DNA, Chromosomes and Genes – An overview
April 8, 2008 by Team ASO
Akhila Satchidanandam
Have we ever realized that we know why our laptop works the way it does and why it conks when it does, better than how our own inner systems work and why we look and behave the way we do? Well, the aim of this article is to explain in the simplest way possible, very basic functional unit of life that makes us what we are – DNA, genes and chromosomes. DNA, genes and chromosomes are commonly misunderstood as three separate entities yet, they are not synonymous.
Living organisms are made up of cells is a known fact. Each of these cells encompasses a viscous substance called cytoplasm and organelles within the cell membrane. Organelles are tiny membrane bound or naked bodies that perform functions like ingestion (taking in nutrients), metabolism (processing of nutrients), storing energy, excretion etc. for the cell’s survival. Of these organelles, the most important is the nucleus that harbors genetic material of the organism. Every cell’s nucleus of the same organism contains genetic material that is physically identical.
DNA (Deoxyribose Nucleic Acid):
DNA is formed by nucleotides arranged consecutively, in two opposite strands and these two strands are wound around each other. Just imagine two paralleled flower garlands wound around each other, the flowers being the nucleotides. That is how DNA looks if we were able to see it. “Uh? Nucleotides??”…. Well, hang on… I’m getting there.
So, what are nucleotides?
There are four different molecules – adenine (A), guanine (G) called
Purines and thymine (T), cytosine(C) called Pyrimidines broadly termed nitrogenous bases. Each of these nitrogenous bases when attached to a sugar called deoxyribose is called a nucleoside. A nucleoside when attached to a phosphate group is called a nucleotide. So, the two strands of DNA are these nucleotides arranged linearly. In other words, nucleotides are the monomers of DNA (Deoxyribose Nucleic Acid). The entire human genome occupies about 3 billion nucleotides.
How does the DNA maintain its integrity?
The two strands formed by nucleotides are inter wound in the form of a double helix and the two strands are bound to each other by means of base pairing i.e. if there is an adenine on one strand, it will as a rule have its complementary thymine on the other strand at the same locus enabling the two bases to pair, likewise guanine and cytosine are complementary bases that pair. A-T OR T-A and G-C OR C-G.
What are the factors that keep this double stranded DNA intact without letting the bases to scatter away? Between two complementary bases on opposite strands which pair, is the hydrogen bond and between two adjacent nucleotides on the same strand is a phosphodiester bond which holds the DNA intact within the nucleus.
A single cell itself is microscopic. So, imagine the size of DNA. But as a matter of fact, in terms of length, it can stretch to about 5 miles. Given its length, it has to be highly flimsy to be accommodated within the nucleus. The beauty here is that, it has been so well protected that it hasn’t disintegrated and has been passed on generation after generation for millions of years now.
Chromosomes:
Chromosomes are inter-coiled, condensed DNA. For easy understanding, water becomes ice as a result of certain external factors like temperature. Similarly, chromosomes are a modified version of DNA under the influence of enzymes. Chromosomes have a constriction called centromere and two arms (chromatids) on either side of the centromere. Centromere is highly condensed part of the chromosome, and seperates the chromosome into two arms.
Why does DNA have to become chromosomes?
Chromosomes are formed just before cell division, be it in somatic cells (body cells) or reproductive cells. Somatic cell division takes place through a process called mitosis and reproductive cell division through a process called meiosis. DNA coils, thickens and becomes highly dense to form chromosomes.
DNA forms 23 such chromosomes during cell division – 22 autosomes + 1 sex chromosome. 22 autosomes are alike for all organisms of the same species. The sex chromosome would be X-chromosome for females and Y-chromosome for males. X and Y chromosomes carry specific sex determining factors which are responsible for development of sex organs at birth and secondary sex characters at puberty. All the 23 chromosomes would be identical in all the cells of the same organism.
During cell division, an equal amount of genetic material has to be passed on to the daughter cell. For this reason, each chromosome replicates during the dividing phase resulting in two copies of a chromosomal set and are therefore referred to as diploid cells (46 chromosomes) while cells with one copy of chromosomes in the interphase stage (dormant, non dividing phase) are referred to as haploid (23 chromosomes).
Genes:
Genes, in simple terms are the coding regions of the DNA. Ok, now what are coding regions? Does the DNA also have non coding regions? If yes, what role do they play?
The DNA comprises of short or long stretches of nucleotides randomly spread along the DNA called genes, which carry “information” in the form of “triplet codes” or codons. Each of these triplet codes represents an amino acid (sub-units of proteins). These genes or coding regions get transcribed and translated to proteins and enzymes required for all biochemical functions in the body. However, it is a fact that, only 30% of the DNA features coding regions and 70% of non-coding or junk DNA, which is also being passed from generation to generation. The actual function of this non-coding region though not definitely known, if it were useless, it wouldn’t have been preserved through so many lineages. Several reasons have been hypothesized for the existence of such large chunks of non-coding region of which the most widely accepted and maintained theory is that, this junk DNA confers stability to the genes and protects the genes.
