What Are The 4 Bases That Makeup A Single Nucleotide

Chapter 9: Introduction to Molecular Biology

9.1 The Structure of Deoxyribonucleic acid

Learning Objectives

By the finish of this section, yous will be able to:

• Draw the construction of Dna
• Describe how eukaryotic and prokaryotic DNA is bundled in the cell

In the 1950s, Francis Crick and James Watson worked together at the Academy of Cambridge, England, to determine the structure of Dna. Other scientists, such every bit Linus Pauling and Maurice Wilkins, were besides 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 X-rays shot through a crystal of the substance. The patterns give of import data nigh the construction of the molecule of interest. In Wilkins' lab, researcher Rosalind Franklin was using X-ray crystallography to understand the structure of Dna. Watson and Crick were able to slice together the puzzle of the Deoxyribonucleic acid molecule using Franklin's data (Figure nine.ii). Watson and Crick as well had key pieces of information available from other researchers such equally Chargaff'due south rules. Chargaff had shown that of the iv kinds of monomers (nucleotides) present in a Deoxyribonucleic acid molecule, two types were always nowadays in equal amounts and the remaining two types were also e'er nowadays in equal amounts. This meant they were always 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 Deoxyribonucleic acid.

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 Ten-ray diffraction pattern of Deoxyribonucleic acid, which helped to elucidate its double helix structure. (credit a: modification of work past Marjorie McCarty; b: modification of work by NIH)

Now permit's consider the structure of the ii types of nucleic acids, deoxyribonucleic acrid (Deoxyribonucleic acid) and ribonucleic acid (RNA). The building blocks of Dna are nucleotides, which are fabricated up of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base (Figure 9.3). There are four types of nitrogenous bases in Deoxyribonucleic acid. Adenine (A) and guanine (Chiliad) are double-ringed purines, and cytosine (C) and thymine (T) are smaller, single-ringed pyrimidines. The nucleotide is named according to the nitrogenous base it contains.

Figure ix.3 (a) Each Deoxyribonucleic acid nucleotide is made upwards of a sugar, a phosphate grouping, and a base of operations.

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

The phosphate group of 1 nucleotide bonds covalently with the sugar molecule of the next nucleotide, and then on, forming a long polymer of nucleotide monomers. The sugar–phosphate groups line upwardly in a "backbone" for each single strand of Deoxyribonucleic acid, and the nucleotide bases stick out from this backbone. The carbon atoms of the 5-carbon sugar are numbered clockwise from the oxygen as 1′, ii′, 3′, four′, and 5′ (1′ is read as "one prime"). The phosphate grouping is attached to the 5′ carbon of one nucleotide and the three′ carbon of the next nucleotide. In its natural state, each Deoxyribonucleic acid molecule is actually composed of two unmarried strands held together along their length with hydrogen bonds betwixt the bases.

Watson and Crick proposed that the Deoxyribonucleic acid is made up of ii strands that are twisted around each other to form a right-handed helix, called a double helix. Base-pairing takes place between a purine and pyrimidine: namely, A pairs with T, and Yard pairs with C. In other words, adenine and thymine are complementary base pairs, and cytosine and guanine are also complementary base pairs. This is the basis for Chargaff's rule; because of their complementarity, at that place is as much adenine equally thymine in a DNA molecule and as much guanine equally cytosine. Adenine and thymine are connected by two hydrogen bonds, and cytosine and guanine are connected by iii hydrogen bonds. The ii strands are anti-parallel in nature; that is, one strand will accept the three′ carbon of the sugar in the "upward" position, whereas the other strand will take the five′ carbon in the upward position. The bore of the DNA double helix is uniform throughout because a purine (two rings) always pairs with a pyrimidine (one band) and their combined lengths are e'er equal. (Figure nine.4).

Effigy 9.4 DNA (a) forms a double stranded helix, and (b) adenine pairs with thymine and cytosine pairs with guanine. (credit a: modification of piece of work past Jerome Walker, Dennis Myts)

The Structure of RNA

There is a second nucleic acrid 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 sugar, and a phosphate group. In the case of RNA, the five-carbon sugar is ribose, not deoxyribose. Ribose has a hydroxyl group at the 2′ carbon, different deoxyribose, which has only a hydrogen atom (Figure 9.5).

