Chapter 6: Nucleic acids and Protein synthesis

Nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are made up of Nucleotides. Therefore, DNA and RNA are polynucleotide.


  • Nucleotides are made up of three components:
    • A nitrogen-containing base (also known as a nitrogenous base)
    • A pentose sugar (containing 5 carbon atoms)
    • A phosphate group

The components of DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine.

The component of and RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil.


Adenosine triphosphate (ATP) is the energy-carrying molecule that provides the energy to drive many processes inside living cells. ATP is another type of nucleic acid and hence it is structurally very similar to the nucleotides that make up DNA and RNA.


A polynucleotide is a combination of nucleotide monomers which are connected to each other through covalent bonds. A single polynucleotide molecule consists of 14 or more monomers of nucleotide in a chain structure. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are examples of polynucleotides.

DNA Structure

It is a double helix composed of two polynucleotides joined together by hydrogen bonds between complementary bases. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases–adenine (A), cytosine (C), guanine (G), and thymine (T). Adenine and Thymine join together by 2 hydrogen bonds and Cytosine and Guanine join together by 3 hydrogen bonds. DNA is the information molecule. It stores instructions for making other large molecules, called proteins.

RNA Structure

RNA consists of ribose nucleotides (nitrogenous bases appended to a ribose sugar) attached by phosphodiester bonds, forming strands of varying lengths. The nitrogenous bases in RNA are adenine, guanine, cytosine, and uracil, which replace thymine in DNA.

DNA Replication

The semi-conservative replication of DNA ensures genetic continuity between generations of cells meaning that genetic information is passed on from one generation from next. DNA replication occurs during S-phase of the mitosis. The hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands are broken.  In this process, the hydrogen bonds between the bases break, allowing free nucleotides to fall into position opposite their complementary ones on each strand of the original DNA molecules. Adjacent nucleotides are then linked, through their phosphates and sugars to form new strands. Two completely new molecules are thus formed from old one.

Protein Synthesis

Protein synthesis is the process of creating protein molecules. In biological systems, it involves amino acid synthesis, transcription, translation, and post-translational events. Transcription is the transfer of genetic instructions in DNA to mRNA in the nucleus. It includes three steps: initiation, elongation, and termination. After the mRNA is processed, it carries the instructions to a ribosome in the cytoplasm.

Translation occurs at the ribosome, which consists of rRNA and proteins. In translation, the instructions in mRNA are read, and tRNA brings the correct sequence of amino acids to the ribosome. Then, rRNA helps bonds form between the amino acids, producing a polypeptide chain.

After a polypeptide chain is synthesized, it may undergo additional processing to form the finished protein.