Monthly Archives: September 2015

Laboratory experiments, computer explorations, and an interactive exhibition for students, educators, and the public.

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The DNALC has teamed up with the Breakthrough Prize and Khan Academy to design a cutting-edge $100,000 Science Lab as part of the $400,000 grand prize for this new competition. Move fast! The deadline is October 7, 2015.

Join the world-renowned Cold Spring Harbor Laboratory as a Faculty Teaching Fellow at DNA Learning Center Asia China's first hands-on genetics education center.

Gene Screen was named the National Human Genome Research Institute's and Smithsonian Institution's August 2015 "Resource of the Month" on Unlocking Life's Code!

The DNALC family of websites includes 2- and 3-D animations, scientist interviews (including 12 Nobel laureates!), historical images, and teaching aids.

Roll over a screenshot for info. Click to launch site.

The Eugenics Archive includes 2,400 photographs, pedigrees, letters, brochures, manuscripts, and articles from archives in the US and UK. Essays by science historians put the vast image database into context.

Discover the concepts and experiments that define the fields of genetics and molecular biology. This animated primer features the work of over 100 scientists and researchers. Includes animations, scientist video interviews, biographies, photo galleries, and links.

Animations present the cause, inheritance, and treatment of 15 common genetic disorders. Interviews with doctors, researchers, healthcare providers, and individuals affected by a disorder offer personal perspectives. Partner site to DNA from the Beginning.

DNAi looks back on the discovery of the DNA structure with a vast collection of interviews and interactive 2- and 3-D animations. myDNAi provides teacher-oriented resources including guides, activities, lesson plans, and the online Lesson Builder.

Explore cancer biology, cancer pathways, and the molecular basis of diagnosis & treatment. Use Teacher Center to inform students on how molecular and cellular genetics are integrated into cancer prevention, diagnosis, and treatment.

The maize genome sequence is just the most recent of experiments with corn stretching back 10,000 years. Weed to Wonder tells the story how human ingenuity transformed a common Mexican weed (Teosinte) into a modern food and fuel wonder (corn).

Learn About SMA, developed with support from the Spinal Muscular Atrophy Foundation, is a resource for SMA patients, families and researchers, including stories of living with SMA and the science behind emerging therapies.

G2C Online is modeled on principles of neural networking, which examines current research to discover the molecular and cellular basis of human thinking. Use network maps to explore our library of 750+ items, including an interactive 3-D brain.

These sites are built around laboratory and bioinformatics experiments and were often developed in collaboration with Cold Spring Harbor Laboratory scientists.

Roll over a screenshot for info. Click to launch site.

Analyze your DNA with the DNALC's bioinformatics tools: Sequence Server, Allele Server, and Simulation Server. Compare your DNA with DNA from other humans and other species stored in a database maintained by the DNALC.

Kits developed in collaboration with Carolina Biological Supply Company provide lab protocols online and for download and accompanying resources.

Learn about plant genomes by using bioinfomatics to analyze newly sequenced genes in rice and maize. Animated tutorials lead into step-by-step analysis of a gene model using Apollo, research software for gene annotation.

Gene Boy is a simple, multi-function, sequence analysis tool. Enter your own sequence or use a preloaded sequence to perform several analyses and tranformations on the sequence.

Find out what DNA tells us about human origins. Use your own DNA to explore relationships between different ethnic populations. Complete with laboratory protocols and information on how to send samples to the DNALC for sequencing.

Bring students up-to-date on advances in plant genetics and genomics by integrating laboratory experiences with online bioinformatics resources.

Use this lab notebook with experiment- and bioinformatics curriculum on RNAi in C. elegans. Has laboratory protocols, photos and videos of C. elegans mutants, and a check-out system for C. elegans mutants and E. coli feeding strains.

The Urban Barcode Project (UBP) is a science competition spanning the five boroughs of New York City made possible by funding from the Alfred P. Sloan Foundation. In the project, student research teams use DNA barcoding to explore biodiversity in NYC.

Lab Center provides pre- and post-visit activities for laboratory field trips. Video intros, online and downloadable protocols, animations, and videos enhance student understanding. Password-protected teacher resources also included.

DNA Barcoding 101 includes experimental method and supporting resources for using DNA barcoding to identify plants or animals - or products made from them.

DNA Subway is a bioinformatics workspace that makes high-level genome analysis available to biology students and educators. Predict and annotate genes in up to 100,000 basepairs of DNA and prospect entire plant genomes.

