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.