Draw the structures that comprise the lariat branch point formed during mRNA splicing: the invariant A, its \(5^{\prime}\) - \(\mathrm{R}\) neighbor, its \(3^{\prime}-\mathrm{Y}\) neighbor, and its \(2^{\prime}\) - \(G\) neighbor.

The process of splicing messenger RNA (mRNA) is pivotal in the gene expression pathway. Here, the lariat branch point structure plays a crucial role, ensuring that non-coding sequences, known as introns, are efficiently removed while coding sequences, or exons, are joined together.

At its core, the lariat structure is a looped intermediate that forms when a specific adenine nucleotide in the intron attacks the 5' splice site. This reaction results in a 'branch point' where the intron is covalently linked to the adenine at a 2'-5' phosphodiester bond. This crucial adenine is the invariant nucleotide known for its constancy throughout various mRNA splicing events.

The 'lariat' refers to the loop-like shape created, akin to a cowboy's lasso. During the splicing process, the branch point aids in bringing the splice sites close together. This proximity allows the precise excision of introns and the ligation of exons in what can be likened to a highly choreographed molecular dance.

The invariant adenine residue is the linchpin in the lariat branch point structure during mRNA splicing. This particular adenine's role is non-negotiable; hence it's termed invariant due to its unwavering presence across splicing events.

In terms of structure, adenine is a purine base symbolized by the letter 'A'. It's one of the four nucleobases in RNA and is distinguished by its two-ringed shape, an amine group at position 6, and a hydrogen at the 1' nitrogen. What makes this adenine invariant is its key role as the nucleophile that attacks the 5' splice site during splicing, initiating the formation of the lariat.

It's critical for students to visualize the invariant adenine correctly in order to understand its significance. The specificity of this reaction relies on the nucleotide's unique chemical structure, which enables it to perform a precise function that other nucleotides cannot mimic. This emphasizes the exquisite molecular selection that governs biological processes such as splicing.

Nucleobase neighbors refer to the nucleotides adjacent to the invariant adenine residue in the lariat branch point structure. These neighbors not only provide structural context but also participate actively in splicing chemistry.

In the given exercise, the neighbors are identified as the following: on the 5' side, a guanine residue symbolized by 'R' indicating it's a purine which typically would be either adenine or guanine. Here it specifically signifies guanine. On the 3' side, there's a nucleobase symbolized by 'Y', which stands for any pyrimidine and could be cytosine (C), uracil (U), or thymine (T). And, at the 2' position, we find another guanine (G) which plays a part in forming the lariat.

Understanding the roles and structures of these nucleobase neighbors is crucial, as their positions and interactions with the invariant adenine affect the splicing mechanism's efficiency and accuracy. Any irregularities or mutations in these neighboring bases can potentially lead to errors in splicing, underscoring the need for precision in this molecular process.