Written by Josh Carl Vince B. Partosa

The human body consists of organs, each having specific functions. Cells are what make tissues, and tissues then form organs. Similarly, cells have organs of their own called organelles. An important organelle is ribosomes. It is important in the synthesis of protein, a molecule needed for growth and metabolism.

The ribosomes produce protein for both inside and outside of the cell. Without these important structures, metabolism and growth would come to a halt.

These structures assemble amino acids to form proteins. They read and follow the molecular instructions found in ribonucleic acid (RNA). This process is what you call translation.

Within the cell, there are two types of ribosomes. These are membrane-bound and free ribosomes.

You can find membrane-bound ribosomes on the rough endoplasmic reticulum (RER). The attached ribosomes are the reason for the RER’s rough appearance, hence, the name.

They are responsible for producing enzymes, a protein important in metabolism. They speed up chemical reactions in the human body. These reactions can be anabolic (grow and build) or catabolic (break down). One example of enzymes produced by bound ribosomes is digestive enzymes. These are the ones that aid in breaking down the food you eat.

To add, these bound structures also make proteins for cell membranes.

In contrast, free ribosomes are not attached to any structure. You can find them free-floating in the cytosol of the cell. They make proteins that constitute part of the cytoplasm which the cell uses.

To understand the importance of ribosomes, you should know the importance of proteins.

Proteins perform several biological functions. Besides growth and metabolism, they also have other roles.

Some proteins are hormones which are chemical messengers. Meanwhile, some transport or store nutrients. Protein (like keratin, collagen, and elastin) also provides structure. They also maintain fluid balance and proper pH across organ systems.

Furthermore, they keep the immune system healthy and are a form of energy source.

Together, these vital functions make protein an important biomolecule of the human body.

How is a ribosome made?

By now, you have learned that ribosomes synthesize protein. But what synthesizes these protein-producing organelles? How are they made? The answer lies within the nucleus. The nucleolus forms a complex of protein and ribosomal RNA (rRNA) into a ribosome.

The ribosomal structure is not bound by a membrane, unlike many organelles. They consist of 2 parts, the large and the small subunits. In eukaryotes, such as in the human body, these subunits contain about half protein and half rRNA.

In ribosome biogenesis, the part of the cell to look at is the nucleus and the cytoplasm.

Within the nucleus is the nucleolus. It is a specific region where the synthesis of ribosomes takes place.

The cytoplasm contains proteins. As discussed before, ribosomes make proteins that are used in and out of the cell. Some proteins present in the cell’s cytoplasm become part of ribosome biogenesis. You call these biomolecules ribosomal proteins.

These ribosomal proteins go to the nucleolus through the nuclear pores. They go to the nucleus because they contain nuclear localization signals(NLS). NLS is part of the protein’s sequence which mediates their transport to the nucleus.

Once the protein reaches the nucleolus, they combine with the ribosomal RNA. This is a type of RNA that combines with proteins to make the ribosome. The DNA sections of some chromosomes encode this.

Together, they assemble a ribosomal subunit. These units then leave the nucleus through the pores. They unite only for protein synthesis in the cytoplasm. When there is no production of protein, the pairs separate into individual units.

How does the genetic code get to a ribosome?

The previous information has established that ribosome makes protein under RNA instruction. This whole process involves 2 important major steps: transcription and translation.

This journey of the genetic code to the said organelle is complex and controlled. Transcription and translation comprise the process of gene expression.

Transcription occurs within the nucleus. This step involves the replication of a gene’s DNA sequence to make an RNA molecule. It occurs in 3 main stages: initiation, elongation, and termination.

1. Initiation occurs when the enzyme RNA polymerase binds to the promoter, a region of a gene. As a result, the DNA unwinds so the enzyme can ‘‘read’’ the bases in the template strand. This is one of the 2 strands of DNA.

2. Elongation is the part where the mRNA strand gets added with nucleotides. The enzyme RNA polymerase will then read the uncoiled strand of DNA. It then proceeds to create the messenger RNA (mRNA) using complementary base pairs.

The resulting mRNA strand is almost identical to the non-template strand. They differ in that RNA has Uracil instead of Thymine in its nucleotide sequence.

3. Termination is the last stage of transcription. It occurs when the RNA polymerase crosses a stop sequence in the gene. As the mRNA strand is complete, it detaches from DNA.

In eukaryotes, there are extra steps such as end modifications and splicing. End modifications increase the stability while splicing provides the correct sequence.

