The other day I was having a conversation with a friend of mine who had some background in computer science. The conversation shifted towards my research and the following question came up: What is the amount of digital information stored in a human genome? I started searching in the deep dark corners of my brain, but I realized that I simply did not know the answer. So I decided to do the math to estimate how much information is stored in our genome.
Laying out the information storage capacity of the genome
The human genome contains the complete genetic information of the organism as DNA sequences stored in 23 chromosomes (22 autosomal chromosomes and one X or Y sex chromosome), structures that are organized from DNA and protein. A DNA molecule consists of two strands that form the iconic double-helix “twisted ladder”, whose backbone, which made of sugar and phosphate molecules, is connected by rungs of nitrogen-containing bases. DNA is composed of 4 different bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). These bases are always paired in such a way that Adenine connects to Thymine, and Cytosine connects to Guanine. These pairings produce 4 different base pair possibilities: A-T, T-A, G-C, and C-G. The haploid human genome (containing only 1 copy of each chromosome) consists of roughly 3 billion of these base pairs grouped into 23 chromosomes. A human being inherits two sets of genomes (one from each parent), and thus two sets of chromosomes, for a total of 46 chromosomes, representing the diploid genome, which contains about 6×10^9 base pairs.
Comparing the genome to computer data storage
In order to represent a DNA sequence on a computer, we need to be able to represent all 4 base pair possibilities in a binary format (0 and 1). These 0 and 1 bits are usually grouped together to form a larger unit, with the smallest being a “byte” that represents 8 bits. We can denote each base pair using a minimum of 2 bits, which yields 4 different bit combinations (00, 01, 10, and 11). Each 2-bit combination would represent one DNA base pair. A single byte (or 8 bits) can represent 4 DNA base pairs. In order to represent the entire diploid human genome in terms of bytes, we can perform the following calculations:
6×10^9 base pairs/diploid genome x 1 byte/4 base pairs = 1.5×10^9 bytes or 1.5 Gigabytes, about 2 CDs worth of space! Or small enough to fit 3 separate genomes on a standard DVD!
Data storage across the whole organism
Some interesting question could follow. For example, how many megabytes of genetic data are stored in the human body? For simplicity’s sake, let’s ignore the microbiome (all non-human cells that live in our body), and focus only on the cells that make up our body. Estimates for the number of cells in the human body range between 10 trillion and 100 trillion. Let us take 100 trillion cells as the generally accepted estimate. So, given that each diploid cell contains 1.5 GB of data (this is very approximate, as I am only accounting for the diploid cells and ignoring the haploid sperm and egg cells in our body), the approximate amount of data stored in the human body is:
1.5 Gbytes x 100 trillion cells = 150 trillion Gbytes or 150×10^12 x 10^9 bytes = 150 Zettabytes (10^21)!!!
Sexual information exchange
Along the same lines, how much genetic data is exchanged during human reproduction?Each sperm cell in a human male is heterogametic and haploid, meaning that it contains only one of two sex chromosomes (X or Y) and only one set of the 22 autosomal chromosomes. Thus, each sperm contains about 3 billion bases of genetic information, representing 750 Mbytes of digital information. The average human ejaculate contains around 180 million sperm cells. So, that’s 180 x 10^6 haploid cells x 750 Mbytes/haploid cell = 135 x10^9 Mbytes=135000 Terabytes!!!! Following this idea even further, while 13500 Tbytes are transferred, only one sperm cell will fuse with an egg, using only 750 Mbytes of data, combining it with another 750 Mbytes of data from the egg. Thus, essentially 99.9999…% of the data transferred during sexual reproduction is lost in the pipeline … Whether the remaining fraction of information will result in anything constructive is up to good parenting.
Having worked out the above numbers, a whole bunch of other curious questions can be asked. Have you ever wondered about the data capacity of our biological organism? What is the rate of data transmission during cell division? The rate of data transmission during gamete fusion? The rate of data transmission when human lymphocytes circulate through the bloodstream? What amount of data is destroyed daily by apoptosis? What amount of data is created daily? How does this compare to the rate of data transfer via an optical fiber?
Please feel free to contribute your own dubious calculations and questions below!