Y Chromosome UPSC NOTE

 Y Chromosome

  • Specialised chromosomes determine birth sex in humans and other mammals. 

  • Females have a pair of X chromosomes, whereas males have a single X and a much smaller Y chromosome.

  • The Y chromosome is male-determining because it bears a gene called SRY, which directs the development of a ridge of cells into a testis in the embryo. 

  • The embryonic testis make male hormones, and these hormones direct the development of male features in a baby boy.

  • Without a Y chromosome and a SRY gene, the same ridge of cells develops into an ovary in XX embryos. 

  • Female hormones then direct the development of female features in the baby girl.

  • The Y chromosome is very different from X and the 22 other chromosomes of the human genome. 

  • Y chromosome is smaller and bears few genes (only 27 compared to about 1,000 on the X).

  • These genes include SRY, a few genes required to make sperm, and several genes that seem to be critical for life – many of which have partners on the X. 

  • Many Y genes (including the sperm genes RBMY and DAZ) are present in multiple copies. 

  • Some Y genes occur in weird loops in which the sequence is inverted and genetic accidents that duplicate or delete genes are common.

  • The Y also has a lot of DNA sequences that don’t seem to contribute to traits. 

  • This “junk DNA” is comprised of highly repetitive sequences that derive from bits and pieces of old viruses, dead genes and very simple runs of a few bases repeated over and over.

  • This last DNA class occupies big chunks of the Y that literally glow in the dark; you can see it down the microscope because it preferentially binds fluorescent dyes.

Sequencing of Y Chromosome

  • It’s only recently that new technology has allowed sequencing of bases along individual long DNA molecules, producing long-reads of thousands of bases. 

  • These longer reads are easier to distinguish and can therefore be assembled more easily, handling the confusing repetitions and loops of the Y chromosome.

  • The Y is the last human chromosome to have been sequenced end-to-end, or T2T (telomere-to-telomere). 

  • Even with long-read technology, assembling the DNA bits was often ambiguous, and researchers had to make several attempts at difficult regions – particularly the highly repetitive region.

Implications from the sequencing

  • A few new genes have been discovered, but these are extra copies of genes that were already known to exist in multiple copies. 

  • The border of the pseudoautosomal region (which is shared with the X) has been pushed a bit further toward the tip of the Y chromosome.

  • We now know the structure of the centromere (a region of the chromosome that pulls copies apart when the cell divides), and have a complete readout of the complex mixture of repetitive sequences in the fluorescent end of the Y.

  • Some groups will now examine the details of Y genes. 

  • They will look for sequences that might control how SRY and the sperm genes are expressed, and to see whether genes that have X partners have retained the same functions or evolved new ones.

  • Others will closely examine the repeated sequences to determine where and how they originated, and why they were amplified. 

  • Many groups will also analyse the Y chromosomes of men from different corners of the world to detect signs of degeneration, or recent evolution of function.



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Learnerz IAS | Concept oriented UPSC Classes in Malayalam: Y Chromosome UPSC NOTE
Y Chromosome UPSC NOTE
Learnerz IAS | Concept oriented UPSC Classes in Malayalam
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