Genes Matter
DNA, DeoxyriboNucleic Acid (DNA) is a complex molecule found in every cell of our body, containing the instructions necessary to create and maintain life. It is mainly made up of four chemical substances: adenine (A), thymine (T), guanine (G) and cytosine (C), which are joined in a very specific way: A with T, C with G. Along these sequences of “letters”, we can differentiate certain fragments known as genes, which contain the information necessary to give rise to proteins that, as we will see below, are necessary for the correct functioning of the organism.
The complete set of DNA, i.e. our genetic material, is called the genome. With few exceptions, every cell in the body contains a copy of our complete genome. If we think about it, we might imagine that all of our DNA is a single, very long molecule. However, this is not the case, since it is divided into a series of unequal segments which, thanks to other molecules, are compacted and form what we know as chromatin. When cells divide, chromatin reaches the highest degree of compactness, forming chromosomes.
Chromosomes, in detail
As you have just seen, chromosomes are compacted packages of DNA. In humans, and in other organisms, they are not found free in the cell, but are inside a structure called the cell nucleus.
DNA contains the instructions necessary to create and maintain life, so, as you can imagine, each organism contains a different number of chromosomes. For example, a dog’s cells contain 78 chromosomes, and an elephant’s cells contain 56.
A curious fact: one of the organisms with the most chromosomes that we know of are ferns, which have more than 100 chromosomes, and the one with the least, the ant Myrmecia pilosula, which has only one.
How many chromosomes do we humans have?
Well, in our case, each human cell has 46 chromosomes, organized in 23 pairs. That is, we have 2 copies of each chromosome, one from our father and one from our mother. This occurs in all the cells of our body, except in the sexual cells, that is to say, in the eggs and spermatozoa, which contain only 23 chromosomes.
This makes it possible that, when both cells (egg and sperm) come together, a zygote is formed which will give rise to an embryo whose cells will have the 23 pairs of chromosomes, of which one copy is from each parent.
Are chromosomes the same?
Each chromosome contains information with different instructions for the maintenance and functioning of the organism and, therefore, they are different from each other.
In the case of humans, as we have seen, we have 23 pairs: 22 of them are called autosomal chromosomes, while the 23rd pair is called the sex chromosome. This is different in each sex:
– In the case of females the 23rd chromosome is composed of two X (XX) copies.
– In the case of males, chromosome 23 consists of one X and one Y copy (XY).
How are chromosomes formed?
As we have seen, DNA molecules are very long, so the formation of chromosomes is essential for the cell to be able to divide. As is logical to think, both resulting cells must have the same information, so in dividing cells the genetic information is duplicated before forming two daughter cells.
DNA is shaped like a double helix. Imagining a spiral staircase can help you visualize it. It is located in the cell nucleus, where the double helix folds around proteins called histones and forms a structure called the nucleosome.
Nucleosomes and chromatin
These nucleosomes (double helix folded around a set of proteins) gradually pack together and form a structure that resembles a string of pearls, which continues to compact even more, until it generates another structure called chromatin, in which other molecules (proteins and RNA) participate. Chromatin, when it reaches its maximum degree of compaction, gives rise to chromosomes.
When the cell divides, as mentioned above, the genetic information is duplicated and X-shaped chromosomes are formed, which are visible under the microscope. If we take a picture of the nucleus, and then arrange the chromosomes in pairs, we can obtain what is called a karyotype, in which the chromosomes look like this:
https://www.genome.gov/genetics-glossary/Karyotype
But if we want to take a closer look at a chromosome, this would be the structure we would find:
https://www.genome.gov/genetics-glossary/Centromere
Types of chromosomes in function of the centromere
As we can see in the image, when the cell is going to divide, the chromosomes are formed by two sister chromatids, which contain the same information. Both are joined by a constriction called the centromere which, as we can see, divides the chromatids into arms. The length of the chromatid arms depends on where the centromere is located on the chromosome. Depending on this, there are different types of chromosomes, which we explain below:
Centromeres have an essential function, since during cell division they participate in the correct alignment and distribution of chromosomes to the daughter cells.
Telomeres, structures related to aging
Telomeres are found at the ends of chromosomes. These structures are involved in chromosomal stability, as they prevent chromosomes from breaking or being damaged. This function is very important, as studies show that as cells divide, telomere length decreases. This telomere attrition is associated with aging.
Pathologies associated with chromosomes
Have you ever heard that Down syndrome is due to a chromosomal abnormality? In fact, this happens because sometimes errors occur in the sex cells or during embryonic development, which cause alterations in the chromosomes, which can cause the pregnancy not to be carried to term, or the newborn to present some pathology.
