Covers protein synthesis, from the building blocks of proteins and the roles of ribosomes to the path of nascent chains through the rough and smooth endoplasmic reticulum and the Golgi apparatus, then synthesis for other locations. Proteasomes follow: their function, and the ubiquitin-proteasome degradation mechanism step by step, including the ring structure and ATPase-driven unfolding. Includes four pictures and a note on ER-associated degradation.
Topic 27 of 97 in the MU Sofia syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 3 sections, about 922 words (6 min of reading), with 14 flashcards.
In this note
Protein synthesis. Proteasomes.
Protein Synthesis
Proteasomes
Protein Synthesis
Definition & Basic Components:
Proteins are fundamental molecules of living organisms, acting as enzymes, blood components, and muscle tissue, among other roles.
They are polymers of linked amino acid monomers joined by peptide bonds. There are about 20 different amino acids that naturally occur in proteins.
The base elements of proteins are Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).
Protein synthesis is the process by which cells create new proteins. It involves DNA Transcription and Translation.
The nucleus houses the genetic material (DNA) that directs protein synthesis and also contains the molecular machinery to synthesize and process all types of RNA.
RNA is an essential carrier molecule for amino acids used in protein synthesis. Restricting protein synthesis to the cytoplasm helps ensure that newly made RNA molecules do not become involved in translation before processing is complete.
The Role of Ribosomes:
Ribosomes are the primary sites of biological protein synthesis.
They link amino acids together in the order specified by messenger RNA (mRNA) molecules.
Ribosomes consist of two major components: a small ribosomal subunit (which reads RNA) and a large subunit (which joins amino acids to form a polypeptide chain). Both subunits are complexes of rRNA and many proteins.
In eukaryotic cells, rRNA molecules are synthesized within the nucleus, and their proteins are synthesized in the cytoplasm before entering the nucleus to associate with rRNAs. The assembled subunits then leave the nucleus and enter the cytoplasm for protein synthesis.
Typically, many ribosomes occupy a single mRNA to form polyribosomes (or polysomes).
Ribosomes assemble the protein chain from amino acids transported by transfer RNA (tRNA). This process ensures the correct translation of the nucleic acid sequence into an amino acid sequence.
Cytoplasmic regions with an abundance of polyribosomes stain intensely with hematoxylin and basic dyes, indicating active protein synthesis.
The Role of the Endoplasmic Reticulum (ER):
The ER is a convoluted membranous network extending from the nucleus to the cell membrane, enclosing channels and sacs called cisternae. It is a major site for biosynthesis of proteins and lipids.
Rough Endoplasmic Reticulum (RER):
Characterised by ribosomes (polyribosomes) attached to its cytosolic surface.
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