Understanding Genetic Transcription & Code

Genetic Transcription Process

TRANSCRIPTION: The process of copying a part of the genetic message from its original form (DNA) to another (RNA) that can be used directly for protein synthesis.

Transcription forms a strand of RNA whose sequence is complementary to the DNA template strand. Consequently, its sequence is the same as the non-template DNA strand (with uracil replacing thymine).

Transcription requires:

• A DNA strand serving as template
• Ribonucleotide triphosphates
• Enzymes (primarily RNA polymerase)

Stages of Transcription

Initiation

The RNA polymerase recognizes specific short sequences of nitrogenous bases (promoter) to which it binds. RNA polymerase opens the DNA double helix to allow incorporation of ribonucleotides.

Elongation

Addition of successive ribonucleotides to form the RNA strand. RNA polymerase moves along the DNA template strand, reading it in the 3' to 5' direction, while synthesizing RNA in the 5' to 3' direction.

In eukaryotes, after approximately 30 ribonucleotides are added, a 5' cap is added, formed by methylguanosine triphosphate. This cap serves as a recognition signal for ribosomes during translation.

Termination

RNA polymerase recognizes specific DNA signals indicating the termination of transcription. At the end, the DNA double helix reforms.

RNA Maturation

In prokaryotes, mRNA can be used directly after transcription, but rRNA and tRNA are transcribed as longer precursors that require processing (cutting).

In eukaryotes, the primary RNA transcript (pre-mRNA) contains both introns and exons. Maturation involves removing the introns and splicing the exons together.

The Genetic Code

The genetic code has the following features:

• It consists of the linear sequence of nitrogenous bases (codons).
• Between successive codons, there are no spaces or gaps, nor do they share bases.
• Codons are read in the 5' to 3' direction.
• It is universal for almost all living things, indicating a single evolutionary origin. There are exceptions (e.g., mitochondria, some protozoa) where the code is slightly different.
• It is slightly degenerate, meaning most amino acids are encoded by more than one codon. Usually, the difference is in the third nucleotide. This is an advantage because a change in a base may not alter the amino acid sequence.
• There is no ambiguity; one codon encodes only one specific amino acid.
• The same mRNA molecule can have multiple initiation codons, potentially allowing the synthesis of more than one polypeptide from a single mRNA.