DNA Structure
Figure 2 Structure of the DNA Molecule
The DNA molecule is made of two polynucleotide strands spiraled around each other to form a double helix. The polynucleotide strands are composed of a five-carbon sugar molecule, phosphate group, and nitrogenous bases. (thymine, adenine, cytosine, guanine)The strands are connected by nitrogenous base pairs (A-T and C-G) which are held together by a hydrogen bond. The bases adenine and thymine will always pair and the bases guanine and cytosine will always pair on the DNA molecule.
DNA, or deoxyribonucleic acid, is a large molecule that holds the genetic information of our entire human design, a blueprint of the human form and function; the plan for whom we are and what we are likely to become. DNA is encoded with the instructions for the construction of all our physical structures and the plethora of their functions. A spool-like protein, called histone, bundles this massive amount of DNA in extremely condensed, tightly packed chromosomes that enable it to fit within the nucleus of the cell. If you lined up the entire DNA from all the cells in your body, its length would be 6,000 million miles long! 4
The Nucleotide
Figure 3 A Nucleotide
A nucleotide is a DNA unit composed of a five-carbon sugar, a phosphate group, and a nitrogenous base. The sequence of these units forms the DNA molecule.
The structure of DNA was established by James Watson and Francis Crick and published in the journal Nature in 1953. This work determined that DNA is arranged in a double helix, meaning it is composed of two polynucleotide strands in a ladder-shaped spiral. When untwisted, the DNA spiral can be compared to a ladder. (see Figure 2) The "rails" of the ladder, or the DNA backbone, are made of phosphates and sugar molecules. The "rungs" of the ladder are made of four different nitrogenous bases. A nucleotide is a unit composed of a five-carbon sugar, a phosphate group, and a nitrogenous base.(see Figure 3) Each base is attached to the sugar molecule part of the "rail." These four chemical bases are the building blocks of the DNA molecule. They are A (adenine), T (thymine), G (guanine), and C (cytosine). Each base is paired with its partner base on the DNA molecule by a hydrogen bond. Adenine is always paired with thymine and guanine is always paired with cytosine (i.e., A-T and C-G). Consequently, these pairings are called base pairs in DNA language. These base pairs are the "rungs" on the DNA ladder that join the two rails or two DNA strands. There are about three billion nucleotides, or base pairs, in the human genome in every cell. If the ladder was climbed, from one end of the DNA molecule to the other, the rungs on the DNA spiral-ladder could be read in order: this sequence (or order) is the language of DNA. The unique sequencing of the bases is the way DNA encodes the information it carries. Further, since these four bases will always pair with their complimentary base, one is able to determine the complementary sequence of bases of the opposite rail (or strand) of DNA from the sequence on the other strand. Functional sections of the DNA are called genes. 5 6 These genes are composed of a set of three base sequences called codons. Here is a more specific example. Imagine that you were walking up the DNA ladder. With each step, you would encounter a different base pair that is making the ladder rung. A gene is the sequence of bases that you would see as you walk up the ladder. Let us read the sequence by focusing on one of the ladder rails. (Remember that we only need to read one, since the other complement rail is automatically known from the A-T and G-C base pairing.) Looking at the top rail in Figure 4, and walking from left to read, we could read our gene sequence: ACTCTTC.
Figure 4 The DNA Molecule
The structure of DNA molecule, when unwound looks like a ladder. It is composed of two supporting "rails" made of sugar and phosphate molecules. The "rungs" of the ladder are the complementary base pairs. The base pairs are held together by hydrogen bonds. A sequence of three bases forms a codon, or gene.
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