By Dr Ananya Mandal, MD
The human genome is made up of about 20,000 genes located on one of the 23 chromosome pairs found in the nucleus or on long strands of DNA located in the mitochondria.
The Human Genome Project and genetic research
To date, about 12,800 genes have been mapped to specific locations (loci) on each of the chromosomes. This database was begun as part of the Human Genome Project. The project was officially completed in April 2003 but the exact number of genes in the human genome is still unknown.
It will still take many years to find all the genes as well as understand the need and use of the non-coding DNA. Genetic research also focuses on the effects of the environment on genes and their expression.
The genetic code
Each gene has its own specific location on the chromosome or on the mitochondrial DNA. The gene performs a single important function. These serve as blueprints for a physical, physiological or mental trait. The genes tell how a person looks like hair, eye, skin color, shape of the body and features, height etc.
The DNA code is made up of very long chains of four basic building blocks (nucleotide bases):
The chromosome contains two of the DNA chains running in opposite directions; the bases pair up to form the rungs of a ladder twisted into a double helical structure.
Adenine (A) can only pair with base Thymine (T), and vice versa; and base Guanine (G) can only pair with base Cytosine (C), and vice versa. There are nearly three billion of these base pairs of DNA to make the human genome. Within the DNA, three of these four chemical ‘letters’ A, G, C and T (a triplet) form a codon. A particular codon codes for an amino acid and sequences of amino acids code for a protein.
DNA that makes up the genes is often called ‘coding DNA’. Between two genes is a region of DNA that does not code for any protein. This is called the ‘non-coding DNA’ and was initially termed ‘junk DNA’ as it appeared that this DNA did not contain the information required for proteins.
Now new research shows that this region may not be junk and may be important for certain functions. That role is still largely unknown but they may play a regulatory role in gene expression. Studies of this non-coding DNA are useful for forensic investigations like paternity suits and criminal investigations etc.
The genetic code codes for proteins. The information of the DNA is ‘translated’ into a chain of amino acids that forms a protein. These proteins form the building blocks for structures within the cells and ultimately the whole body. Proteins also form enzymes and other chemicals that perform various functions in the body. Each gene can code for different proteins and thus the number of proteins known to exist in the cells is more than the number of genes.
All genes are not expressed or do not code for any protein. This could be organ specific for example a liver cell expresses different genes than kidney cells.
The environment also plays a role in determining the ultimate trait. The phenotype of an organism thus depends on the interaction of genetics with the environment. The environment for example, has a role in effects of the human genetic disease phenylketonuria. The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine. This leads to toxic build-up of an intermediate molecule leading to mental retardation and seizures. Persons with phenylketonuria mutation on a strict diet that avoids this amino acid may remain normal and healthy.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)