As stated by Alberts et al. "a chemical bond is chemical affinity between two atoms that holds them together"  . There are various different types of chemical bonds that occur in living cells and organisms, but perhaps the most prominent are covalent bonds, ionic bonds, hydrogen bonds and disulphide bridges.
A covalent bond is a chemical bond in which two atoms share a pair of electrons and it is the attractive interaction in this type of bonding between the nuclei of both atoms that holds the atoms together. It is perhaps one of the strongest types of chemical bonds that occur and is much stronger than hydrogen bonds and, either similar strength or stronger than ionic bonds. Disulphide bonds are a type of covalent bond and these are present in the tertiary structure of proteins as a 'disulphide bridge' forms between two thiol groups between cysteine residues in order to help maintain this tertiary structure which is responsible for the final specific shape that the protein assumes.
An ionic bond is a type of chemical bonding which involves the electrostatic attraction between oppositely charged ions. A few well known molecules in which ionic bonding is present are NaCl, MgO, KCl, Al2O3 and CaCl2.
A hydrogen bond is a chemical bond that is generally weaker than both covalent and ionic bonding and is one in which an electropositive hydrogen atom is bound to a highly electronegative atom. However, a hydrogen bond is not a true bond and is just a very strong dipole-dipole interaction between atoms. It is hydrogen bonding that is responsible for the particularly high boiling point of water (100oC). Hydrogen bonding is also very prominent in molecules such as DNA and other proteins. It is hydrogen bonds that form between the nucleotide base pairs on opposite, complementary DNA strands, therefore joining them together. Two hydrogen bonds form between the bases adenine and thymine and three form between the bases cytosine and guanine. As previously stated it is hydrogen bonds that are present in the secondary structure of proteins to form alpha helices and beta sheets
- ↑ Bruce Alberts (2007), Molecular Biology of the Cell, 5th edition, New York, Garland Science