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The ribosome
Learning objectives: understand the following concepts
Overview about ribosomes
Ribosomes: molecular machines made of __ and ribosomal ___.
Physiological role: synthesizing ___ from amino acids.
The number of ribosomes in a cell depends on the activity of the cell.
Ribosomes are freely suspended in the cytoplasm or attached to the
endoplasmic reticulum forming the ___ endoplasmic reticulum.
Both prokaryotes and eukaryotes contain multiple copies of rRNA genes with strong promoters.
On average, in a mammalian cell, there can be about 10 million ribosomes.
Additional overview of ribosomes
• The ribosome is responsible for the final step of translating genetic codes into functional ___.
• This molecular machine consists of two universally conserved subunits, the ___ subunit (__) and the ___ subunit (__), both of which are composed of rRNAs and ribosomal proteins.
• In eukaryotes, the mature subunits are the ___ (SSU) and the __ (LSU), whereas in prokaryotes they are called __ (SSU) and ___ (LSU)
• The biogenesis of ribosomes is a central cellular event that consumes significant amounts of metabolites and ___.
• In addition to rRNA and r-protein components, the eukaryotic ribosomal subunit assembly requires >350 ___-ribosomal factors.
• In eukaryotes, both the 40S and the 60S subunits are produced from a single pre-ribosomal particle known as the __ that is assembled in the ___.
• The mature eukaryotic 80S ribosomes contain -5500 nucleotides of rRNA (SSU has 18S rRNA, while LSU has 5S, 5.8S, and 25S rRNA) and more than 80 proteins.
**Ratio = __ rRNA and __ proteins
Slide showing composition of prokaryotic and eukaryotic ribosomes with molecular mass, number of nucleotides
and proteins
Ribosome assembly in prokaryotes
• Binding of ribosomal proteins to the __ rRNA and RNase __ and __ digestion
Additional ribosomal protein binding and maturation to ___
• Assembly of the 50S subunit follows a similar principle.
Note that the pre-ribosomal RNA is transcribed from the ___ gene.
Overview of eukaryotic ribosome synthesis and maturation
Co-transcriptional binding of early trans-acting factors to the 35S rRNA forms the
__ pre-ribosome.
Cleavage at the __ site separates the pre-60S subunit (blue) and the pre-40S subunit (green), which undergo independent maturation
pathways in the nucleoplasm and, eventually, in the cytoplasm.
Final maturation in the ___ allows the joining of both subunits to form the functional __ ribosome.
Export of pre-ribosomes through the NPC
(Nuclear Pore Complex) Channel
All transport events between the nucleus and cytoplasm occur through the __ ___ ___ (__), which are composed of approximately __ proteins.
The Ribosome is a ribozyme
• The ribosome is __-__ rRNA and __-__ ribosomal protein
• rRNA is considered to be responsible for the overall ___, for positioning tRNAs on mRNA, and for the formation of ___ bonds between amino acids.
• The r-proteins are generally found on the surface and fill the __ and __ to stabilize the RNA __.
• The ___ provides the framework on which the tRNAs are accurately matched to the codons of mRNA, while the __ catalyzes the formation of peptide bonds.
• The SSU and the LSU are ___when not actively synthesizing proteins, and they join on an mRNA molecule.
• To carryout coordinated movements required for efficient translation, a ribosome contains __ RNA binding sites one for mRNA and three for tRNAs (the A site, the P site, and the E site).
Prokaryotes - rRNA forms skeleton of ribosomes through secondary and tertiary structures
Note that __-___ within each rRNA leads to the formation of secondary
and tertiary structures that serve as the skeleton for r-protein binding and
ribosome formation
The 30S SSU composition of PROKARYOTES
Interface view (mainly __):
-16S rRNA colored gray
-SSU ribosomal proteins colored distinctly
and labeled (S plus a numeral)
Solvent view:
-Note that the SOLVENT surface is occupied mainly by ___
-SSU ribosomal proteins colored distinctly and labeled (S plus a numeral)
The 50S LSU composition of PROKARYOTES
Interface view:
-Note that the interface is mainly
___ (gray)
Solvent view:
-The solvent surface is mainly
___ (colored)
-LSU proteins are labeled as L plus a numeral
Eukaryotes - rRNA forms skeleton of ribosomes through secondary and tertiary structures
Note that __ or __ and ___ base-pair to form secondary and tertiary structures that serve as the skeleton for r-protein binding and ribosome
formation
The 40S SSU composition of EUKARYOTES
Interface surface is mainly occupied by the 18S ___ (gray) with some
colored proteins
The solvent surface is occupied mainly by ___ colored distinctively and labeled (S plus numeral).
e in the label refers to eukaryotic specific proteins while the rest are common between prokaryotes and eukaryotes
The 60S LSU composition of EUKARYOTES
Interface surface is mainly occupied by the 28S ___ (gray) with some
colored proteins
The solvent surface is occupied mainly by ___ colored distinctively and labeled (L plus numeral).
e in the label refers to eukaryotic specific proteins while the rest are common between prokaryotes and eukaryotes.