Figure 9.5 The difference 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 by 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 function. These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)—molecules that are involved in the production of proteins from the Deoxyribonucleic acid code.

How Dna Is Arranged in the Cell

Dna is a working molecule; it must exist replicated when a cell is prepare to divide, and information technology must be "read" to produce the molecules, such as proteins, to carry out the functions of the cell. For this reason, the Deoxyribonucleic acid is protected and packaged in very specific ways. In add-on, Deoxyribonucleic acid molecules tin can be very long. Stretched terminate-to-end, the Deoxyribonucleic acid molecules in a single human prison cell would come to a length of about 2 meters. Thus, the Dna for a cell must be packaged in a very ordered way to fit and function within a construction (the prison 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 ix.six). Most prokaryotes contain a single, round chromosome that is found in an area in the cytoplasm called the nucleoid.

Effigy ix.6 A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the cytoplasm in an expanse called the nucleoid.

The size of the genome in one of the well-nigh well-studied prokaryotes, Escherichia coli, is 4.6 million base of operations pairs, which would extend a distance of almost 1.6 mm if stretched out. Then how does this fit within a small bacterial jail cell? The Dna is twisted across the double helix in what is known equally supercoiling. Some proteins are known to be involved in the supercoiling; other proteins and enzymes assistance in maintaining the supercoiled structure.

Eukaryotes, whose chromosomes each consist of a linear Deoxyribonucleic acid molecule, employ a dissimilar type of packing strategy to fit their Deoxyribonucleic acid inside the nucleus. At the well-nigh basic 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 side by side one past a short strand of Dna that is costless of histones. This is also known equally the "beads on a string" structure; the nucleosomes are the "beads" and the brusk lengths of Deoxyribonucleic acid between them are the "string." The nucleosomes, with their DNA coiled around them, stack compactly onto each other to grade a 30-nm–wide cobweb. This fiber is further coiled into a thicker and more meaty construction. 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 700 nm in width, and are plant in association with scaffold proteins.

In interphase, the stage of the cell wheel between mitoses at which the chromosomes are decondensed, eukaryotic chromosomes have two distinct regions that tin exist distinguished by staining. In that location 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 incorporate genes that are active, with Dna packaged around nucleosomes just non further compacted.

Effigy 9.7 These figures illustrate the compaction of the eukaryotic chromosome.

Concept in Action

Watch this animation of Deoxyribonucleic acid packaging.

Section Summary

The model of the double-helix structure of DNA was proposed by Watson and Crick. The Deoxyribonucleic acid molecule is a polymer of nucleotides. Each nucleotide is composed of a nitrogenous base, a 5-carbon carbohydrate (deoxyribose), and a phosphate group. There are four nitrogenous bases in DNA, 2 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 betwixt the phosphate group of i and the deoxyribose carbohydrate of the side by side. 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 effectually each other in a shape called a double helix. Ribonucleic acid (RNA) is a second nucleic acrid institute 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. Various RNA molecules function in the procedure of forming proteins from the genetic code in Dna.

Prokaryotes contain a single, double-stranded circular chromosome. Eukaryotes contain double-stranded linear DNA molecules packaged into chromosomes. The DNA helix is wrapped effectually proteins to course nucleosomes. The poly peptide coils are farther coiled, and during mitosis and meiosis, the chromosomes become even more than greatly coiled to facilitate their motility. Chromosomes have two distinct regions which can be distinguished past staining, reflecting different degrees of packaging and adamant by whether the Deoxyribonucleic acid in a region is being expressed (euchromatin) or not (heterochromatin).

Glossary

deoxyribose: a five-carbon carbohydrate molecule with a hydrogen atom rather than a hydroxyl group in the 2′ position; the carbohydrate component of DNA nucleotides

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

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

phosphate group: a molecular group consisting of a central phosphorus atom bound to 4 oxygen atoms

Source: https://opentextbc.ca/biology/chapter/9-1-the-structure-of-dna/

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