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DNA Learning Center

Monty Rakusen/Photodisc/Getty Images

Popularized by its use in high-profile criminal investigations and paternity cases, DNA, deoxyribonucleic acid, is most commonly used to prove a relationship to an individual. New tests created in recent years, however, have also turned DNA into a popular tool for determining ancestry and predicting health and genetic traits. As DNA is passed down from one generation to the next, some parts remain almost unchanged, while other parts change greatly.

This creates an unbreakable link between generations and it can be of great help in reconstructing our family histories.

While it can't provide you with your entire family tree or tell you who your ancestors are, DNA testing can:

DNA tests have been around for many years, but it is only recently that the cost of genetic testing has finally come down into the realm of possibility for the average individual interested in tracing their roots. Some homeDNA test kits can often be ordered for less than $100 (cost varies by test)andusually consist of a cheek swab or a spit collection tube to easily collect a sample of cells from the inside of your mouth. You send back the sample through the mail and within a month or two you receive the results - a series of numbers that represent key chemical "markers" within your DNA.

These numbers can then be compared to results from other individuals to help you determine your ancestry.

Confused with all the talk of markers, mutations and haplotypes? See Genetic Basics: Understanding the Clues Found in Our DNA

There are threebasic types of DNA tests available for genealogical testing. Each has a different purpose and one can only be used by males.

Autosomal DNA (atDNA)(All lines, available for both men and women) - Available for both men and women, this test surveys 700,000+ markers on all 23 chromosomes to look for connections along all of your family lines (maternal and paternal). The test results provide some information about your ethnic mix (thepercentage of your ancestry that comes from Central Europe, Africa, Asia, etc.), as well as as helps to identify cousins (1st, 2nd, 3rd, etc.) on any of your ancestral lines. Autosomal DNA only survives recombination (the passing down of DNA from your various ancestors) for an average of 57 generations, so this test is most useful for connecting with genetic cousins and connecting back to more recent generations of your family tree.

mtDNA Tests(direct maternal line, available forboth men and women)- Mitochondrial DNA (mtDNA) is contained in the cytoplasm of the cell, rather than the nucleus. This type of DNA is passed by a mother to both male and female offspring without any mixing, so your mtDNA is the same as your mother's mtDNA, which is the same as her mother's mtDNA. mtDNA changes very slowly, so iftwo people have an exact match in their mtDNA, then there is a very good chance they share a common maternal ancestor, but it is hard to determine if this is a recent ancestor or one who lived hundreds of years ago. It is important to keep in mind with this test that a male's mtDNA comes only from his mother and is not passed on to his offspring.

Example: The DNA tests that identified the bodies of the Romanovs, the Russian imperial family, utilized mtDNA from a sample provided by Prince Philip, who shares the same maternal line from Queen Victoria.

Y-DNA Tests(direct paternal line, available for males only)- The Y chromosome in the nuclear DNA can also be used to establish family ties. The Y chromosomal DNA test (usually referred to as Y DNA or Y-Line DNA) is only available for males, since the Y chromosome is only passed down the male line from father to son. Womendon't have a Y chromosome!Tiny chemical markers on the Y chromosome create a distinctive pattern, known as a haplotype, that distinguishes one male lineage from another. Shared markers can indicate relatedness between two men, though not the exact degree of the relationship. Y chromosome testing is most often used by individuals with the same last name to learn if they share a common ancestor.

Example: The DNA tests supporting the probability that Thomas Jefferson fathered the last child of Sally Hemmings were based on Y-chromosome DNA samples from male descendants of Thomas Jefferson's paternal uncle, since there were no surviving male descendants from Jefferson's marriage.

Markers on both mtDNA and Y chromosome tests can also be used to determine an individual's haplogroup, a grouping of individuals with the same genetic characteristics. This test may provide you with interesting information about the deep ancestral lineage of your paternal and/or maternal lines.

Next > What Can I Expect to Learn From the Tests?

See the article here:
Ancestry DNA Tests for Genealogists - Information

Deoxyribonucleic acid (DNA) in a living cell contains the master blueprint for the production of proteins and for the replication of itself. It is the repository for the hereditary information. DNA accomplishes protein production by first providing a pattern for the production of mRNA, a process called transcription. The RNA then contains the information from the DNA to manufacture a protein, a process called translation. Some proteins are structural, but some are control proteins called enzymes. These enzymes are employed in the production of proteins, even copies of themselves, so enzymes are used to make other enzymes. Each protein, including enzymes, is made according to a pattern of nucleotides along a segment of the DNA called a "gene". A single living cell contains thousands of enzymes.

