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Pseudogene – Definition, Types and Significance

A pseudogene is a non-functional DNA sequence that resembles a functional gene but cannot produce a working protein due to mutations or structural changes.

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Things worth knowing about "Pseudogene"

A pseudogene is a non-functional DNA sequence that resembles a functional gene but cannot produce a working protein due to mutations or structural changes.

What Is a Pseudogene?

A pseudogene is a segment of DNA that closely resembles a functional gene in sequence but is unable to produce a functional protein due to accumulated mutations or structural alterations. The term derives from the Greek word pseudo (false, apparent) combined with gene. Pseudogenes are considered molecular relics of evolution and are found in the genomes of virtually all known organisms.

Origins of Pseudogenes

Pseudogenes arise through several distinct mechanisms:

Duplication and divergence: A functional gene is copied during genome duplication. Over evolutionary time, the resulting copy accumulates mutations until it loses its original function.
Retrotransposition: Processed or retrotransposed pseudogenes are created when a mature messenger RNA (mRNA) is reverse-transcribed into DNA by the enzyme reverse transcriptase and reinserted at another location in the genome. These pseudogenes typically lack introns and their own promoter regions.
Point mutations: Individual nucleotide changes within an existing gene can introduce a stop codon or shift the reading frame, thereby inactivating the gene.

Types of Pseudogenes

Duplicated Pseudogenes

These arise from faulty gene duplication events. They retain introns and regulatory sequences but are inactivated by mutations.

Processed Pseudogenes (Retropseudogenes)

These originate from the genomic reintegration of mRNA copies. Because they derive from processed mRNA, they characteristically lack introns and promoter regions.

Unitary Pseudogenes

These are genes that remain functional in other species but have been inactivated by mutation in a specific lineage. A well-known example is the GULO gene, which encodes an enzyme for ascorbic acid (Vitamin C) synthesis in many animals but exists as a pseudogene in humans and other primates.

Significance in Medicine and Genetics

Although pseudogenes were historically considered genomic junk, modern research has revealed that they can have genuine biological relevance:

Regulation of gene expression: Some pseudogene transcripts can act as non-coding RNA molecules and regulate the expression of their functional counterparts, for example by sequestering microRNAs (miRNAs).
Diagnostic importance: Knowledge of pseudogenes is essential in molecular diagnostics, as pseudogenes can cause misinterpretation in genetic tests when mistakenly analyzed as active genes.
Evolutionary research: Pseudogenes serve as molecular clocks, providing insight into the evolutionary history of organisms.
Disease relevance: Failure to distinguish pseudogenes from functional genes can be clinically significant, for example in the diagnosis of congenital adrenal hyperplasia caused by mutations in the CYP21A2 gene, which has a highly similar pseudogene counterpart.

Pseudogenes in the Human Genome

More than 10,000 pseudogenes have been identified in the human genome to date. With the advancement of next-generation sequencing technologies and bioinformatic tools, their number and functional diversity are becoming increasingly well understood. Databases such as GENCODE and PseudoPipe systematically catalog human pseudogenes.

References

Balakirev, E. S. & Ayala, F. J. (2003). Pseudogenes: Are They Dead or Alive? Annual Review of Genetics, 37, 123-151.
ENCODE Project Consortium (2012). An integrated encyclopedia of DNA elements in the human genome. Nature, 489(7414), 57-74.
Poliseno, L. et al. (2010). A coding-independent function of gene and pseudogene mRNAs regulates tumour biology. Nature, 465(7301), 1033-1038.

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