How do nitrogen bases bond together

There are two classes of nitrogen bases called purines double-ringed structures and pyrimidines single-ringed structures. The four bases in DNA's alphabet are:. Watson and Crick discovered that DNA had two sides, or strands, and that these strands were twisted together like a twisted ladder -- the double helix. The sides of the ladder comprise the sugar-phosphate portions of adjacent nucleotides bonded together.

The phosphate of one nucleotide is covalently bound a bond in which one or more pairs of electrons are shared by two atoms to the sugar of the next nucleotide. The hydrogen bonds between phosphates cause the DNA strand to twist.

The nitrogenous bases point inward on the ladder and form pairs with bases on the other side, like rungs. Each base pair is formed from two complementary nucleotides purine with pyrimidine bound together by hydrogen bonds. The base pairs in DNA are adenine with thymine and cytosine with guanine.

A hydrogen bond is a weak chemical bond that occurs between hydrogen atoms and more electronegative atoms, like oxygen, nitrogen and fluorine.

The participating atoms can be located on the same molecule adjacent nucleotides or on different molecules adjacent nucleotides on different DNA strands. Hydrogen bonds do not involve the exchange or sharing of electrons like covalent and ionic bonds.

The weak attraction is like that between the opposite poles of a magnet. Hydrogen bonds occur over short distances and can be easily formed and broken. Nucleotides that compose DNA are called deoxyribonucleotides. The three components of a deoxyribonucleotide are a five-carbon sugar called deoxyribose , a phosphate group, and a nitrogenous base , a nitrogen-containing ring structure that is responsible for complementary base pairing between nucleic acid strands Figure 1.

A nucleoside comprises the five-carbon sugar and nitrogenous base. Figure 1. The deoxyribonucleotide is named according to the nitrogenous bases Figure 2. The nitrogenous bases adenine A and guanine G are the purines ; they have a double-ring structure with a six-carbon ring fused to a five-carbon ring.

The pyrimidines , cytosine C and thymine T , are smaller nitrogenous bases that have only a six-carbon ring structure. Figure 2. Nitrogenous bases within DNA are categorized into the two-ringed purines adenine and guanine and the single-ringed pyrimidines cytosine and thymine.

Thymine is unique to DNA. Phosphodiester bonding between nucleotides forms the sugar-phosphate backbone , the alternating sugar-phosphate structure composing the framework of a nucleic acid strand Figure 3. During the polymerization process, deoxynucleotide triphosphates dNTP are used. To construct the sugar-phosphate backbone, the two terminal phosphates are released from the dNTP as a pyrophosphate. The two unused phosphate groups from the nucleotide triphosphate are released as pyrophosphate during phosphodiester bond formation.

Pyrophosphate is subsequently hydrolyzed, releasing the energy used to drive nucleotide polymerization. Figure 3. By the early s, considerable evidence had accumulated indicating that DNA was the genetic material of cells, and now the race was on to discover its three-dimensional structure. Around this time, Austrian biochemist Erwin Chargaff [1] — examined the content of DNA in different species and discovered that adenine, thymine, guanine, and cytosine were not found in equal quantities, and that it varied from species to species, but not between individuals of the same species.

Figure 4. The X-ray diffraction pattern of DNA shows its helical nature. Other scientists were also actively exploring this field during the midth century. Unfortunately, by then Franklin had died, and Nobel prizes at the time were not awarded posthumously. Work continued, however, on learning about the structure of DNA. Figure 5. Watson and Crick proposed that DNA is made up of two strands that are twisted around each other to form a right-handed helix. The sugar and phosphate of the polymerized nucleotides form the backbone of the structure, whereas the nitrogenous bases are stacked inside.

These nitrogenous bases on the interior of the molecule interact with each other, base pairing. The asymmetrical spacing of the sugar-phosphate backbones generates major grooves where the backbone is far apart and minor grooves where the backbone is close together Figure 6.

These grooves are locations where proteins can bind to DNA. Figure 6. Watson and Crick proposed the double helix model for DNA. Base pairing takes place between a purine and pyrimidine. The base pairs are stabilized by hydrogen bonds; adenine and thymine form two hydrogen bonds between them, whereas cytosine and guanine form three hydrogen bonds between them.

Figure 7. Hydrogen bonds form between complementary nitrogenous bases on the interior of DNA. In the laboratory, exposing the two DNA strands of the double helix to high temperatures or to certain chemicals can break the hydrogen bonds between complementary bases, thus separating the strands into two separate single strands of DNA single-stranded DNA [ ssDNA ].

This process is called DNA denaturation and is analogous to protein denaturation, as described in Proteins. At the metaphase stage of mitosis, when the chromosomes are lined up in the center of the cell, the chromosomes are at their most compacted.

They are approximately nm in width, and are found in association with scaffold proteins. In interphase, the phase of the cell cycle between mitoses at which the chromosomes are decondensed, eukaryotic chromosomes have two distinct regions that can be distinguished by staining. There is a tightly packaged region that stains darkly, and a less dense region. The darkly staining regions usually contain genes that are not active, and are found in the regions of the centromere and telomeres.

The lightly staining regions usually contain genes that are active, with DNA packaged around nucleosomes but not further compacted. Concept in Action. Watch this animation of DNA packaging. The DNA molecule is a polymer of nucleotides.

Each nucleotide is composed of a nitrogenous base, a five-carbon sugar deoxyribose , and a phosphate group. There are four nitrogenous bases in DNA, two purines adenine and guanine and two pyrimidines cytosine and thymine. A DNA molecule is composed of two strands. Each strand is composed of nucleotides bonded together covalently between the phosphate group of one and the deoxyribose sugar of the next.

From this backbone extend the bases. The bases of one strand bond to the bases of the second strand with hydrogen bonds. Adenine always bonds with thymine, and cytosine always bonds with guanine. The bonding causes the two strands to spiral around each other in a shape called a double helix. Ribonucleic acid RNA is a second nucleic acid found in cells. RNA is a single-stranded polymer of nucleotides.

It also differs from DNA in that it contains the sugar ribose, rather than deoxyribose, and the nucleotide uracil rather than thymine. Prokaryotes contain a single, double-stranded circular chromosome. Eukaryotes contain double-stranded linear DNA molecules packaged into chromosomes.