Question

What tells the ribosome where to start translation and stop translation??

Answer 1

The mRNA is the message sent from the nucleus to the ribosome, the organelle of protein synthesis. Like a foreign language, the genetic code of the mRNA message must then be translated so that the ribosomes make the correct protein. The process of reading the code of a mRNA to make a protein is called translation.

Translation

The mRNA, which is transcribed from the DNA in the nucleus, carries the directions for the protein-making process. mRNA tells the ribosome (Figure below) how to create a specific protein.

Ribosomes translate RNA into a protein with a specific amino acid sequence. The tRNA binds and brings to the ribosome the amino acid encoded by the mRNA.

The process of reading the mRNA code in the ribosome to make a protein is called translation: the mRNA is translated from the language of nucleic acids (nucleotides) to the language of proteins (amino acids). Sets of three bases, called codons, are read in the ribosome, the organelle responsible for making proteins.

The following are the steps involved in translation:

• mRNA travels to the ribosome from the nucleus.

The following steps occur in the ribosome:

• The base code in the mRNA determines the order of the amino acids in the protein. The genetic code in mRNA is read in “words” of three letters (triplets), called codons. Each codon codes for an amino acid. There are 20 amino acids used to make proteins, and different codons code for different amino acids. For example, GGU codes for the amino acid glycine, while GUC codes for valine.
• tRNA reads the mRNA code and brings a specific amino acid to attach to the growing chain of amino acids. The anticodon on the tRNA binds to the codon on the mRNA. Each tRNA carries only one type of amino acid and only recognizes one specific codon. For example, a GGC anticodon will bind to a CCG codon, and a CGA anticodon will bind to a GCU codon.
• tRNA is released from the amino acid.
• Three codons, UGA, UAA, and UAG, indicate that the protein should stop adding amino acids. They are called stop codons and do not code for an amino acid. Once tRNA comes to a stop codon, the protein is set free from the ribosome.

The following chart (Figure below) is used to determine which amino acids correspond to which codons.

This chart shows the genetic code used by all organisms. For example, an RNA codon reading GUU would encode for a valine (Val) according to this chart. Start at the center for the first base of the three base codon, and work your way out. Notice that more than one codon may encode for a single amino acid. For example, glycine (Gly) is encoded by a GGG, GGA, GGC, and GGU. Notice there are 64 codons. Of the 64 codons, three are stop codons.

Summary

• Translation is the process of reading the mRNA code in the ribosome to make a protein.
• Sets of three bases on the mRNA, called codons, are read in order to select the correct amino acid for building a protein.