Navigation Links
Ribosome


Figure 1: 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 ribosome is an organelle composed of rRNA (synthesized in the nucleolus) and ribosomal proteins. 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 another organelle called the endoplasmic reticulum. Since ribosomes are ribozymes, it is thought that they might be remnants of the RNA world.

Overview

Ribosomes consist of two subunits (Figure 1) that fit together (Figure 2) and work as one to translate the mRNA into a polypeptide chain during protein synthesis (Figure 3). Each subunit consists of one or two very large RNA molecules (known as ribosomal RNA or rRNA) and multiple smaller protein molecules. Experiments have shown that the rRNA are the crucial components in protein synthesis, and that one aspect of the process, peptide transfer, can occur in the presence of rRNA alone, albeit at a slower rate. This suggests that the protein components of ribosomes act as a scaffold that may enhance the ability of rRNA to synthesise protein.

The structure and function of ribosomes, and their attendant molecules, known as the translational apparatus, has been of ongoing research interest since the mid 20th century on through the early 21st century. A triennial conference is held to discuss the ribosome. In 1999, the conference was held in Elsinore, Denmark. The 2002 conference was held in Queenstown, New Zealand [1].


Figure 2 : Large (1) and small (2) subunit fit together

Free ribosomes

Free ribosomes occur in all cells, and also in mitochondria and chloroplasts in eukaryotic cells. Several free ribosomes can associate on a single mRNA molecule to form a polyribosome or polysome. Free ribosomes usually produce proteins that are used in the cytosol or in the organelle they occur in.

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.

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.[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.]

Figure 3 : Translation (1) of mRNA by a ribosome (2) into a polypeptide chain (3). The mRNA begins with a start codon (AUG) and ends with a stop codon (UAG).

In Figure 3, 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, several ribosomes are working parallel on a single mRNA.

See also: protein assembly



'"/>


(Date:9/21/2014)... dioxide emissions, the main contributor to global warming, are set ... of 40 billion tonnes. , The 2.5 per cent projected ... Carbon Project, which is co-led in the UK by researchers ... University of East Anglia and the College of Engineering, Mathematics ... comes ahead of the New York Climate Summit, where world ...
(Date:9/21/2014)... developed a protein therapy that disrupts the process that ... sites, travel through the blood stream and start aggressive ... known as metastasis, can cause cancer to spread with ... succumb to cancer fall prey to metastatic forms of ... bioengineering who describes a new therapeutic approach in ...
(Date:9/19/2014)... the sake of fairness per se but in order ... Frans de Waal, PhD, and Sarah Brosnan, PhD, co-authors ... is defined as a negative reaction to unequal outcomes. ... , Their conclusion comes after the co-authors reviewed more ... it is the evolution of forestalling partner dissatisfaction with ...
Breaking Biology News(10 mins):CO2 emissions set to reach new 40 billion ton record high in 2014 2CO2 emissions set to reach new 40 billion ton record high in 2014 3Stanford researchers create 'evolved' protein that may stop cancer from spreading 2Stanford researchers create 'evolved' protein that may stop cancer from spreading 3Stanford researchers create 'evolved' protein that may stop cancer from spreading 4Evolution of responses to (un)fairness 2Evolution of responses to (un)fairness 3
... New research shows that nearly 1 in 5 cases of ... are caused by not one but multiple strains of the ... of Minnesota Medical School report their findings today in the ... journal published by the American Society for Microbiology. "Koch,s ...
... of researchers led by Jeffrey Peng, assistant professor of ... is using Nuclear Magnetic Resonance (NMR), to move drug ... drugs and their targets. The research, which was ... contributes to the growing attention given toward the shape-shifting ...
... Spanish . , Researchers at the Centre ... the Universidad de Kentucky (USA), have discovered a new way ... pathogen agents responsible for transmissible spongiform encephalopathies or TSEs) by ... control prion disease in deer, which has become an epidemic ...
Cached Biology News:Unexpectedly high rate of multiple strains in fungal infection 2Nuclear magnetic resonance aids in drug design 2Researchers embark on work to control the prion epidemic affecting deer in the USA 2Researchers embark on work to control the prion epidemic affecting deer in the USA 3
Other biology definitionOther Tags