Transcription & Translation | From DNA to RNA to Protein

Transcription is the process where a section of DNA is copied or transcribed into RNA. This is explained using an analogy of taking a picture of a recipe page from a cookbook, capturing only a fraction of the total information.

Transcription involves three main steps: initiation, elongation, and termination. Initiation begins when RNA polymerase finds a promoter sequence on the DNA, signaling where and when transcription should start. The RNA polymerase then separates the double-stranded DNA and begins transcribing one strand, the template strand, into RNA.

During elongation, RNA polymerase reads the template DNA strand and builds a complementary RNA strand by adding appropriate nucleotides. The resulting RNA strand is nearly identical to the non-template DNA strand, with the key difference being that thymine (T) in DNA is replaced by uracil (U) in RNA.

Termination occurs when RNA polymerase reaches a specific DNA sequence called the Terminator sequence, signaling the end of transcription. Upon reaching this sequence, the newly formed RNA strand detaches from the DNA template. Afterward, the RNA undergoes modifications before leaving the nucleus for translation.

Translation is the process of synthesizing a protein based on an RNA blueprint. This process happens on ribosomes, which can be found either attached to the rough endoplasmic reticulum or floating freely in the cytoplasm. Continuing the recipe analogy, translation is like making the cookies using the recipe on your phone.

Before diving deeper into translation, it's essential to understand codons. Codons are sequences of three nucleotides in RNA that code for a specific amino acid, which are the building blocks of proteins. RNA contains the nucleotides U, C, A, and G. There are 61 known codons, each specifying one of 20 amino acids.

Once the RNA leaves the nucleus, it attaches to a ribosome, and translation starts. The ribosome reads the RNA strand until it encounters the 'AUG' sequence, which is the start codon, initiating protein synthesis. Transfer RNA (tRNA) molecules bring specific amino acids to the ribosome, matching their anticodons to the codons on the mRNA. For example, the start codon AUG pairs with a tRNA carrying the anticodon UAC and the amino acid methionine. This process continues, linking amino acids to form a polypeptide chain.

Protein synthesis ceases when the ribosome reaches one of three stop codons: UAA, UAG, or UGA. These stop codons do not code for an amino acid but instead signal the ribosome to stop translation, leading to the release of the newly formed polypeptide chain for further modification before it becomes a functional protein.