Molecular Biology Topics

GENES

Written by MicroDok

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. Thus 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. Generally, genes are sequence of nucleotides found in the chromosome or plasmid of an organism, and which encodes 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; and 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; and gene expression is controlled in vivo at several levels by some transcriptional, translational and post-translational factors. Generally, proteins are produced through the direction of the genetic instruction encoded by genes; and these synthesized proteins are what actually perform the most of life’s functions in the body, and 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.

Fig. 1. Central dogma of molecular biology

The Central Dogma. The central dogma shows how genetic information flow in the cell of an organism. 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.

It is 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; and 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.

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 (Figure 1). Haemoglobin is a protein that is found in the red blood cells (RBCs); and its main function is to transport oxygen from the lungs to the body tissues. Chromosomes are threadlike structures that carries the genes (i.e. sections or units of the DNA), and they are found in the nucleus of a cell. 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 of the 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 2).

Autosomes are chromosomes that are not sex chromosomes, and they include the 22 pairs of body or somatic cells of a normal human cell. The genome of human beings is mainly distributed along 23 pairs of chromosomes that comprises of 22 autosomes or autosomal pairs of chromosomes and a pair of sex chromosomes that comprises of 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; and the sex of the child or offspring is determined by each of the chromosome in the pair of sex chromosome especially the XY chromosome.

Figure 2: 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.

Genes in the DNA code for proteins; and 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; and 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. Each possible set of three nucleotides (i.e. a codon) in the DNA is what specifies one particular amino acid, which in combination with other amino acids forms a protein. For example, AAA specifies the amino acid lysine while GCT specifies alanine.

Table 1: The Twenty Essential Amino Acids

Amino acid Abbreviation
Alanine ALA
Arginine ARG
Asparagines ASN
Aspartic acid ASP
Cysteine CYS
Glutamine GLN
Glutamic acid GLU
Glycine GLY
Histidine HIS
Isoleucine ILE
Leucine LEU
Lysine LYS
Methionine MET
Phenylalanine PHE
Proline PRO
Serine SER
Threonine THR
Tryptophan TRP
Tyrosine TYR
Valine VAL

A codon is a triplet of three nucleotides in the mRNA that codes for a specific amino acid during the synthesis of a protein molecule. It is a sequence of nucleotides formed by triplet of bases (e.g. TAC is the codon for the amino acid tyrosine). Codon provides the genetic information which causes a specific amino acid to be produced in a cell. There are various types of codons, and each performs a specific function during protein synthesis in the cell of an organism. A start codon is the codon that signals translation, and it tells the mRNA when to initiate the peptide chain formation or elongation. It is the first codon of an mRNA transcript that is translated by the ribosomes during protein synthesis; and start codons are also known as initiator codons because they signal and initiate the translation process.

AUG is a start codon, and it also codes for the amino acid, methionine. Apart from signaling the incorporation of the amino acid methionine into the growing polypeptide chain, the start codon AUG also signals the start of translation during protein synthesis in the cell. Stop codons are triplet of nucleotide sequence that signals the end of translation. They are also known as termination or nonsense codons because they bring protein synthesis in the cell to an end. Examples of stop codons include UAG, UAA, and UGA.

In the genetic code, there are 64 possible codons; and this comprises of 3 nucleotides in each codon with 4 possible bases which are adenine, guanine, cytosine and thymine or uracil (Figure 3).

Fig. 3. Structures of adenine, guanine, cytosine, thymine and uracil

The language of the gene is created when these bases (i.e. A, G, C, and T) are genetically arranged or set in the DNA just the same way the 26 English alphabets are ordered to create meaningful words. These four bases which includes adenine (A), guanine (G), cytosine (C), and thymine (T), and which are usually found in the mRNA of the cell can generate 64 possible triplet combinations (e.g. UAG). Out of this 64 possible triplet combinations or codons, 61 triplet combinations encode the 20 essential amino acids (Table 1) while the remaining three triplet combinations (UAG, UGA & UAA) are generally known as stop or nonsense codons because they signify the termination of the polypeptide chain formation during protein synthesis in the ribosome.

Mathematically, 43 = 64 possible codons; where “4” refer to the bases (i.e. A, G, C & T) and “3” refer to the triplet combinations or codons.

REFERENCES

Alberts B, Bray D, Johnson A, Lewis J, Raff M, Roberts K and Walter P (1998). Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. Third edition. Garland Publishing Inc., New York.

Alberts B.M, Johnson A, Lewis J, Raff M.C, Roberts K and Walter P (2002). Molecular Biology of the Cell (4th Edn). Garland, New York. A good general reference volume.

Dale J (2003). Molecular genetics of bacteria. Jeremy W. Dale and Simon Park (4th eds.). John Wiley & Sons Ltd, West Sussex, UK.

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

Twyman R.M (1998). Advanced Molecular Biology: A Concise Reference. Bios Scientific Publishers. Oxford, UK.

About the author

MicroDok

Leave a Comment