the bases are bonded to what part of the backbone of the dna molecule?

Affiliate 9: Introduction to Molecular Biology

9.i The Structure of DNA

Learning Objectives

By the end of this section, you volition be able to:

• Draw the construction of DNA
• Describe how eukaryotic and prokaryotic DNA is arranged in the jail cell

In the 1950s, Francis Crick and James Watson worked together at the University of Cambridge, England, to determine the structure of DNA. Other scientists, such as Linus Pauling and Maurice Wilkins, were also actively exploring this field. Pauling had discovered the secondary construction of proteins using X-ray crystallography. 10-ray crystallography is a method for investigating molecular structure by observing the patterns formed past Ten-rays shot through a crystal of the substance. The patterns give important information about the structure of the molecule of involvement. In Wilkins' lab, researcher Rosalind Franklin was using Ten-ray crystallography to understand the structure of DNA. Watson and Crick were able to piece together the puzzle of the DNA molecule using Franklin's data (Effigy 9.ii). Watson and Crick also had key pieces of information available from other researchers such every bit Chargaff's rules. Chargaff had shown that of the iv kinds of monomers (nucleotides) present in a Deoxyribonucleic acid molecule, two types were ever present in equal amounts and the remaining two types were also always present in equal amounts. This meant they were e'er paired in some way. In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine for their work in determining the structure of Dna.

Figure 9.2 Pioneering scientists (a) James Watson and Francis Crick are pictured here with American geneticist Maclyn McCarty. Scientist Rosalind Franklin discovered (b) the X-ray diffraction pattern of Dna, which helped to elucidate its double helix structure. (credit a: modification of work past Marjorie McCarty; b: modification of work by NIH)

At present let's consider the structure of the two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The building blocks of DNA are nucleotides, which are made up of three parts: a deoxyribose (five-carbon sugar), a phosphate grouping, and a nitrogenous base of operations (Figure 9.iii). There are four types of nitrogenous bases in Dna. Adenine (A) and guanine (G) are double-ringed purines, and cytosine (C) and thymine (T) are smaller, single-ringed pyrimidines. The nucleotide is named according to the nitrogenous base information technology contains.

Effigy ix.3 (a) Each Dna nucleotide is fabricated up of a saccharide, a phosphate group, and a base of operations.

Figure 9.3 (b) Cytosine and thymine are pyrimidines. Guanine and adenine are purines.

The phosphate grouping of one nucleotide bonds covalently with the sugar molecule of the adjacent nucleotide, and so on, forming a long polymer of nucleotide monomers. The sugar–phosphate groups line upwards in a "courage" for each single strand of DNA, and the nucleotide bases stick out from this courage. The carbon atoms of the five-carbon sugar are numbered clockwise from the oxygen as ane′, 2′, three′, iv′, and five′ (one′ is read as "i prime"). The phosphate group is fastened to the five′ carbon of ane nucleotide and the 3′ carbon of the side by side nucleotide. In its natural state, each Deoxyribonucleic acid molecule is actually composed of two single strands held together along their length with hydrogen bonds between the bases.

Watson and Crick proposed that the DNA is made up of 2 strands that are twisted around each other to grade a right-handed helix, called a double helix. Base-pairing takes identify between a purine and pyrimidine: namely, A pairs with T, and M pairs with C. In other words, adenine and thymine are complementary base pairs, and cytosine and guanine are too complementary base pairs. This is the footing for Chargaff'south rule; because of their complementarity, there is as much adenine as thymine in a Dna molecule and as much guanine as cytosine. Adenine and thymine are connected by two hydrogen bonds, and cytosine and guanine are connected by iii hydrogen bonds. The two strands are anti-parallel in nature; that is, i strand will accept the 3′ carbon of the sugar in the "upward" position, whereas the other strand will take the 5′ carbon in the upwards position. The bore of the DNA double helix is uniform throughout because a purine (two rings) always pairs with a pyrimidine (one ring) and their combined lengths are always equal. (Figure 9.four).

Figure ix.4 DNA (a) forms a double stranded helix, and (b) adenine pairs with thymine and cytosine pairs with guanine. (credit a: modification of work by Jerome Walker, Dennis Myts)

The Structure of RNA

There is a second nucleic acid in all cells called ribonucleic acid, or RNA. Like DNA, RNA is a polymer of nucleotides. Each of the nucleotides in RNA is made up of a nitrogenous base, a five-carbon saccharide, and a phosphate group. In the case of RNA, the five-carbon sugar is ribose, not deoxyribose. Ribose has a hydroxyl group at the two′ carbon, unlike deoxyribose, which has merely a hydrogen cantlet (Effigy 9.5).

Figure 9.5 The departure between the ribose found in RNA and the deoxyribose found in Dna is that ribose has a hydroxyl grouping at the 2′ carbon.

