Sister chromatids

Chromosomes during mitosis

Human karyotype with bands and sub-bands

Condensation and resolution of human sister chromatids in early mitosis

Sister chromatids are two identical copies of a single replicated chromosome that are connected by a centromere. Originating during the S phase of the cell cycle, sister chromatids are crucial for the processes of cell division, specifically during mitosis and meiosis. They ensure that each daughter cell receives an identical set of genes from the parent cell, maintaining genetic continuity through cellular generations.

Formation and Structure

Sister chromatids are formed during the S phase of the cell cycle. During this phase, the entire DNA of a cell is replicated, resulting in two identical copies of each chromosome. These copies, now called sister chromatids, remain attached to each other at a region known as the centromere. The attachment is facilitated by cohesin proteins, which form a complex to hold the chromatids together. This cohesion is essential for the proper segregation of chromosomes during cell division.

Role in Cell Division

Mitosis

In mitosis, sister chromatids play a critical role in ensuring that each daughter cell receives an exact copy of the parent cell's genome. During metaphase of mitosis, sister chromatids align at the cell's equatorial plate, attached at their centromeres to the mitotic spindle fibers. In anaphase, the cohesin proteins are cleaved, allowing the sister chromatids to separate and move to opposite poles of the cell, now referred to as daughter chromosomes. The process ensures that each daughter cell inherits a complete set of chromosomes.

Meiosis

During meiosis, the role of sister chromatids is slightly different but equally crucial. Meiosis consists of two consecutive cell divisions, meiosis I and meiosis II, but only one round of DNA replication. In meiosis I, homologous chromosomes (each consisting of two sister chromatids) pair up and may exchange genetic material in a process called crossing over, which increases genetic diversity. The homologous chromosomes are then separated into two cells. In meiosis II, the sister chromatids of each chromosome are separated, similar to mitosis, resulting in four genetically distinct haploid cells.

Genetic Stability and Errors

The accurate segregation of sister chromatids is vital for genetic stability. Errors in their separation can lead to aneuploidy, a condition where cells have an abnormal number of chromosomes, which can cause genetic disorders or diseases such as Down syndrome or cancer. Mechanisms like the spindle assembly checkpoint ensure that cells do not proceed with division until all chromosomes are properly attached to the spindle apparatus, minimizing segregation errors.

Research and Clinical Significance

Understanding the mechanisms governing the formation, cohesion, and separation of sister chromatids has significant implications in genetics, developmental biology, and medicine. Research in this area can lead to better understanding of genetic diseases, fertility issues, and the development of cancer therapies targeting cell division processes.