GENES AND GENE EXPRESSION

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Genes are sections of the deoxyribonucleic acid (DNA) that codes for the synthesis of a specific protein sequence in a cell. A gene is generally the genetic unit or sequence on a chromosome which direct the synthesis of a particular protein sequence as wells as the characteristics of the synthesized protein molecule. When the trait that a gene controls is always passed on from the parent to their offspring’s and even from one generation to another, such a gene is known as a dominant gene. On the other hand, when the characteristics is not continuously expressed in the offspring but only appears if both parents have contributed the same form of the gene, such genes are generally known as recessive genes. Dominant genes and recessive genes are the two main types of genes that govern the physical characteristics or phenotypes of living organisms. The physical trait of living organisms is governed by two sets of genes (i.e. the dominant and recessive genes). The resulting phenotypes of organisms are usually governed by dominant genes in cases where one of the genes is recessive and the other is dominant. Recessive genes are only expressed in an individual when both of the genes are recessive.

Figure 1: The Central Dogma. The central dogma shows how genetic information flow in the cell of an organism. Photo courtesy: https://www.microbiologyclass.com

Generally, genes are sequence of nucleotides found in the chromosome or plasmid of an organism. They encode a functional polypeptide chain or ribonucleic acid (RNA) molecule inclusive of tRNA, rRNA and mRNA that directs protein synthesis in the cell or whole organism. It is vital to note that the genetic information in the DNA of a cell is required for the biosynthesis of protein molecules which are unique to the cell. This is achievable through transcription and translation as exemplified by the central dogma of molecular biology (Figure 1). A gene is said to be expressed in a cell or whole organism when it is transcribed (i.e. copied) into messenger ribonucleic acid (mRNA) and then translated into proteins. Gene expression is controlled in vivo at several levels by some transcriptional, translational and post-translational factors. The control of gene expression in the cell is critical because it ensures that the correct type of protein molecules is synthesized at the correct time and in the right cell. The control of gene expression also saves useful energy for the cell and prevents the production of unwanted proteins. Generally, proteins are produced through the direction of the genetic instruction encoded by genes. These synthesized proteins are what actually perform the most of life’s functions in the body. They also make up the majority of the cellular structures in the body of living organisms. And this is why proteomics, the study of the entire protein complement that a cell or genome expresses at any given time (known also as the proteome) is using high-throughput techniques to understand the biological systems of the human body in view of unraveling the mysteries behind non-infectious diseases such as cancer. Most of the cellular processes that occur inside the cell of an organism are spurred and directed by the proteins, but the instructions to perform these functions actually come from the DNA, and this is exemplified by the central dogma of molecular biology (Figure 1).

Transcription is the process of transferring or copying the genetic information encoded in the DNA into a strand of mRNA. During transcription, the RNA polymerase reads from the DNA strand complementary to the RNA molecule to construct the complimentary mRNA which encodes or carry the genetic code required for gene expression in the cell (i.e. the biosynthesis of particular proteins). Translation is the process by which the cellular machinery reads the genetic code encoded by the mRNA and then creates a polypeptides chain required for protein synthesis in the ribosome. Itis the phase of protein synthesis in which information in the mRNA is used to guide the sequence of amino acids or polypeptide chain assembled by the ribosomes. It ensures that the right type of protein molecules (as encoded by the genetic code of the DNA) are synthesized in the cell. Transcription (which occurs in the nucleus) and translation (which occur in the ribosome or endoplasmic reticulum) are the two stages in which the genes or genetic information stored in the DNA are expressed in a cell. Genetic code is the code in the nucleotide sequence of nucleic acids (DNA and RNA) that contains the information required for the synthesis of particular proteins in the cell of an organism.

Haemoglobin is a protein that is found in the red blood cells (RBCs). Its main function is to transport oxygen from the lungs to the body tissues. Chromosomes are threadlike structures that carry the genes (i.e. sections or units of the DNA). They are found in the nucleus of a cell (Figure 2). The chromosome is a structure that contains DNA which carries genetic information that is vital to both the prokaryotic and eukaryotic cells. Chromosomes are usually paired, and a normal human cell contains 46 chromosomes (because chromosomes exist as 23 pairs) which consist of 22 pairs of autosomes (that comprises somatic or body cells) and two sex chromosomes (which are the gametes) designated as “Y” chromosome(for sperm cells)and “X” (for egg or ova) chromosome (Figure 3). Autosomes are chromosomes that are not sex chromosomes. They include the 22 pairs of body or somatic cells of a normal human cell.

Figure 2. A generalized structure of the chromosome. Made up of proteins (especially histones) and DNA, the prime job of the chromosome is to ensure that the DNA is accurately copied and distributed in the cell during cell division. Prokaryotic chromosomes are circular while those of eukaryotic cells are linear. Photo courtesy: https://www.microbiologyclass.com

The genome of human beings is mainly distributed along 23 pairs of chromosomes that comprises 22 autosomes or autosomal pairs of chromosomes and a pair of sex chromosomes that comprises two “X” chromosomes (i.e. XX) for females and one “X” and one “Y” chromosomes (i.e. XY) for males. One chromosome in each pair of the sex chromosomes is inherited from the father while the other member of the pair is inherited from the mother. The sex of the child or offspring therefore is determined by each of the chromosome in the pair of sex chromosome especially the XY chromosome.

Figure 3. Schematic illustration of the human chromosome. The sex of the child is determined by the XY chromosome. The “X” chromosome is the genetic marker for female sex or gender while the “Y” chromosome is the genetic marker for male gender. Photo courtesy: https://www.microbiologyclass.com

Genes in the DNA code for proteins. It is the gene that directs the cell in what particular order to assemble the amino acids which eventually becomes the building blocks of protein molecules. The cell of an organism must use one nucleotide or more in the DNA to spell out or specify each of the amino acid in a particular protein. This is because there are 20 possible essential amino acids and only four (4) possible bases (i.e. guanine-G, adenine-A, thymine-T and cytosine-C) that are mainly involved in the protein synthesis process as well as in other genetic processes. The 20 essential amino acids are shown in Table 1.

Table 1. The Twenty Essential Amino Acids

Amino acidAbbreviation
AlanineALA
ArginineARG
AsparaginesASN
Aspartic acidASP
CysteineCYS
GlutamineGLN
Glutamic acidGLU
GlycineGLY
HistidineHIS
IsoleucineILE
LeucineLEU
LysineLYS
MethionineMET
PhenylalaninePHE
ProlinePRO
SerineSER
ThreonineTHR
TryptophanTRP
TyrosineTYR
ValineVAL

Further reading

Cooper G.M and Hausman R.E (2004). The cell: A Molecular Approach. Third edition. ASM Press.

Das H.K (2010). Textbook of Biotechnology. Fourth edition. Wiley edition. Wiley India Pvt, Ltd, New Delhi, India.

Davis J.M (2002). Basic Cell Culture, A Practical Approach. Oxford University Press, Oxford, UK. 

Mather J and Barnes D (1998). Animal cell culture methods, Methods in cell biology. 2rd eds, Academic press, San Diego.

Noguchi P (2003).  Risks and benefits of gene therapy.  N  Engl J Med, 348:193-194.

Sambrook, J., Russell, D.W. (2001). Molecular Cloning: a Laboratory Manual, 3rd edn. Cold Spring Harbor Laboratory Press, New York.

Tamarin Robert H (2002). Principles of Genetics. Seventh edition. Tata McGraw-Hill Publishing Co Ltd, Delhi.     

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