Download Translation

Transcription and Translation
Transcription
• the process by which chemical information
encoded in DNA is copied into mRNA
(messenger RNA)
• transcription is classically described in three
distinct steps: initiation, elongation and
termination.
• Initiation
– an enzyme binds to specific sequence of
bases on a DNA strand called the promoter
region and begins to break H+ of the
complementary bases, unwinding the DNA.
• a segment of DNA of around 17 base pairs
stays unwound throughout the
transcription process.
• Elongation
– an enzyme helps free floating ribonucleotides add
to the unwound strand
OH
OH
OH
– each ribonucleotide is inserted into the
growing RNA strand following the rules of
base pairing
• for each C encountered on the DNA strand, a G
is inserted in the RNA
• for each G, a C
• for each T, an A
• however, each A on the DNA guides the insertion of
the pyrimidine uracil, U instead of A
• Termination
– when the enzyme reaches a specific sequence
of nucleotides, it and its newly transcripted
RNA are released from the DNA.
• finally, two major processes are done to the
new mRNA strand before it's ready to be used
to make protein:
– introns are regions of mRNA that do not
code for proteins and must be removed by
spliceosomes leaving only exons or coding
regions that code for proteins
• this is now considered mature mRNA
– a cap and tail are added to the ends for
stability/protection
Splicing Out Introns Movie
Translation
• process of converting the information in
mRNA into a protein.
• 3 main parts- Initiation, Elongation,
Termination
University of Colorado Translation Movie
1. Initiation
• an initiation complex is formed when:
– small subunit of the ribosome binds to a site
on the mRNA
– it proceeds “downstream” until it encounters
the start codon AUG.
– here it is joined by the large ribosomal
subunit and a special initiator tRNA.
– in eukaryotes (cells with a nucleus), the
initiator tRNA with the anticodon UAC
carries the amino acid methionine (Met) that
binds to the first site on the large ribosomal
unit
2. Elongation
• the process of adding more a.a. to the protein
– another tRNA covalently bonded with its
amino acid base pairs with the next codon on
the mRNA at the next site on the ribosome
– the preceding amino acid methionine (Met)
is then bonded to the incoming amino acid
– the first tRNA is released and the ribosome
moves one codon downstream
3. Termination
• the point where the translating ribosome reaches a
stop codon
– stop codons are UAA, UAG, and UGA because
none of the tRNAs have anticodons that are
complimentary to these three codons
– instead, a protein binds to the mRNA and releases
the polypeptide from the ribosome which splits
back into its two subunits
McGraw Hill Movies.