Which portions of dna are connected by covalent bonds

Note that there are two strands: one shown in blue, one in yellow. Other examples of a helix include yarn, a phone cord, or a spiral staircase. Each chain of the double helix is made up of repeating units called nucleotides. A single nucleotide is composed of three functional groups: a sugar , a triphosphate, and a nitrogenous nitrogen-containing base , as shown below. Note that in the figures drawn in this unit, each unlabeled vertex of a structure represents a carbon atom.

The sugar found in DNA is a variant of the five-carbon sugar called ribose. The structure of ribose is drawn below.

Each carbon of ribose is numbered as shown. In an aqueous environment, like inside the cell, the phosphate groups are negatively charged, as drawn in the figure above. Although each nucleotide in DNA contains identical sugar and phosphate groups, there are four different bases and thus four different nucleotides that can be incorporated into DNA. The four bases are adenine , cytosine , guainne , and thymine , and their structures are shown below.

This nucleosome is linked to the next one by a short strand of DNA that is free of histones. This fiber is further coiled into a thicker and more compact structure.

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.

Nucleotides are the building blocks of polymers called polynucleotides. Each nucleotide monomer consists of a pentose five-carbon sugar, to which is attached two other groups; a phosphate group and a nitrogenous base. The nitrogenous base is either a double ringed structure known as a purine or single ringed structure known as a pyrimidine. There are five common nitrogenous bases; adenine, guanine, thymine, cytosine and uracil. Nucleotides are joined together by covalent bonds between the phosphate group of one nucleotide and the third carbon atom of the pentose sugar in the next nucleotide.

This produces an alternating backbone of sugar - phosphate - sugar - phosphate all along the polynucleotide chain. The simplest of the polynucleotides is a single chain in which the pentose sugar is always ribose.

The name of this polynucleotide comes from the sugar r ibo n ucleic a cid, abbreviated to the three letters RNA. Adenine, guanine, cytosine and uracil are the four nitrogenous bases always found in RNA.