Explain how and when mRNA is modified

mRNA, or messenger RNA, is modified during and after its synthesis in a process known as RNA processing. This process occurs in eukaryotic cells and is essential for the proper functioning of mRNA. Here's a step-by-step explanation of how and when mRNA is modified:

1. Transcription: The process begins with the transcription of DNA into mRNA in the nucleus of the cell. This is done by an enzyme called RNA polymerase.

2. 5' Capping: Once the mRNA has been transcribed, it undergoes a modification at the 5' end, known as 5' capping. This involves the addition of a modified guanine nucleotide. This cap protects the mRNA from degradation, assists in the export of the mRNA from the nucleus to the cytoplasm, and is involved in initiating translation (the process of protein synthesis).

3. Splicing: The next modification is splicing. Eukaryotic genes are composed of exons, which are coding sequences, and introns, which are non-coding sequences. During splicing, the introns are removed from the mRNA, and the exons are joined together. This process is carried out by a complex of proteins and RNA molecules known as the spliceosome.

4. 3' Polyadenylation: The final modification is the addition of a poly(A) tail at the 3' end of the mRNA, a process known as polyadenylation. This tail, made up of a string of adenine nucleotides, protects the mRNA molecule from enzymatic degradation in the cytoplasm and aids in transcription termination, export of the mRNA from the nucleus, and translation.

5. Export: After these modifications, the mRNA is transported out of the nucleus and into the cytoplasm, where it will be translated into a protein by the ribosome.

These modifications to the mRNA occur during and after its synthesis, and they are crucial for the correct processing and expression of genetic information.