Ribosomes

Cells need proteins to speed up biological processes. Cells rely on ribosomes to make proteins, which is why ribosomes are called protein builders or protein synthesizers. They connect one amino acid at a time. As you know, the amino acid is the building block of protein.

What are ribosomes?

They are a complex cellular mechanism, a cell organelle used to translate the genetic code into amino acid chains. Amino acids, long-chain, serve as a protein in the cells. In other words, they are micro machine which is responsible for making proteins. They are special because they exist in both eukaryotes and prokaryotes. (1, 2, 3, and 4)

Ribosomes Structure

Ribosomes are organelles and they are found in the human cell, animals, and plant cells. They are located in the cell’s cytosol while others are free-floating to the membrane of the coarse endoplasmic reticulum. Ribosomes are primarily made up of two structures:

• Ribonucleic acid (RNA)
• Proteins

Both proteins and RNA components are nearly equal in number. A prokaryotic cell has thousands of ribosomes. Eukaryotic cells such as in human have millions of ribosomes. The ribosomes in eukaryotic cells are bigger when compared with prokaryotic.

Ribosomes have large and small subunits and they come together around the mRNA molecule during the translation phase. Each subunit contains proteins and RNA or ribosomal RNA, which varies in terms of lengths. They are also surrounded by proteins that form ribosomes. The ribosomal RNAs secure both mRNA and tRNA thereby speeding up the formation of peptide bonds between amino acids. Ribosomes small subunit help hold mRNA in place during the translation of ribosome into protein. As with the large subunit, there are different sites involves various parts of the protein synthesis process. (3, 4, 5, and 6)

Let us take a look at the subunits in detail:

1. Small ribosomal subunit – It consists of one ribosomal RNA (rRNA) and about 21 proteins (prokaryotes) and about 33 proteins (eukaryotes).
2. Large ribosomal subunit
   1. Prokaryotes (bacteria) – They have two ribosomal RNAs; one small and one large and a total of about 31 proteins.
   2. Eukaryotes – They have three ribosomal RNAs; one large and two small and about 49 proteins. However, before the subunits are used they need to be synthesized in the nucleolus first and exported to the cytoplasm. (5, 6, and 7)

Where are ribosomes found?

Every single cell contains ribosomes including prokaryotic and eukaryotic cells. Every plant, animal, and human contains millions of ribosomes. Their specific location is in the cytoplasm although some of them are attached to the endoplasmic reticulum.

Types of ribosomes

• Free-floating ribosomes – They are ribosomes that float freely in the cytoplasm. They do not attach themselves to other organelles. Such ribosomes make structural proteins and are present in a cluster of 5 or 10 in rapidly growing cells such as the ones found in the pancreas and the brain. Free-floating ribosomes manufacture proteins that are needed to create the cytoplasm.
• Ribosomes found in the endoplasmic reticulum/membrane-bound ribosomes – The purpose of these ribosomes is to produce various types of enzymes in the body. They also manufacture proteins needed to create cell membranes. Endoplasmic reticulum that contains ribosomes are called rough endoplasmic reticulum. The one that does not contain ribosomes is called smooth endoplasmic reticulum. (6, 7, 8, 9, and 10)

Ribosome synthesis

The synthesis of the ribosome is a complex process and requires coordination between gene-encoding ribosomal proteins and ribosomal RNAs. There is an orderly pathway that needs to be followed. In eukaryotic cells, the assembly and synthesis of rRNA take place in the nucleolus; a structure inside the nucleus.

The synthesis of protein through ribosomes has three steps:

1. Initiation – Initiation factor proteins deliver messenger RNAs to the small subunit. The first tRNA line sup and guide its association with the large subunit. It lines up the messenger RNA in the proper place and prepares it for special initiator tRNA.
2. Elongation – Once the ribosomes are set up, they start to read along the messenger RNA strand resulting in the building of protein one amino acid at a time.
3. Termination – During the synthesis phase, the ribosome meets a stop codon instructing it to wrap up the process of creating protein. Release factor proteins will force the ribosome to release the finished protein. (10, 11, 12, and 13)

What is the function of ribosomes?

• Protein production – The primary function of ribosomes is to produce proteins. Proteins are essential in various cellular functions. They are helpful through the following ways:
  • They serve as a catalyst which speeds up the reaction time.
  • They serve as fibers which provide support.
  • Some proteins perform special task like muscle contraction in cells.
  • Recognize sequence in the DNA and bind them with the aid of other proteins.
• Machinery – Ribosomes act as machinery that connects different parts.
• Assembly line – Ribosomes act as an assembly line that read mRNA and finds the corresponding tRNA. They also attach amino acids to the binding site.
• Translation –Ribosomes primary role has something to do with translation; creating a whole range of proteins needed by the organism, especially in converting enzymes to hormones to parts of cells and muscles. (3, 5, 9, and 12)

What will happen if there are no ribosomes?

Ribosomes are significant for without them various vital bodily processes will not take place. Without ribosomes, no proteins will be created. Enzymes won’t be able to carry out chemical reactions, hormones won’t be able to transmit signals, and antibodies will not be effective in protecting the body against foreign invaders such as bacteria and viruses. (12 and 13)

Do prokaryotes have ribosomes?

Prokaryotes have ribosomes. As a matter of fact, all cells have ribosomes, especially those going to live for very long and need to synthesize proteins. Prokaryotes or prokaryotic cells lack a nucleus and organelles such as mitochondria and endoplasmic reticulum. An example of a prokaryotic cell is bacteria. (2, 13)

Quick facts about ribosomes

• The word “rib” in ribosome is derived from RNA (ribonucleic acid) that contains the instructions on creating proteins.
• All living cells have ribosomes.
• Ribosomes are created inside the nucleolus of the nucleus and are sent outside the nucleus once they are ready.
• Ribosomes are attached to the rough endoplasmic reticulum (ER) and float freely in the cell’s cytosol.
• Ribosomes differ from other organelles because they are not surrounded by a protective membrane.
• It was in 1974 when ribosomes were discovered by Albert Claude, George Emil Palade, and Christina de Duve.
• The people who discover ribosomes won a Nobel Prize.
• The basic function of ribosomes is to synthesize proteins in the cells.
• The size of the ribosome is somewhere between 30 and 30 nanometers.
• The absence of ribosomes results in the absence of protein in the body.
• The manufacturing of protein takes place through the process called translation.
• Ribosomes read and translate information encoded in the messenger RNA. Such type of RNAs contains the instructions template produced by the DNA. The process of translation is needed for the production of protein.
• The unit of the ribosome is based on its Svedberg value/S value, which is based on the sedimentation rate in the centrifuge.
• Ribosomes in eukaryotic cells have a Svedberg value of 80, which corresponds to 40s and 60s subunits.
• Ribosomes in prokaryotic cells have 70 Svedberg value, which corresponds to 30s and 50s subunit.
• The Svedberg unit is not additive. The sedimentation rate of the molecule primarily depends on the size and shape and not mainly on the molecular weight.
• Ribosomes are ribozymes because they use RNA for their reaction and not mainly the protein.

A ribosome may be one of the smallest structures in the cell but it is one of the most important parts of the cell. It is an organelle responsible for the production of protein. Without protein, it would be impossible for every living organism to function properly. (1, 4, 9, 14, and 15)