Transcript Polypeptide Synthesis - Fairfax Senior High School

From Genes to Proteins
Transcription in prokaryotes
 Simple process
 No nucleus; ribosomes lie close to the DNA in
cytoplasm
 1st part of the mRNA attaches directly to ribosomes
 The rest is being transcribed
 Assembly of aa into proteins begins right away
Transcription in eukaryotes
 Transcription MUCH MORE complex!!!!
 DNA wound around protein spools
 Before transcription happens…
 Spools unwound
 Enzyme cannot begin process-able to attach to start
signal region(replication fork)
 Once new mRNA is transcribed from DNA template
mRNA breaks into pieces. Why?
Eukaryotic transcription
 mRNA reassembles to form finished mRNA
 Portions of the original is left out.
 Finished strand is shorter
 Why does the original mRNA molecule break up into
pieces?
 What is discarded when the pieces are spliced back
together?
What is happening?
What next?
 During mRNA processing
 Introns are cut out and exons are spliced together
 Events occur in the cell
 Introns: intervening sets of nucleotides (junk)
 Exons: code for aa, because they are expressed
 Once mRNA is processed, where do it go?
 What happens to the mRNA molecule after processing
is complete?
Translation
 Removing introns from mRNA is known as mRNA
processing.
 During Translation-processed mRNA attaches to free
ribosomes or attached ribosomes on RER
 Polypeptide synthesis begins
 mRNA carries genetic info in sequence of codons
 Particular order of of aa
Protein synthesis
 To manufacture protein requires deciphering the info
and assembling in an particular order
 Synthesis of a polypeptide chain (small protein) from
info carried by mRNA is called TRANSLATION!!!!!
 How is mRNA “language” translated into protein
“language”
20 amino acids/64 possible
 Combinations
 Job bringing aa to ribosomes is performed by tRNA
 2 parts[ anticodon & aa]
 Anticodon-a set of 3 bases that is specific to the
type of aa the tRNA carries.
 Each have a different anticodon
 Binds ONLY to the complementary mRNA codon
Codons & anticodons joining
mRNA codons  amino acids
Joining codons & anticodons
 Depends on proper translation
 Ribosome attaches to 1st codon on mRNA strand
 tRNA approaches carrying aa
 If tRNA recognizes the 1st mRNA codon, 2 join
 tRNA w/aa moves along mRNA strand, exposing the
next mRNA codon
 Again, proper tRNA w/its aa joins mRNA strand
Forming a polypeptide
 Once 1st two aa are in place, 1st tRNA is then released
 Leaving it free to pick up another aa
 Process continues-ribosomes moves along mRNA
 Polypeptide chain grows
 Aa added to a chain at 15/s
 One of 3 codons stop the translation process
 No tRNA to match up
 Translation stops polypeptide complete
 Breaks away from the assembly line
summary
 DNA stores instructions for polypeptide synthesis
 The DNA instructions transcribed mRNA, carrying
info from the nucleus into cytoplasm
 Ribosomes & tRNA interact w/mRNA to translate the
info & build polypeptides
proteins
 They are made to order
 Synthesized when needed, in quantity needed
 Only certain genes are expressed at one time
 Kinds of proteins synthesized by a cell differentiate it
from other kinds of cell in an organism
 Protein in a muscle cell is different in nerve cell. Get
it?