The Structure and Function of DNA

The Structure and Function of DNA Getting the Idea DNA is a large macromolecule that is central to life. DNA in each cell directs all the cell's activities. It is also responsible for the shape and features of the entire organism. Although DNA controls many complicated structures and functions, in some ways this molecule is very simple. The code it contains, which directs so many life processes, is written in a language that uses only four letters. Genetics and DNA Genetics is the branch of biology concerned with heredity, the passing of characteristics from parent to offspring. Each characteristic of an organism, such as cell structure or body shape, is called a trait. Organisms inherit most of their traits from their parents. The genetic information that determines these traits is contained in nucleic acids. Recall that nucleic acids are large organic molecules made up of carbon, hydrogen, oxygen, nitrogen, and phosphorus. Each nucleic acid is made up of smaller units called nucleotides. A nucleotide consists of a five-carbon sugar molecule bonded to a nitrogenous base and a phosphate group. Cells contain two types of nucleic acids—DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Each nucleic acid is named for the sugar it contains: deoxyribose in DNA and ribose in RNA. You will learn more about RNA in the next lesson. DNA and RNA both contain fivecarbon sugar molecules. Although the specific sugars in DNA and RNA differ, the carbon atoms of both sugars are arranged in a ring. DNA carries the cell's genetic information. It also contains instructions for cellular activity and for making the proteins organisms need for survival. In eukaryotic cells, most of the DNA is located in the nucleus, where it is coiled into structures called chromosomes. Recall that prokaryotic cells do not have nuclei. In prokaryotes, DNA floats freely in the cytoplasm. Some of this DNA may be in the form of a circular ring called a plasmid. The Structure of DNA Each molecule of DNA is made up of linked nucleotides. The nucleotides are all the same, except for the nitrogenous bases they contain. Each nucleotide includes one of four nitrogenous bases. The four bases in DNA are adenine, thymine, guanine, and cytosine. Scientists often refer to each base by the first letter in its name. Thus, A = adenine, T = thymine, G = guanine, and C = cytosine. A molecule of DNA is shaped like a twisted ladder. This shape is called a double helix. The sides of the ladder are two strands that spiral around an imaginary axis. As shown in the diagram, these strands are made up of alternating phosphate and sugar molecules that are joined together by chemical bonds. The "rungs" of the twisted ladder shape are composed of pairs of nitrogenous bases. The bases in DNA always pair in the same way: adenine with thymine, A-T or T-A, and cytosine with guanine, C-G or G-C. The nucleotides in each pair are known as complementary bases. They are held together by weak hydrogen bonds. The sequence of bases from rung to rung along the ladder stores the genetic information contained in the DNA molecule. An organism's traits are based on the proteins present during its development and throughout its life. Cells use the sequence of nucleotides in DNA as a set of instructions for making proteins. However, the proteins are not made directly by DNA. Instead, the information in DNA is copied from the DNA molecule by RNA. The RNA then carries the information to the cell's ribosomes, where proteins are made. You will learn more about this process in the next lesson. DNA Replication Cells are living structures. They must be able to reproduce to make new cells like themselves. Recall from Lesson 5 that the cell cycle is a continuous process in which cells grow, make copies of their chromosomes, and divide to form daughter cells. Before a cell divides, its DNA makes a copy of itself in a process called replication. The DNA molecule replicates during the S phase of the cell cycle, so that each daughter cell receives an exact copy of the parent cell's DNA. In the first step of replication, the DNA molecule is unzipped down the middle by two enzymes that break the weak hydrogen bonds between the complementary bases. The nucleotides separate, breaking the DNA molecule into two complementary halves. The sequence of bases on each half is used to construct a duplicate DNA molecule. When the double helix separates into two strands, complementary sequences of bases are exposed. For example, if one strand "reads" ACTTG, then its complementary strand "reads" TGAAC, as shown below. After the DNA molecule unzips, enzymes in the cytoplasm join nucleotides together to make matching DNA strands. The DNA-synthesizing enzymes move nucleotides into the proper positions so that A pairs with T and G pairs with C to form two new complementary strands. The result is two new DNA molecules. Each is made up of half of the original DNA molecule and a newly formed matching half. Finally, an enzyme called DNA polymerase checks the arrangement of bases in each new DNA strand, decreasing the chance that the copy contains an error. Each new molecule is identical to the original molecule of DNA. The diagram below shows this process. Chromosomes and Genes A molecule of DNA can be quite long. The nucleus of a single human cell contains more than 1 meter of DNA. To fit inside the nucleus, long sections of DNA are tightly coiled into chromosomes. Chromosomes are structures that contain the genetic information that is passed down from one generation to the next. Every species has a characteristic number of chromosomes in its cells. An organism has many more inherited traits than chromosomes. A trait is determined by one or more small sections of a chromosome called genes. Each chromosome contains hundreds or thousands of genes. Each gene codes for a specific protein. The diagram shows the relationship among genes, chromosomes, and DNA. The genetic information of each species differs from that of every other species. However, the DNA of all organisms has some things in common. The more similar the genetic codes of two organisms, the more closely related the organisms are. The similarity is measured by determining how many genes the organisms share and how alike the shared genes are. Similar genes code for the same proteins, which may be reflected in shared traits. For example, a bird shares more genes with a reptile than with a fern plant or an insect. Even within a species, the genetic code of each organism is likely to differ from that of any other organism. These genetic differences explain why individual organisms have unique combinations of traits.