De novo or "orphan" genes, whose sequence shows no similarity to existing genes, are extremely rare. Estimates of the number of de novo genes in the human genome range from 18 to 60. Such genes are typically shorter and simpler in structure than most eukaryotic genes, with few if any introns. Two primary sources of orphan protein-coding genes are gene duplication followed by extremely rapid sequence change, such that the original relationship is undetectable by sequence comparisons, and formation through mutation of "cryptic" transcription start sites that introduce a new open reading frame in a region of the genome that did not previously code for a protein. COUNT 5
QSN1 What is a gene whose sequence shows no similarity to existing genes called? ANS1 De novo or "orphan" genes QSN2 What is the estimate of the number of orphan genes in the human genome? ANS2 18 to 60 QSN3 How do the lengths of orphan genes compare to most eukaryotic genes? ANS3 typically shorter QSN4 How does the structure of orphan genes compare to most eukaryotic genes? ANS4 simpler in structure QSN5 What is one primary source of orphan protein-coding genes? ANS5 gene duplication followed by extremely rapid sequence change