Cytology · MU Pleven

Topic 17. Non-membranous cell organelles – ribosomes, polyribosomes

Explains ribosomes as the sites of protein synthesis, covering their composition of rRNA and proteins, the 80S and 70S forms, and how free and bound ribosomes produce different protein destinations. It also describes how ribosomal subunits are made in the nucleolus, polyribosome formation on mRNA, and the stages of transcription and translation.

Inside: 2 sections · 2 labelled pictures · 14 flashcards · 4 min

Topic 17 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 2 sections, about 571 words (4 min of reading), with 14 flashcards.

In this note

  1. TOPIC 17. Non-membranous cell organelles – ribosomes, polyribosomes
  2. Protein synthesis – transcription and translation

TOPIC 17. Non-membranous cell organelles – ribosomes, polyribosomes

Ribosomes (polyribosomes – intensely basophilic)

Ribosomes are small cell organelles which are responsible for protein synthesis. They only work when attached to an mRNA molecule/sequence and help to catalyse protein translation.

Ribosomes are composed of 4 segments of rRNA (65%) and 80 different proteins (35%).

Found in cytoplasm of eukaryotes (80S) and prokaryotes (70S) Ribosomes are composed of two subunits in eukaryotes, the 80s unit and the 40s unit which join together when attached to mRNA.

The small ribosomal subunit reads the mRNA, and the large subunit joins amino acids delivered by the tRNA to form a polypeptide chain.

Each subunit is composed of one or more ribosomal RNA (rRNA) molecules and a variety of ribosomal proteins. They are held together by Mg²⁺ ions. Ribosomal subunits are synthesised by the nucleolus. The ribosomes work with tRNA and mRNA to translate protein coding genes into proteins. The large and small subunits of ribosomes come together by binding an mRNA strand and normally, many ribosomes occupy a single mRNA strand, to form polyribosomes

Two types of ribosomes in cell, they can be free or bound to the RER:

Free ribosomes

produce proteins that stay within the cell. Free ribosomes usually synthesise cytosolic proteins and cytoskeletal proteins and proteins for import into the nucleus, mitochondria and peroxisomes.

Bound ribosomes

produce proteins which are exported to the outside of the cell. Bound ribosomes produce proteins that are to be incorporated into membranes, stored in lysosomes or eventually excreted from the cell by exocytosis are made of polyribosomes attached to the RER. The proteins produced by these ribosomes are separated during translation into the interior of the RER’s membrane cisternae.

Cells which have high rates of protein synthesis have many ribosomes and a prominent nucleolus.

Ribosome synthesis:

  1. 1. rRNA molecules of both subunits are synthesised in the nucleus. The ribosomal proteins are synthesised in the cytoplasm, but then enter the nucleus and associate with the rRNAs assembling the large and small ribosomal subunits which then leave the nucleus into the cytoplasm, ready for action in translation.
  2. 2. The compact core of each ribosome contains the rRNA molecules that provide structural support to the ribosome and help position the tRNAs

in the correct reading frame

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