By Dr Ananya Mandal, MD
DNA is deoxyribonucleic acid. It is located in the nuclei of cells, which make up the body. Consequently, DNA can be considered as one of the building blocks of the body.
To understand the exact structure and function of the body it is essential to know what a cell is and how it is structured.
The cell is the basic structure of the body. The human body is built of billions and trillions of cells. Cells of different organs vary according to their function.
Each cell contains the hereditary material and can make copies of themselves by reproducing and multiplying. After a specific life span the old cells die off.
Parts of the cell are called organelles. Human cells contain the following major parts:
Nucleus – This is central part of the cell that carries the blue print for the cell functioning and tells the cell when to grow, reproduce and die. It also houses DNA (deoxyribonucleic acid).
Mitochondria – These are the powerhouses of the cell and produce energy for the various activities of the cell.
Cytoplasm – This is a jelly like fluid within the cell in which the other organelles float.
Endoplasmic reticulum (ER) – This helps in processing the molecules (e.g. proteins) created by the cell.
Ribosomes – These lie over the ER and process the genetic instructions or the blueprints within the DNA and create new proteins. These can also float freely in the cytoplasm.
Lysosomes and peroxisomes – These help in digesting foreign bacteria that invade the cell, rid the cell of toxic substances
Cell membrane – This is the outer lining of the cell.
Within the nucleus the DNA strands are tightly packed to form chromosomes. During the cell division the chromosomes are visible.
Each chromosome has a constriction point called the centromere from where two arms are formed. The short arm of the chromosome is labelled the “p arm.” The long arm of the chromosome is labeled the “q arm.”
Each pair of chromosome is shaped differently by the location of the centromere and the size of the p and q arms.
Humans normally have 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females.
The 23rd pair is called the sex chromosomes and differs between males and females. Females have two copies of the X chromosome or XX, while males have one X and one Y chromosome.
Genes are hereditary material that lies within the cell nucleus. Genes, which are made up of DNA, act as instructions to make molecules called proteins.
The Human Genome Project has estimated that humans have between 20,000 and 25,000 genes. Every person has two copies of each gene, one inherited from each parent. These are mostly similar in all people but a small number of genes (less than 1 percent of the total) are slightly different between people and this forms the basis of paternity tests and DNA analysis.
Where is DNA found?
DNA, or deoxyribonucleic acid, is the hereditary material that lies within the nucleus of all cells in humans and other living organisms. Most of the DNA is placed within the nucleus and is called nuclear DNA. However, a small portion of DNA can also be found in the mitochondria and is called mitochondrial DNA or mtDNA.
What is DNA made of?
DNA contains four chemical bases:
DNA base pairs
DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule.
DNA in humans contains around 3 billion bases and these are similar in two persons for about 99% of the total bases. These bases are sequenced differently for different information that needs to be transmitted. This is similar to the way that different sequences of letters form words and sequences of words form sentences.
Nucleotides and the double helix
A base, sugar, and phosphate in combination is called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. This looks like a twisted ladder and the base pairs form the rungs of the ladder and the sugar and phosphate molecules form the sides of the ladder.
How does DNA replicate itself?
The DNA can make copies of itself. Both the strands of the DNA open up and make a copy of each and become two DNA stands. Thus each new DNA has one copy of the old DNA from where the copy is made.
The mitochondria contain small amount of DNA. This genetic material is known as mitochondrial DNA or mtDNA.
Each cell contains hundreds to thousands of mitochondria that lie within the cytoplasm. Mitochondrial DNA contains 37 genes that help it to function normally. Thirteen of these genes provide instructions for making enzymes involved in energy production by oxidative phosphorylation. The rest of the genes help in making molecules called transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs) that help in protein synthesis.