Within your body, the cells reproduce continuously to create new cells that will replace the old ones.
During this replication, a single cell is divided into two, separating the content of the mother cell in half, both the cytoplasm and the cell membrane, and forming two daughter cells.
The dividing mother cell must provide both daughter cells with a complete set of chromosomes. To do this, the stem cell must duplicate its chromosomes before cell division. This duplication is carried out during the synthesis phase of the cell cycle.
The chromosomal duplication is then part of a duplicated chromosome.
This process involves the production of one or more copies of any DNA fragment, sometimes even a gene or even a complete chromosome.
Duplications have been necessary for the evolution of the human genome (and many other organisms).
Typically, duplications arise from an event called recombination, which occurs between misaligned homologous chromosomes during meiosis (formation of germ cells).
Human cells contain approximately 2 meters of DNA that must be duplicated without errors before each cell division to produce two identical daughter cells.
Surprisingly, these cells manage to accomplish this task most of the time, but sometimes errors occur, and if they are not repaired efficiently, they can cause mutations and genomic instabilities.
DNA replication plays a vital role in developing cancer, genetic diseases, and aging.
DNA replication is one of the fundamental processes in life, and it is very well preserved throughout evolution.
This fact facilitates significant studies of the DNA replication process in model systems.
Rearrangements of the chromosomes include deletions of DNA sequences and duplications of segments, which can range from thousands to hundreds of thousands of bases.
On the one hand, specific structural characteristics of the genome, also known as genome architecture, can make several regions fragile and prone to events such as chromosome breakage, often resulting in translocations, deletions, and duplications.
Often, these alterations occur due to errors during cell division when the chromosomes are aligned.
Homologous recombination between areas of repeated DNA sequences often creates deletions and duplications because they commonly involve more than one gene. The disorders caused by this large deletion and duplication mutations are often severe.
Types of duplications
In chromosomal duplications, additional copies of a chromosomal region are formed, which results in different numbers of gene copies within that area of the chromosome.
If duplicate sections are adjacent to the original, the process is known as tandem duplication, while if unduplicated regions separate them, it is known as displaced duplication.
The cell cycle
The cell cycle is the complete life cycle of the body cells and consists of two main phases: interphase and mitosis.
The interface is the non-division phase; it starts with interval 1. The new cell grows and carries out its functions in the body; the S phase, or synthesis, when the chromosomes are replicated; and interval 2, when the cell grows more and prepares to divide.
Once the chromosomes have replicated, the cell contains twice the average number of chromosomes until the cell divides into mitosis.
During mitosis, the duplicated chromosomes are aligned, and the cell is divided into two daughter cells, each with an identical copy of the complete chromosomal package of the stem cell.
The complete history of how chromosomes replicate is complex. Still, a simplified way to think about this replication of the synthesis phase is the decompression of a strand of yarn (DNA) in two of the two halves.
The half strand of decompressed DNA is paired with a newly formed filament medium.
Because both halves receive a new half chain, the cell ends with a double set of chromosomes.
Thanks to various enzymes and RNA molecules, decompressing and forming a complementary half-strand is carried out.
This DNA synthesized during the Synthesis phase makes two identical copies, forming a paired chromatid.
These chromatids are linked by a kinetochore protein that keeps the pair together until mitosis.
With its double chromosome package, the cell continues to grow and function through interval 2.
At the end of the interface, the cell forms structures called microtubules, which separate the chromatids when hooked to the kinetochore and give way to mitosis.
Mitosis consists of four main events: prophase, metaphase, anaphase, and telophase.
During the prophase, the stem cell nucleus separates, exposing the chromatids.
In the metaphase, the chromatids are aligned along the center of the cell, and the microtubules join them. The microtubules then separate the chromatids in the anaphase.
During the final phase of mitosis, the telophase, the cell is pinched in two (like number 8). Each daughter cell forms a nucleus around its complete set of chromosomes, finally separating.
Mitosis only occurs in somatic cells, which make up the body.
The gametes, the ovule, or the sperm cells that fuse during the sexual reproductive phase replicate their chromosomes during the synthesis phase but suffer a double division during meiosis to end up with only half the chromosome package.
Effects of duplications
Duplications can affect the phenotype by altering the gene dose.
For example, the amount of protein synthesized is often proportional to the number of copies of genes present so that additional genes can lead to an excess of synthesized proteins.
Because most embryonic developmental processes rely heavily on carefully balanced protein levels, duplications that produce additional gene copies can lead to developmental defects.
Similar to the effect of other rearrangements. However, duplications can also provide the raw material for evolution by producing new copies of genes that are free to mutate and assume other functions.
Gene families, such as the gene family of human globin, attest to the role of duplication in evolution.
Several globin genes (protein fraction of hemoglobin) have emerged from a single ancestral precursor, which makes individual genes available to take on specialized functions, with some genes activated during embryonic and fetal development and others that become active in the adult organism.
Deletions, duplications, and diseases
Chromosomal deletions are deletions that involve the loss of DNA sequences.
The phenotypic effects of the deletions depend on the size and location of the deleted sequences in the genome.
Chromosomal deletions cause several human disorders, and, in general, their phenotypes are more severe than those caused by duplications.
Among the disorders, elimination or informers are better characterized by the syndrome cri du chat, Prader-Willi syndrome, Smith-Magenis syndrome, Williams-Beuren syndrome, and hereditary neuropathy with pressure palsies.
Genetic disease, such as Charcot-Marie-Tooth type I, is produced by chromosomal duplication.