The 12 parts of chromosomes (characteristics and functions)
- This is the number of chromosomes that make up the human genome. Each and every one of our cells has 23 pairs of chromosomes in their nucleus, 22 autosomal pairs and 1 sexual pair (X and Y), of which half come from the father and the other half, from the mother.
Human beings are the result of the interaction between the 30,000 genes in our genome and the environment, which determines genetic expression. But be that as it may, these genes are scattered across chromosomes, a vital concept in biology and cytogenetics.
Chromosomes are each of the highly organized structures of DNA and proteins that contain most of an individual's genetic information, being especially important for cell division to culminate with a faithful distribution of genes.
But what exactly are chromosomes? What is your function? What parts are they made of? If you want to find the answer to this and many other questions, you have come to the right place. In today's article we will dive into the secrets of chromosomes, the key structures of genetics.
What are chromosomes
"Chromosome" is a concept that comes from the Greek chroma (color) and soma (body), alluding to how these cellular structures are stained dark using the dyes in cytogenetics laboratories. But beyond this interesting etymological origin, let's see what exactly they are.
Chromosomes are, in essence, highly ordered bundles of DNA found inside the nucleus of cells. These are structures with the appearance of a thread (which changes depending on what phase of the cell cycle we are in) located within the cell nucleus that contain most of the genetic information of that individual.
In this sense, chromosomes are each one of the highly organized structures that, being formed by DNA and proteins that allow their cohesion (the most recognized form is the one that occurs during division, when the DNA has to be packed as much as possible and acquire their traditional X morphology), serve as gene packaging regions.
Each chromosome is made up of proteins combined with a single DNA molecule (a succession of nucleotides) and it is these proteins that determine its degree of compaction. And it is that surprising as it may seem, if we put it online, our genome would measure approximately 2 meters. And this only that of a cell. If we put together all the DNA of all our cells, it would measure more than 100,000 million km.
These chromosomes, through the action of histone-type proteins (small proteins with a positive charge, which facilitate their binding to DNA), allow it to compact into a tangle of DNA strands that fit inside the microscopic nucleus of our cells. We have to condense 2 meters of DNA into a nucleus with a size of about 2 micrometers (one millionth of a meter). And even when it is time to divide the cell, this tangle begins an amazing condensation process to give rise to chromosomes with their characteristic X shape.
Humans are diploid, which means that our genome is made up of pairs of chromosomes: half from the father and half from the mother. We have 23 pairs of homologous chromosomes, which have the same genes located in the same place as their "partner" but with different genetic information. In these 46 total chromosomes the 30,000 genes that give rise to our genetic information are condensed.
Either way, these chromosomes are essential so that throughout the cell cycle, DNA remains intact, is evenly distributed, and can be condensed enough to fit into the nucleus of the cell. By packaging the DNA into these structures, we ensure that, during mitotic division, it is copied and distributed properly.
When there are problems in their morphology or in the total number of chromosomes (because they have not been distributed well), those known as chromosomal abnormalities or mutations arise, which are alterations in the structure of chromosomes or modifications in the normal number of this that they can lead to different types of diseases.
What is the structure of chromosomes?
Recapitulating, a chromosome is a structure present in the nucleus of the cell where DNA associates with histone-type proteins that allow a sufficient condensation of nucleic acids to contain, in an intact and uniform way, the genetic information of an individual. And now that we've understood this, we are more than ready to see what parts chromosomes are made of.
1. Chromosomal matrix
The chromosomal matrix is a substance present within the film (an outer membrane that we will discuss at the end) that, in principle, is the medium that contains the cromonema, which we will analyze below.
We say "in principle" because, although its existence is plausible, it has not been confirmed by electron microscopy studies and some scientists doubt that there really is a matrix as such. Either way, it would be, to understand us, a kind of "jelly" that covers the chromosomes.
A chromonema is each of the filaments that make up chromatids (each one of the two longitudinal units of the chromosome), being filamentous structures composed of DNA and proteins. Each cromonema consists of about 8 microfibrils and each one of them, of a double helix of DNA.
