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Overview of the Respiratory System

Author(s): Nancy Moreno, PhD, Deanne Erdmann, MS, and Sonia Rahmati Clayton, PhD
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The Human Genome (II)

The haploid human genome (23 chromosomes) contains ~3 billion base pairs of DNA. The diploid human genome (23 pairs of chromosomes) contains ~6 billion base pairs of DNA.

Only 1.5-3% of the human genome codes for proteins. It is believed that much of the rest of the human genome serves regulatory and structural functions. The exact number of genes carried by humans is not yet known, but is likely to be somewhere between 35,000 and 100,000.

Humans are more than 99% identical to each other at the DNA level. Only identical twins possess identical genomic DNA. It is the <1% variability in our DNA that makes each of us unique. In recent years, the variability in human DNA has been used as a fossil record to study human history and trace the migration of humans out of Africa as we spread across and populated the Earth. What we have learned from our DNA is clear: every human on the planet today is related to every other human. We all are descended from the same trunk of the human family tree: brothers and sisters separated only by time and geography.

The photograph in this slide is of twin stacks containing a total of eighty-nine Houston telephone books, spiraled to mimic the shape of the DNA double helix. Together the two stacks of telephone books contain approximately the same number of characters as the haploid human genome (23 chromosomes) contains base pairs.