A ribosome is a cell organelle that makes proteins from messenger RNA (mRNA) by linking amino acids together. This process is called translation. When the amino acid chain is complete, the ribosome releases it into the cellular cytoplasm to be folded into a functional protein. Ribosomes are capable of synthesizing peptide strands at a rate of 200 per minute, taking as much as two or three hours for a very large protein.
A ribosome is made of two parts, or subunits that lock together to carry out its function. The larger of the two subunits is about twice the size of the smaller subunit. The larger subunit’s function is primarily catalytic, while the smaller subunit performs decoding of the mRNA.
Where are ribosomes made?
Ribosomes are synthesized in the nucleolus, a structure within the nucleus of a cell. They may be found in many locations within the cell. Free ribosomes are located in the cytoplasm. Membrane-bound ribosomes can be found on the many membranes in cell, particularly the endoplasmic reticulum. Ribosomes give the endoplasmic reticulum its characteristic rough appearance under a microscope.
Because the ribosomes’ function is production of proteins, they are mainly found in cells that are active in synthesizing proteins, such as the pancreas and brain. Proteins may be synthesized for use within the cell or transported out of the cell.
Regulation of ribosomes
Ribosomes can be regulated in both eukaryotic and prokaryotic cells by repressor proteins that block translation on specific mRNA sequences. Another method of regulating translation in eukaryotic cells is modulation of initiation factors. This occurs through phosphorylation of the initiation factor by regulatory protein kinases.
Mitochondria and chloroplasts have their own ribosomes, which resemble prokaryotic ribosomes. This similarity supports theories that mitochondria and chloroplasts evolved from prokaryotic ancestors.
Ribosomal function is so crucial to the survival of an organism that it was once believed any functional mutation in a ribosome would be fatal. However, a number of genetic diseases have now been connected to ribosomal mutations.
Mutations of the small subunit
Treacher Collins syndrome is an autosomal dominant disorder that causes abnormalities of the ears, eyes and bones of the face. It is caused by mutations in the gene encoding the protein Treacle, which is involved in the transcription and methylation of the gene for 18S ribosomal RNA, a component of the smaller subunit of the ribosome. Mutations in UTP14, a protein essential for 18S rRNA maturation, lead to male infertility. North American Indian childhood cirrhosis is linked to mutations in Cirhin, a protein required for optimal transcription of ribosomal DNA. Bowen-Conradi syndrome, an autosomal recessive disorder that is fatal by the age of one, is caused by a mutation in EMG1, a protein required for synthesis of the 40S ribosome subunit.
Mutations of the large subunit
Mutations of the RBM28, a protein essential for synthesis of the 60S subunit, can cause alopecia, neurological defects and endocrinopathy syndrome (ANE syndrome). Shwachman-Bodian syndrome is another autosomal recessive disorder caused by a ribosome mutation. One of its symptoms is pancreatic insufficiency caused by replacement of the pancreas with adipose tissue. It also causes hematologic defects, skeletal abnormalities and short stature, among other symptoms. It is due to mutations of SBDS, a protein that is required for maturation and export of the 60S subunit.
Ribosomal defects contribute to primary open angle glaucoma and several diseases are connected with mutations of small nucleolar ribonucleoproteins and mutations of the ribosomal proteins.
Ribosomal function is highly complex and essential for the survival of the cell. Ribosomes are found in eukaryotic and prokaryotic organisms and in mitochondria and chloroplasts. They have highly conserved similarities in structure and protein producing function.