In transcription, the single-stranded genetic material made is mRNA. Its name describes its function. It delivers the message from the DNA in the nucleus into the cytoplasm.

Translation occurs in the cytoplasm. The mRNA exits the nucleus through its membrane’s pores and connects to the ribosome. Each codon or sequence of 3 nucleotides codes for an amino acid, the building block of protein.

Another RNA comes into the picture, and it is the transfer RNA (tRNA). The tRNA recognizes a complementary mRNA calling for its amino acid.

It then binds its anticodon to the codon. An anticodon is a trinucleotide sequence that complements the codon. The usual start codon which begins translation is the AUG sequence.

As the ribosome moves along the mRNA, the growing protein chain gets 1 new amino acid. Each time, this is accompanied by the release of the tRNA into the cytoplasm. This assembly of protein continues till the organelle meets a stop codon. It can be either of the 3 sequences: UAG, UAA, or UGA.

To aid in understanding this whole process, you can view the figure below.

Do ribosomes make DNA?

You have learned that ribosomes make protein. But what other substances do ribosomes make? Does it produce DNA? The answer to this question is “No.” The ribosomes do not make deoxyribonucleic acid.

Ribosomes make a protein or amino acid chains. The building blocks of DNA are nucleotides which make up nucleic acids. Although both amino acids and nucleic acids have “acid” in their names, they are not the same. They are different in structure and function.

Amino acids consist of carboxyl and amino groups. Nucleic acids have sugar, nitrogenous base, and a phosphate group. Amino acids form protein while nucleic acids are genetic material.

Because there is no protein in DNA, you can conclude that ribosomes do not make DNA.

If they do not make DNA, what does?

In DNA synthesis or replication, DNA copies itself. Rather than being “produced,” it is replicated. This is important as the new cell needs to be an accurate and precise replica. Otherwise, it might malfunction.

DNA replication for eukaryotes takes place in the nucleus. It occurs during the Synthesis (S) phase before mitosis and cell division. It is a set of very complex mechanisms.

This process requires enzymes like DNA polymerase, primase, helicase, and topoisomerase.

Recalling what you have read before, ribosomes make proteins. Enzymes are proteins. Thus, you can say that ribosomes are indirectly involved in DNA synthesis.

Do ribosomes make lipids?

Ribosomes do not make DNA. But do they make lipids? The answer to this is the same as the former question. No. They do not make lipids.

To reiterate, they make proteins. Moreover, proteins and lipids are also different in both structure and function. So, what organelle makes lipids?

It is the smooth endoplasmic reticulum (ER). It plays a big role in the synthesis of lipids using enzymes. It produces phospholipids and cholesterol that form the membranes.

The smooth ER also plays a role in steroid hormone production and detoxification.

Are ribosomes eukaryotic or prokaryotic?

Eukaryotes and Prokaryotes have characteristics that make them unique from each other. Are ribosomes found only in prokaryotes or eukaryotes? Both of these types of cells contain ribosomes!

This organelle is in both prokaryotes and eukaryotes. This affirms that the ribosome is a characteristic that evolved early on. It is most likely present in the common progenitor of both cell types.

Although the organelle is present in both, they have their fair share of differences.

By this time, you know that the nucleolus is the site for eukaryotic ribosome biogenesis. In the case of prokaryotes, biogenesis takes place in the cytoplasm.

Prokaryotes have 70S ribosomes while eukaryotes have larger 80S ones. The S refers to the Svedberg unit, a unit for sedimentation rate. This rate is a measure of the time it will take for a particle to sediment from a solution when centrifuged.

Do viruses have ribosomes?

You can find ribosomes in the cells of living organisms. A virus is something that is not alive. It does not have a cell. Thus, it having those organelles would be unlikely. Viruses do not have ribosomes.

Due to their abnormal reproduction capabilities, you may think they are alive. But they are not. They are parasites in the sense that they hijack cells for reproduction.

Without a host cell, they cannot reproduce. Because they lack ribosomes, they cannot make proteins. Hence, they must use the ribosomes of their host cell to translate viral mRNA into viral proteins.

Despite being alive, it is interesting to note that they contain nucleic acids. This may either be DNA or RNA. Nucleic acids, if you recall, are genetic material found in the cell.

Many speculate that viruses are a form of proto life–that they preceded cellular life. But their inability to survive without a host makes it very unlikely to be the case. Some scientists say that viruses began as segments that became parasitic.

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