Specifically, in the case of Down syndrome, a trisomy occurs, that is, the cells of people with this syndrome generally have 3 copies of chromosome 21, so they have a total of 47 chromosomes instead of the expected 46. This is what is known as a numerical chromosomal abnormality, which in this case is caused by a gain, but we can also find cases of monosomy, in which the loss of a chromosome occurs, as in Turner syndrome, where one copy of the X chromosome is lost.
In addition to numerical abnormalities, structural abnormalities can also occur, where a fragment of a chromosome is broken and relocated, lost or duplicated, among others, resulting in:
https://www.genome.gov/about-genomics/fact-sheets/Chromosome-Abnormalities-Fact-Sheet
Chromosomal abnormalities
Types of DNA
We can make different classifications depending on the location or functionality. As we have been discussing throughout the post, the genetic material is found in the cell nucleus, but we can also differentiate another type of DNA, the mitochondrial DNA. As its name suggests, this DNA is located in the mitochondria, which are the organelles responsible for providing energy to the cells. Unlike nuclear DNA, this DNA is small (it is composed of about 16,500 base pairs) and is not organized in segments, but the double strand that composes it is supercoiled and closes on itself, giving it a circular shape. It is composed of 37 genes that code for proteins that perform their function within the mitochondria.
On the other hand, when we talk about nuclear DNA we must differentiate between two types of DNA; coding DNA and non-coding DNA. Let’s look at them in detail.
Non-coding DNA
Surprising as it may seem, non-coding DNA represents about 99% of our entire genome. But we must not confuse non-coding with non-functional. In other words, these DNA sequences do not give rise to proteins, but they have multiple other functions, as they participate in gene activation and DNA packaging, among others.
Coding DNA and genes
As we explained at the beginning, within the DNA sequence we find specific regions known as genes. It is in these regions where we find the coding DNA, that is, the sequences that contain the information to form other molecules called proteins that have different functions, such as, for example:
From genes to proteins
The process by which a gene gives rise to a protein can be divided into two main phases, and is part of the central tenet of molecular biology:
In many cases, the synthesis of the protein does not end here, but needs modifications so that it can acquire its final conformation and, with it, can perform its function.
The promoter, the key to turning a gene on or off
This process of protein synthesis does not occur randomly, nor with all genes at the same time in the organism. It is important to know that genes, apart from introns and exons, have a sequence called promoter, which is located before the gene sequence where transcription begins. The promoter is the sequence necessary for a gene to be activated, since the transcription process starts at this point.
The process of gene expression is highly regulated as cells synthesize and stop protein synthesis when appropriate. This is called gene regulation, which is the process of activation and deactivation of genes. It is a very complex mechanism, where different molecules participate and different processes are carried out. One of the most studied is epigenetics.
Epigenetics
DNA shows epigenetic marks such as DNA methylation or histone modification, among others. These change the way genes are expressed. Any type of cell has specialized epigenetic patterns.
There are also proteins that activate or deactivate certain genes by binding to areas near them, called transcription factors. If transcription factors promote transcription, they are called activators, whereas if they decrease transcription, they are called repressors.
As we have already seen, not all species and organisms have the same number of chromosomes, and the same is true for the number of genes. While humans have approximately 20,000 genes, for example, the fruit fly contains about 13,500, and a recent study has sequenced a tiny crustacean, the water flea (Daphnia pulex), indicating that it has about 31,000 genes.
Genes, responsible for our distinctive traits
Now that we know what a gene is and how the information it contains gives rise to the processes and mechanisms of our organism, let’s talk about the phenotype, which we can define as the observable traits of a person.
As we have seen, genes give rise to proteins that carry out a large number of functions in our organism. These functions are not only metabolic, but the color of our eyes, our height, and even our predisposition to eat between meals depends on the information contained in our genes.
Environmental factors in gene expression
Although we can relate a genotype to a trait, in general all these factors are multifactorial. That is, genetics plays a very important role in determining these traits, but there are other external factors such as diet, smoking or exercise, which affect the way in which this trait is displayed.
0.1%, the small number that makes us unique
As you know, 99.9% of our genome is the same in all humans, but that 0.1% is what makes us unique. In terms of traits, have you ever wondered why some people have a sweet tooth, or why others are better at endurance sports? Well, it is this small percentage that also marks these differences between us.
We hope this article has helped you to better understand how genes and chromosomes work, which, as you have seen, are fundamental in defining who we are.
Can you imagine being able to read all the letters that make up your genetic information and know curious facts about you, such as whether you have a greater predilection to eat between meals, as well as your risk of certain diseases? In short, can you imagine being able to know all the information that your genes contain about you?
Today, it is possible: you now have the opportunity to read the big book of your genes, something that was unthinkable just a few years ago. With the myGenome test from Veritas you can sequence your entire genome and find out what your genes say about you. Can there be more valuable information than that?
Veritas Scientific Department
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