Biology Textbook - VUNMC 2000

Chapter 3. CELLULAR LEVEL OF THE ORGANIZATION OF LIFE

3.5. PROTEIN SYNTHESIS

The Central dogma of molecular biology states that Genetic information flows from DNA through mRNA to protein.

Each PLANT AND ANIMAL species has a unique set of Proteins characteristic only of it. Even in individuals of the same species, including humans, proteins vary in their properties.

The set of proteins forms The basis of individual and species Specificity. Hereditary information regarding Cell/13.html">Protein Structure is stored in DNA molecules. Although DNA is the carrier of all genetic information in The Cell, it does not directly participate in Protein Synthesis. DNA molecules are localized within the Chromosomes of The Nucleus, whereas the assembly of protein molecules takes place in the Cytoplasm on Ribosomes. Information from the nucleus is delivered to the ribosomes via an intermediary, known as Messenger RNA (mRNA).

To translate The nucleotide sequence of DNA and mRNA into the Amino Acid Sequence of the synthesized protein molecule, a special "cipher," or Genetic Code, is used. METABOLISM/28.html">The Genetic Code is a system for recording information in mRNA molecules, reflected in the nucleotide sequence that determines the order of Amino Acids in protein molecules. The information is "transcribed" in the nucleus from the DNA molecule to the mRNA.

3.5.1. PROPERTIES OF THE GENETIC CODE

1. Triplet nature. Each amino acid is encoded by a sequence of three NUCLEOTIDES, referred to as a triplet or codon.

2. Degeneracy. Each amino acid is encoded by more than one codon, with the exception of Methionine and Tryptophan, each of which is encoded by a single triplet. To code for the 20 amino acids, 61 nucleotide combinations are used. The AUG triplet, which encodes methionine, is designated as the start codon; protein synthesis begins with it. Three codons (UAA, UAG, UGA) carry information for the termination of PROTEIN SYNTHESIS AND are termed termination triplets.

3. Universality. Across all organisms on Earth, the same triplets encode the same amino acids.

4. Unambiguity. Each triplet encodes only one specific amino acid.

5. Colinearity - the correspondence between The amino acid sequence in the synthesized protein molecule and The sequence of triplets in the mRNA (Table 5).

Class="center">Table 5. Messenger RNA Codons

Note. The first nitrogenous Base of the triplet is located in the left vertical Column, the second in the top horizontal row, and the third in the right vertical column. The desired amino acid is found at the intersection of the three bases' lines.

Amino acids are designated as follows: Ala - Alanine, Arg - Arginine, Asn - asparagine, Asp - aspartic acid, Val - valine, His - Histidine, Gly - Glycine, Gln - glutamine, Glu - glutamic acid, Ile - isoleucine, Leu - leucine, Lys - Lysine, Met - methionine, Pro - Proline, Ser - Serine, Tyr - Tyrosine, Thr - Threonine, Trp - tryptophan, Phe - phenylalanine, Cys - Cysteine.