Virtually every cell in your body contains DNA or the genetic code that makes you you. DNA carries the instructions for the development, growth, reproduction, and functioning of all life.
Differences in the genetic code are the reason why one person has blue eyes rather than brown, why some people are susceptible to certain diseases, why birds only have two wings, and why giraffes have long necks.
Amazingly, if all of the DNA in the human body was unraveled, it would reach to the sun and back more than 300 times.
In this article, we break down the basics of DNA, what it is made of, and how it works.
What is DNA?
In short, DNA is a long molecule that contains each person’s unique genetic code. It holds the instructions for building the proteins that are essential for our bodies to function.
DNA instructions are passed from parent to child, with roughly half of a child’s DNA originating from the father and half from the mother.
The structure of DNA
DNA’s double helix.
DNA is a two-stranded molecule that appears twisted, giving it a unique shape referred to as the double helix.
Each of the two strands is a long sequence of nucleotides or individual units made of:
- a phosphate molecule
- a sugar molecule called deoxyribose, containing five carbons
- a nitrogen-containing region
There are four types of nitrogen-containing regions called bases:
- adenine (A)
- cytosine (C)
- guanine (G)
- thymine (T)
The order of these four bases forms the genetic code, which is our instructions for life.
The bases of the two strands of DNA are stuck together to create a ladder-like shape. Within the ladder, A always sticks to T, and G always sticks to C to create the “rungs.” The length of the ladder is formed by the sugar and phosphate groups.
Packaging DNA: Chromatin and chromosomes
The complete set of chromosomes in a human male.
Image credit: National Human Genome Research Institute
Most DNA lives in the nuclei of cells and some is found in mitochondria, which are the powerhouses of the cells.
Because we have so much DNA (2 meters in each cell) and our nuclei are so small, DNA has to be packaged incredibly neatly.
Strands of DNA are looped, coiled and wrapped around proteins called histones. In this coiled state, it is called chromatin.
Chromatin is further condensed, through a process called supercoiling, and it is then packaged into structures called chromosomes. These chromosomes form the familiar “X” shape as seen in the image above.
Each chromosome contains one DNA molecule. Humans have 23 pairs of chromosomes or 46 chromosomes in total.
Chromosome 1 is the largest and contains around 8,000 genes. The smallest is chromosome 21 with around 3,000 genes.
What is a gene?
Each length of DNA that codes for a specific protein is called a gene. For instance, one gene codes for the protein insulin, the hormone that helps control levels of sugar in the blood. Humans have around 20,000–30,000 genes, although estimates vary.
Our genes only account for around 3 percent of our DNA, the remaining 97 percent is less well understood. The outstanding DNA is thought to be involved in regulating transcription and translation.
How does DNA create proteins?
For genes to create a protein, there are two main steps:
Transcription: The DNA code is copied to create messenger RNA (mRNA). RNA is a copy of DNA, but it is normally single-stranded. Another difference is that RNA does not contain the base thymine (T), which is replaced by uracil (U).
Translation: The mRNA is translated into amino acids by transfer RNA (tRNA).
mRNA is read in three-letter sections called codons. Each codon codes for a specific amino acid or building block of a protein. For instance, the codon GUG codes for the amino acid valine.
There are 20 possible amino acids.
What is a telomere?
Telomeres are regions of repeated nucleotides at the end of chromosomes.
They protect the ends of the chromosome from being damaged or fusing with other chromosomes.
They have been likened to the plastic tips on shoelaces that stop them from becoming frayed.
As we age, this protective region steadily becomes smaller. Each time a cell divides and DNA is replicated, the telomere becomes shorter.
In a nutshell
Chromosomes are tightly coiled strands of DNA. Genes are sections of DNA that code individual proteins.
Put another way, DNA is the master plan for life on earth and the source of the wonderful variety we see around us.