Molecular Biology: The Flow of Genetic Information

Flow of Genetic Information

The transcript originates within the nucleus. To assemble amino acids into proteins, a molecule is required to act as an intermediary between DNA and ribosomes; this intermediary is mRNA. The formation of RNA is called transcription.

During translation, a peptide chain is synthesized using the information contained in mRNA, involving rRNA and tRNA, which carry amino acids to the ribosomes. The genetic transfer from parents to offspring occurs through the following steps:

• Complementarity of nitrogenous bases.
• Replication and transcription into mRNA.
• Translation by tRNA and rRNA.

Due to replication mechanisms, some viruses have altered the standard flow of genetic information, utilizing RNA-replicase to copy RNA or reverse transcriptase to perform reverse transcription. Consequently, the central dogma of molecular biology is redefined as follows:

Transcription (DNA Synthesis)

This process occurs in the nucleus and requires:

• DNA Template: Of the two DNA strands, only one (the template strand) is transcribed, while the other (the coding strand) is not.
• Enzymes: Catalyzed by RNA polymerases.
• Ribonucleotides: A, G, C, and U, joined by ester bonds between phosphoric acid and the OH group.

Transcription consists of three stages: initiation, elongation, and termination, followed by RNA splicing.

Initiation

It begins when RNA polymerase recognizes specific DNA signals called promoter sites. These sequences of nitrogenous bases bind to the RNA polymerase, causing the double helix to open.

Elongation

Ribonucleotides are added to form RNA. RNA polymerase moves along the DNA template strand in the 3' to 5' direction, while RNA synthesis occurs in the 5' to 3' direction. In eukaryotes, a cap formed by methyl-guanosine-phosphate is added to the 5' end as a recognition signal for reading.

Termination

RNA polymerase recognizes termination signals indicating the end of the transcript.

• Prokaryotes: A palindromic base sequence (G-rich followed by several T's) forms an RNA loop, favoring DNA separation.
• Eukaryotes: The cutting signal is a polyadenylation sequence (AAUAAA). After separation, an enzyme (poly-A polymerase) inserts a poly-A tail at the 3' end, which is essential for maturation and transport out of the nucleus.