Another way to organized these ideas is in terms of the "central dogma" of molecular biology.

Though a useful organizing structure, the "central dogma" has numerous exceptions. For example, retroviruses use "reverse transcription" to construct DNA from RNA. In general, not every gene gets expressed all the way to the construction of proteins. Some RNAs have other tasks to do, such as the ribosomal RNA and other specifically tasked RNAs with specific tasks in the cell.

Biochemical concepts

Chemistry concepts

See the rest here:
DNA - Georgia State University

Remains of unidentified soldiers who died aboard battleship Oklahoma during 1941 Pearl Harbor attack are being reexamined and subjected to DNA testing in order to confirm their identities; severe damage done to bodies on Oklahoma, one of two ships in battle damaged beyond repair, has made identification process difficult. MORE

Hannah Landenmark study in journal PLoS Biology calculates global biodiversity using new metric of amount of DNA found on Earth; study reports Earth contains around 50 trillion trillion trillion DNA base pairs. MORE

David Rosenberg, who was given up for adoption by his mother Margaret Erle Katz in 1962, relocated her with help of a DNA testing kit and the Internet, only to have less than a month with her before he succumbed to thyroid cancer; growing number of adoptees are locating birth parents using similar tools. MORE

Atlanta jury awards more than $2 million to Atlas Logistics Group Retail Services workers who were required to submit to DNA samples in effort to identify out who had been defecating in warehouse. MORE

DNA matching Richard W Matt and David Sweat, murderers who escaped from Clinton Correctional Facility in upstate New York, is found in cabin 15 miles from prison; Matt and Sweat were discovered missing from prison on June 6. MORE

DNA evidence extracted from elephant dung, tissue and hair could help researchers to identify origins of illegal ivory and target poachers. MORE

Researchers have created fuller picture of how Ebola virus spread and evolved during 2014 outbreak by sequencing genomes of viruses from over 400 patients in Sierra Leone, Guinea and Liberia. MORE

Study published in journal Nature finds that Kennewick Man, 8,500-year-old skeleton that was discovered in Washington state riverbed, has no European DNA, and that he was likely most closely related to Native Americans; remains became subject of controversy as various groups attempted to claim them. MORE

London-based skincare store GeneU uses DNA testing to match treatments to each customer. MORE

Two studies in journal Nature present largest examinations ever done of ancient European DNA, extracted from 170 skeletons found in countries from Spain to Russia; both studies show Europeans descend from three groups who entered area at different stages in history. MORE

Atlanta judge rules that Atlanta warehouse owner Atlas Logistics Group Retail Services violated Genetic Information Nondiscrimination Act when it tested DNA of two men to determine if they had been leaving piles of feces around workplace; decision in unusual case shows scope of law goes beyond its original intent. MORE

Mat Johnson Lives essay describes taking DNA test in order to clear up question of how black he really is, and having one done for his mother as well; notes that there are issues of identity that cannot be resolved by finding out details of one's racial heritage. MORE

Researchers are making progress in using immunotherapy to fight cancer, or using DNA mutations inherent in immune system to beat cancer at its own game; questions remain, however, as to why treatments work better against some cancers than others and why some patients do not respond. MORE

National Institute of Standards and Technology announces new reference standards that could be used to ensure accuracy of DNA testing; marks important step towards new era in the use of genetic medicine. MORE

Profile of Dr Jennifer A Doudna, biochemist at University of California, Berkeley, who is fighting for control of potentially highly profitable intellectual property rights to what is known as the Crispr-Cas9 genome editing technique; Doudna, who helped discover the process, is also actively working to deepen scientific community's understanding of bioethics concerns that have arisen from the revelation of this new way to alter DNA. MORE

Dr Love Dalen study published online in Current Biology reports that researchers have sequenced DNA of woolly mammoth and have determined that species most likely died out on an isolated island from inbreeding. MORE

Law enforcement authorities arrest Joseph Giardala at Los Angeles International Airport and return him to New York to face charges after DNA evidence links him to 1995 rape case in West Village; case is latest effort by Manhattan District Attorney Cyrus Vance Jr to use DNA to solve cold cases. MORE

