Question

Even if we know where the genes are present in a genome, it is not entirely clear how to count them. Give two reasons for this problem.

Hint:

While the human genome contains only about 20,000 protein-coding genes (fewer than originally expected), the human proteome is much larger, containing over 100,000 distinct proteins due to alternative splicing and post-translational modifications.

Answer 1

Step 1: Concept
Identifying and counting the exact number of genes in a sequenced genome is a major bioinformatics challenge because the molecular definition of a gene is complex.

Step 2: First Reason --- Alternative Splicing
In eukaryotic genomes, most primary transcripts (pre-mRNA) undergo alternative splicing, where exons are joined in different combinations. A single genetic locus can produce multiple distinct mRNAs, which are translated into different functional proteins. This alternative splicing makes it difficult to define a gene simply as "one region encoding one protein."

Step 3: Second Reason --- Pseudogenes and Non-coding RNAs
1. Pseudogenes: Genomes contain numerous non-functional gene copies (pseudogenes) that have accumulated mutations (stop codons, frameshifts) over evolutionary time. Distinguishing functional genes from pseudogenes using sequence analysis alone is highly challenging.
2. Non-coding RNAs: Many genes do not code for proteins at all. Instead, they transcribe functional non-coding RNA molecules (such as microRNAs, lncRNAs, snRNAs, and rRNAs). Since these genes lack open reading frames (ORFs), automated gene-prediction software often misses them.
Final Answer: Counting genes is challenging because (1) alternative splicing allows one gene to produce multiple distinct proteins, and (2) genomes contain many non-functional pseudogenes and genes encoding non-coding RNAs, which are difficult to identify and classify.