Two DNA chains are held together by: Bbl6epute onuH OTBeT: a. phosphodiester bond b. covalent bond c. disulfide bridge d. ionic bond

## Step 1<br />The question is asking about the type of bond that holds together the two strands of DNA. DNA, or deoxyribonucleic acid, is the molecule that carries the genetic instructions for the development, functioning, and reproduction of all known living organisms. It is composed of two strands that are held together by bonds between the nitrogenous bases.<br /><br />## Step 2<br />The options provided are:<br />a. Phosphodiester bond<br />b. Covalent bond<br />c. Disulfide bridge<br />d. Ionic bond<br /><br />## Step 3<br />The phosphodiester bond is a type of covalent bond that links the phosphate group of one nucleotide to the sugar of another, forming the backbone of the DNA strand. This bond is not responsible for holding the two strands of DNA together.<br /><br />## Step 4<br />The covalent bond is a strong bond that holds the atoms within each strand of DNA together. This bond is not responsible for holding the two strands of DNA together.<br /><br />## Step 5<br />The disulfide bridge is a type of covalent bond that forms between two cysteine residues in a protein, providing stability to the protein's structure. This bond is not responsible for holding the two strands of DNA together.<br /><br />## Step 6<br />The ionic bond is a type of bond that forms between oppositely charged ions. In the case of DNA, the ionic bond forms between the positively charged hydrogen atoms of the nitrogenous bases and the negatively charged oxygen atoms of the sugar-phosphate backbone. This bond is responsible for holding the two strands of DNA together.