Bases are held together by hydrogen bonds, and the DNA backbone is held together by phosphodiester bonds.

​Double Helix Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are held together by bonds between the bases, adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.

hydrogen bonds

hydrogen bonding

Hydrogen bonds occur between the two strands and involve a base from one strand with a base from the second in complementary pairing. These hydrogen bonds are individually weak but collectively quite strong.

The two strands bind through the bonding of the bases of each nucleotide i.e. the bases from one strand bond to the bases of the second strand of DNA. This makes the two chains run anti-parallel to each other and gives the DNA the spiraling structure that makes the double helix.

The structure of the DNA helix is stabilized by van der Waals forces, hydrogen bonds between complementary organic bases (a base pair), and hydrophobic interactions between the nitrogenous bases and the surrounding sheath of water.

The main bonding in DNA which renders the double helix structure so stable is that of hydrogen bonds. Between the complementary base pairs, hydrogen bonds connect the two strands of the helix. There are 3 H bonds between Guanine and Cytosine and 2 between Adenine and Thymine.

B form

The double-helix shape allows for DNA replication and protein synthesis to occur. In these processes, the twisted DNA unwinds and opens to allow a copy of the DNA to be made. In DNA replication, the double helix unwinds and each separated strand is used to synthesize a new strand.

Although usually single-stranded, some RNA sequences have the ability to form a double helix, much like DNA. Gehring said identifying the double-helical RNA will have interesting applications for research in biological nanomaterials and supramolecular chemistry.

Each DNA molecule is actually a pair of strands wound together, forming a double helix. To make a protein from a gene, a cell must separate the strands and read their sequence. To reproduce, a cell must rip the strands apart and build new counterparts for each one.

The rapid growth in our understanding over the past 60 years, including the delivery of genomes for a range of species including humans, has affected all of us at some level. This knowledge has brought improved medical treatments, new drugs and better disease diagnosis.

Francis Crick

Mutational effects can be beneficial, harmful, or neutral, depending on their context or location. Most non-neutral mutations are deleterious. In general, the more base pairs that are affected by a mutation, the larger the effect of the mutation, and the larger the mutation’s probability of being deleterious.

Created by Rosalind Franklin using a technique called X-ray crystallography, it revealed the helical shape of the DNA molecule. Watson and Crick realized that DNA was made up of two chains of nucleotide pairs that encode the genetic information for all living things.

Watson and Crick’s model erroneously placed the bases on the outside of the DNA molecule with the phosphates, bound by magnesium or calcium ions, inside. One of the key characteristics of science is that it relies on evidence.

One claim was that during the race to uncover the structure of DNA, Jim Watson and Francis Crick either stole Rosalind Franklin’s data, or ‘forgot’ to credit her. Neither suggestion is true. The model the Cambridge duo put forward did not simply describe the DNA molecule as a double helix.

Franklin is best known for her work on the X-ray diffraction images of DNA while at King’s College London, particularly Photo 51, taken by her student Raymond Gosling, which led to the discovery of the DNA double helix for which Francis Crick, James Watson, and Maurice Wilkins shared the Nobel Prize in Physiology or …

On February 28, 1953, Cambridge University scientists James D. Watson and Francis H.C. Crick announce that they have determined the double-helix structure of DNA, the molecule containing human genes.

Rosalind

On 6 May 1952, at King´s College London in London, England, Rosalind Franklin photographed her fifty-first X-ray diffraction pattern of deoxyribosenucleic acid, or DNA.

The image was tagged “photo 51” because it was the 51st diffraction photograph that Franklin and Gosling had taken. It was critical evidence in identifying the structure of DNA.

Rosalind Franklin discovered the density of DNA and, more importantly, established that the molecule existed in a helical conformation. Her work to make clearer X-ray patterns of DNA molecules laid the foundation for James Watson and Francis Crick’s suggestion that DNA is a double-helix polymer in 1953.

Franklin’s biographer, Brenda Maddox, called her “the Dark Lady of DNA”, based on a disparaging reference to Franklin by one of her coworkers, and also because although her work on DNA was crucial to the discovery of its structure, her contribution to that discovery is little known.

Franklin, whose lab produced the photograph that helped unravel the mystery of DNA, received no credit for her role until after her death. At the time of her death, she was working on the molecular structure of viruses with her colleague Aaron Klug, who received a Nobel Prize for the work in 1982.

Rosalind Franklin’s short scientific carrier produced brilliant contri- butions to the structure of carbon, DNA, and helical and spherical vi- ruses. At 30, she was a recognized authority who switched from car- bon to DNA research and, a few years later, to nucleic-acid-protein complexes known as viruses.

Hydrogen bonds

In the structures for the complementary base pairs given in the graphic on the left, notice that the thymine – adenine pair interacts through two hydrogen bonds represented as (T=A) and that the cytosine-guanine pair interacts through three hydrogen bonds represented as (C=G).

​Base Pair Attached to each sugar is one of four bases–adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.

The strongest hydrogen bond type is the one involving a bond between oxygen, nitrogen and fluorine, with a hydrogen atom. Since in the choices, only water shows a O-H bond, therefore this is the strongest hydrogen bonding. The answer is water.

HF (boiling point = 19.4 degrees Celsius) has the strongest intermolecular forces.

two types

There are three different types of intermolecular forces in terms of strength. They are (strongest to weakest) hydrogen bonding, dipole-dipole and Van der Waals’ forces.

As a result, it is more difficult to deform the surface of water than the surface of ethyl alcohol. Therefore, since water molecules on a liquid surface are harder to push down on the surface tension is higher for water than for ethyl alcohol.

Hydrogen bonding is so strong among dipole-dipole interactions because it itself is a dipole-dipole interaction with one of the strongest possible electrostatic attractions. Remember that hydrogen bonding cannot occur unless hydrogen is covalently bonded to either oxygen, nitrogen, or fluorine.