Chromatids are a component of chromosomes. They contain a DNA double strand and play a role in mitosis and meiosis. Diseases such as Down syndrome are associated with errors in the division of chromatids and chromosomes.
What is a chromatid?
Living things with nucleated cells are also called eukaryotes. Their genes and genetic information sit in chromosomes. These macromolecule complexes are wrapped in a coat of proteins. The complex of DNA and proteins is also called a chromatid. Each chromatid contains a double strand of DNA and its associated proteins. Depending on the cell cycle phase of the cell, a chromosome corresponds to either exactly one or two chromatids. Chromatids are thus the identical longitudinal halves of metaphase chromosomes. The longitudinal halves are interconnected by the so-called centromere. In meiosis, the second meiotic division takes place. In this process, the cleavage halves of the two-chromatid chromosomes separate. This results in chromosomes with only one chromatid, which are also known as one-chromatid chromosomes. The changes in the chromatid during the cell cycle play a crucial role in cell division.
Anatomy and structure
Each human chromosome consists of two distinct halves. In the simple case, it contains a DNA double helix, also known as a DNA molecule. In reality, the DNA molecule is two single-stranded molecules. Histones and other proteins are attached to the double strand of DNA. The DNA, histones and proteins form the chromatin as a complete package. The chromatid is formed from the double strand with the progressive addition of proteins. Thus, immediately after a nuclear division, the chromosome becomes a single-chromatid chromosome. At some point in the cell cycle, during any cell growth with the goal of cell division, the DNA must duplicate so that the daughter nuclei each contain one copy of the entire genetic material. After doubling, the chromosome has an identical DNA double strand that is spatially separated and individually sheathed by proteins. This creates sister chromatids or two-chromatid chromosomes.
Function and tasks
Nuclear division is also known as mitosis. It takes place at the two identical sister chromatids of a chromosome, each running parallel to the other and separated by a thin gap. At the centromere, the chromosome is at its narrowest at this moment in the cell cycle, but still connects the sister chromatids. During the transition from mitotic metaphase to mitotic anaphase, the sister chromatids separate completely. This gives rise to two daughter chromosomes, each of which is distributed to a newly forming nucleus. The chromosomes in the newly formed nuclei thus again correspond to a single chromatid. The chromatid thus always contains only one double strand of DNA. In contrast, depending on the phase of the cell cycle, a chromosome can also contain two DNA double strands and thus up to two chromatids. Polytic chromosomes are an exception in this respect, as they can contain more than a thousand DNA double strands. The chromatids are divided into two arms by their centromere. Depending on the centromere position, there is talk of metacentric, acrocentric or submetacentric chromosomes. The former carry the centromere in the middle. Acrocentric chromosomes carry it at the end, with the shorter arm being extremely small. In humans, for example, this is the case on chromosomes 13, 14, 15, 21, 22 and on the Y chromosome. Submetacentric chromosomes carry the centromere between the middle and the end. Their shorter arm is called the p-arm. Their longer arm is the q arm. The chromosome ends are also called telomeres and contain an extremely short and identically repeated sequence of DNA, which in humans corresponds to TTAGGG. At this point, chromosomes shorten with each doubling. In humans, the short arms of acrocentric chromosomes contain the genetic material of ribosomal RNA. When cells do not divide further and reach G0 phase, they remain identical with their chromatid. If further division is desired, chromosomes grow in G1 phase. In S phase, the DNA duplicates and the DNA double strand is present in duplicate. The resulting sister chromatids are present in meiosis in direct proximity to the homologous chromosome of the other parent.Thus, sister chromatid exchange can occur, causing chromatids to break off at the same level and interchange with parts of the homologous chromosome. These processes are known as crossing over or recombination. In the prophase of mitosis, condensation of chromosomes occurs. The threads of the sister chromatids are disentangled and are now adjacent to each other. In the subsequent metaphase, the chromosomes remain as two-chromatid chromosomes. In anaphase, they are separated as the spindle apparatus pulls the chromatids in opposite directions.
Diseases
Defects can occur in the chromatid division processes. These defects cause various diseases. One of the best known of such diseases is Down syndrome, also known as trisomy-21. The basis of this disease is chromosomal damage. In more than 90 percent of cases, the damage is independent of hereditary diseases. About one in 700 newborns suffers from trisomy 21. In this disease, chromosome 21 has not doubled, but tripled. This results in additional chromosomes or chromosome segments. Instead of 46 chromosomes, trisomy sufferers have 47 chromosomes. Such chromosomal defects are related to a lack of chromosome segregation. During the formation and maturation of sperm and eggs, one chromosome was not segregated. The cause of this non-separation or non-disjunction has not yet been determined. Crossing-over or recombination of homologous sister chromatids can also result in errors that can cause malformations or other pathological abnormalities. Either chromosomes are present in a higher number in such an error or they are present in a lower number, which sometimes even limits the survivability of the embryo. In other cases, the chromosome pieces are incorrectly assembled, resulting in useless chains. This can be the case, for example, due to an insertion of certain chromosome pieces in the wrong place. Deletion, on the other hand, is when a chromosome piece is completely deleted and lost. Depending on the localization of the deletion, this phenomenon can also affect viability.