The two chromonemes are closely linked, forming what appears to be a single spiral filament about 800 Å (an angstrom is one millionth of a millimeter) wide. When the cell needs it, these roll up and form the chromomers.
Chromomers are granules that accompany the cromonema along its length. They are a kind of knot that is perceived as denser regions within the filament and, being always in the same position within the chromosome, they seem to be important when it comes to transporting genes during division.
The centromere is the waist of the chromosome. It is the narrow region of the chromosome that separates the short arms from the long ones. Regardless, despite what its name may indicate, it is not always exactly in the center. It is a primary constriction in which the two chromonemes unite and divide the chromosome into two sections or arms, which we will discuss later.
When the centromere is right in the center (there is almost no difference between short and long arms), we speak of a metacentric chromosome. When it is slightly above or below the center, submetacentric chromosome. When it is very far from the center, it has an acrocentric chromosome. And when it is practically at the end of the chromosome, the telocentric chromosome. There are also special cases in which there may be two (diccentric) or more centromeres (polycentric) and even absence of this centromere (acentric).
Telomeres are the ends of chromosomes. They are highly repetitive non-coding sequences, which means that the genes they present do not code for proteins. They are regions of the chromosome that do not provide genetic information, but they are essential to give it resistance and stability.
And it is in them that we find, in part, the genetic origin of aging. With each cell division, these telomeres get shorter, as chromosomes inevitably lose portions of their ends. And this reduction in telomeres is what, due to loss of chromosomal stability, causes cell lines to die. If we could find a way to avoid telomere shortening (something that, to this day, is pure science fiction), we would be opening the door to an incredibly high life span.
The kinetochore is a protein region that arises in the prometaphase of the cell cycle and consists of a structure located in the centromere. The kinetochore is the anchoring site for the microtubules of the mitotic spindle, thus being a fundamental piece so that, through this anchoring, the microtubules place the chromosomes aligned in the vertical center of the cell in order to bring half to one pole of the cell and the other half to the other pole.
7. Secondary constrictions
As we have said, the centromere is the primary constriction. But homologous chromosomes often have other additional constrictions known as "secondary", representing approximately 0.3% of the DNA of the chromosome. They are found at the ends of the arms, generally in regions where the genes responsible for transcription as RNA are located, being necessary for the formation of the nucleolus, which is why they are also known as “nucleolar organization regions”.
Satellites are regions that have some chromosomes and that consist of terminal chromosomal structures beyond secondary constrictions. In other words, satellites are distal segments separated from the rest of the chromosome by one of the secondary constrictions we have seen before.
In the human genome, chromosomes 13, 14, 15, 21, 22 and Y have satellites that, being associated with secondary constrictions, are in the same place, so they are useful as markers to identify specific chromosomes.
Chromatids are each of the two longitudinal units of the chromosome. A chromatid is attached to its sister through the centromere. In this sense, a chromatid is each of the “bar”-shaped chromosomal structures that is found on one of the two sides of the centromere. Therefore, it is a vertical division.
In other words, a chromatid is half of a duplicated chromosome, since sister chromatids are identical copies formed after DNA replication of a chromosome that are joined by a shared centromere. Furthermore, in a horizontal plane, each chromatid can be divided into two arms: one above the centromere and one below. And since there are two chromatids, we have a total of four arms on the chromosome that we will now look at.
10. Short arm
The short arms of a chromosome are the horizontal divisions of its chromatids. Except in perfectly metacentric chromosomes (with the centromere right in the center), there will always be arms that, due to the horizontal plane of division, are smaller. In this sense, chromosomes always tend to have two shorter arms (one from each chromatid) that are designated by the letter p.
11. Long arm
That there are short arms implies that there must also be long ones. And so it is. In chromosomes not perfectly metacentric, each chromatid has one arm longer than the other. These two long arms (one from each chromatid) are designated by the letter what.
12. Chromosome film
The chromosome film is an envelope that covers all the structures that we have seen. It is a very thin outer membrane of the chromosome and made up of achromatic substances, that is, they do not have color. In the same way that it happened with the matrix, we are not convinced that such a film exists.