What Are Chromosomes and What Do They Do?

Chromosomes are biological structures that compact DNA into genes. In addition to being the molecular units in charge of keeping DNA grouped and codified, they also store it. It is in doing so they can transmit genetic information from parents to offspring of each new generation.

The formation of chromosomes can be threads, double helices or small rings. Their shape changes depending on the phase of the cell cycle. These chromosomes are made up of:

• Deoxyribonucleic acid (DNA)
• Various proteins such as histones

All living beings have a specific number of chromosomes for their species inside their cells. This is known as chromosome endowment. It regulates the characteristics and functioning of individuals of the same species. For example:

• Prokaryotic cells: the chromosomes are scattered in the cytoplasm.
  
  
• Eukaryotic cells: the chromosomes are found inside the cell nucleus in the form of chromatin. This is an association of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) that has the appearance of thin, tangled threads. In our related article, discover the difference between eukaryotic and prokaryotic cells.

For the cell division process to develop successfully, it is necessary that the chromosomes remain intact. When genetic material is divided or grouped irregularly or deficiently, mutations occur that can have serious consequences in the subsequent organisms. These include problems in embryonic development or diseases such as leukemia.

Human beings have 46 chromosomes distributed in 23 pairs. 22 pairs make up the non-sex chromosomes known as autosomes. The remaining pair is the sex chromosomes, known as heterochromosomes. Heterochromosomes determine the sex of the offspring in the following ways:

• Chromosomes ‘X’ and ‘X’ = female.
• Chromosomes ‘X’ and ‘Y’ = male.

Gametes or sex cells do not have a complete chromosome load. In fact, they are the only human cells like this. These only have half the chromosomes. When two of them unite (egg and sperm respectively) the resulting cells have a full chromosome load inherited in equal parts by both parents.

As we have already stated, chromosomes are molecules. We understand more about what this means by making a comparison of molecules and atoms.

Chromosomes have four fundamental parts. These are the chromatids, the centromere, the arms and the telomeres. Below we discover what each of these parts of chromosomes do.

Chromatids

Chromatids are identical coiled strands of DNA. They are made up of microfilaments called chromonemes and small granules or knots called chromomeres. Two longitudinal chromatids with the same type and number of genes form a linear chromosome.

Centromere

The centromere is the narrow, condensed region that joins the two chromatids of a linear chromosome. In addition, it helps keep chromosomes correctly aligned during cell division and allows them to interact with spindle fibers during mitotic and meiotic anaphases. It also correctly performs the respective chromosome movements of these phases. Learn more about cell division by learning the difference between mitosis and meiosis.

Above them are protein structures called kinetochores. These anchor to the microtubules of the mitotic spindle during mitosis and allow both halves of the chromosome to be brought to opposite poles of the cell.

Arms

The arms are the two segments resulting from the horizontal division of the chromatids by the centromere. In acrocentric and submetacentric chromosomes the arms have different lengths. To facilitate their study, the shorter arms are designated with the letter ‘P’ and the longer arms are designated with the letter ‘Q’.

Telomeres

Telomeres correspond to the ends of linear chromosomes. They are stretches of DNA that do not encode any protein or provide genetic information. Their function is to protect the ends of the chromosomes and prevent them from fraying and disorganizing.

Learn more about the makeup of organisms by finding out the difference between plant and animal cells.

We have already detailed the difference between autosomes and heterochromosomes. However, there are other criteria into which the different types of chromosomes can be classified. These are according to the position of its centromere, its shape and its function. We look at the different types of chromosomes using these criteria below:

Types of chromosomes according to the position of the centromere

The centromere is the narrowest, most condensed region of a chromosome. It joins its sister chromatids and separates it into two sections with arms of specific lengths. Depending on the position of its centromere, a chromosome can be:

• Telocentric chromosome: one whose centromere is located at one of its ends, meaning the chromosome has only one arm.
  
  
• Acrocentric chromosome: one whose centromere is much closer to one telomere than to the other, meaning the chromosome has one very short arm and one very long one.
  
  
• Submetacentric chromosome: one whose centromere is located very close to the center of the chromosome, but slightly closer to one end than the other.
  
  
• Metacentric chromosome: one whose centromere is located exactly in the center of the chromosome. It forms two arms of equal lengths.

Types of chromosomes according to their shape

Depending on its physical shape, a chromosome can be linear or circular:

• Linear chromosomes: those that have linear chains of DNA that are arranged in pairs within cells. Most of the chromosomes in eukaryotic cells are of this type.
  
  
• Circular chromosomes: consist of a single circular DNA molecule and some associated proteins. They are smaller than linear chromosomes and are found in prokaryotic organisms, as well as the mitochondria and chloroplasts of eukaryotic organisms.

Types of chromosomes according to their function

In organisms that reproduce sexually there are two types of chromosomes that are either sexual or non-sexual.

• Somatic chromosomes: also called autosomal chromosomes or autosomes, these are all those that are not involved in determining the sex of individuals. In humans, chromosome pairs 1 through 22 are autosomal chromosomes.
  
  
• Sex chromosomes: also called heterochromosomes or allosomes, these are those that are responsible for determining the sex of individuals. In the case of the human being, the pair of chromosomes 23 is the one that dictates the sex of the people.

DNA and RNA are two concepts that are often confused with each other. Our related article discovers the differences between RNA and DNA.

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