Transcript Transcription - Santa Susana High School

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