Metabolism and Energy Transformation in Body Cells - Renata Armenakovna Petrosova 2004

The genetic code halted
The genetic code

The Specificity of each Cell is determined by its set of Proteins. Accounting for half of The Cell's total mass, proteins perform diverse and numerous Functions, driving cell growth, development, and differentiation, while maintaining cellular Structure and function.

All extensive hereditary information is encoded in DNA AS A linear sequence of four types of NUCLEOTIDES: adenine (A), thymine (T), guanine (G), and cytosine (C). Thus, Genetic information is written in a four-letter alphabet. The nucleotide sequence in DNA dictates its corresponding informational content. The number of possible distinct sequences in a DNA molecule is 4ⁿ, where п is the number of nucleotides in a single strand of the DNA molecule. The value of п is exceptionally large. For instance, animal cellular DNA typically contains 3 · 10⁹ nucleotides. How does the cell translate the nucleotide sequence of DNA into the Amino Acid Sequence of a protein?

The rules governing the Translation of a nucleotide sequence in a nucleic acid into The amino acid sequence of a protein are known as the Genetic Code. It was deciphered in the 1960s.

Through a series of experiments and mathematical calculations, it was determined that a single amino acid is encoded by three nucleotides. There are 4 types of nucleotides found in the DNA molecule. If an amino acid were specified by a single nucleotide, only 4 Amino Acids could be encoded. However, this is insufficient, as the cell utilizes 20 different amino acids. Assuming an amino acid is encoded by a combination of two nucleotides, 4² = 16, meaning only 16 amino acids could be coded. Consequently, 4 amino acids would be left out of proteins. Therefore, a single amino acid must be encoded by a triplet of nucleotides: 4³ = 64.

64 codons are more than enough to specify 20 amino acids. In this case, each amino acid is encoded not by just one, but potentially by several codons. Experimental testing confirmed these hypotheses. Numerous studies have established the following key Properties of the genetic code.

1. The code is triplet — each amino acid corresponds to a combination of three nucleotides. There are 64 such combinations, known as codons. Of these, 61 are sense codons—meaning they correspond to the 20 amino acids—while 3 are nonsense stop codons, which do not code for amino acids but instead mark the boundaries between genes.

2. The code is unambiguous — each triplet specifies one and only one amino acid.

3. The code is degenerate — Most amino acids are encoded by more than one codon. For example, the amino acid Glycine has 4 codons: CCA, CCG, CCT, and CCC. More frequently, Amino acids are specified by 2 codons.

4. The code is universal — All living organisms share essentially the same genetic code for amino acids.

5. The code is non-overlapping — there are no gaps or spaces between codons.

6. The code is non-overlapping — the terminal nucleotide of one codon cannot serve as the starting nucleotide of the next.

Genetic Code Table

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