How do genes code for the amino acid sequence of polypeptides?

Sourav PanNovember 8, 2024

Answered

Genes code for the amino acid sequence of polypeptides through a well-defined process that involves transcription and translation, utilizing the genetic code. Here’s a detailed explanation of how this process works:

1. The Genetic Code

Definition

The genetic code is a set of rules that defines how the nucleotide sequences in DNA and RNA correspond to specific amino acids in proteins. It is based on triplet codons, where each codon consists of three nucleotides.

Codon Structure

• There are 64 possible codons (4 nucleotides raised to the power of 3), but only 61 codons specify amino acids; the remaining three are stop codons that signal the end of protein synthesis.
• Each amino acid is coded for by one or more codons, making the code degenerate (i.e., multiple codons can encode the same amino acid).

2. Transcription

Process Overview

1. DNA to mRNA: The process begins with transcription, where a gene’s DNA sequence is copied into messenger RNA (mRNA). This occurs in the nucleus of eukaryotic cells.
2. RNA Polymerase: An enzyme called RNA polymerase binds to the promoter region of the gene and unwinds the DNA strands. It then synthesizes a single strand of mRNA by adding complementary RNA nucleotides to the growing strand, following base pairing rules (A-U and C-G).

Result

• The resulting mRNA molecule carries the genetic information from DNA in a sequence of nucleotides that will be translated into an amino acid sequence.

3. Translation

Process Overview

1. Ribosome Assembly: The mRNA leaves the nucleus and binds to a ribosome, which serves as the site of protein synthesis.
2. tRNA Role: Transfer RNA (tRNA) molecules bring specific amino acids to the ribosome. Each tRNA has an anticodon that is complementary to a corresponding codon on the mRNA.
3. Codon Recognition: The ribosome reads the mRNA codons in a 5′ to 3′ direction, matching each codon with its corresponding tRNA anticodon.

Peptide Bond Formation

• As each tRNA binds to its respective codon, the ribosome catalyzes the formation of peptide bonds between adjacent amino acids, linking them together into a growing polypeptide chain.
• This process continues until a stop codon is reached, which signals termination of translation and release of the newly synthesized polypeptide.

4. Summary of Key Steps

• Transcription: DNA → mRNA
• Translation: mRNA → Amino Acid Sequence
• Each triplet codon in mRNA corresponds to one specific amino acid, allowing for precise assembly of polypeptides based on genetic instructions.