Chinese researchers try and fail to permanently alter DNA in human embryos; widely-predicted failure follows outcry from scientific community, which had called for halt to research until it could be proved safe and until ethical debate could proceed; researchers were attempting to alter DNA in manner that would be passed on to future generations. MORE

DNA-matching web sites have brought to forefront stories of babies illegally sold by Gertrude Pitkanen of Butte, Mont, to adoptive couples through the 1950s; baby-sellers like Pitkanen, midwife who also performed illegal and sometimes fatal abortions, are part of mid-20th century American subculture; so-called Gertie's Babies, who have found relatives through sites, say they are unique as they never found living biological parents due to how securely secrets were kept. MORE

DNA study published in online digital library bioRxiv reports that geneticists in Great Britain have found Armenians are mix of ancient populations that existed from 3000 to 2000 BC; period confirms date given by fifth century historian Movses Khorenatsi for founding of Armenia. MORE

Scientific community is engaged in debate about whether majority of any given genome's DNA actually serves purpose; recent research has caused some to question idea, and debate is in many ways latest skirmish in intellectual battle that has been going on for 200 years. MORE

Study in journal Science concludes that wheat has existed in Great Britain for 8,000 years, according to DNA evidence, suggesting that it made its way from Neolithic farmers of Southern Europe to Mesolithic hunter-gatherers of Britain. MORE

Scientists are developing tools that allow investigators to create facial image of suspect using DNA found at crime scene; process, known as forensic DNA phenotyping, has raised concerns among some who question technology's accuracy or warn that it could increase racial profiling or lead to privacy violations. MORE

Study in journal Current Biology shows that mice embryos injected with bits of human DNA grow brains 12 percent larger than embryos injected with chimpanzee genes, demonstrating role that gene sequence HARE5 plays in human brain development. MORE

Scientists are finding that they can confirm presence of animal species in a particular location and track their movements by collecting animal's DNA that was shed in environment; DNA samples may persist in area for weeks, allowing researchers to detect invasive and resurgent species, animal diversity, and, perhaps eventually, to measure animal populations. MORE

Pres Obama plans to request hundreds of millions of dollars to fund research into medical treatments attuned to specific patients' DNA and other characteristics; some experts consider so-called 'personalized medicine' or 'individualized medicine' to be forefront of field. MORE

California's First District Court of Appeal strikes down state law requiring collection of DNA from anyone arrested on suspicion of committing a felony, citing state Constitution's ban on unreasonable search and seizure. MORE

Nobel laureate James D Watson, a pioneer in DNA science, will auction his prize medal and donate much of proceeds to educational institutions; move is part of Watson's effort to redeem himself after making offensive remarks about race in 2007 that tarnished his reputation. MORE

DNA analysis of bite marks on harbor porpoises shows that gray seals are often responsible for wounds. MORE

Research shows that obtaining DNA swab from fang mark of snake bite can accurately identify type of snake, helping medical professionals to administer correct antivenin. MORE

Office of Chief Medical Examiner holds New York City Missing Persons Day to help identify the roughly 1,200 bodies they have received since about 1990 that they have been unable to identify; relatives gather to offer DNA samples, which will be used to create a corresponding forensic number that is then crosschecked against genetic information in a city database and others maintained by municipalities across the country. MORE

Study in journal Nature reports that scientists have reconstructed genome of man who lived 45,000 years ago, by far the oldest genetic record ever obtained from modern humans; research provides new clues to expansion of modern humans from Africa into Europe and Asia, as well as strong evidence that early humans interbred with Neanderthals. MORE

Experts working to identify the victims of Malaysia Airlines Flight 17 say they have positively identified 173 individuals from DNA samples. MORE

Food and Drug Administration approves first screening test for colon cancer that uses patients DNA to help spot potentially deadly tumors and growths. MORE

Italian police link Massimo Giuseppe Bossetti to 2011 murder of 13-year-old Yara Gambirasio after embarking on country's largest DNA dragnet, taking genetic samples from nearly 22,000 people; DNA testing also unexpectedly revealed that Bossetti was the illegitimate son of a man who had died in 1999, setting off debate over risks of privacy violations in criminal investigations involving DNA searches. MORE

Dr Bryan Sykes study in Proceedings of the Royal Society B performs first rigorous genetic analysis of three dozen hair samples that collectors claim came from undiscovered living humanoids, such as the Yeti or Sasquatch; study reveals that hairs came from range of known creatures, from humans to dogs. MORE

