USC Scientists Find DNA Replication Begins When the Double Helix Melts
12 12 2016 For years, scientists have puzzled over what prompts the intertwined double helix DNA to open its two strands and then start replication. Knowing this could be the key to understanding how organisms — from healthy cells to
12/12/2016 – For years, scientists have puzzled over what prompts the
intertwined double-helix DNA to open its two strands and then start
replication. Knowing this could be the key to understanding how organisms —
from healthy cells to cancerous tumors — replicate and multiply for their
USC scientists believe they have solved the mystery. Replication
is prompted by a ring of proteins that bond with the DNA at a special location
known as “origin DNA.” The ring tightens around the strands and melts them to
open up the DNA, initiating replication.
This all takes place at a nano level that is impossible to see
with the naked eye. A strand of DNA is only about one nanometer in size — not
even close to the width of a human hair which is roughly equivalent to 100,000
The researchers made their discovery by studying SV40, a
cancerous virus. The virus hijacked the DNA replication process with a ring of
proteins called a “helicase” that mimicked the rings of proteins that prompt
genetic replication in healthy cells.
The findings were published on Dec. 6 in the journal eLife.
“Understanding the mechanisms of origin DNA opening or melting
allows us to learn this fundamental process of genetic duplication,” said
corresponding author Xiaojiang Chen, a professor of biological sciences and
chemistry at the USC Dornsife College of Letters, Arts and Sciences and
director of the college’s Center of Excellence in NanoBiophysics. “The knowledge
we have gained may be applicable for future intervention of this process to
block the replication of viral pathogens and cancer cells.”
When the origin DNA melts, the double helix divides into
separate strands, Chen explained. Those DNA strands then become the template
for faithful duplication of other strands — a Xerox copy of their parental DNA.
As soon as replication is complete, one double helix DNA now becomes two exact
copies of the same double helix.
“DNA replication is critical for heredity and survival,” said
Chen, who also is affiliated with the Norris Comprehensive Cancer Center at the
Keck School of Medicine at USC. “The origin DNA’s opening is an essential step
for DNA replication in our cells and for some tumor viral pathogens to
replicate and spread.”
Why is origin DNA so special? Regular DNA sequences contain the
A, T, G and C nucleotides, more or less in equal ratio. But origin DNA
sequences contain more A and T nucleotides than usual.
To prompt replication, the scientists used a helicase from a
“Large Tumor Antigen” or Large T. The antigen comes from the SV40 virus linked
to human cancers such as brain and bone cancers, mesothelioma and lymphoma. The
six proteins from Large T comprise a “helicase” that mimics the structure of
the healthy cells’ helicases.
The scientists obtained a 3-D view of the atomic structure of
the helicase using X-ray crystallography, a technique for examining
nano-biomolecules and their structures at the atomic level that has been
refined over centuries. Chen said the images revealed that the proteins
surrounding the DNA had attached to it, then tightened like a vice until the
bonds between the two strands of the double helix broke — or melted — the
Although the scientists used a cancerous virus to study
replication, healthy cells replicate in a similar way, Chen said.
Other co-authors were Dahai Gai, also of USC Dornsife, Damian
Wang of the Keck School of Medicine at USC and Shu-Xing Li of USC Dornsife’s
Center for Excellence in NanoBiophysics.
The study was funded by a National Institutes of Health grant
caption: This ribbon diagram depicts a magnified view of genetic
replication in the making, as a group of proteins surround the DNA strands.
Each of the six proteins is in distinct color, with origin DNA in the center.
Credit: Xiaojiang Chen, USC.]