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"Dark Heart of the Human Genome" – Surprising influence of Neanderthals



  Neanderthal vs. Human

"It is the heart of the darkness of the genome, we warn students against going there," said Charles Langley, a professor of evolution and ecology at the University of California, Davis. Geneticists exploring the dark heart of the human genome have discovered large pieces of Neanderthals and other ancient DNA. The results open up new possibilities to study how chromosomes behave during cell division and how they have changed during human evolution.

The central region of the chromosomes, the centromere, contains DNA that has remained largely unaltered for hundreds of years. Researchers at UC Davis and the Lawrence Berkeley Laboratory have found out for thousands of years. Part of this DNA comes from Neanderthals or other relatives or ancestors of humans from the migration of modern humans from Africa.

  The central region of the chromosomes, the centromere,

centromere sit in the middle of the chromosomes, the pinched "waist" in the image of a chromosome from a biology textbook. The centromere anchors the fibers that disassemble the chromosomes during cell division. This means that they are of great importance in understanding cell division dysfunctions that lead to cancer or genetic defects.

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However, centromere DNA contains many repetitive sequences, and scientists were unable to map this region properly. "It's the heart of the darkness of the genome, we warn students against going there," said Langley, senior author on a paper describing the work that will be published in an upcoming issue of eLife magazine.

Langley and colleagues Sasha Langley and Gary Karpen of the Lawrence Berkeley Laboratory and Karen Miga of UC Santa Cruz concluded that there could be haplotypes – groups of genes that are inherited together in human evolution – that stretch across Extend parts of our genome and even across the centromere.

because the centromere is not involved in the "crossover" process that occurs when cells divide to form sperm or egg cells. During crossover, paired chromosomes line up side by side and their limbs intersect. Sometimes they cut and splice DNA between them so that the genes can be mixed. In the vicinity of centromeres, however, the transitions fall to zero. Without this mixing in each generation, centromeres could preserve intact portions of the DNA intact.

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The researchers looked for inherited single nucleotide polymorphisms – inherited variations in a single letter of DNA – that would enable them to map haplotypes in the centromere.

First they showed that they could identify centromeric haplotypes or "cenhaps" in Drosophila fruit flies.

This finding has two implications, Langley said. First, researchers who can distinguish chromosomes from one another by means of their centromeres can begin to perform functional tests to determine if these differences affect the inherited piece of DNA. For example, during egg formation, four chromatids are made up of two chromosomes, but only one creates it into the egg. Scientists want to know: Are certain centromere haplotypes transmitted more frequently? And are some haplotypes more likely to be involved in errors?

Second, researchers can use centromeres to study lineage and evolutionary lineage.

With respect to human DNA, the researchers studied centromere sequences from the 1000 Genome Project, a public catalog of human variation. They discovered haplotypes that span the centromeres in all human chromosomes.

In the X chromosome of these genome sequences, they found several receding centromeric haplotypes that represented lines half a million years old. Throughout the genome, most of the diversity is found among African genomes, which is consistent with the recent spread of humans outside the African continent. One of the oldest centromere haplotype lineages was not supported by these early emigrants.

In chromosome 11, they found strongly divergent haplotypes of Neanderthal DNA in non-African genomes. These haplotypes ranged between 700,000 and one million years ago, when ancestors of Neanderthals separated from other human ancestors. The centromere of chromosome 12 also contains an even older, archaic haplotype, apparently derived from an unknown relative.

This Neanderthal DNA on chromosome 11 could still influence the differences in our sense of smell. The cells that respond to taste and odor carry odor receptors triggered by specific chemical signatures. Humans have about 400 different genes for olfactory receptors. Thirty-four of these genes are located within the chromosome 11 centromere haplotype. The centromeric Neanderthal haplotypes and a second ancient haplotype account for about half of the variation of these odor receptor proteins.

From the work of others it is known that genetic variations in odor receptors can influence the sense of taste and smell, but the functional effects of the differences noted in this study have yet to be discovered and their effects on taste and odor analyzed.

The Daily Galaxy over UC Davis

Credit: Thanks to Philipp Gunz (CC BY-NC-ND 4.0) [19659027] (function (d, s, id) {
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