The genes coding for human hemoglobins are located on chromosomes 11 and 16. As illustrated above, chromosome 11 contains the epsilon chain gene, two gamma chain genes (symbolized by Gγ and Aγ), a delta chain gene, and a beta chain gene. Chromosome 16 contains a zeta gene and two alpha chain genes (α1 and α2). There are also DNA segments that are structurally similar to the expressed hemoglobin genes. These pseudogenes were probably functional in the past but are now incapable of generating a product because of mutations they contain. These pseudogenes are represented by black boxes in the diagram above. The globin genes on chromosome 16 (the alpha-like cluster) and on chromosome 11 (the beta-like cluster) are thought to have arisen by the duplication of a single ancestral gene some 450 million years ago. The table shows the segmental organization of the transcription units (TUs) from the globin genes. All contain 5′ and 3′ ends (which are not translated), three exons (E1, E2, E3), and two introns (I1 and I2) that are spliced out of the mature mRNA. The numbers represent base pairs. The major difference between the two gene clusters is that the second introns of the beta-like genes contain about six times as many base pairs as the second introns of alpha-like genes. Transcription begins 40 to 50 base pairs upstream of E1, and ends about 100 base pairs downstream from E2. Subsequently the 5′ end receives a methylated cap (q.v.), and the 3′ end loses a segment of about 25 nucleotides but gains a tail of 50 or more adenylic acids. See Chronology, 1961, Ingram; 1976, Efstratiadis et al.; 1977, Tilghman et al.; 1979, Fritsch et al.; 1985, Saiki et al.; 1986, Constantini et al.; compound heterozygote, gene, hereditary persistence of hemoglobin F, myoglobin gene, polyadenylation, posttranscriptional processing, sickle-cell anemia, thalassemias, transcription unit.
Subjects: Genetics and Genomics.