Just as we might translate a work from Spanish into English, genetic translation converts the language of nucleotides into the language of amino acids (fig. 4.6). This job is done by ribosomes, which are found mainly in the cytosol and on the rough ER and nuclear envelope. A ribosome consists of two granular subunits, large and small, each made of several rRNA and enzyme molecules.

The mRNA molecule begins with a leader sequence of bases that are not translated to protein but serve as a binding site for the ribosome. The small ribosomal subunit binds to it, the large subunit joins the complex, and the ribosome begins pulling the mRNA through it like a ribbon, reading bases as it goes. When it reaches the start codon, AUG, it begins making protein. Since AUG codes for methionine, all proteins begin with methionine when first synthesized, although this may be removed later.

Translation requires the participation of 61 types of transfer RNA (tRNA), one for each codon (except stop codons). Transfer RNA is a small RNA molecule that turns back and coils on itself to form a cloverleaf shape, which is then twisted into an angular L-shape (fig. 4.7). One end of the L includes three nucleotides called an anticodon, and the other end has a binding site specific for one amino acid. Each tRNA picks up an amino acid from a pool of free amino acids in the cytosol. One ATP molecule is used to bind the amino acid to this site and provide the energy that is used later to join that amino acid to the growing protein. Thus, protein synthesis consumes one ATP for each pep-tide bond formed.

When the small ribosomal subunit reads a codon such as CGC, it must find an activated tRNA with the corresponding anticodon; in this case, GCG. This particular tRNA would have the amino acid alanine at its other end. The ribosome binds and holds this tRNA and then reads the next codon—say GGU. Here, it would bind a tRNA with anticodon CCA, which carries glycine.

The large ribosomal subunit contains an enzyme that forms peptide bonds, and now that alanine and glycine are side by side, it links them together. The first tRNA is no longer needed, so it is released from the ribosome. The second tRNA is used, temporarily, to anchor the growing peptide to the ribosome. Now, the ribosome reads the third codon—say GUA. It finds the tRNA with the anticodon CAU, which carries the amino acid valine. The large subunit adds valine to the growing chain, now three amino acids long. By repetition of this process, the entire protein is assembled. Eventually, the ribosome reaches a stop codon and is finished translating this mRNA. The polypeptide is turned loose, and the ribosome dissociates into its two subunits.

One ribosome can assemble a protein of 400 amino acids in about 20 seconds, but it does not work at the task alone. After the mRNA leader sequence passes through one ribosome, a neighboring ribosome takes it up and begins translating the mRNA before the first ribosome has finished. One mRNA often holds 10 or 20 ribosomes together in a cluster called a polyribosome (fig. 4.8). Not only is each mRNA translated by all these ribosomes at once, but a cell may have 300,000 identical mRNA molecules undergoing simultaneous translation. Thus, a cell may produce over 150,000 protein molecules per second—a remarkably productive protein factory! As much as 25% of the dry weight of liver cells, which are highly active in protein synthesis, is composed of ribosomes.

Many proteins, when first synthesized, begin with a chain of amino acids called the signal peptide. Like a molecular address label, the signal peptide determines the protein's destination—for example, whether it will be sent to the rough endoplasmic reticulum, a peroxisome, or a mitochondrion. (Proteins used in the cytosol lack signal peptides.) Some diseases result from errors in the signal peptide, causing a protein to be sent to the wrong address,

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(5 The preceding tRNA hands off the growing peptide to the new tRNA, and the ribosome links the new amino acid to the peptide.

Free amino acids

@ tRNA binds an amino acid; binding consumes 1 ATP.

Free tRNA



— J-i

  • 5 The preceding tRNA hands off the growing peptide to the new tRNA, and the ribosome links the new amino acid to the peptide.
  • tRNA binds an amino acid; binding consumes 1 ATP.

tRNA is released from the ribosome and is available to pick up a new amino acid and repeat the process.

@ tRNA anticodon binds to complementary mRNA codon.


@ tRNA anticodon binds to complementary mRNA codon.

tRNA is released from the ribosome and is available to pick up a new amino acid and repeat the process.

Ribosome binds mRNA

Ribosome binds mRNA

@ mRNA leaves the nucleus.


@ mRNA leaves the nucleus.

(7 After translating the entire mRNA, ribosome dissociates into its two subunits.

Ribosomal subunits rejoin to repeat the process with the same or another mRNA.

Figure 4.6 Translation of mRNA.

Why would translation not work if ribosomes could bind only one tRNA at a time?

such as going to a mitochondrion when it should have gone to a peroxisome, or causing it to be secreted from a cell when it should have been stored in a lysosome. Gunter Blobel of Rockefeller University received the 1999 Nobel Prize for Physiology or Medicine for discovering signal peptides in the 1970s.

Figure 4.9 summarizes transcription and translation and shows how a nucleotide sequence translates to a hypothetical peptide of 6 amino acids. A protein 500 amino acids long would have to be represented, at a minimum, by a sequence of 1,503 nucleotides (3 for each amino acid, plus a stop codon). The average gene is probably around 1,200 nucleotides long; a few may be 10 times this long.

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  • ty marshall
    Where are ribosomal subunits assmbled in E.R or cytosol?
    7 years ago

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