A ribosome is a cellular structure that assembles proteins by linking together amino acids based on genetic instructions from messenger RNA (mRNA). Ribosomes are tiny, granular organelles found in both eukaryotic and prokaryotic cells. They are found inside the cytosol of the cell. Ribosomes are composed of two subunits, one larger and one smaller, each of which is made up of proteins and RNA molecules.
Characteristics of Ribosomes
Ribosomes a cellular structures that take part in protein synthesis in all living organisms. The characteristics of ribosomes are as follows:
- Ribosomes were first observed by George Palade (1953) under the electron microscope.
- Ribosomes are found in both prokaryotic and eukaryotic organisms.
- It is a kind of complex molecular machine that produces proteins from amino acids during a process of protein synthesis, also called translation.
- The ribosome translates the genetic information stored in messenger RNA into proteins.
- Ribosomes have specific binding sites for molecules carrying out protein synthesis.
- Ribosomes are found in two regions of the cell: scattered throughout the cytoplasm and attached to the endoplasmic reticulum in some cases, and form the rough endoplasmic reticulum.
- Prokaryotes possess 70S ribosomes, consisting of a small subunit (30S) and a large subunit (50S). Eukaryotes possess 80S ribosomes, with a small subunit (40S) and a large subunit (60S).
- They read the genetic code carried by messenger RNA (mRNA) and use it to assemble amino acids into a specific sequence, ultimately forming proteins.
Location of Ribosomes
Ribosomes are present in the cytosol or attached to the endoplasmic reticulum in both plant and animal cells. They play an important role in translating DNA into proteins.
- Some ribosomes are permanently associated with the rough endoplasmic reticulum; their association depends on the specific proteins they help to produce.
- In animal or human cells, there can be as many as 10 million ribosomes.
- Multiple ribosomes can be linked to the same mRNA strand, a structure known as a Polysome.
Structure of Ribosomes
The structure of the ribosome is described as follows:
- Ribosomes consist of both ribonucleic acid (RNA) and protein. The RNA component is called ribosomal RNA (rRNA), and the protein component consists of various ribosomal proteins.
- Ribosomes consist of two subunits – a small subunit and a large subunit, and these subunits work together for the process of protein synthesis.
- The small subunit reads the genetic information and binds to mRNA. The large subunit catalyses peptide bond formation and binds to the aminoacylated tRNAs.
- The ribosome has specific binding sites for different molecules involved in protein synthesis. These include: A (aminoacyl) site: The site where aminoacylated tRNA molecules are accepted. P (peptidyl) site: contains the tRNA which carries the growing peptide chain. E (exit) site: the site where deacylated tRNA molecules remain before leaving the ribosome.
- The ribosomes, when attached to the endoplasmic reticulum, are called the rough endoplasmic reticulum.
- Bound and free ribosomes are similar in structure, and they are involved in protein synthesis.
Functions of Ribosomes
Ribosomes have two principal functions, which involve decoding the messages and the formation of peptide bonds.
- Ribosomes participate in the creation of proteins, and the DNA makes RNA by DNA transcription.
- The mRNA is converted into proteins by the process of translation.
- The mRNA is organised in the nucleus and is moved to the cytoplasm for the process of protein synthesis.
- The ribosomal subunits in the cytoplasm are bound around mRNA polymers. The tRNA then integrates proteins.
- The proteins organised in the cytoplasm are used in the actual cytoplasm, and the proteins synthesised by bound ribosomes are moved to external cells.