What smaller elements make up this complex molecule, how are these elements arranged, and how is information extracted from them? This unit answers each of these questions, and it also provides a basic overview of the process of DNA discovery.
This page appears in the following eBook. Aa Aa Aa. Deoxyribonucleic acid , more commonly known as DNA , is a complex molecule that contains all of the information necessary to build and maintain an organism. All living things have DNA within their cells. In fact, nearly every cell in a multicellular organism possesses the full set of DNA required for that organism. Key Concepts DNA chromosomes. Topic rooms within Genetics Close. No topic rooms are there. Browse Visually.
Other Topic Rooms Genetics. Student Voices. Aa Aa Aa. What components make up DNA? Figure 1: A single nucleotide contains a nitrogenous base red , a deoxyribose sugar molecule gray , and a phosphate group attached to the 5' side of the sugar indicated by light gray. Opposite to the 5' side of the sugar molecule is the 3' side dark gray , which has a free hydroxyl group attached not shown.
Figure 2: The four nitrogenous bases that compose DNA nucleotides are shown in bright colors: adenine A, green , thymine T, red , cytosine C, orange , and guanine G, blue. Although nucleotides derive their names from the nitrogenous bases they contain, they owe much of their structure and bonding capabilities to their deoxyribose molecule. The central portion of this molecule contains five carbon atoms arranged in the shape of a ring, and each carbon in the ring is referred to by a number followed by the prime symbol '.
Of these carbons, the 5' carbon atom is particularly notable, because it is the site at which the phosphate group is attached to the nucleotide. Appropriately, the area surrounding this carbon atom is known as the 5' end of the nucleotide. Opposite the 5' carbon, on the other side of the deoxyribose ring, is the 3' carbon, which is not attached to a phosphate group. This portion of the nucleotide is typically referred to as the 3' end Figure 1.
When nucleotides join together in a series, they form a structure known as a polynucleotide. At each point of juncture within a polynucleotide, the 5' end of one nucleotide attaches to the 3' end of the adjacent nucleotide through a connection called a phosphodiester bond Figure 3. It is this alternating sugar-phosphate arrangement that forms the "backbone" of a DNA molecule. Figure 3: All polynucleotides contain an alternating sugar-phosphate backbone. This backbone is formed when the 3' end dark gray of one nucleotide attaches to the 5' phosphate end light gray of an adjacent nucleotide by way of a phosphodiester bond.
How is the DNA strand organized? Figure 4: Double-stranded DNA consists of two polynucleotide chains whose nitrogenous bases are connected by hydrogen bonds. Within this arrangement, each strand mirrors the other as a result of the anti-parallel orientation of the sugar-phosphate backbones, as well as the complementary nature of the A-T and C-G base pairing.
Figure Detail. Figure 6: The double helix looks like a twisted ladder. How is DNA packaged inside cells? Figure 7: To better fit within the cell, long pieces of double-stranded DNA are tightly packed into structures called chromosomes. What does real chromatin look like? Compare the relative sizes of the double helix, histones, and chromosomes. Figure 8: In eukaryotic chromatin, double-stranded DNA gray is wrapped around histone proteins red. Figure 9: Supercoiled eukaryotic DNA. How do scientists visualize DNA?
Figure This karyotype depicts all 23 pairs of chromosomes in a human cell, including the sex-determining X and Y chromosomes that together make up the twenty-third set lower right.
Watch this video for a closer look at the relationship between chromosomes and the DNA double helix. What are karyotypes used for? Who is James Watson? What do we know about Francis Crick? Topic rooms within Genetics Close.
To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies. Each DNA sequence that contains instructions to make a protein is known as a gene.
The size of a gene may vary greatly, ranging from about 1, bases to 1 million bases in humans. Genes only make up about 1 percent of the DNA sequence. DNA sequences outside this 1 percent are involved in regulating when, how and how much of a protein is made. DNA's instructions are used to make proteins in a two-step process. First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA.
Next, the information contained in the mRNA molecule is translated into the "language" of amino acids, which are the building blocks of proteins. This language tells the cell's protein-making machinery the precise order in which to link the amino acids to produce a specific protein.
This is a major task because there are 20 types of amino acids, which can be placed in many different orders to form a wide variety of proteins. But nearly a century passed from that discovery until researchers unraveled the structure of the DNA molecule and realized its central importance to biology.
For many years, scientists debated which molecule carried life's biological instructions. Most thought that DNA was too simple a molecule to play such a critical role. Instead, they argued that proteins were more likely to carry out this vital function because of their greater complexity and wider variety of forms. By studying X-ray diffraction patterns and building models, the scientists figured out the double helix structure of DNA - a structure that enables it to carry biological information from one generation to the next.
Despite his scientific achievements, Dr. Scientist use the term "double helix" to describe DNA's winding, two-stranded chemical structure. This shape - which looks much like a twisted ladder - gives DNA the power to pass along biological instructions with great precision.
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