List three types of alternative splicing patterns and how theylead to the production of different protein isoforms.

Alternative splicing is a regulated process during gene expression that results in a single gene encoding multiple proteins. This process occurs when different exons of a pre-mRNA transcript are joined together, resulting in different mRNA molecules that can be translated into different protein isoforms, thus increasing the diversity of the proteome.

Exon skipping is a common type of alternative splicing where an exon (a nucleotide sequence within a gene) is skipped or excluded during splicing. In this pattern, exons may be included or excluded, depending on certain regulatory signals or sequences. This results in mRNA molecules with different combinations of exons, which then leads to the production of several protein isoforms. For example, let's consider a gene with exons A, B, C, and D. Through exon skipping, we can have the following combinations of exons: 1. A-B-C-D 2. A-C-D 3. A-B-D In this example, three different mRNA transcripts will produce three different protein isoforms.

Alternative 5' or 3' splice sites refer to the selection of alternative sites for splicing at the boundaries of exons and introns. In this pattern, a single exon can be extended or shortened by selecting different splice sites, leading to different sizes of the mRNA molecule and protein isoforms. For example, let's consider a gene with exons A, B, and C, and a 3' splice site exists in two locations within exon B: 1. A-B1-C 2. A-B2-C In this example, depending on the 3' splice site selected within exon B, two different mRNA transcripts will produce two different protein isoforms.

Intron retention is another type of alternative splicing where an intron (a non-coding region of the gene) is not removed from the pre-mRNA molecule. This retained intron makes the mRNA molecule larger, which can then be translated into a protein isoform with additional amino acids. For example, let's consider a gene with exons A and B with an intron, I, between them. During regular splicing, intron I would be removed, producing the following mRNA sequence: A-B. However, when intron retention occurs, the mRNA sequence would result in A-I-B. This would lead to the production of a different protein isoform. In conclusion, alternative splicing patterns, like exon skipping or cassette exon, alternative 5' or 3' splice sites, and intron retention, play significant roles in generating diverse protein isoforms from a single gene.