DNA is one of the most intriguing molecules out there, as it contains all the information necessary to make us who we are. Currently we know in detail its structure, how it replicates, the elements that make it up, and its fundamental units or genes. In this article, we are going to explore some fun facts about this fascinating molecule.
1. DNA in figures
Virtually every cell in our body (with the exception of red blood cells) contains a DNA sequence 3.2 billion base pairs in length. Although it is a very precise measurement, it is difficult to understand, so we are going to translate it into more understandable formats.
DNA is made up of four bases, called Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). If we wrote a book with the 3.2 billion letters (A, C, T and G) that make up DNA and printed it out, how long would it be?
We will take as a reference one of the most famous works in literature, "Don Quixote de la Mancha", which is composed of 381,104 words, which have an average of 4.3 letters. Doing the calculation, we obtain that Don Quixote de la Mancha has 1,638,747 letters. Dividing the number of DNA letters by the number of Don Quixote letters, we will verify that the length of the book written from our genetic code is equivalent to the length of two thousand copies of Don Quixote de la mancha.
Even more shocking, if we decided to write the book of our DNA, assuming a typing speed of 60 words per minute, and working eight hours a day every day, it would take fifty years.
Now let's talk about lengths. A one-millimeter-long piece of DNA contains three million bases, or letters. If we unrolled and unpacked the DNA of a single cell, we would obtain a sequence of two meters in length. If we unwrap the total DNA of a human being (estimated to have about 38 trillion cells), its length would be more than 100,000 million kilometers, and it would weigh only 0.18 grams. An extension equivalent to making about 26,000 trips from the Earth to the Moon (round trip).
2. Human genome project
In 1990 the Human Genome Project was founded, the objective of which was to determine the sequence of base pairs that make up DNA and to identify all the genes found in that sequence, from a physical and functional point of view. The project was endowed with three billion dollars (curiously, as many dollars as our DNA has base pairs) and was established to achieve it within fifteen years.
Parallel to the public body, a private company called Celera started its own Human Genome Project. The race that was established between the two competitors, together with the advancement of sequencing techniques, allowed the final results of the complete genome were presented in 2003, two years ahead of schedule. These final results describe the sequence of three billion base pairs of our genome, as well as three million genetic variants and between twenty and twenty-five thousand different genes.
17 years have passed since the human genome project was published, and Today, sequencing the entire genome of an individual costs less than a thousand dollars, and it takes about 24 hours to complete.
3. Who do we look like?
Comparative genomics is a branch of genomics that is based on comparing the sequences of the complete genome of different species, with the aim of locating regions of similarities and regions of differences. The identification of DNA sequences that have been conserved, that is, that have remained common in many different organisms over millions of years, makes it possible to locate genes classified as essential.
However, when we compare our genome with the genomes of other species of animals and plants we can take some surprises. For instance, humans share about 60% of our genes with the fruit fly, Drosophila melanogaster, and also with bananas, and we have fewer genes than a broccoli.
4. Genetic fingerprinting and forensic medicine
Genetic fingerprinting is a technique used to identify individuals of the same species through their DNA. It is based on the fact that no two humans are genetically identical, since even monozygotic twins are not. The average difference between the genomes of the different individuals is about twenty million base pairs, which represents 0.5 % of the total genome.
Forensic medicine is the branch of medicine in charge of determining the cause and circumstances of some deaths, and identifying perpetrators of crimes or assaults. Genetic fingerprinting is used in forensic medicine to identify suspects through blood, hair, saliva, or semen samples. In 1987, the first crime was solved using the genetic fingerprint technique, and Colin Pitchfork was convicted of the Narborough murders in 1983 and the Enderby murders in 1986.
The genetic fingerprinting technique is also used in paternity testing and compatibility studies in organ donation.
5. Why do sex-linked diseases affect men more?
Monogenic sex-linked diseases are those caused by the alteration of a single gene, which is also located on the sex chromosomes (X and Y). However, these chromosomes are very different from each other. The X chromosome is three times the size of the Y, and contains over a thousand genes, while the Y contains just 75. This means that when we speak of monogenic diseases linked to sex, we are actually referring to diseases caused by a gene located on the X chromosome.
As we all know, women (XX) inherit an X chromosome from their mother and an X chromosome from their father. Males (XY), on the other hand, inherit the X chromosome from their mother and the Y chromosome from their father. This fact will cause as long as men inherit a genetic mutation located on the X chromosome they express the disease (either dominant or recessive). In contrast, women, when it comes to a recessive disease, can mask the effect of the mutation with the second (healthy) copy of the same gene located on the other X chromosome.
Therefore, all monogenic recessive diseases, whose causative gene is located on the X chromosome, will affect males more than females. Some examples of these diseases that affect men more than women are hemophilia, red-green color blindness, congenital night blindness, high blood pressure, Duchenne muscular dystrophy, and also Fragile X syndrome.
- Rosa García-Verdugo, “Genetics is you”. Editorial Paidós, 2020.
- Sergio Parra, "That WAS NOT in my GENETICS BOOK". Editorial Guadalmazán, 2020.
- Siddhartha Mukherjee, "The gene: a personal story." Editorial Penguin Random House, 2017.
- Antonio Frías Delgado, "Frequency distribution of the length of words in Spanish: diachronic and stylometric aspects", University of Cádiz.