BIOLOGY Lecture Notes - Golden Pages 2003

1. THE CELL

Ribosomes

Ribosomes are complex spherical particles composed of ribosomal Cell/22.html">RNA and Protein. There are two known types of high-molecular-weight rRNA, with sedimentation coefficients of 16-18 S and 22-28 S, respectively. The values of these constants depend on the source of the ribosomes. Higher constant values are characteristic of ribosomes from higher organisms. Each of these types of ribosomal RNA forms a complex with protein to yield a larger ribosomal subunit. In turn, these subunits join together in a 1:1 ratio to form a functionally active ribosome with a sedimentation coefficient of 80 S.

rRNAs are synthesized in the Cell Nucleus and nucleolus using a DNA template. The various types of rRNA differ from one another in both molecular weight and nucleotide composition, which is why they form complexes with different Proteins. Ribosomal proteins are synthesized in the Cytoplasm and then transported into the nucleoli, where ribosomal subunits spontaneously assemble by combining proteins with their respective rRNAs. Some ribosomal proteins perform catalytic Functions. The assembled ribosomal subunits are transported into the cytoplasm through the nuclear pores. Ribosomes are found in the cytoplasm either free or attached to the membranes of The Endoplasmic reticulum (ER). Specific receptor proteins exist for ER membrane-bound ribosomes. Protein Synthesis for export out of The Cell occurs on the ER membrane, whereas proteins required by the cell itself are synthesized on free ribosomes in the cytoplasm.