What connects the base of one nucleotide to its complementary base on the other strand
Chapter 9: Introduction to Molecular Biology
nine.1 The Structure of DNA
Learning Objectives
By the end of this department, you will be able to:
- Describe the structure of DNA
- Depict how eukaryotic and prokaryotic Dna is arranged in the cell
In the 1950s, Francis Crick and James Watson worked together at the University of Cambridge, England, to decide 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 by 10-rays shot through a crystal of the substance. The patterns give important data about the structure of the molecule of interest. In Wilkins' lab, researcher Rosalind Franklin was using Ten-ray crystallography to understand the construction of Dna. Watson and Crick were able to piece together the puzzle of the DNA molecule using Franklin's data (Figure 9.ii). Watson and Crick likewise had key pieces of data available from other researchers such every bit Chargaff's rules. Chargaff had shown that of the iv kinds of monomers (nucleotides) present in a DNA molecule, two types were e'er present in equal amounts and the remaining two 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 piece of work in determining the structure of Dna.
Now allow'southward 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 (5-carbon sugar), a phosphate grouping, and a nitrogenous base (Effigy nine.3). In that location are four types of nitrogenous bases in Deoxyribonucleic acid. 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 it contains.
The phosphate group of one nucleotide bonds covalently with the sugar molecule of the next nucleotide, and and then on, forming a long polymer of nucleotide monomers. The carbohydrate–phosphate groups line upward in a "backbone" for each single strand of DNA, and the nucleotide bases stick out from this backbone. The carbon atoms of the five-carbon carbohydrate are numbered clockwise from the oxygen every bit 1′, ii′, 3′, 4′, and 5′ (1′ is read as "ane prime"). The phosphate group is attached to the 5′ carbon of 1 nucleotide and the 3′ carbon of the adjacent nucleotide. In its natural state, each DNA molecule is actually composed of two single strands held together along their length with hydrogen bonds betwixt the bases.
Watson and Crick proposed that the DNA is made upwards of two strands that are twisted around each other to course a right-handed helix, called a double helix. Base-pairing takes place between a purine and pyrimidine: namely, A pairs with T, and G 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'southward dominion; considering of their complementarity, there is equally much adenine every bit thymine in a Deoxyribonucleic acid molecule and as much guanine as cytosine. Adenine and thymine are continued by ii hydrogen bonds, and cytosine and guanine are continued by three hydrogen bonds. The two strands are anti-parallel in nature; that is, 1 strand volition have the 3′ carbon of the sugar in the "upward" position, whereas the other strand will accept the 5′ carbon in the upward position. The diameter of the DNA double helix is uniform throughout because a purine (two rings) e'er pairs with a pyrimidine (one band) and their combined lengths are ever equal. (Effigy ix.four).
The Construction of RNA
In that location is a 2d 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 v-carbon sugar, and a phosphate group. In the case of RNA, the five-carbon carbohydrate is ribose, not deoxyribose. Ribose has a hydroxyl group at the two′ carbon, different deoxyribose, which has merely a hydrogen atom (Figure 9.5).
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 footing 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 code.
How DNA Is Arranged in the Jail cell
DNA is a working molecule; information technology must exist replicated when a cell is ready to divide, and it must be "read" to produce the molecules, such equally proteins, to carry out the functions of the cell. For this reason, the Dna is protected and packaged in very specific ways. In add-on, Dna molecules tin can be very long. Stretched finish-to-end, the Deoxyribonucleic acid molecules in a single homo jail cell would come to a length of about 2 meters. Thus, the Dna for a cell must be packaged in a very ordered mode to fit and function within a construction (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 (Figure 9.vi). Near prokaryotes comprise a unmarried, round chromosome that is plant in an area in the cytoplasm called the nucleoid.
The size of the genome in one of the most well-studied prokaryotes, Escherichia coli, is 4.half dozen one thousand thousand base of operations pairs, which would extend a distance of well-nigh one.6 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 every bit supercoiling. Some proteins are known to exist involved in the supercoiling; other proteins and enzymes assistance in maintaining the supercoiled structure.
Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a dissimilar blazon of packing strategy to fit their DNA inside the nucleus. At the virtually bones level, Dna is wrapped effectually proteins known as histones to grade structures called nucleosomes. The Deoxyribonucleic acid is wrapped tightly around the histone cadre. This nucleosome is linked to the side by side one by a short strand of DNA that is free of histones. This is besides known every bit the "beads on a string" structure; the nucleosomes are the "beads" and the short lengths of DNA between them are the "cord." The nucleosomes, with their DNA coiled around them, stack compactly onto each other to form a thirty-nm–wide fiber. This fiber is further coiled into a thicker and more meaty structure. At the metaphase stage of mitosis, when the chromosomes are lined up in the heart of the jail cell, the chromosomes are at their nearly compacted. They are approximately 700 nm in width, and are plant in clan 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 dumbo 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 unremarkably contain genes that are active, with Dna packaged around nucleosomes simply not further compacted.
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 equanimous of a nitrogenous base, a five-carbon sugar (deoxyribose), and a phosphate group. At that place are iv nitrogenous bases in Deoxyribonucleic acid, two purines (adenine and guanine) and two 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 1 and the deoxyribose carbohydrate of the adjacent. 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 screw around each other in a shape called a double helix. Ribonucleic acid (RNA) is a 2d nucleic acid found in cells. RNA is a single-stranded polymer of nucleotides. It as well differs from DNA in that information technology contains the sugar ribose, rather than deoxyribose, and the nucleotide uracil rather than thymine. Diverse RNA molecules function in the process of forming proteins from the genetic code in Dna.
Prokaryotes contain a single, double-stranded circular chromosome. Eukaryotes comprise 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 greatly coiled to facilitate their movement. Chromosomes have two distinct regions which can be distinguished by staining, reflecting different degrees of packaging and determined by whether the DNA in a region is being expressed (euchromatin) or non (heterochromatin).
Glossary
deoxyribose: a five-carbon sugar molecule with a hydrogen atom rather than a hydroxyl group in the 2′ position; the sugar component of Dna nucleotides
double helix: the molecular shape of DNA in which two strands of nucleotides wind effectually each other in a screw shape
nitrogenous base: a nitrogen-containing molecule that acts as a base; often referring to one of the purine or pyrimidine components of nucleic acids
phosphate group: a molecular group consisting of a cardinal phosphorus atom leap to four oxygen atoms
Source: https://opentextbc.ca/biology/chapter/9-1-the-structure-of-dna/
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