— By Ximena Rodriguez and Amit U Sinha
Why do some people have blue eyes and others have brown eyes? Why does the human population have varied blood types? Why do certain people have congenital diseases? And what have these got to do with our gene structure?
The answers to these questions point to our genes — the physical and functional units of heredity present in our cells — responsible for controlling our characteristics and the functioning of our system. To understand how genes work and perform this control, it is important to know their structure.
The remarkable results of DNA sequence analysis in recent years have revealed despite having a thousand times more DNA than bacteria, human beings only have twenty percent more genes than any bacteria. This exceptional revelation in the field of genetics highlights the vast majority of human DNA is undoubtedly nonfunctional. This paradox of gene density in humans caused much of an initial trepidation among the research community. For instance, the rudimentary question of “why not more complex organisms such as humans would have more DNA” can activate the thinking cap of even an amateur.
However, incredible advancements in the domain of genetics and genomics, predominantly DNA sequence analysis, suggests the gene content of an organism has little to do with the organism’s complexity. Recent statistics tell us that human gene density comprises approximately 100,000 expressed genes, which are translated from 100,000 separate mRNAs — but only a fraction of them are expressed at an instant in the cell.
Genes are the basic units of heredity carrying the essential information necessary for the survival and reproduction of all organisms. Thus, understanding gene structure of a human is crucial for understanding the complex mechanisms of gene expression, annotation, and function. The completion of the Human Genome Project in 2003 equipped the researchers with powerful tools such as blotting, PCR, SAGE, DNA sequence analysis, and others, to identify the gene structure of humans.
In terms of functional structure, each gene in the human body contains several working parts, each of which has its unique role in gene expression. However, two main functional units, the coding region, and the promoter region, are adherent to each gene as indicated by the advancements in DNA sequencing and analysis techniques. The promoter region is involved in controlling the location and time of gene expression in each tissue. For instance, the promoter region of the globin gene is involved in expressing itself in erythroid cell and not in the cardiac cells. This tissue-specific expression of the gene is facilitated by the structural elements, called nucleotide sequences that allow the gene to be expressed only in its specified cell. These elements are also referred to as cis-acting elements because they are present in the same region of DNA as of gene. These cis-acting elements are physically involved in the transcription of the gene as they bind these proteins.
The gene’s coding region is involved in specifying the structure of a gene protein. This coding region is essential in directing the erythroid cells to direct the erythroid cell in synthesizing the amino acids for the production of the β-globin protein. The coding region of the gene also contains its unique sequence of nucleotides, which aids in gene expression and translation of proteins.
The structural consideration of the human gene sees nucleotides as the fundamental, repeating units of the DNA. The composition of nucleotides is invariant in different human genes, but all of them chiefly consists of a phosphate group attached with a five-carbon deoxyribose sugar and a variable section called the base. Purines [Adenine (A) and Guanine (G)] and Pyrimidine [Thymine (T) and Cytosine (C)] appear in the nucleotides of DNA which are connected through definite phosphate groups.
Such structural organization of the gene, allows the bases to interact with each other through hydrogen bonding. However, it should be kept in mind that the base-pairing interaction is exclusive; denoting that A only interacts with T and G only interacts with C. DNA sequence and analysis visualization techniques reveal that these base pairs are aligned in such a manner that the base of each complementary strand of DNA faces each other. Thus, the base pairing of DNA inside each gene is complementary which allow genes to facilitate DNA in making an exact replica of itself.
Pyrosequencing, which is a modern DNA sequence and analysis technique, has proved to be useful in highlighting the structure of various genes in human. For instance, each gene in the human cell does not have continuous coding regions but is interrupted by sequences of DNA, which are not transcribed into mature DNA. These regions are called introns and exons, respectively.
The structure of the human gene comprises several nested sequence elements with each of the code having a direct role in the gene expression. The lengths and sequences of each of this region inside the human chromosome may vary, but they may account to the production of same proteins. The gene also contains the open reading frame (ORF) which indicates the 5′ to 3′ direction of the coding or sense strand. The 5′ to 3′ direction specifies the position of the carbon atoms of the backbone of the ribose sugar.
The DNA sequence and analysis techniques, specifically NGS also indicates the presence of regulatory sequences, which are present at the extremities of the gene. In most of the human genes, these regions are located either near the promoter or separated by numerous kilobases near the splicers or enhancers. As the name suggests, the promoter region is located at the 5′ end of the gene and comprises a unique promoter sequence that indicates the starting site for the process of transcription. The promoter region of the gene essentially functions to bind RNA polymerase and other proteins for replication of DNA or RNA.
In short, genes are definite lengths of DNA with specific sequences that account for the synthesis of protein. Getting an overview of the structure of genes is the first step towards the understanding of genetics, genomics, gene expression, cellular biology, heredity and other domains of biology.
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Author Bio: Written by Ximena Rodriguez, a freelance writer with an interest in the fields of healthcare, science and lifestyle. Reviewed by Amit U Sinha, PhD (Machine Learning and Genomics), founder and CEO of Basepair, an online NGS analysis platform. Amit is an expert in genomics and bioinformatics, with over a decade of experience in the field. Amit worked as an investigator at Memorial Sloan Kettering Cancer Center, has held research faculty positions at the Dana Farber Cancer Institute and Harvard Medical School. Amit’s work focuses on leveraging technology to improve healthcare research.
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