Ribosomes are the protein-synthesizing machines of the cell.

external image ch2_ribosome_proteinbig.jpg
Ribosomes are responsible for assembling the proteins of the cell. Ribosomal subunits are synthesized by the nucleolus. Depending on the protein production level of a particular cell, ribosomes may number in the millions.

external image ribo.gifWhat are their distinguishing characteristics?

Ribosomes are typically composed of two subunits: a large subunit and a small subunit. These two units join together when the ribosome attaches to messenger RNA to produce a protein in the cytoplasm (cyto-).

external image riboboth.gif
Small and large ribosome subunits, Image courtesy of The Virtual Cell. Ribosome attached to endoplasmic reticulum, Image courtesy of The Virtual Cell.
Ribosome Structure and Function
Ribosome Structure and Function

All living things contain ribosomes and is made 60% of Ribosomal RNA and 40% protein

Role of the Ribosome

The route from the DNA code to the protein.

DNA-RNA.jpg (25692 bytes)
DNA-RNA.jpg (25692 bytes)
Before cell division, the DNA in our chromosomes replicates so each daughter cell has an identical set of chromosome. In addition, the DNA is responsible for coding for all proteins. Each amino acid is designated by one or more set of triplet nucleotides. The code is produced from one strand of the DNA by a process called "transcription". This produces mRNA which then is sent out of the nucleus where the message is translated into proteins. This can be done in the cytoplasm on clusters of ribosomes, called "polyribosomes". Or it can be done on the membranes of the rough endoplasmic reticulum. The cartoon to the left shows the basic sequence of transcription and translational events.

What happens at the site of the ribosome?

tRNA.jpg (51285 bytes)
tRNA.jpg (51285 bytes)
The code is actually translated on structures that are also made in the nucleus, called Ribosomes. These ribosomes provide the structural site where the mRNA sits. The amino acids for the proteins are carried to the site by "transfer RNAs,". In the cartoon to the left, these are shown as blue molecules. Each transfer RNA (tRNA) has a nucleotide triplet which binds to the complementary sequence on the mRNA (see the three letters at the bottom of each molecule).
The tRNA carries the amino acid at its opposite end. One can trace and detect binding of a particular tRNA-amino acid complex to the mRNA by labeling that amino acid. It will bind to its tRNA. In the case to the left, Phenylalanine is bound to the tRNA which carries the complementary base code AAA (adenine-adenine-adenine). This triplet code would bind to the complementary sequence on mRNA UUU (uracil X3). The mRNA is shown as a green arrow. This cartoon shows the selective binding site on the mRNA which is attached in the ribosome. It also shows the tRNA carrying the Phenylalanine bound at the site In this particular assay which uses a polyuracil mRNA, only phenylalanine-bearing tRNA is bound and detected on the filter.

ribosome1.jpg (37988 bytes)
ribosome1.jpg (37988 bytes)

The cartoon shows the initiation of this process. It begins with the small subunit of the ribosome bound to the mRNA. An initiator tRNA is attracted to the region (carrying a methionine. It binds to the triplet code AUG.
This then attracts the large ribosomal subunit which will bind to the small subunit. Note that it has an A site and a P site. These are different binding sites for the tRNAs. The cartoon below describes the next phase in the process.


ribosome2.jpg (35070 bytes)
ribosome2.jpg (35070 bytes)
In this cartoon, note that the initiator tRNA complex has moved to the P site. This leaves the A site open for the next tRNA. In this case, we have Proline, which carries the complementary code GGC. Note that its binding site on the mRNA is CCG. After binding to the A site, the peptide bond between the methionine and proline forms. The empty tRNA carrying the MET leaves and the tRNA carrying the Proline moves to the P site. The ribosome moves to the next triplet code from 5' to the 3' direction (note arrow on mRNA). The tRNAs are moving from 3' to the 5' direction as the ribosome reads the code

translation1.jpg (28543 bytes)
translation1.jpg (28543 bytes)
The ribosome continues to read the code from the 5' to the 3' and amino acids are added to the growing peptide chain. This one shows the tRNA carrying the glycine amino acid coded by CCA. Its complementary bases are GGU.
This continues until the stop codon is reached. This is highlighted in red in this figure and the next figure.
The following cartoon shows what happens when the stop codon is reached.

