10.2: Polynucleotides
- Explain the structure of a polynucleotide
- Explain the primary structure of DNA and RNA.
DNA Polynucleotide
Deoxyribonucleic acid (DNA) is the nucleic acid that stores genetic information. Nucleotides are joined together through the phosphate group of one nucleotide linked to the OH group on the third carbon atom of the sugar of a second nucleotide. This bond that connects one nucleotide to another is called the 3',5'-phosphodiester bond . The process is repeated to produce a long nucleic acid chain. The backbone of the chain consists of alternating phosphate and sugar units (2-deoxyribose in DNA ). The purine and pyrimidine bases branch off this sugar-phosphate backbone. The structure of a DNA polynucleotide is shown in figure 10.2.1.
Figure \(\PageIndex{1}\): A DNA polynucleotide.
There are 20 different kinds of amino acids. However there are only 4 different nucleotides in DNA. For amino acid sequences in the primary structure of the protein, the convention is to write the amino acids in order starting with the N-terminal amino acid. In writing nucleotide sequences for the primary structure of DNA, the convention is to write the one-letter abbreviations for the bases, starting with the nucleotide having a free phosphate group, which is known as the 5′-end, and indicate the nucleotides in order. For DNA, a lowercase d is often written in front of the sequence to indicate that the monomers are deoxyribonucleotides. The final nucleotide has a free OH group on the 3′ carbon atom and is called the 3'-end. Note the absence of uracil and presence of thymine in DNA.
The sequence of in the DNA segment shown in figure 10.2.1 is written as: 5′-dC-T-G-A-3′.
RNA Polynucleotide
Ribonucleic acid (RNA) is the nucleic acid responsible for using the genetic information encoded in DNA to produce the proteins found in living organisms. In RNA too nucleotides are joined together through the phosphate group of one nucleotide linked to the OH group on the third carbon atom of the sugar of a second nucleotide. This bond that connects one nucleotide to another is called the 3',5'-phosphodiester bond.The process is repeated to produce a long nucleic acid chain. The backbone of the chain consists of alternating phosphate and sugar units (ribose in RNA ). The purine and pyrimidine bases branch off this backbone. The structure of a RNA polynucleotide is shown in figure 10.2.2.
Figure \(\PageIndex{2}\): A RNA polynucleotide.
There are only 4 different nucleotides in RNA. In writing nucleotide sequences for the primary structure of RNA, the convention is to write the one-letter abbreviations for the bases, starting with the nucleotide having a free phosphate group, which is known as the 5′-end, and indicate the nucleotides in order. The final nucleotide has a free OH group on the 3′ carbon atom and is called the 3'-end. Note the absence of thymine and presence of uracil in RNA.
The sequence of in the RNA segment shown in figure 10.2.2 would be written 5′-G-U-A-C-3′.
Summary
Nucleotides are joined together through the phosphate group of one nucleotide linked to the OH group on the third carbon atom of the sugar of a second nucleotide. The process is repeated to produce a long nucleic acid chain. The backbone of the chain consists of alternating phosphate and sugar units. The purine and pyrimidine bases branch off this backbone.