gene expression: from gene to protein - Coggle Diagram
gene expression: from gene to protein
the synthesis of RNA using a DNA template
initiation this occurs in the beginning of transcription. it occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. this signals the DNA to unwind so the enzyme can "read" the bases in one of the DNA strands. the enzyme is now ready to make a strand of mRNA with a complementary sequence of bases
elongation is the addition of nucelotides to the mRNA strand. RNA polymerase reads the unwound DNA strand and builds the mRNA molecule, using complementary base pairs. there is a brief time during this process when the newly formed RNA is bound to the unwound DNA. during this process, an adenine in the DNa binds to an uracil in the RNA
termination is the ending of transcription, and occurs when RNA polymerase crosses a stop sequence in the gene. the mRNA strand is complete, and it detaches from DNA
17.8 this general depiction of transcription applies to both bacteria and eukaryotes, but the details of termination differ. in a bacterium, the RNA transcript is immediately usable as mRNA, in a eukaryote, the RNA transcript must first undergo processing.
17.9 the initiation of transcription at a eukaryotic promoter. in eukaryotic cells, proteins called transcription factors mediate the initiation of transcription by RNA polymerase II
17.10 RNA polymerase moves along the DNA template strand, joining complementary RNA nucleotides to the 3' end of the growing RNA transcript. behind the polymerase, the new RNA peels away from the template strand, which re-forms a double helix with the nontemplate strand
the synthesis of a polypeptide using the genetic information encoded in an mRNA molecule. there is a change of "language" from nucleotides to amino acids
initiation the ribosomes assembles around the target mRNA. the first tRNA is attached at the start codon
elongation the tRNA transfers an amino acid to the tRNA corresponding to the next codon. the ribosome then moves to the mRNA codon to continue the process, creating an amino acid chain
termination when a peptidyl tRNA encounters a stop codon, then the ribsome folds the polypeptide into its final structure
17.19 the first components to associate with each other during initiation stage of translation are mRNA, a tRNA bearing the first amino acid of the polypeptide, and the same ribosomal subunit
17.20 in the elongation stage of translation, amino acids are added one by one to the previous amino acid at the c-terminus of the growing chain. each addition involves several proteins called elongation factors and occurs in the three-step cycle described in this figure
after synthesis of a eukaryotic primary RNA transcript, the removal of portions of the transcript/introns that will not be included in the mRNA and the joining together of the remaining portions/exons
figure 17.12 shows where large portions of the RNA molecules are removed and the remaining portions are reconnected.
most eukaryotic genes are spilt into segments: they have introns interspersed among the exons (the regions included in the mRNA).
in RNA splicing, introns are removed and exons joined. RNA splicing is typically carried out by spliceosomes, but in some cases, RNA alone catalyzes its owns splicing.
structure of tRNA
the tRNA molecule has a distinctive folding structure with three hairpin loops that form the shape of a three-leafed clover.
one of these hairpin loops contains a sequence called anticodon, which can recognize and decode an mRNA codon
each tRNA has its corresponding amino acid attached to its end
function of signal recognition peptide
signal peptides function to prompt a cell to translocate the protein, usually to the cellular membrane.
in prokaryotes, signal peptides direct the newly synthesized protein to the secYEG protein-conducting channel, which is present in the plasma membrane
one role is to function as the usual signal peptide to translocate the protein downstream of the internal signal peptide
the other role is to function as a stop-transfer signal to create a transmembrane protein with the second signal peptide anchoring the protein in the mebrane
a cluster of ribosome linked together by a molecule of messenger of RNA and forming the site of protein synthesis