A ribosome is an organelle composed of rRNA and ribosomal proteins (known as a Ribonucleoprotein). It translates mRNA into a polypeptide chain (e.g., a protein). It can be thought of as a factory that builds a protein from a set of genetic instructions. Ribosomes can float freely in the cytoplasm (the internal fluid of the cell) or bind to the endoplasmic reticulum, or to the nuclear envelope. Since ribosomes are ribozymes, it is thought that they might be remnants of the RNA world.
The structure and function of the ribosomes and associated molecules, known as the translational apparatus, has been of research interest since the mid 20th century and is a very active field of study today.
Ribosomes consist of two subunits that fit together and work as one to translate the mRNA into a polypeptide chain during protein synthesis. Each subunit consists of one or two very large RNA molecules (known as ribosomal RNA or rRNA) and multiple smaller protein molecules. Crystallographic work has shown that there are no ribosomal proteins close to the reaction site for polypeptide synthesis. This suggests that the protein components of ribosomes act as a scaffold that may enhance the ability of rRNA to synthesise protein rather than directly participating in catalysis.
Free ribosomes Free ribosomes occur in all cells, and also in mitochondria and chloroplasts in eukaryotic cells. Free ribosomes usually produce proteins used in the cytosol or organelle in which they occur. As the name implies, they are not bound to anything within the cell.
Membrane bound ribosomes When certain proteins are synthesized by a ribosome, it can become "membrane-bound", associated with the membrane of the nucleus and the rough endoplasmic reticulum (in eukaryotes only) for the time of synthesis. They insert the freshly produced polypeptide chains directly into the ER, from where they are transported to their destinations. Bound ribosomes usually produce proteins that are used within the cell membrane or are expelled from the cell via exocytosis.
Structure and function The ribosomal subunits of prokaryotes and eukaryotes are quite similar. However, prokaryotes use 70S ribosomes, each consisting of a (small) 30S and a (large) 50S subunit, whereas eukaryotes use 80S ribosomes, each consisting of a (small) 40S and a bound (large) 60S subunit. However, the ribosomes found in chloroplasts and mitochondria of eukaryotes are 70S. [The unit S means Svedberg units, a measure of the rate of sedimentation of a particle in a centrifuge, where the sedimentation rate is associated with the size of the particle. Svedberg units are not additive - two subunits together can have Svedberg values that do not add up to that of the entire ribosome]. Also, the 70S ribosomes are vulnerable to some antibiotics that the 80S ribosomes are not. This helps pharmaceutical companies create drugs that can destroy a bacterial infection without harming the animal/human host's cells.
Both ribosomal subunits (small and large) assemble at the start codon (the 5' end of the mRNA). The ribosome uses tRNA (transfer RNAs which are RNA molecules that carry an amino acid and present the matching anti-codon, according to the genetic code, to the ribosome) which matches the current codon (triplet) on the mRNA to append an amino acid to the polypeptide chain. This is done for each triplet on the mRNA, while the ribosome moves towards the 3' end of the mRNA. Usually in bacterial cells, several ribosomes are working parallel on a single mRNA. This is what we call polyribosome or polysome.
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.
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.
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.
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?
A group of ribosomes in action 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.
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