Introduction:
- A gene is a unit of hereditary information that is passed down from parent to offspring. Genes are made up of DNA, which stands for deoxyribonucleic acid. DNA is a long, double-stranded molecule that is composed of four chemical units called nucleotides.
- The sequence of these nucleotides contains the instructions for the production of proteins, which are complex molecules that perform a wide range of functions in the body. Proteins play important roles in many biological processes, including the structure and function of cells, tissues, and organs. They are also involved in the regulation of the body’s metabolism and immune system.
- Genes are found on chromosomes, which are long, thread-like structures that are located inside the nucleus of cells. There are 23 pairs of chromosomes in the human genome, which is the complete set of genetic material present in an individual. Each chromosome contains hundreds or thousands of genes, and each gene has a specific role in determining an individual’s characteristics, such as their physical appearance, health, and behavior.
Features:
- Inheritance: Genes are passed down from parent to offspring and play a role in determining an individual’s characteristics and traits.
- Locus: A gene is located at a specific position, called a locus, on a chromosome.
- Alleles: A gene can have multiple versions, called alleles, which can result in different traits or characteristics.
- Expression: Genes are expressed, or turned on, in order to produce proteins. The level of gene expression can vary depending on the cell type and the environment.
- Regulation: The expression of genes is regulated at various levels, including transcription, translation, and post-translational modification.
- Mutation: A change in the DNA sequence of a gene is called a mutation. Mutations can result in changes in the protein produced by a gene, which can alter its function.
- Polymorphism: There can be variations in the DNA sequence of a gene among individuals within a population. These variations, called polymorphisms, can result in differences in traits and characteristics.
Structures:
- The structure of a gene includes both the DNA sequence that encodes the instructions for the production of a protein and the regulatory elements that control the expression of the gene.
- The DNA sequence of a gene is made up of four nucleotide bases: adenine (A), cytosine (C), guanine (G), and thymine (T). These bases are arranged in a specific order, and the sequence of bases determines the sequence of amino acids in the protein produced by the gene.
- The regulatory elements of a gene include promoters, enhancers, and silencers. Promoters are located at the start of a gene and control the initiation of transcription, the process by which the instructions in DNA are copied into RNA. Enhancers are DNA sequences that can increase the expression of a gene. Silencers are DNA sequences that can reduce gene expression.
- In addition to the DNA sequence and regulatory elements, a gene also includes introns and exons. Introns are non-coding regions of DNA that are present within the gene. Exons are the coding regions of DNA that are transcribed into RNA and then translated into protein.
- In prokaryotes, such as bacteria, the genetic material is organized into a single, circular chromosome. The chromosome is composed of DNA and is located in the cell’s cytoplasm. Prokaryotic genes are usually transcribed and translated as a single unit, and they do not have introns. An operon is a unit of genetic regulation in prokaryotes that consists of a group of genes and the regulatory sequences that control the expression of those genes.
Fig: Eukaryotic and Prokaryotic Gene Structure
- In eukaryotes, such as animals and plants, the genetic material is organized into multiple linear chromosomes that are located in the nucleus. Eukaryotic genes are usually transcribed into mRNA and then spliced to remove introns before being translated into a protein. Eukaryotic genes are often regulated at the level of transcription, and the expression of eukaryotic genes can be regulated by a variety of mechanisms, including the use of transcription factors and the modification of chromatin structure.
Types:
Structural genes
These genes encode the instructions for the production of proteins. The proteins produced by structural genes can be structural components of cells and tissues, enzymes that catalyze chemical reactions, hormones that regulate various processes in the body, or other proteins with specialized functions.
Regulatory genes
These genes control the expression of other genes. They can either increase or decrease the production of proteins encoded by structural genes. Regulatory genes are important for coordinating the activity of different genes and ensuring that they are expressed at the right time and in the right place.
Housekeeping genes
These genes are essential for the basic functions of cells and are expressed at a constant level in most tissues. Examples of housekeeping genes include those that encode ribosomes, which are involved in protein synthesis, and those that encode enzymes involved in energy production and metabolism.
Pseudogenes
These are non-functional copies of genes that have been inactivated due to mutations. Pseudogenes do not produce proteins and do not have a known function.
Oncogenes:
These genes play a role in the development of cancer. When oncogenes are mutated or overexpressed, they can cause normal cells to become cancerous.
Tumor suppressor genes
These genes help to prevent the development of cancer by regulating cell growth and division. When tumor suppressor genes are mutated or inactive, the risk of cancer increases.
Gene expression:
Gene expression is the process by which a gene’s instructions are used to produce a protein. It is an essential process that occurs in all cells and is important for the development, growth, and function of an organism.
Gene expression begins with transcription, the process by which the instructions in DNA are copied into RNA. RNA is a molecule that is similar to DNA but is composed of a single strand of nucleotides. The RNA molecule produced during transcription is called messenger RNA (mRNA), and it contains a copy of the instructions for the production of a specific protein.
Fig: Gene expression
Translation is the process by which the instructions in mRNA are used to produce a protein. Translation occurs on ribosomes, which are complex molecules that read the instructions in mRNA and synthesize proteins. The process of translation involves the assembly of amino acids, the building blocks of proteins, in the correct order based on the sequence of nucleotides in the mRNA molecule.
Gene expression is regulated at various levels, including transcription, translation, and post-translational modification. Changes in the level of gene expression can affect the amount of protein produced and, in turn, the function of the protein.
Functions:
- Determining physical characteristics: Genes play a role in determining an individual’s physical characteristics, such as eye color, hair color, and facial features.
- Regulating development: Genes play a role in the development of an organism, including the development of specific tissues and organs.
- Maintaining homeostasis: Genes play a role in maintaining homeostasis, or the balance of various functions in the body. This includes the regulation of metabolism, the immune system, and other processes.
- Responding to environmental changes: Genes play a role in the response of an organism to changes in the environment. This includes the ability to adapt to different conditions, such as changes in temperature or the availability of nutrients.
- Producing proteins: Genes encode the instructions for the production of proteins, which perform a wide range of functions in the body. Proteins can be structural components of cells and tissues, enzymes that catalyze chemical reactions, hormones that regulate various processes in the body, or other proteins with specialized functions.
References:
- Pearson, H., 2006. What is a gene?. Nature, 441(7092), pp.398-402.
- Rheinberger, H.J. and Müller-Wille, S., 2018. The Gene. In The Gene. University of Chicago Press.
- Old, R.W. and Primrose, S.B., 1981. Principles of gene manipulation: an introduction to genetic engineering (Vol. 2). Univ of California Press.
- Hall PA, Reis-Filho JS, Tomlinson IP, Poulsom R. An introduction to genes, genomes and disease. J Pathol. 2010 Jan;220(2):109-13.