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Sister chromatids are two identical copies of the same chromosome, formed by DNA replication , attached to each other by a structure called a centromere . Chromatids are each of the two longitudinal units of a chromosome that has already been duplicated. During cell division, each of the sister chromatids separate from each other, and each daughter cell receives a copy of the chromosome.
Chromosomes are located within the cell nucleus of animals and plants, where there are also proteins combined with a single molecule of deoxyribonucleic acid (DNA). That DNA is passed on to the descendants, and each of them will have their own unique characteristics (except in the case of identical twins).
Differences between sister chromatids and homologous chromatids
Diploid organisms have two copies of each chromosome, each of which is received from each parent through their gametes. These pairs are called homologous chromosomes, and they carry information for the same genes, but they are not exactly identical in their DNA sequence. This system is common in organisms that reproduce sexually, and ensures that the population is varied and genetically rich.
During synthesis, that is, the S phase of the cell cycle, all the DNA in the cell is duplicated, so that each chromosome now has an exact copy, as well as having a homologous pair. During mitosis, the DNA condenses to form visible chromosomes, and these two identical copies, or sister chromatids, attach to each other, giving rise to the “X-shape.”
While sister chromatids are exact copies of each other, non-sister chromatids come from homologous chromosomes. That is, they code for the same genes, but they are not genetically identical.
Occasionally, genetic material is exchanged between non-sister chromatids during meiosis, allowing new gene variants to be passed on to offspring. This is called crossing over or genetic recombination .
Sister chromatid functions
DNA replication allows cells to maintain their genetic content through generations. The primary function of sister chromatids is to pass a complete set of chromosomes to the daughter cells formed as a result of cell division.
During mitosis, the chromatids are attached to each other through the centrometre, which is a stretch of DNA that gives rise to complex enzyme proteins. At the end of mitosis, a series of biochemical reactions separate the two sister chromatids, which move to opposite ends of the dividing cell; immediately a new cell membrane is formed between them and the cycle of mitosis ends with the formation of two daughter cells. Both cells are genetically identical to the stem cells. Mitosis is the process of cell division by which two identical cells are obtained from a single cell.
In addition to this basic function, sister chromatics play an important role in maintaining the integrity of the genome by repairing DNA that might eventually become damaged. Compared to pairs of homologous chromosomes, sister chromatids are very close to each other and, thanks to the intervention of certain enzymes and their self-replicating capacity, DNA replicates in areas where the sequence has been damaged, recovering the original order of its bases and, consequently, the genetic information.
Sources
- Editors, B. (2016). Sister Chromatids. Retrieved 22 March 2021, from https://cutt.ly/5xxtMQH