New fertility treatment could avoid certain hereditary diseases by altering genetic makeup of egg, raising issues of whether or not such a process is ethical or even safe. MORE

Study published in journal Nature Communications reports that researchers are able to determine where strain of malaria originated by using DNA 'bar code' of 23 short snips from genes of parasites. MORE

Researchers at University of California, San Francisco, use DNA sequencing to pinpoint, within 48 hours, what had been causing 14-year-old Joshua Osborn's brain to swell for weeks; breakthrough holds great promise for diagnostics, but experts say it will be years before method is in widespread use; case is reported in the New England Journal of Medicine. MORE

Adam Liptak Sidebar column; South Carolina Supreme Court will decide whether to hear appeal of Billy Wayne Cope, who claims his confessions in 2001 rape and murder of his daughter were coerced; DNA tests identified a different man, James Sanders, as the perpetrator. MORE

Scientists at Scripps Research Institute create first living organism with artificial DNA, taking significant step toward altering the fundamental alphabet of life; accomplishment could lead to new antibiotics, vaccines and other products, though a lot more work needs to be done before this is practical; research, published online in journal Nature, is bound to raise safety concerns and questions about whether humans are playing God. MORE

Study in journal PLoS One reports that Harvard researchers generated hemihelix, helix that changes direction midway, while working with complex rubber band; unusual shape is variation on helix often associated with DNA strand. MORE

Op-Ed article by evolutionary geneticist Svante Paabo warns against using sequenced genomes of Neanderthals to re-create Neanderthal individuals; contends from an ethical perspective such an idea should be condemned, and argues that using stem cells to create cells and tissues in test tubes for research is far more ethically defensible and technically feasible. MORE

Scientists have revealed that unusual repeating DNA sequence occurring next to gene in common bacterium are part of sophisticated immune system used to fight viruses; molecular system, known as Crispr, may provide scientists with power to edit genome and make precise changes to DNA of humans, essentially rewriting code of life. MORE

Municipal administration in Naples, Italy, decides to take DNA samples of dog waste on the sidewalks so that they may track down owners who do not pick up after their dogs; cutting-edge campaign is attempt to address widespread urban problem. MORE

British scientists will grind up some of King Richard IIIs bones to try to sequence his genetic code, about a year and a half after discovery of his corpse. MORE

British scientists will grind up some of King Richard IIIs bones to try to sequence his genetic code, about a year and a half after discovery of his corpse. MORE

Anne Eisenberg Novelties column holds that as technology becomes more sophisticated, genomic sequencing will inevitably expand into the world of newborns, but the process has both medical and ethical implications. MORE

Two studies show how the legacy of Neanderthals endures 30,000 years after their extinction, finding Neanderthal genes in skin and hair that may have helped humans evolve; findings appear in journals Nature and Science. MORE

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DNA - News - Science - The New York Times

DNA, or deoxyribonucleic acid, is like a blueprint of biological guidelines that a living organism must follow to exist and remain functional. RNA, or ribonucleic acid, helps carry out this blueprint's guidelines. Of the two, RNA is more versatile than DNA, capable of performing numerous, diverse tasks in an organism, but DNA is more stable and holds more complex information for longer periods of time.

DNA and RNA are nucleic acids. Nucleic acids are long biological macromolecules that consist of smaller molecules called nucleotides. In DNA and RNA, these nucleotides contain four nucleobases sometimes called nitrogenous bases or simply bases two purine and pyrimidine bases each.

DNA is found in the nucleus of a cell (nuclear DNA) and in mitochondria (mitochondrial DNA). It has two nucleotide strands which consist of its phosphate group, five-carbon sugar (the stable 2-deoxyribose), and four nitrogen-containing nucleobases: adenine, thymine, cytosine, and guanine.

During transcription, RNA, a single-stranded, linear molecule, is formed. It is complementary to DNA, helping to carry out the tasks that DNA lists for it to do. Like DNA, RNA is composed of its phosphate group, five-carbon sugar (the less stable ribose), and four nitrogen-containing nucleobases: adenine, uracil (not thymine), guanine, and cytosine.

In both molecules, the nucleobases are attached to their sugar-phosphate backbone. Each nucleobase on a nucleotide strand of DNA attaches to its partner nucleobase on a second strand: adenine links to thymine, and cytosine links to guanine. This linking causes DNA's two strands to twist and wind around each other, forming a variety of shapes, such as the famous double helix (DNA's "relaxed" form), circles, and supercoils.

