Chromatid: Definition, Replication of a Chromosome, Mitosis and Meiosis

It is a copy of a newly copied chromosome that is still attached to the original chromosome by a single centromere.

Before replication, a chromosome is made up of a DNA molecule. After replication, each chromosome is made up of two DNA molecules; In other words, DNA replication  increases the amount of DNA, but does not increase the number of chromosomes.

The two identical copies, each making up half of the replicated chromosome, are called chromatids. During the later stages of cell division, these chromatids split longitudinally to become individual chromosomes.

Color pairs are normally genetically identical and are said to be homozygous; however, if mutations occur, they will differ slightly, in which case they are heterozygous.

Chromatid pairing should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome.

The chromonema is the fiber-like structure in prophase in the primary stage of DNA condensation.

In metaphase, they are called chromatids.

Sister chromatids

A sister chromatid refers to the identical copies (chromatids) formed by the replication of a chromosome, with both copies joined by a common centromere .

In other words, a sister chromatid can also be said to be “half” of the duplicated chromosome. A pair of sister chromatids is called a dyad. A complete set of sister chromatids is created during the synthesis phase of the interface, when all the chromosomes in a cell are replicated.

The two sister chromatids are separated from each other in two different cells during mitosis or during the second division of meiosis.

Compare sister chromatids to homologous chromosomes, which are the two different copies of a chromosome that diploid organisms (such as humans) inherit, one from each parent.

Sister chromatids are generally identical (having the same alleles, also called variants or versions, of genes) because they are derived from an original chromosome.

An exception is towards the end of meiosis, after the crossover has occurred, because the sections of each sister chromatid may have been interchanged with the corresponding sections of the homologous chromatids that they pair with during meiosis.

Homologous chromosomes may or may not be the same as each other because they are derived from different parents.

There is evidence that sister chromatids are the preferred template for DNA repair in some species . The cohesion of the sister chromatid is essential for the correct distribution of genetic information between daughter cells and the repair of damaged chromosomes.

Defects in this process can lead to aneuploidy and cancer, especially when checkpoints do not detect DNA damage or when misconnected mitotic spindles are not working properly.

Mitosis

Mitotic recombination is primarily the result of DNA repair processes that respond to spontaneous or induced damage.

Homologous repair of recombinations during mitosis is largely limited to the interaction between close sister chromatids that are present in a cell after DNA replication but before cell division.

Due to the special close relationship they share, sister chromatids are not only preferred over distant homologous chromatids as substrates for repair, but they have the ability to repair more DNA damage than homologues.

Meiosis

Studies with the budding yeast Saccharomyces cerevisiae indicate that sister-to-sister recombination occurs frequently during meiosis, with up to one-third of all recombination events occurring between sister chromatids.