I know that hydrogen bond connects the nucleotides of each strand, but what type of bond holds the two strands together? is it covalent?
No, the bond that holds the two strands of DNA together is not covalent. The two strands of DNA are held together by a type of bonding called hydrogen bonding. The hydrogen bonds form between complementary nitrogenous bases on the two DNA strands. In DNA, the nitrogenous bases adenine (A) and thymine (T) can form two hydrogen bonds, while guanine (G) and cytosine (C) can form three hydrogen bonds. These hydrogen bonds between the base pairs provide the necessary stability for the DNA double helix structure.
To understand why the hydrogen bonds hold the two strands together instead of covalent bonds, let's briefly explain the difference between the two. Covalent bonds involve the sharing of electrons between atoms, resulting in the formation of strong chemical bonds. In contrast, hydrogen bonds are relatively weak interactions between a hydrogen atom and an electronegative atom (such as oxygen or nitrogen) in a different molecule or within the same molecule.
In the case of DNA, the hydrogen bonding between the nitrogenous bases is crucial because it allows for the separation of the two strands during replication and transcription processes. The hydrogen bonds can be easily broken, facilitating the unwinding and separation of the double helix. Conversely, they can be reformed, allowing the two separated strands to come back together and form the double helix structure once again.