End of translation

Clusters of ribosomes may sit on a mRNA and make proteins, each making a strand of polypeptides. These clusters are called polyribosomes. When they are free in the cytoplasm, they are called free polyribosomes (linked by the mRNA). Or, they may bind to rough endoplasmic reticulum.external image rer4.jpg
Ribosomes are visualized as small (20 X 30 nm) ribonucleoprotein particles. They are formed from two subunits. As you learned in the lecture on the nucleolus , the subunits are produced in the nucleolus in organizing centers on certain chromosomes. The two ribosomal subunits leave the nucleus separately through the nuclear pores . The pores are structured to allow transit of only the subunits. Whole ribosomes are formed outside in the cytoplasm. This prevents protein synthesis from occurring in the nucleus. Why might this be important?
The above photograph shows a group of ribosomes in action. They are connected by a strand of messenger RNA which runs between the large and small subunits. They read the 3 nucleotide code for an amino acid and the appropriate transfer RNA brings the amino acid to the growing polypeptide chain. In this photograph, we see the growing peptide chain radiating at right angles to the mRNA. It extends from the base of the large ribosomal subunit.

external image ribos1.jpgThe left hand view of this cartoon shows the free polyribosomes connected by the mRNA. They are arranged in rosettes and these can be seen in the cytoplasm in conventional electron micrographs. The right hand view shows the arrangement of polyribosomes on the rough endoplasmic reticulum. Note that the growing polypeptide chain (which projects down from the large subunit) is inserted through the membrane and into the cisterna of the rough endoplasmic reticulum.
Return to Menu external image ribos2.jpg
This cartoon shows the binding site on the rough endoplasmic reticulum. The membrane of the rough endoplasmic has a receptor that binds the larger subunit of the ribosome. Next to the receptor is a pore that allows newly synthesized proteins to enter and be stored initially in the rough endoplasmic reticulum cisterna or lumen. Note that the ribosomes are still connected to one another outside the rough endoplasmic reticulum by the mRNA which runs between the large and small subunits.
external image rer5.jpgThis electron micrograph shows a high magnification of a longitudinal section through the rough endoplasmic reticulum. The electron dense ribosomes are on its outside surface. Inside the sac (cisterna) is flocculent material, the newly synthesized proteins. The details of ribosomal structure cannot be appreciated in this micrograph. They look like small irregular balls on the outside of the membrane. Note that the sacs of rough endoplasmic reticulum are bridged by a junction. This is shown diagrammatically in the following figure.
external image rer2.jpg
The cartoon in this figure shows the rough endoplasmic reticulum with a bridge adjoining two sacs. In this way, the sacs communicate and proteins fill the spaces all over the cell. They even communicate with the inside of the nuclear envelope. Recall that the outside membrane of the nuclear envelope is studded with ribosomes and is part of the rough endoplasmic reticulum. An immunocytochemical labeling protocol, such as that found in the above figure, will delineate the reticulum filled with the newly synthesized proteins.

Ribosomes are complexes of RNA and protein that are found in all cells.
Ribosomes from bacteria, archaea and eukaryotes, the three domains of life.

Ribosomes are the most numerous organelles in a cell. Unlike most other organelles, robosomes are not surrounded by a membrane. Each ribosome is an assemblage of two organic compounds- protein and RNA. In the cell's nuclues, proteins and RNAs are made into ribosomes, then taken to the cytosol. Some ribosomes remain free within the cytosol while others attach to an organelle called the endoplasmic reticulum. Ribosomes are used in the synthesis of proteins. Proteins that are needed in the cytosol are made by the ribosomes that are in the cytosol. Proteins that need to go to the membrane or have to leave the cell are made from the ribosomes that are attached to other organelles.

Ribosomes are small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.

Ribosomes - Protein Construction Teams

Cells need to make proteins. Those proteins might be used as enzymes or as support for other cell functions. When you need to make proteins, you look for ribosomes. Ribosomes are the protein builders or the protein synthesizers of the cell. They are like construction guys who connect one amino acid at a time and build long chains.

