Draw a Circle Around the Phosphate Group

Affiliate nine: Introduction to Molecular Biology

nine.1 The Construction of DNA

Learning Objectives

By the terminate of this section, you will exist able to:

• Depict the structure of Deoxyribonucleic acid
• Describe how eukaryotic and prokaryotic DNA is arranged in the jail 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 as Linus Pauling and Maurice Wilkins, were also actively exploring this field. Pauling had discovered the secondary structure of proteins using X-ray crystallography. X-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 important information about the structure of the molecule of interest. In Wilkins' lab, researcher Rosalind Franklin was using X-ray crystallography to understand the structure of Deoxyribonucleic acid. Watson and Crick were able to piece together the puzzle of the Deoxyribonucleic acid molecule using Franklin's data (Effigy ix.2). Watson and Crick also had primal pieces of information available from other researchers such equally Chargaff'due south rules. Chargaff had shown that of the four kinds of monomers (nucleotides) nowadays in a Deoxyribonucleic acid molecule, two types were ever nowadays in equal amounts and the remaining ii types were also always present 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 Dna.

Effigy nine.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 piece of work past NIH)

Now permit's consider the structure of the two types of nucleic acids, dna (Deoxyribonucleic acid) and ribonucleic acid (RNA). The edifice blocks of DNA are nucleotides, which are made upward of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base (Figure nine.three). 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 co-ordinate to the nitrogenous base information technology contains.

Figure ix.iii (a) Each DNA nucleotide is made up of a sugar, a phosphate group, and a base.

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

The phosphate group of one nucleotide bonds covalently with the saccharide molecule of the next nucleotide, and so on, forming a long polymer of nucleotide monomers. The sugar–phosphate groups line upward in a "courage" for each single strand of Dna, and the nucleotide bases stick out from this backbone. The carbon atoms of the five-carbon sugar are numbered clockwise from the oxygen as 1′, 2′, three′, 4′, and 5′ (1′ is read equally "ane prime"). The phosphate group is fastened to the 5′ carbon of ane nucleotide and the 3′ carbon of the adjacent nucleotide. In its natural land, each Dna molecule is actually composed of ii single strands held together along their length with hydrogen bonds between the bases.

Watson and Crick proposed that the Deoxyribonucleic acid is fabricated up of 2 strands that are twisted around each other to class a right-handed helix, called a double helix. Base-pairing takes place betwixt a purine and pyrimidine: namely, A pairs with T, and Grand 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 footing for Chargaff's rule; because of their complementarity, there is every bit much adenine equally thymine in a DNA molecule and as much guanine as cytosine. Adenine and thymine are continued by two hydrogen bonds, and cytosine and guanine are connected by three hydrogen bonds. The ii strands are anti-parallel in nature; that is, one strand will have the three′ carbon of the saccharide in the "upwards" position, whereas the other strand will accept the 5′ 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 (ane band) and their combined lengths are ever equal. (Figure 9.4).

Figure 9.4 Deoxyribonucleic acid (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 Construction of RNA

There is a 2nd nucleic acid in all cells chosen 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 of operations, a 5-carbon sugar, and a phosphate group. In the example of RNA, the five-carbon sugar is ribose, not deoxyribose. Ribose has a hydroxyl grouping at the ii′ carbon, different deoxyribose, which has only a hydrogen cantlet (Effigy 9.5).

Figure nine.5 The difference betwixt the ribose establish in RNA and the deoxyribose constitute 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 exercise not contain thymine, which is instead replaced by uracil, symbolized past a "U." RNA exists equally a single-stranded molecule rather than a double-stranded helix. Molecular biologists have named several kinds of RNA on the ground 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 DNA lawmaking.

How Deoxyribonucleic acid Is Arranged in the Cell

Deoxyribonucleic acid is a working molecule; it must exist replicated when a cell is ready to dissever, 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 Dna is protected and packaged in very specific ways. In addition, Dna molecules can be very long. Stretched terminate-to-end, the Deoxyribonucleic acid molecules in a single human cell would come to a length of about 2 meters. Thus, the Deoxyribonucleic acid for a cell must exist packaged in a very ordered way to fit and function within a structure (the jail cell) that is non visible to the naked middle. The chromosomes of prokaryotes are much simpler than those of eukaryotes in many of their features (Effigy 9.6). Near prokaryotes comprise a single, round chromosome that is found in an area in the cytoplasm called the nucleoid.

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

The size of the genome in ane of the most well-studied prokaryotes, Escherichia coli, is 4.vi million base pairs, which would extend a altitude of near 1.half-dozen mm if stretched out. So how does this fit inside a small bacterial jail cell? The DNA is twisted beyond the double helix in what is known as supercoiling. Some proteins are known to exist involved in the supercoiling; other proteins and enzymes help in maintaining the supercoiled construction.

Eukaryotes, whose chromosomes each consist of a linear DNA molecule, apply a different blazon of packing strategy to fit their Dna inside the nucleus. At the most basic level, DNA is wrapped effectually proteins known as histones to course structures called nucleosomes. The Deoxyribonucleic acid is wrapped tightly around the histone core. This nucleosome is linked to the next one by a brusk strand of Dna that is gratis of histones. This is as well known as the "beads on a cord" structure; 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 cobweb. This cobweb is further coiled into a thicker and more than compact construction. 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 found in clan with scaffold proteins.

In interphase, the phase of the cell cycle between mitoses at which the chromosomes are decondensed, eukaryotic chromosomes take two distinct regions that can exist distinguished by staining. There is a tightly packaged region that stains darkly, and a less dumbo region. The darkly staining regions usually comprise genes that are not active, and are plant in the regions of the centromere and telomeres. The lightly staining regions usually contain genes that are active, with DNA packaged around nucleosomes simply not farther compacted.

Figure ix.7 These figures illustrate the compaction of the eukaryotic chromosome.

Concept in Action

Lookout man 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 of operations, a v-carbon saccharide (deoxyribose), and a phosphate group. In that location are iv nitrogenous bases in DNA, two purines (adenine and guanine) and two pyrimidines (cytosine and thymine). A Deoxyribonucleic acid molecule is composed of ii 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 ever bonds with thymine, and cytosine always bonds with guanine. The bonding causes the two strands to spiral around each other in a shape chosen a double helix. Ribonucleic acid (RNA) is a second nucleic acid constitute 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 function in the procedure of forming proteins from the genetic code in Deoxyribonucleic acid.

Prokaryotes contain a unmarried, double-stranded circular chromosome. Eukaryotes contain double-stranded linear DNA molecules packaged into chromosomes. The DNA helix is wrapped around proteins to form nucleosomes. The protein coils are further coiled, and during mitosis and meiosis, the chromosomes become even more than greatly coiled to facilitate their motion. Chromosomes have two distinct regions which tin be distinguished by staining, reflecting different degrees of packaging and determined by whether the DNA in a region is existence expressed (euchromatin) or not (heterochromatin).

Glossary

deoxyribose: a five-carbon sugar molecule with a hydrogen atom rather than a hydroxyl grouping in the 2′ position; the sugar component of Dna nucleotides

double helix: the molecular shape of Deoxyribonucleic acid in which two strands of nucleotides air current around each other in a screw shape

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

phosphate group: a molecular group consisting of a cardinal phosphorus cantlet bound to iv oxygen atoms

shirkworgird2000.blogspot.com

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

Share this post