1. Eukaryotic mRNA (messenger RNA) is transcribed in the nucleus as a precursor mRNA and is subsequently processed to form a mature mRNA that can be translated in the cytoplasm. Describe (make a schematic drawing) the different steps/processes that are required to produce a functional mRNA, ready to be translated, from a precursor mRNA.
The ends of eukaryotic mRNAs are modified during and after transcription through addition of a 5’ cap and a 3’ poly(A) tail to make the mRNA more stable and more easily recognized for e.g. splicing and translation. The different processing steps (capping, polyadenylation, and splicing) are shown below.
2. Describe briefly (no translation factors included) the different steps (initiation, elongation, termination) of translation. What roles do RNA molecules (apart from mRNA) play in the protein synthesis machinery?
Initiation: In prokaryotes, the 16S ribosomal RNA in the small subunit has a
sequence that is complementary to a short sequence (Shine-Dalgarno sequence)
in the mRNA just upstream of the translation start codon (AUG).This
“anchors” the ribosome to the translation start site and makes sure that the
translation starts at the right AUG. The first tRNA molecule, charged with a
formylated methionine (f-Met), binds to the AUG via its anticodon. The large
ribosomal subunit binds to the complex.
Eukaryotic translation is similar to
prokaryotic. However, there is no Shine-Dalgarno sequences in eukaryotic
mRNAs, instead the 5’
cap of the mRNA (see question 1.) is recognized by the ribosome, which then
"scans" the mRNA until it finds the first AUG and starts translating.
Elongation: The tRNA with an anticodon complementary to the next
codon in the mRNA sequence enters, carrying its amino acid. The first amino
acid is connected to the second. mRNA moves, leaving the A site empty for a
new charged tRNA. Upon binding of a new tRNA (next cycle) the first tRNA
leaves the E site.
Termination: A release factor (protein) binds to the A site/stop codon, causing
dissociation of the translation machinery and release of the newly
synthesized protein. The different "parts" (i.e.
ribosomal subunits, tRNAs, and mRNA) can be reused in translation of
another protein.
The ribosomal RNAs (rRNAs) perform
the actual catalytic function of the ribosome, namely the linking together of amino
acids to form the new protein, the ribosomal proteins are only speeding up
the process.
Transfer RNAs (tRNAs) function as adapter molecules between the nucleotides in the mRNA sequence and the amino acids. Each tRNA is charged with an amino acid and contains three nucleotides (anticodon) that are complementary to a mRNA codon, and can thereby bring each amino acid to the corresponding codon.
Translation (prokaryotes)
3. If you know
the amino acid sequence for a protein, is it then possible to tell the
nucleotide sequence for the gene that encodes it? Discuss your answer.
Since 18 out of the 20 amino acids are encoded by more than one codon (2-6
codons/amino acid), this would only be possible if the protein consisted of
only methionines (only encoded by ATG) and tryptophans (only encoded by TGG),
something which is not likely to
occur.