RNA nucleotides contain the nitrogenous bases adenine, cytosine, and guanine. However, they do not contain thymine, which is instead replaced past uracil, symbolized by a "U." RNA exists as a single-stranded molecule rather than a double-stranded helix. Molecular biologists have named several kinds of RNA on the basis of their office. These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)—molecules that are involved in the production of proteins from the DNA lawmaking.

How Deoxyribonucleic acid Is Arranged in the Cell

DNA is a working molecule; it must be replicated when a jail cell is gear up to divide, and it must be "read" to produce the molecules, such as proteins, to deport out the functions of the prison cell. For this reason, the Dna is protected and packaged in very specific ways. In add-on, Dna molecules tin be very long. Stretched end-to-cease, the DNA molecules in a single human cell would come up to a length of nearly 2 meters. Thus, the Deoxyribonucleic acid for a cell must be packaged in a very ordered manner to fit and function within a structure (the jail cell) that is not visible to the naked eye. The chromosomes of prokaryotes are much simpler than those of eukaryotes in many of their features (Effigy nine.6). Most prokaryotes contain a single, circular chromosome that is establish in an surface area in the cytoplasm called the nucleoid.

Figure 9.6 A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the cytoplasm in an area chosen the nucleoid.

The size of the genome in one of the most well-studied prokaryotes, Escherichia coli, is 4.half dozen million base pairs, which would extend a distance of most ane.6 mm if stretched out. So how does this fit inside a small bacterial cell? The DNA is twisted across the double helix in what is known as supercoiling. Some proteins are known to be involved in the supercoiling; other proteins and enzymes aid in maintaining the supercoiled structure.

Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a unlike type of packing strategy to fit their DNA within the nucleus. At the most bones level, Dna is wrapped around proteins known as histones to form structures called nucleosomes. The Dna is wrapped tightly around the histone core. This nucleosome is linked to the next one past a brusque strand of Dna that is free of histones. This is also known equally the "beads on a string" construction; the nucleosomes are the "beads" and the short lengths of Dna between them are the "string." The nucleosomes, with their Dna coiled around them, stack compactly onto each other to form a 30-nm–wide fiber. 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 jail cell, the chromosomes are at their most compacted. They are approximately 700 nm in width, and are institute in association with scaffold proteins.

In interphase, the phase of the jail cell bicycle betwixt mitoses at which the chromosomes are decondensed, eukaryotic chromosomes have two distinct regions that tin can be distinguished by staining. In that location is a tightly packaged region that stains darkly, and a less dense region. The darkly staining regions unremarkably contain genes that are not active, and are establish in the regions of the centromere and telomeres. The lightly staining regions normally contain genes that are active, with Dna packaged around nucleosomes but non further compacted.

Figure 9.seven These figures illustrate the compaction of the eukaryotic chromosome.

Concept in Action

Watch this animation of DNA packaging.

Section Summary

The model of the double-helix structure of DNA was proposed by Watson and Crick. The Dna molecule is a polymer of nucleotides. Each nucleotide is composed of a nitrogenous base, a five-carbon sugar (deoxyribose), and a phosphate grouping. At that place are four nitrogenous bases in DNA, two purines (adenine and guanine) and 2 pyrimidines (cytosine and thymine). A Dna molecule is composed of ii strands. Each strand is composed of nucleotides bonded together covalently between the phosphate group of ane and the deoxyribose sugar of the next. From this courage extend the bases. The bases of i strand bond to the bases of the 2nd strand with hydrogen bonds. Adenine ever bonds with thymine, and cytosine always bonds with guanine. The bonding causes the two strands to spiral effectually 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. Information technology also differs from Dna in that it contains the saccharide ribose, rather than deoxyribose, and the nucleotide uracil rather than thymine. Various RNA molecules part in the procedure of forming proteins from the genetic code in Deoxyribonucleic acid.

Prokaryotes contain a single, double-stranded circular chromosome. Eukaryotes contain double-stranded linear DNA molecules packaged into chromosomes. The Deoxyribonucleic acid helix is wrapped around proteins to grade nucleosomes. The poly peptide coils are further coiled, and during mitosis and meiosis, the chromosomes get even more greatly coiled to facilitate their motion. Chromosomes have two singled-out regions which tin can be distinguished by staining, reflecting dissimilar degrees of packaging and determined past whether the DNA in a region is being expressed (euchromatin) or not (heterochromatin).

Glossary

deoxyribose: a five-carbon sugar molecule with a hydrogen atom rather than a hydroxyl group in the ii′ position; the sugar component of DNA nucleotides

double helix: the molecular shape of Dna in which two strands of nucleotides air current around each other in a spiral shape

nitrogenous base: a nitrogen-containing molecule that acts as a base; oft referring to one of the purine or pyrimidine components of nucleic acids

phosphate group: a molecular group consisting of a central phosphorus atom jump to four oxygen atoms

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