In RNA, adenine and uracil (not thymine) link together, while cytosine still links to guanine. As a single stranded molecule, RNA folds in on itself to link up its nucleobases, though not all become partnered. These subsequent three-dimensional shapes, the most common of which is the hairpin loop, help determine what role the RNA molecule is to play as messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA).

DNA provides living organisms with guidelinesgenetic information in chromosomal DNAthat help determine the nature of an organism's biology, how it will look and function, based on information passed down from former generations through reproduction. The slow, steady changes found in DNA over time, known as mutations, which can be destructive, neutral, or beneficial to an organism, are at the core of the theory of evolution.

Genes are found in small segments of long DNA strands; humans have around 19,000 genes. The detailed instructions found in genesdetermined by how nucleobases in DNA are orderedare responsible for both the big and small differences between different living organisms and even among similar living organisms. The genetic information in DNA is what makes plants look like plants, dogs look like dogs, and humans look like humans; it is also what prevents different species from producing offspring (their DNA will not match up to form new, healthy life). Genetic DNA is what causes some people to have curly, black hair and others to have straight, blond hair, and what makes identical twins look so similar. (See also Genotype vs Phenotype.)

RNA has several different functions that, though all interconnected, vary slightly depending on the type. There are three main types of RNA:

DNA's genes are expressed, or manifested, through the proteins that its nucleotides produce with the help of RNA. Traits (phenotypes) come from which proteins are made and which are switched on or off. The information found in DNA determines which traits are to be created, activated, or deactivated, while the various forms of RNA do the work.

One hypothesis suggests that RNA existed before DNA and that DNA was a mutation of RNA. The video below discusses this hypothesis in greater depth.

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DNA vs RNA - Difference and Comparison | Diffen

An Interactive Animated Nonlinear Tutorial by Eric Martz Adapted for using Jmol instead of Chime, by Angel Herrez Part of Biomodel website by Angel Herrez, Univ. de Alcal (Spain)

Disponible tambin en espaol. Tambm disponvel em portugus. Auch verfgbar auf Deutsch. Disponible aussi en franais.

This version works in any Java-compatible browser. Java Virtual Machine must be installed (JVM, included with some operating systems or available in Sun's Java website). More tutorials on DNA and proteins, in English, Spanish, etc., are indexed at molvisindex.org.

If you prefer using Chime for molecular models, the page using it is still available, with equivalent content and functionality.

This tutorial is designed to complement Biology or Biochemistry and Molecular Biology books, so it is not by itself a complete introduction to DNA structure. Please, check the original source for more recent versions. You can also read the version history.

You can request a copy of this tutorial for off-line use; once you have your own copy on your computer's hard disk, you can use it without an Internet connection (and it will run faster).

Methods, Acknowledgements, and References.

Do you know there are more tutorials at MolviZ.Org? Feedback/Requests to or .

More about Jmol: Jmol home page.

The use of this work is subjected to the conditions stated on the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License (details)

This DNA Tutorial is being used in the Biomedical Sciences Curriculum developed by Project Lead The Way, Inc., a not-for-profit corporation that provides curriculum at no charge to high schools and middle schools located in the United States.

This DNA Tutorial (English and Spanish versions) has been included in BioMolecular Explorer 3D, version 2, a website+CD-ROM designed to give high school biology teachers easy access to interactive 3D structures of biologically significant molecules.

This DNA Tutorial (English and Spanish versions) has been included in the live bootable DVD Xplora Knoppix, by Xplora - The European Science Education Gateway.

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DNA Structure - Contents page

Despite what you may have seen in some textbooks, DNA is not built like a twisted ladder. The helix, or spiral, is an inherent feature of the DNA molecule. Notice, for instance, that in the picture below, that the groove on the left side of the picture is much larger than the right side. This is because the paired bases in the center meet each other at an angle.

DNA is a very large molecule; the image here shows only a tiny fraction of the typical molecule. If an entire molecule of DNA from the virus "bacteriophage lambda" were shown at this scale, the image would be 970 meters high. For the bacterium Escherichia coli, the image would be 80 kilometers long. And for a typical piece of DNA from a eukaryote cell, the image would stretch for 1600 kilometers, about as far as it is from Dallas to Washington, D. C.! Obviously such a large molecule is not fully stretched out inside the cell, but is wound around proteins called histones which protect the DNA.

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DNA - University of California Museum of Paleontology