Lecture 18 Flashcards
What are polysomes?
Polyribosomes (or polysomes) also known as ergosomes are a cluster of ribosomes, bound to a mRNA molecule. Many ribosomes read one mRNA simultaneously, progressing along the mRNA to synthesize the same protein
What is tRNA wobble pairing?
There are 64 possible combinations of 3 nucleotides to make 64 different codons. 3 of these are stop codons. Therefore, there should be 61 tRNA molecules, each pertaining to a specific codon. However, most organisms have around 50 tRNA molecules. Therefore, certain tRNA molecules must be specific to multiple codons. In this case, a tRNA molecule can either complentarily pair or uncomplementarily pair to a nucleotide as a “wobble” function to make up for a lack of specific tRNA molecules
What is aminoacyl tRNA synthetase?
Aminoacyl tRNA synthetase is an enzyme that simply attaches the appropriate, or “cognate,” amino acid onto the corresponding tRNA. This is sometimes called “charging” or “loading” the tRNA with the amino acid. Once the tRNA is charged, a ribosome can transfer the amino acid from the tRNA onto a growing peptide, according to the genetic code.
What is the Shine-Dalgarno sequence?
The Shine-Dalgarno (SD) sequence is a ribosomal binding site in prokaryotic mRNA, generally located around 8 bases upstream of the start codon AUG. The Shine-Dalgarno sequence exists both in bacteria and archaea, being also present in some chloroplast and mitochondrial transcripts. The six-base consensus sequence is AGGAGG. It helps recruit the ribosome to the mRNA to initiate protein synthesis by aligning it with the start codon.
What is the Iron Response Element (IRE)?
Iron response element or Iron-responsive element (IRE) is a short conserved stem-loop which is bound by iron response proteins. When iron concentration is low, IRPs bind the IRE in the ferritin mRNA and cause reduced translation rates. In contrast, binding to multiple IREs in the 3’ UTR of the transferrin receptor (involved in iron acquisition) leads to increased mRNA transcription rates.
Describe a eukaryotic mRNA strand prior to translation
1) The coding region, aka open reading frame (ORF), cistron, or coding sequence (CDS), begins with an initiation codon (AUG, for Met) and ends with a stop codon (any of the 3)
2) The coding region is flanked by non-coding regions, the 5’-UTR and 3’-UTR, which often contain regulatory signals
3) The mRNA termini are modified with a cap at the 5’ end and a polyA tail at the 3’ end
Describe initiation of translation
1) Binding of ribosomes to mRNA
2) Ribosomes locate initiation site
Describe elongation of translation
Moving stepwise along mRNA, codon by codon, in a 5’-to-3’ direction. The following events are repeated at each step:
1) Positioning correct AA-tRNA for each codon in the A site
2) Transferring the nascent chain from the peptidyl-tRNA in the P site to the alpha amino group of the AA on the AA-tRNA in the A site
3) Translocating by 1 codon in the 5’-to-3’ direction, thereby shifting the now-extended peptidyl-tRNA into the P site
Describe termination of translation
Hydrolyzing the completed polypeptide from its linkage to the tRNA that brought in the C-terminal AA (amino acid)
What is the central dogma of molecular biology?
DNA –> RNA –> Protein. Nucleic acids can convert between each other and to protein. In other words, DNA –> DNA, DNA –> RNA, RNA –> RNA, RNA –> DNA, RNA –> Protein, DNA –> Protein. However, Protein cannot convert to RNA, DNA, nor other protein.
Translation occurs in which direction?
5’ –> 3’ end of strands. N-terminus (amino) to C-terminus ends (carboxy)
What is the start codon?
AUG (Methionine)
What are the stop codons?
UAA, UAG, & UGA
What are five characteristics of the genetic code?
1) Co-linear triplet code
2) Nearly universal - variations in mitochondria, mycoplasma, & ciliates
3) Degenerate (or redundant)
4) Non-overlapping
5) Unpunctuated - although some codons are signals
What does a suppressor mutation do to a protein?
It suppresses the stop codon in the mRNA coding for a protein, making the protein longer
