Page 22 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

PART II What Genes Are and What They Do
6

chapter

DNA Structure,

Replication, and

Recombination

The double-helical structure of DNA provides an explanation for
the transmission of genetic information from generation to
generation over billions of years.
© Adrian Neal/Getty Images RF

chapter outline

• 6.1 Experimental Evidence for DNA as the
Genetic Material
• 6.2 The Watson and Crick Double Helix Model
of DNA
FOR NEARLY 4 BILLION YEARS, the double-stranded DNA • 6.3 Genetic Information in Nucleotide Sequence
molecule has served as the bearer of genetic information. • 6.4 DNA Replication
It was present in the earliest single-celled organisms and • 6.5 Homologous Recombination at the DNA Level
in every other organism that has existed since. Over that
long period of time, the hardware—the structure of the • 6.6 Site-Specific Recombination
molecule itself—has not changed. In contrast, evolution
has honed and vastly expanded the software—the pro-
grams of genetic information that the molecule stores, expresses, and transmits from
one generation to the next.
Under special conditions of little or no oxygen, DNA can withstand a wide range
of temperature, pressure, and humidity and remain relatively intact for hundreds, thou-
sands, even tens of thousands of years. Molecular sleuths have retrieved the evidence:
38,000-year-old DNA from a Neanderthal skeleton (Fig. 6.1). Amazingly, this ancient
DNA still carries readable sequences—shards of decipherable information that act as
time machines for the viewing of genes in this long-vanished species. Comparisons
with homologous DNA segments from living people make it possible to identify the
precise mutations that have fueled evolution.
For example, comparisons of Neanderthal and human DNA have helped anthro-
pologists settle a long-running debate about the genetic relationship of the two. The
evidence shows that Neanderthals and our own species, Homo sapiens, last shared a
common ancestor between 600,000 and 800,000 years ago. Neanderthal ancestors
migrated to Europe about 400,000 years ago while our own ancestors remained in
Africa. The two groups remained out of contact until 40,000 years ago, when Homo
sapiens first arrived in Europe. Within a few millennia, the Neanderthals were extinct.
However, their recently recovered DNA suggests that during the 10,000 years that
Neanderthals shared Europe with Homo sapiens, some interbreeding took place; 1–4%
of the genomes of modern non-Africans can be traced to Neanderthals.
Francis Crick, co-discoverer of DNA’s double-helical structure and a leading
twentieth-century theoretician of molecular biology, wrote that “almost all aspects
of life are engineered at the molecular level, and without understanding molecules,

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