By Dr Ananya Mandal, MD
Deoxyribonucleic acid (DNA) is the chemical information database that carries the complete set of instructions for the cell as to the nature of the proteins produced by it, its life span, maturity, function and death. Genes are the working subunits of DNA. Each gene contains a particular set of instructions, usually coding for a particular protein or for a particular function.
Deoxyribonucleic Acid (DNA). Image Credit: genome.gov
Every human cell has 46 molecules of double-stranded DNA. This DNA is coiled and supercoiled to form chromosomes. Each chromosome has around 50 to 250 million bases.
Image Credit: genome.gov
Human cells contain two sets of chromosomes, one set inherited from the mother and one from the father. The egg from the mother contains half of the 46 (23) and the sperm from the father carries the other half 23 of 46 chromosomes. Together the baby has all 46 chromosomes.
There are 22 pairs of autosomes and 1 pair of sex chromosomes. Females have an XX chromosome while men have an XY chromosome.
DNA resides in the core, or nucleus, of each of the body's trillions of cells. Every human cell (with the exception of mature red blood cells, which have no nucleus) contains the same DNA.
The DNA is a double, stranded spiral forming a double helix. Each strand is made up of millions of chemical building blocks called bases. There are only four types of bases making up the DNA - adenine, thymine, cytosine, and guanine. The order of these bases are changed with permutation and combination in a sequence and unique sequences code for proteins.
The concept is similar to combination of alphabets to form words that further combine to form sentences.
The DNA in each chromosome constitutes many genes. The DNA also contains large sequences that do not code for any protein and their function is not known. The gene of the coding region encodes instructions that allow a cell to produce a specific protein or enzyme. There are nearly 50,000 and 100,000 genes with each being made up of hundreds of thousands of chemical bases.
In order to make proteins, the gene from the DNA is coped by each of the chemical bases into messenger RNA (ribonucleic acid) or mRNA. The mRNA moves out of the nucleus and uses cell organelles in the cytoplasm called ribosomes to form the polypeptide or amino acid that finally folds and configures to form the protein.
The human genome
All the DNA in the cell makes up the human genome. There are about 20,000 important genes located on one of the 23 chromosome pairs found in the nucleus or on long strands of DNA located in the mitochondria.
The DNA in the genes make up only around 2% of the genome. For some years now each of the sequences and genes discovered are carefully recorded as to their specific location, sequences etc. The whole information is stored in a database that is publicly accessible.
Nearly 13000 genes have been mapped to specific locations (loci) on each of the chromosomes. This information was initiated by the work done as part of the Human Genome Project. The completion of the project was celebrated in April 2003 but the exact number of genes and numerous other genes in the genome of humans is as yet unknown.
Genetic switches and non-coding DNA regions
The genes that contain the information to make the necessary proteins are therefore ‘switched on’ in some of the specialized cells while the remaining genes are ‘switched off’. For example, the genes that are ‘switched on’ in kidney cells are different to those that are ‘switched on’ in brain cells because the cells of the brain have different roles and make different proteins.
In addition to the Human genome project, more information is needed to find what each of the genes as well as the vast amounts of non-coding regions do. These non-coding regions form nearly 90% of the chromosome and earlier much of it was termed “junk DNA” as it appeared that this DNA did not contain the information for gene products that the cells use and produce.
Now it is increasingly clear that the non-coding DNA has a very important role to play. That role is still largely unknown but is likely to include regulating which genes are ‘switched on’ or ‘switched off’ in each cell.
The non-coding regions of the DNA is also important for forensic investigations and determining biological relationships – paternity etc.
Promoter regions, exons & introns of genes
A gene can have more than one promoter, resulting in RNAs that may vary in lengths. Some genes may have "strong" promoters that bind the transcription machinery well, and others have "weak" promoters that bind poorly. Weak ones allow for less transcription to protein than strong ones. Other possible regulatory regions include enhancers. These enhancers may help the weak promoters.
Many prokaryotic genes are organized into operons. These sequences are genes that have products with related functions. Long stretches of DNA that are coded to proteins are called introns and non-coding regions are called exons.
Genes & mutations
Around 20,000 genes in the cell guide the growth, development and health of the animal or human. The genetic information contained in the DNA is in the form of a chemical code, called the genetic code. The code is similar in many ways and in most of the sequences across all living organisms.
An ''allele'' is one variant of that gene. In many cases, all people would have the gene, but certain people will have a specific allele of that gene, which results in the trait. This could be a simple trait like hair or eye color.
There are, however, variations in the genetic code that makes each individual unique. Most variations are harmless. However, variations to the genetic information can sometimes mean that some proteins are not produced properly, produced in the wrong amounts or not produced at all.
Variations that make the gene faulty are called mutations. SNPs or single nucleotide polymorphisms are changes in a single base or single letter in the sequence and may code of a different protein altogether making it akin to a genetic mutation.
Mutations of genes that are important for functions in the body can lead to a genetic condition that may affect growth or health of the individual. Some mutations do not directly cause disease but may make a person more susceptible to developing a genetic condition.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)