What is Pre mRNA ?

 Pre-mRNA (precursor messenger RNA) is made through a process called transcription, which occurs in the nucleus of eukaryotic cells. Here’s how it happens step by step:

1. Initiation

• RNA polymerase II binds to the promoter region of a gene, typically at the TATA box.
• Transcription factors help RNA polymerase recognize and attach to the promoter.
• The DNA strands unwind, exposing the template strand (also called the antisense strand).

2. Elongation

• RNA polymerase moves along the DNA template strand (3' to 5'), synthesizing pre-mRNA in the 5' to 3' direction.
• Ribonucleotides (A, U, C, G) pair with their complementary DNA bases (U replaces T in RNA).
• RNA polymerase catalyzes phosphodiester bonds between nucleotides to form the growing pre-mRNA strand.

3. Termination

• Transcription continues until RNA polymerase reaches a termination signal (polyadenylation signal: AAUAAA).
• The pre-mRNA is released, and RNA polymerase detaches from the DNA.

4. Pre-mRNA Processing (Before Becoming Mature mRNA)

Before pre-mRNA can be used for protein synthesis, it undergoes processing:

1. 5' Capping

   • A modified guanine (7-methylguanosine cap) is added to the 5' end to protect against degradation and help with ribosome recognition.

2. Splicing

   • Introns (non-coding regions) are removed, and exons (coding regions) are joined together by a complex called the spliceosome.

3. 3' Polyadenylation

   • A poly-A tail (100-250 adenine nucleotides) is added to the 3' end to protect mRNA stability and help with export from the nucleus.

Final Step: Mature mRNA

• Once processed, the pre-mRNA becomes mature mRNA and leaves the nucleus for translation in the cytoplasm.