Chromatid

Chromosome

Human karyotype with bands and sub-bands

Condensation and resolution of human sister chromatids in early mitosis

Chromatid refers to one of two identical halves of a replicated chromosome. During cell division, chromosomes are replicated to ensure that each new cell receives a full set of chromosomes. A chromatid is thus one copy of a duplicated chromosome, which is joined to the other copy by a single centromere. Before replication, one chromosome is composed of a single DNA molecule. Post-replication, it consists of two identical DNA molecules; each is called a chromatid. When a cell is preparing to divide, the centromere serves as the attachment point for the two sister chromatids.

Structure and Function

The structure of a chromatid includes a long chain of DNA wound around proteins called histones, forming a structure known as chromatin. This chromatin further coils and folds to form the chromatid. The primary function of chromatids is to ensure accurate DNA replication and distribution during cell division. In mitosis, the sister chromatids are separated and pulled to opposite ends of the cell, ensuring that each new cell receives an identical set of chromosomes. In meiosis, the process is more complex due to the exchange of genetic material between chromatids in a process known as crossing over, which increases genetic diversity.

Types of Chromatids

There are two main types of chromatids:

• Sister Chromatids: These are the two identical copies of a chromosome that are connected by a common centromere. Sister chromatids are created during the S phase of the cell cycle and are separated during mitosis or meiosis II.
• Non-Sister Chromatids: These refer to the chromatids belonging to homologous chromosomes (chromosomes of the same type but from different parents) that come together during meiosis I. Non-sister chromatids can exchange genetic material through crossing over.

Importance in Genetics

Chromatids play a crucial role in genetics, particularly in the inheritance of traits and genetic variation. The process of crossing over between non-sister chromatids during meiosis I is a source of genetic diversity, as it allows for the recombination of genetic material between homologous chromosomes. This genetic recombination is fundamental to the process of evolution and the survival of species.

Chromatid Cohesion and Separation

The cohesion of sister chromatids is maintained by a complex of proteins known as the cohesin complex. This complex ensures that the sister chromatids remain attached until they are supposed to be separated during cell division. The separation of sister chromatids is triggered by the degradation of cohesin and is regulated by various enzymes and proteins to ensure that each daughter cell receives an identical set of chromosomes.

Clinical Significance

Abnormalities in chromatid cohesion and separation can lead to genetic disorders and diseases. For example, improper separation of chromatids can result in aneuploidy, the presence of an abnormal number of chromosomes in a cell, which is a common cause of genetic disorders such as Down syndrome. Understanding the mechanisms of chromatid cohesion and separation is crucial for the study of genetic diseases and the development of potential treatments.