid abbreviated by the 1st 3 letters of the name

Transcription – Step 1• RNA polymerase II pries apart DNA

– reads the DNA in the 3'-5' direction (sense strand) beginning at the start codon AUG • binds to the DNA prior to the start codon in the promoter region

– TATA Box 5'-TATAAAA-3" (full sequence about a dozen nucleotides long) – used for the attachment of transcription factors that aid in the binding of RNA pol II

» transcription factors + RNA pol II is called the transcription unit

– assembles RNA in the 5'-3' direction • can be assembled by multiple RNA pol II enzymes

– transcription continues until a stop codon (terminator) is reached • stop codons are UAA, UAG, UGA • the pre-mRNA also continues a poly-A tail (AAUAAA) before it is cut free

from the growing strand • RNA pol II falls off of the DNA

Transcription – Step 2• pre-mRNA is now modified

– 5' receives a cap that consists of GTP and around 20-40 nucleotides

– 3' end receives a cap of A (50-250 nucleotides) to extend the poly-A tail • both help protect the mRNA from degradation • are considered NTR - nontranslated regions

– splicing occurs as the introns (non-coding regions) are excised leaving only the exons (coding regions • cut by snRNPs (small nuclear RNA) - specialized

RNA that remove the introns • spliceosome releases the introns and joins the

exon regions • introns may also be removed via self splicing in

some organisms • alternate splicing may give rise to different

proteins from the same transcriptional unit

• The finished length of RNA is now called the mRNA unit