Question

In eukaryotes, what types of modifications occur to pre-mRNAs?

Short answer

In eukaryotes, pre-mRNAs undergo three main types of modifications: 5' capping, 3' polyadenylation, and RNA splicing.

Step-by-step solution

Introduction to pre-mRNA

Pre-messenger RNA (pre-mRNA) is a type of primary transcript and the initial version of mRNA which is directly synthesized from a DNA template during transcription. However, before becoming the mature mRNA that can be translated into protein sequences, pre-mRNA undergoes a series of modifications.

5' capping

The first modification is the addition of a cap to the 5' end of the pre-mRNA. This 'cap' is a modified guanine nucleotide, which protects the pre-mRNA from degradation, assists in ribosome binding during translation, and aids in the export of the mRNA from the nucleus.

3' polyadenylation

The second modification is the addition of a poly-A tail to the 3' end of the pre-mRNA. This tail, which consists of approximately 200 adenine nucleotides, also helps to protect the pre-mRNA from degradation, assists in nuclear export, and aids in translation.

RNA splicing

The third and potentially the most crucial modification is RNA splicing. During splicing, introns, or non-coding sequences, are removed from the pre-mRNA, and exons, or coding sequences, are joined together. This process is conducted by a complex called the spliceosome and results in a mature mRNA molecule consisting only of exons that can be translated into protein sequences.