SBI 4U1
Protein Synthesis Part 1: TRANSCRIPTION
Transcription
 The first step required for gene expression
 Takes place in the nucleus
 Products:
o mRNA (usually leads to a polyeptide gene product)
o tRNA (transfers amino acids to ribosomes)
o rRNA (assembles amino acids into a polypeptide chain in ribosomes)
 enzyme involved
o RNA polymerase (speeds formation of RNA from a DNA template)
SBI 4U1
Steps in Transcription
Its binding site only
recognizes the promoter
region
Consists of a characteristic base-pair
pattern, one that is high in A-T bases
Coding strand
Terminator sequence
Posttranscriptional Modifications (ONLY in Eukaryotes)Primary transcript
SBI 4U1
 Primary transcript needs to be modified (not ready to leave the nucleus)through:
 a process called capping and tailing
o A 5’cap is added to the start of the primary transcript.
o Poly-A tail is added to the 3’end by an enzyme: poly-A polymerase.
 a process called introns excision
o the introns are removed from the primary transcript by spliceosomes.
 mRNA exits the nucleus, but the spliced-out introns stay inside the nucleus
(they are degraded and their nucleotides are recycled)
 mRNA transcript can now be translated by a ribosome into a protein.
About Errors during Transcription
 More errors are made during this process b/c there is no quality control enzyme
to ensure that mRNA transcript is correct
 The errors are not as detrimental to the cell as those that occur during DNA
replication b/c only one gene is transcribed repeatedly to produce hundreds of
transcripts of the same gene.
 If an error is made, the protein is susceptible to degradation once it is
synthesized.
SBI 4U1
 The correct copies of the mRNA transcript will produce adequate amounts of
the required protein
Terms that You Should Know
promoter
upstream
template
strand
coding strand
terminator
sequence
primary
transcript
5’cap
 sequence of DNA that binds RNA polymerase upstream a gene
 region of DNA adjacent to the start of a gene
 the strand of DNA which is used as a guide by RNA
polymerase to build complementary mRNA
 the strand of DNA that is not used for transcription
 sequence of bases at the end of a gene that signals the RNA
polymerase to stop transcribing
 mRNA that has to be modified before exiting the nucleus in
eukaryotic cells
 7-methyl guanosine added to the start of an primary transcript
to protect it from digestion in the cytoplasm and to bind it to the
ribosome as part of the initiation of translation
poly-A
polymerase
Poly-A tail
 Enzyme that adds a string of adenine bases to the end of mRNA
to protect it from degradation later on.
 A string of 200-300 adenine base pairs at the end of an mRNA
transcript
 Particles made of protein and RNA that cut introns from mRNA
primary transcript and joins together the remaining exons
 mRNA that has been modified for exit out the nucleus into the
cytoplasm.
spliceosomes
mRNA
transcript