Translation: Final Stage in Information Transfer

     
 

Content

Principles of Genetics:A Review
Mendel’s Investigations 
Chromosomal Basis of Inheritance 
  - Meiosis: Reduction Division of Gametes
  - Sex Determination
Mendelian Laws of Inheritance
  - Mendel’s First Law
  - Mendel’s Second Law
  - Sex-Linked Inheritance
  - Autosomal Linkage and Crossing Over
  - Chromosomal Aberrations
Gene Theory
Storage and Transfer of Genetic Information
  - Nucleic Acids: Molecular Basis of Inheritance 
  - DNA Coding by Base Sequence
  - Transcription and the Role of Messenger RNA
  - Translation: Final Stage in Information Transfer
  - Regulation of Gene Expression
  - Gene Regulation in Eukaryotes
  - Molecular Genetics
  - Recombinant DNA
  - Genomics
Sources of Phenotypic Variation 
  - Gene Mutations
  - Frequency of Mutations
  - Molecular Systematics
Molecular Genetics of Cancer 
  - Oncogenes and Tumor Suppressor Genes
References 
 
     
How the protein chain is formed. As ribosomes move along messenger RNA, the amino acids are added stepwise to form the polypeptide chain.
Figure 5-19 How the protein chain is formed. As
ribosomes move along messenger RNA, the amino acids
are added stepwise to form the polypeptide chain.
 
Structure of a tRNA molecule. The anticodon loop bears bases complementary to those in the mRNA codon. The other two loops function in binding to the ribosomes in protein synthesis. The amino acid is added to the free single-stranded ®OH end by tRNA synthetase.
Figure 5-20 Structure of a tRNA molecule. The anticodon
loop bears bases complementary to those in the mRNA
codon. The other two loops function in binding to the
ribosomes in protein synthesis. The amino acid is added to
the free single-stranded −OH end by tRNA synthetase.


Translation: Final Stage in Information Transfer
Formation of polypeptide chain on messenger RNA. As ribosome moves down messenger RNA molecule, transfer RNA molecules with attached amino acids enter ribosomes (top). Amino acids are joined together into polypeptide chain, and transfer RNA molecules leave ribosome (bottom).
Figure 5-21 Formation of polypeptide
chain on messenger RNA. As ribosome
moves down messenger RNA molecule,
transfer RNA molecules with attached
amino acids enter ribosomes (top).
Amino acids are joined together into
polypeptide chain, and transfer RNA
molecules leave ribosome (bottom).

The translation process takes place on ribosomes, granular structures composed of protein and ribosomal RNA (rRNA). Ribosomal RNA is composed of a large and a small subunit, and the small subunit comes to lie in a depression of the large subunit to form the functional ribosome (Figure 5-19). The mRNA molecules attach themselves to the ribosomes to form a messenger RNA-ribosome complex. Since only a short section of mRNA molecule is in contact with a single ribosome, the mRNA usually attaches to several ribosomes at once. The entire complex, called a polyribosome or polysome, allows several molecules of the same kind of protein to be synthesized at once, one on each ribosome of the polysome (Figure 5-19).

The assembly of proteins on the mRNA-ribosome complex requires the action of another kind of RNA called transfer RNA (tRNA). The tRNAs are surprisingly large molecules that are folded in a complicated way in the form of a cloverleaf (Figure 5-20). The tRNA molecules collect free amino acids from the cytoplasm and deliver them to the polysome, where they are assembled into a protein. There are special tRNA molecules for every amino acid. Furthermore, each tRNA is accompanied by a specific tRNA synthetase. The tRNA synthetases are enzymes that are necessary to sort and attach the correct amino acid to a site on the end of each tRNA by a process called charging.

On the cloverleaf-shaped molecule of tRNA, a special sequence of three bases (the anticodon) is exposed in just the right way to form base pairs with complementary bases (the codon) in the mRNA. The codons are read and proteins assembled along the mRNA in a 5´ to 3´ direction. The anticodon of the tRNA is the key to the correct sequencing of amino acids in the protein being assembled.

For example, alanine is assembled into a protein when it is signaled by the codon GCG in an mRNA. The translation is accomplished by alanine tRNA in which the anticodon is CGC. The alanine tRNA is first charged with alanine by its tRNA synthetase. The alanine tRNA complex enters the ribosome where it fits precisely into the right place on the mRNA strand. Then the next charged tRNA specified by the mRNA code (glycine tRNA, for example) enters the ribosome and attaches itself beside the alanine tRNA. The two amino acids are united with a peptide bond (with the energy from a molecule of guanosine triphosphate), and the alanine tRNA falls off. The process continues stepwise as the protein chain is built (Figure 5-21). A protein of 500 amino acids can be assembled in less than 30 seconds.

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