Ribosomes floating and on rough endoplasmic reticulum
Ribosomes floating and on rough endoplasmic reticulum
Ribosomes are found in many places around the cell. You might find them floating in the cytoplasm (cytosol). Those floating ribosomes make proteins that will be used inside of the cell. Other ribosomes are found on the endoplasmic reticulum. Endoplasmic reticulum with attached ribosomes is called rough. It looks bumpy under a microscope. Those attached ribosomes make proteins that will be used inside the cell and proteins made for export out of the cell.

Mixing and Matching Amino Acids

Ribosomes involved in protein construction
Ribosomes involved in protein construction

The process of making proteins is quite simple. We just explained that mRNA is made in the nucleus and sent into the cell. The mRNA then combines with the ribosome subunits. Another nucleic acid lives in the cell - tRNA, which stands for transfer RNA. tRNA is bonded to the amino acids floating around the cell. With the mRNA offering instructions, the ribosome connects to a tRNA and pulls off one amino acid. Slowly the ribosome makes a long amino acid chain that will be part of a larger protein.

All living cells contain ribosomes, tiny organelles composed of approximately 60 percent ribosomal RNA (rRNA) and 40 percent protein. However, though they are generally described as organelles, it is important to note that ribosomes are not bound by a membrane and are much smaller than other organelles. Some cell types may hold a few million ribosomes, but several thousand is more typical. The organelles require the use of an electron microscope to be visually detected.

Ribosomes are mainly found bound to the endoplasmic reticulum and the nuclear envelope, as well as freely scattered throughout the cytoplasm, depending upon whether the cell is plant, animal, or bacteria. The organelles serve as the protein production machinery for the cell and are consequently most abundant in cells that are active in protein synthesis, such as pancreas and brain cells. Some of the proteins synthesized by ribosomes are for the cell's own internal use, especially those that are produced by free ribosomes. Many of the proteins produced by bound ribosomes, however, are transported outside of the cell.
In eukaryotes, the rRNA in ribosomes is organized into four strands, and in prokaryotes, three strands. Eukaryote ribosomes are produced and assembled in the nucleolus. Ribosomal proteins enter the nucleolus and combine with the four rRNA strands to create the two ribosomal subunits (one small and one large) that will make up the completed ribosome (see Figure 1). The ribosome units leave the nucleus through the nuclear pores and unite once in the cytoplasm for the purpose of protein synthesis. When protein production is not being carried out, the two subunits of a ribosome are separated.

Ribosomes are packets of RNA and protein that play a crucial role in both prokaryotic and eukaryotic cells. They are the site of protein synthesis. Each ribosome comprises two parts, a large subunit and a small subunit. Messenger RNA from the cell nucleus is moved systematically along the ribosome where transfer RNA adds individual amino acid molecules to the lengthening protein chain.

ribosome structure
ribosome structure

Ribosome structure indicating small subunit (A) and large subunit (B). Side and front view.

1 Head
2 Platform
3 Base
4 Ridge
5 Central protuberance
6 Back
7 Stalk
8 Front

A tiny **organelle** that is the site of **protein synthesis** (protein translation) in the living **cell**. Ribosomes are complex, bead-like structures composed of about 40% **protein** and 60% **ribosomal RNA** (rRNA). In **eukaryotes**, ribosomes are made of four strands of RNA and are often attached to the membranes of the **endoplasmic reticulum** to form rough ER. In **prokaryotes**, they are made of three strands of RNA and occur free in the **cytoplasm**.

Eukaryote ribosomes are produced and assembled in the
**nucleolus**. Three of the four strands are produced there, but one is produced outside the nucleolus and transported inside to complete the ribosome assembly. Ribosomal proteins enter the nucleolus and combine with the four strands to create the two subunits that will make up the completed ribosome. The ribosome units leave the **nucleus** through the nuclear pores and unite once in the cytoplasm. Some ribosomes will remain free-floating in the cytoplasm, creating proteins for the cell's use. Others will attach to the endoplasmic reticulum and produce the proteins that will be "exported" from the cell.

Protein synthesis requires the assistance of two other RNA molecules.
**Messenger RNA** (mRNA) provides instructions from the cellular **DNA** for building a specific protein. **Transfer RNA** (tRNA) brings the protein building blocks, **amino acids**, to the ribosome. Once the protein backbone amino acids are polymerized, the ribosome releases the protein and it is transported to the **Golgi apparatus**. There, the proteins are completed and released inside or outside the cell. For more details of the role played by ribosomes in protein synthesis, see **protein translation**