DNA, often described as the “molecule of life,” holds the key to understanding the core processes governing the existence of all living organisms. At the heart of this biological marvel lies the Central Dogma, a fundamental concept in molecular biology first proposed by Francis Crick in the 20th century. The Central Dogma outlines the flow of genetic information, beginning with DNA as the repository of genetic code and extending to the synthesis of proteins, the workhorses of cellular function. This article explores the intricate dance of molecules and enzymes that underpins the Central Dogma, shedding light on the mechanisms by which DNA is transcribed into RNA and then translated into functional proteins. As we journey through this fundamental biological principle, we gain profound insights into the inner workings of life itself and how genetic information is harnessed to create the rich tapestry of living organisms.

DNA and the Central Dogma – Summary

• DNA is the molecule that contains the genetic information, and can be divided into functional units called genes.
• DNA contains the instructions to make functional cells and organisms, but does not directly participate in carrying out cellular programmes
• DNA instead dictates cellular operations through a 2 step process: first by transcribing its messages to RNA, which is then translated by the protein synthesising machinery
• This is the process of gene expression, and is often referred to as the central dogma.

All the information required to make an entire organism is encoded in an important molecule called DNA (deoxyribonucleic acid). This DNA is housed in the nucleus in eukaryotic cells. In fact, almost every cell in a living thing contains the full set of DNA needed to programme the entire organism!

We inherit our DNA from our parents, and it gets passed on to our offspring. The DNA does not leave the nucleus, and so does not directly participate in the operations of the cell. However, it is the source of all the instructions required for the cell to carry out its functions. DNA does this primarily by directing protein synthesis, through another molecule called RNA (ribonucleic acid). Proteins are used in a variety of processes in the cell, and are key to catalysing all cellular activities. The coordination of these activities allows the cells to function, and consequentially modulates entire tissues, organs and the whole organism.

How does DNA hold the genetic information?

DNA can be thought of as a linear set of instructions that the cell has to read and interpret to make all the components required for its functions. DNA is a linear molecule made up of a sequence of monomer units called nucleotides. There are four possible monomers, with different chemical groups attached that make them unique. They are Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). The order of these monomers specifies the sequence of the DNA, and dictates instructions for the cell. This sequence can be thought of as a language that the cell machinery can read and interpret, allowing the production of molecules crucial for its operations.

The DNA is not interpreted as one extremely long sequence, instead it is divided into units that can be interpreted as individual ‘words’. These segments of DNA are the genes, which specify a functional product. These products are usually polypeptide chains, which are ultimately processed to become functional proteins such as enzymes.

Genes

As described in the previous section, the genome (i.e. all the DNA in the cell) isn’t just interpreted as a long sequence of nucleotides. Instead, it’s divided up into functional units of inheritance called genes. The sequences encoded in the DNA sequence within genes specify certain products that perform functions within the cell. The functional output of genes is usually the primary amino acid sequence of proteins.

It is important to note that lots of the genome consists of sequences that do not code for proteins. These non-coding sequences were initially called ‘junk’ DNA by researchers, because they thought that this non-coding DNA had no functional role, and was redundant in the cell. However, it was later discovered that these sequences have important jobs in the cell, including coding for regulatory RNAs that do not form proteins but still have important roles in regulating other proteins, turning on/off nearby genes and fine tuning the cellular programmes, and in structural roles. In fact, non-coding sequences make up more than 90% of the genome, so it is unlikely that they were merely ‘junk’.

The process of DNA dictating protein synthesis occurs through a two part procedure: (1) the DNA is first transcribed into messenger RNA (mRNA), and (2) the messenger RNA is then translated to an amino acid sequence by the protein synthesis machinery. This is what is referred to as the central dogma.

The central dogma

Gene expression, i.e. the process by which genes achieve their functional output, relies on the effective communication of the coded information held in the genes to the sites of protein manufacture (the ribosomes) in the cytoplasm. The two step processes described above are called transcription (where the DNA is transcribed into a molecule of mRNA) and translation (where the RNA is translated into an amino acid sequences).

Briefly, the information contained within a gene is transcribed into an mRNA. The mRNA can exit the nucleus holding the information contained within the DNA sequence. The mRNA is used as a template by the ribosomes to create an amino acid chain, which is further folded and modified to become the final protein.

An obvious question is why this cannot be simplified to one step? One explanation is simply the fact that the DNA is stored in large complex moleculesthat cannot leave the nucleus because of their size and structure, and thus having a small ‘translatable’ molecule such as mRNA to be able to carry this message to the cytoplasm is a way around this. Secondly, the DNA is a crucial molecule for the long term survival and propagation of the cell, and is therefore kept safe in the nucleus. If the DNA had to leave the nucleus every time a protein was to be made, the risk of damage occurring to the DNA is high.

The following sections will cover the DNA molecule and its transcription and translation in more detail.

Read more DNA Replication in Brief

Frequently Asked Questions

What is DNA?

DNA is a polymer of deoxyribonucleotides present in all living cells. It carries the genetic information of a cell that is passed on to the next generations. This genetic information guides all the processes that are carried out within the cell or by the cell. 

What is the central dogma?

The Central Dogma is a process by which the information present in the DNA in the form of genes is utilized to synthesise proteins in a cell. It is the flow of information from genes in the DNA to ribosomes so that proteins can be made. 

What are the two steps of the central dogma?

The two steps of Central Dogma are transcription and translation. Transcription is the process by which information present in the genes is copied in the form of mRNA. In the process of translation, the information present in the mRNA is used to make proteins. 

Where does translation take place?

The second step of Central Dogma known as translation is carried out by ribosomes that are present in the cytosol. 

References and further reading

http://researchguides.library.vanderbilt.edu/c.php?g=69346&p=816436

Campbell, Neil A., and Jane B. Reece. Biology (8th Edition). San Francisco: Benjamin Cummings, 2007.

https://www.yourgenome.org/facts/what-is-the-central-dogma (Image)