The Human X and Y Chromosomes
Sex chromosomes are structurally differentiated in mammals, but not in birds
Pseudoautosomal (recombining) ends on the X and Y chromosomes
Male-specific region of human Y constitutes roughly 95% of its length
Male specific region (MSY)
Two arms are termed Yp and Yq
Roughly 156 transcription units
78 protein-coding transcription units
60 are members of 9 different MSY-specific gene families
Remaining 18 are all single copy
Two, RPS4Y1 and RPS4Y2 are 93.6% identical in coding regions, but highly divergent in introns
Three classes of sequence
X-transposed
Result of a recent (3-4 Ma), large transposition. Postdates human-chimpanzee divergence.
Y sequences are 99% identical to human Xq21 (X chromosomal) sequence
Broken into two non-contiguous segments in modern Y
Identified only two genes, both with homologs in Xq21
LINE elements account for ca 36% of X-transposed (genome average is 20%)
X-degenerate
Regions identifiably homologous to X chromosome, but non-recombining.
27 single copy (14) or pseudogene (13) homologs of X-linked genes.
Individual genes show 60-96% identity to X genes.
Many of the genes that are expressed are expressed in all tissues
ampliconic
Sequences with strong identity to other regions of the MSY
Evolutionary stratification of X-Y genes
Lahn and Page
Measured Ks -- average silent site divergence between X and Y homologs
Palindromes, inverted repeats
Transcriptionally active tandem arrays
Include a number of spliced, but apparently non-coding transcriptional units
Decay in the absence of sexual recombination
Y-Y gene conversion in ampliconic regions
Centromeres and Telomeres
Amplicon - (DNA) an amplified region
Originally referred to the amplification of DNA by artificial cloning vectors
Extended to refer to the repetitive unit of any stretch of naturally occurring repetitive DNA
Now also used to refer to the amplified products of the Polymerase Chain Reaction (PCR)
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Platypus sex chromosomes:
Carrel, L. 2005. Chromosome chain makes a link. Nature 432:817-818.
Grutzner, F., W. Rens, E. Tsend-Ayush, N. El-Mogharbel, P. C. M. O'Brien, R. C. Jones, M. A. Ferguson-Smith, and J. A. Marshall Graves. 2004. In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes. Nature 432: 913.