Why do some people have blue eyes and others have brown eyes? Why does the human population have diverse 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.
Humans have between 20,000 to 25,000 genes. Every person has two copies of each gene, one copy from each parent. To understand how genes work and perform, let’s get a glimpse into their structure.
Human Gene Structure – In Brief
Genes are the basic structural and functional units of heredity carrying the essential information necessary for the survival and reproduction of organisms.
Gene: any interval along the chromosomal DNA that is transcribed into a functional RNA molecule or that is transcribed into RNA and then translated into a functional protein.
How many genes do we have?
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. And a bacterium is just a one-celled organism. This exceptional revelation in the field of genetics highlights the fact a 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, it got even the most amateur thinker in the field asking the question: “Why not more complex organisms such as humans have more DNA than bacteria?”
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 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.
The gene content of an organism has little to do with the organism’s complexity. Humans only have 20 percent more genes than any bacteria.
A basic grasp of the gene structure of humans 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 human gene structure.
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. For example, 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.
What Are Nucleotides?
A nucleotide is the building block of the nucleic acids — the DNA and the RNA. The structure of the human gene sees nucleotides as the fundamental, repeating units of the DNA or the RNA.
The composition of nucleotides that form the DNA is the same across different human genes, and all of them chiefly consists of a phosphate group attached to a five-carbon deoxyribose sugar and a variable section containing nitrogen called the base.
There are five kinds of bases: Adenine (A), Guanine (G) [together called purines], Thymine (T), Cytosine (C), and Uracil (U) [together called pyrimidines]. In DNA, the bases that appear are A, C, G, T. In comparison, the RNA carries the bases A, C, G, U.
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 these base pairs are aligned in such a manner that the base of each strand of DNA faces another complementary strand. Thus, the base pairing of DNA inside each gene matches each other which allows genes to facilitate DNA 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 humans. 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 codes having a direct role in the gene expression. The lengths and sequences of each of these regions inside the human chromosome may vary, but they may account for the production of the 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 Next Generation Sequencing (NGS) also indicate the presence of regulatory sequences, which are present at the extremities of the gene.
In most 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 the replication of DNA or RNA.
Final Words
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 toward the understanding of genetics, genomics, gene expression, cellular biology, heredity, and other domains of biology.
[Now that you reached here, you could just take a quick look at this amazing science story: 17 Monkeys of Neuroplasticity]
Author Bio: Written by Ximena Rodriguez. Reviewed by Amit U Sinha, PhD (Machine Learning and Genomics), founder and CEO of Basepair – an online Next Generation Sequencing (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.
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