DNA is made up of four nucleotide building blocks: adenine, thymine, cytosine, and guanine. Two strands of nucleotides coil to form a double helix, held together by hydrogen bonds between complementary base pairs. Adenine binds only to thymine and cytosine only binds to guanine. The order of the bases is not important, but the base sequence of one strand specifies the other. Thymine is unique to DNA, while adenine, cytosine, and guanine are also found in RNA.
Deoxyribonucleic acid, or DNA, is what genes are made of. Within a DNA molecule, four different nucleotide building blocks occur. Each contains a five-carbon sugar and a phosphate group, but differs depending on which organic base is attached. The four bases found in a DNA molecule are adenine, thymine, cytosine and guanine.
A DNA molecule consists of two strands of nucleotides that coil around each other to form a double helix. The nucleotide backbone is created by the sugar of one nucleotide binding to the phosphate group of the next. The two strands are held together by hydrogen bonds between the bases of opposite nucleotides. This hydrogen bonding is very specific and occurs only between complementary base pairs.
The structure of each base determines the exact base it will mate with. All four bases have a ring structure that contains carbon and nitrogen atoms, hence they are often called nitrogenous bases. While each has a different chemical structure, they are grouped into two categories based on the number of rings they contain. Adenine and guanine are purine bases and have a double ring structure. Cytosine and thymine have a single ring structure and are pyrimidine bases.
Two important constraints are placed on how cross pegs can be formed between DNA strands in order for hydrogen bonds to form and regular double helix winding to occur. First, purine bases bind only to pyrimidine bases. By having only purine bases bonded with pyrimidine bases, the cross peg length between DNA strands will remain constant. If purine bases could bind with purine bases or pyrimidine bases with pyrimidine bases, the length of the cross peg would change causing the DNA molecule to bend.
Secondly, and more specifically, adenine binds only to thymine and cytosine only binds to guanine. When adenine bonds with thymine, two hydrogen bonds are formed. Three hydrogen bonds are formed between cytosine and guanine. Only these two pairs are capable of forming the hydrogen bonds necessary to maintain the stability of the DNA molecule.
What is not important in the DNA molecule is the order in which the bases appear. This means that there can be four different cross steps: adenine with thymine, thymine with adenine, cytosine with guanine and guanine with cytosine. This is biologically significant as it means that the base sequence of one strand of a DNA molecule specifies the base sequence of the other strand. In other words, the two strands can be separated and exact copies can be made every time a cell divides.
Thymine is unique among the four bases as it is found only in DNA molecules. Adenine, cytosine and guanine are also found in the nucleotides that make up ribonucleic acid or RNA. Within an RNA molecule, thymine is replaced by uracil.
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