Crystal Clear: X-Ray Snapshots Illuminate How Enzymes Stitch Together DNA

By Travis, John | Science News, February 14, 1998 | Go to article overview

Crystal Clear: X-Ray Snapshots Illuminate How Enzymes Stitch Together DNA


Travis, John, Science News


In 1953, James Watson and Francis Crick earned immortality in the annals of science by identifying the three-dimensional shape of deoxyribonucleic acid, better known as DNA--the chemical that makes up genes. In 1968, Watson set down his account of the race to this discovery in The Double Helix, a book whose title succinctly summarizes the structure of DNA.

This helical shape results from two intertwined strands of nucleotides, the building blocks of DNA. Watson and Crick argued that DNA's four nucleotides pair only in certain combinations--an adenine on one strand normally joins only to a thymine on the other strand, and cytosine pairs with guanine.

This complementary nature of the two DNA strands suggested a solution to a major mystery of genetics: How do cells copy their DNA?

"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material," Watson and Crick noted in the remarkably reserved final sentence of their April 25, 1953 report in Nature.

A month later, the scientists made public their proposal that DNA's double helix unwinds its two strands, and the cell then reads the nucleotides on each and fashions the new, complementary strands.

In the years since, researchers have largely confirmed Watson and Crick's hypothesis and have identified the tools that cells employ in this process. Central among them are DNA polymerases, the enzymes that choose appropriate nucleotides and stitch them together into new strands of DNA. Some polymerases repair short spans of damaged DNA, while others duplicate whole genomes at a time.

"These are the enzymes that make copies of the blueprint of life," says Lorena S. Beese of Duke University Medical Center in Durham, N.C.

Now, by shining X rays at crystallized versions of the polymerases, a research group led by Beese and one led by Tom Ellenberger of Harvard Medical School in Boston have obtained high-resolution images of the enzymes' three-dimensional structures. With these pictures in hand, the investigators have begun to clear up the mysteries surrounding how these crucial proteins work.

"The results provide atomic-level detail of an enzyme that is incredibly important for maintaining the stability of genetic information," says Thomas A. Kunkel of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C.

"It's as if we're seeing frames from a movie, and if we get enough different shots, we will eventually get the whole story," adds Catherine M. Joyce of Yale University.

The story should be compelling, because polymerases often lie at the heart of gene mutations that can cause cancer or other diseases.

"One of the easiest ways for a mutation to arise is from a copying error by a DNA polymerase," says Kunkel. "It's really an important human health issue to understand how DNA is copied accurately."

Investigators have for many years created crystals of DNA polymerases and shone X rays through them to reveal the precise locations of the molecules' many atoms. While each polymerase has displayed its own unique shape, the images have inspired researchers to describe DNA polymerases in general as shaped somewhat like an open hand--that is, a palm, a thumb, and a set of fingers.

"Essentially, you have a flat part where the DNA sits and two appendages that kind of wrap around the DNA," explains Ellenberger.

Scientists believe that the palm holds a polymerase's active site, the region where the enzyme catalyzes the chemical reaction that joins a new nucleotide to a DNA strand. The DNA and the nucleotide bind to different regions of the polymerase, but the enzyme somehow brings them together in the active site.

Ellenberger and his colleagues may now have captured this climactic scene on film. In the Jan. 15 Nature, they publish an X-ray picture with all three characters--the polymerase, the target DNA, and the incoming nucleotide--of this genetic drama. …

The rest of this article is only available to active members of Questia

Already a member? Log in now.

Notes for this article

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
One moment ...
Default project is now your active project.
Project items
Notes
Cite this article

Cited article

Style
Citations are available only to our active members.
Buy instant access to cite pages or passages in MLA 8, MLA 7, APA and Chicago citation styles.

(Einhorn, 1992, p. 25)

(Einhorn 25)

(Einhorn 25)

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

Note: primary sources have slightly different requirements for citation. Please see these guidelines for more information.

Cited article

Crystal Clear: X-Ray Snapshots Illuminate How Enzymes Stitch Together DNA
Settings

Settings

Typeface
Text size Smaller Larger Reset View mode
Search within

Search within this article

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

Help
Full screen
Items saved from this article
  • Highlights & Notes
  • Citations
Some of your highlights are legacy items.

Highlights saved before July 30, 2012 will not be displayed on their respective source pages.

You can easily re-create the highlights by opening the book page or article, selecting the text, and clicking “Highlight.”

matching results for page

    Questia reader help

    How to highlight and cite specific passages

    1. Click or tap the first word you want to select.
    2. Click or tap the last word you want to select, and you’ll see everything in between get selected.
    3. You’ll then get a menu of options like creating a highlight or a citation from that passage of text.

    OK, got it!

    Cited passage

    Style
    Citations are available only to our active members.
    Buy instant access to cite pages or passages in MLA 8, MLA 7, APA and Chicago citation styles.

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn, 1992, p. 25).

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences."1

    1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

    Cited passage

    Thanks for trying Questia!

    Please continue trying out our research tools, but please note, full functionality is available only to our active members.

    Your work will be lost once you leave this Web page.

    Buy instant access to save your work.

    Already a member? Log in now.

    Search by... Author
    Show... All Results Primary Sources Peer-reviewed

    Oops!

    An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.