DNA Structure, Transcription, and Translation Explained

Deoxyribonucleic acid, or DNA, is a molecule that contains the instructions an organism needs to develop, live, and reproduce. These instructions are found inside every cell and are passed down from parents to their children.

Structure of DNA

DNA is made up of molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar group, and a nitrogen base. The four types of nitrogen bases are adenine (A), thymine (T), guanine (G), and cytosine (C). The order of these bases is what determines DNA's instructions, or genetic code. Human DNA has around 3 billion bases, and more than 99 percent of those bases are the same in all people, according to the U.S. National Library of Medicine (NLM).

Similar to the way the order of letters in the alphabet can be used to form a word, the order of nitrogen bases in a DNA sequence forms genes, which, in the language of the cell, tells cells how to make proteins. Another type of nucleic acid, ribonucleic acid, or RNA, translates genetic information from DNA into proteins.

Nucleotides are attached together to form two long strands that spiral to create a structure called a double helix. If you think of the double helix structure as a ladder, the phosphate and sugar molecules would be the sides, while the bases would be the rungs. The bases on one strand pair with the bases on another strand: adenine pairs with thymine (A-T), and guanine pairs with cytosine (G-C). Examples of base pairing include A-T, T-A, G-C, and C-G.

Protein Synthesis: Transcription & Translation

The process in which cells make proteins is called protein synthesis. It actually consists of two processes: transcription and translation.

Transcription: DNA to RNA

Transcription takes place in the nucleus. It uses DNA as a template to make an RNA molecule. RNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. Translation reads the genetic code in mRNA and makes a protein.

Transcription is the first part of the central dogma of molecular biology: DNA → RNA. It is the transfer of genetic instructions in DNA to messenger RNA (mRNA). During transcription, a strand of mRNA is made that is complementary to a strand of DNA. The Figure below shows how this occurs.

Translation: mRNA to Protein

Translation proceeds in three phases:

• Initiation: The ribosome assembles around the target mRNA. The first tRNA is attached at the start codon.
• Elongation: The tRNA transfers an amino acid to the tRNA corresponding to the next codon. The ribosome then moves (translocates) to the next mRNA codon to continue the process, creating an amino acid chain.
• Termination: When a stop codon is reached, the ribosome releases the polypeptide.