7.1 Introduction

Learning Objectives

  • Discuss what is meant by “The Central Dogma” of Biology.
  • Describe the Beadle and Tatum experiments which led to the “One gene, One enzyme” hypothesis.
  • Outline the use of genetic screens for mutations and applications to biochemical pathway analysis.

How is the genetic information in DNA (genes) expressed as biological traits, such as the flower colour of Mendel’s peas? The answer lies in what has become known as molecular biology’s Central Dogma. While not all genes code for proteins, most do (Figure 7.1.1). This chapter describes the Central Dogma and some experiments that were used to support this concept.

Image showing a pink cell and DNA to RNA to protein
Figure 7.1.1 Most Genes Code for Proteins – But Not All. They are transcribed into mRNA, which is then translated into polypeptides.

When we think of the word “mutation”, we automatically think of it as something negative or detrimental. However, a mutation, which is a change in the DNA sequence, may have one or more effects on an organism, depending on what it is and in which gene it occurs. While detrimental effects are most common, sometimes mutations can create new features. These mutations give us a tool with which to investigate the gene and the biological processes in which it is involved.

We will first take a look at how scientists perform genetic screening for mutations, and the various consequences of those mutations.

Picture of a Canadian sphynx cat with blue eyes and hairless body
Figure 7.1.2 The Canadian Sphynx Lack Hair Due to a Genetic Mutation. This breed originated in Minnesota, but the Canadian Sphynx line was started in Toronto in 1966 through a selective breeding program from a spontaneous mutation that gave naked kittens. This mutation is inherited in an autosomal recessive manner for the hairlessness gene.

The Central Dogma of Biology describes the concept that genetic information is encoded in DNA in the form of genes (Figure 7.1.3). This information is then transferred as needed, in a process called transcription into a messenger RNA (mRNA) sequence. The information is then transferred again, in a process called translation into a polypeptide (protein) sequence. The sequence of bases in DNA directly dictates the sequence of bases in the RNA, which in turn dictates the sequence of amino acids that make up a polypeptide.

The original core of the Central Dogma is that genetic information is NEVER transferred from protein back to nucleic acids. In certain circumstances, the information in RNA may be converted back to DNA through a process called reverse transcription. As well, DNA, and its information, can also be replicated.

Proteins do most of the “work” in a cell. They (1) catalyze the formation and breakdown of most molecules within an organism, as well as (2) form their structural components, and (3) regulate the expression of genes. By dictating the sequence and thus structure of each protein, DNA directs the function of that protein, which can thereby, affect the entire organism. Thus, the genetic information, or genotype, defines the potential form, or phenotype of the organism. Note, however, that the environment can also influence phenotype.

Simple graphic showing the central dogma from DNA replication, to transcription and then translation
Figure 7.1.3 Central Dogma of Molecular Biology.

In the case of Mendel’s peas, purple-flowered plants have a gene that encodes an enzyme that produces a purple pigment molecule. In the white-flowered plants (a pigment-less mutant), the DNA for this gene has been changed, or mutated, so that it no longer encodes a functional protein. This is an example of a spontaneous, natural mutation in a gene coding for an enzyme in a biochemical pathway.

Life depends on (bio)chemistry to supply energy and to produce the molecules that construct and regulate cells. In 1908, Archibald Garrod described “in-born errors of metabolism” in humans using the congenital disorder, alkaptonuria (black urine disease), as an example of how “genetic defects” (genotype) led to the lack of an enzyme in a biochemical pathway and caused a disease (phenotype). The reason why people with alkaptonuria have black urine is because a chemical, called “alkapton”, makes urine black when exposed to air. In normal people, enzymes catalyze the reaction to break down alkapton, but people who are born with the disease, due to genetic defect, cannot make such enzymes and, therefore, cannot break down alkapton. Garrod’s work gave huge impact to modern genetics as it attempted to explain the biochemical mechanism behind the genes proposed in Mendelian genetics.

Take a look at the video below, Inborn Error of Metabolism: Alkaptonuria, by Walter Jahn (2016) on YouTube, which describes alkaptonuria.

Media Attributions

References

Betts et al. (2013, April 25). Figure 3.29 From DNA to Protein: Transcription through Translation [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-4-protein-synthesis

Jahn, W. (2016, July 16). Inborn error of metabolism: Alkaptonuria (video file). YouTube. https://www.youtube.com/watch?v=T6QDc7TCrv4

License

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Open Genetics by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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