Table of Contents
- Introduction to Sister chromatids
- Sister Chromatids Formation
- Sister Chromatids Separation
- Functions of Sister Chromatid
- Sister chromatids in mitosis
- Sister chromatids in meiosis
- Non-sister Chromatid
- Sister Chromatid vs. Non-sister Chromatid
- References
Introduction to Sister chromatids
- Sister chromatids are two identical copies of the same chromosome that are attached at a structure called the centromere.
- These identical copies are formed during the S phase (synthesis phase) of interphase in the process of cell division in eukaryotic cells, where chromosomes present in the nucleus are replicated.
- The two sister chromatids remain physically connected at the centromere, which is a constricted region of the chromosome.
- As long as the sister chromatids are joined at the centromere, they are considered a single chromosome.
- Once the sister chromatids separate during cell division, each individual chromatid is recognized as a separate chromosome.
- In the process of meiosis, after homologous chromosomes are separated in meiosis I, the sister chromatids are separated from each other during meiosis II.
Sister Chromatids Formation
- The formation of sister chromatids involves two critical processes in the chromosome cycle: the precise duplication of the genome through DNA replication and the subsequent segregation of the two genome copies via chromosomal segregation.
- Sister chromatid cohesion is a vital requirement for proper chromosomal segregation, as it ensures the correct biorientation of chromosomes on the mitotic or meiotic spindle.
- During the G2 phase and early stages of mitosis, including prophase, prometaphase, and metaphase, sister chromatids remain connected as part of one chromosome.
- In most eukaryotic organisms, the nuclear envelope breaks down during these stages of mitosis, and chromosomes attach to both poles of the mitotic spindle.
- Sister chromatids are only separated during anaphase, after all chromosomes have achieved biorientation, enabling the mother cell to divide into two genetically identical daughter cells as chromatids are pulled to opposite spindle poles.
- The continuous physical linkage between sister chromatids from the S phase through metaphase is critical for the successful execution of chromosome segregation.
- This connection, termed sister chromatid cohesion, counteracts the pulling forces exerted by microtubules attached to kinetochores and assists in aligning chromosomes correctly on the mitotic spindle.
- A protein complex called cohesin is essential for maintaining sister chromatid cohesion; it consists of multiple subunits, including Smc1, Smc3, and Scc1 (also known as Rad21 or Mcd1), which together form a distinct ring-shaped structure that holds the sister chromatids together.
Sister Chromatids Separation
- In the late prophase of mitosis, sister chromatids are joined along their entire lengths by axis-linking chromatin or structural bridges.
- These bridges ensure that the sister chromatids compact properly during prometaphase and metaphase, maintaining a parallel, paranemic alignment and preventing helical coiling.
- To enable sister chromatid separation during anaphase, these structural bridges must be removed.
- The separation of sister chromatid axes occurs in three defined steps:
- Global parallel separation
- Peeling apart
- Final resolution
1. Global parallel separation
- About one to two minutes before visible movement of centromeres begins, sister chromatid axes separate in parallel along their lengths, including both chromosomal arms and centromere/kinetochore regions.
- This global separation is driven by inter-sister chromatin pushing forces and occurs uniformly across the entire chromosome complement.
2. Peeling apart
- During this stage, centromere-proximal bridges are extensively stretched by poleward spindle forces.
- These stretched bridges are then removed in a process dependent on the enzyme topoisomerase IIα.
- As centromeres move toward opposite spindle poles, the separation spreads outward from the centromere along the chromosome arms and toward the telomeres.
- Meanwhile, regions far from the centromere that are not yet under tension remain mostly parallel and only slightly separated.
3. Final resolution
- Even after chromatids are widely separated, they may remain subtly linked in the telomere regions for a few minutes, likely due to catenation (intertwining of DNA strands).
- Complete separation is achieved during anaphase and is driven by further pole separation and/or chromatin compaction.
- The cleavage of cohesin is crucial in this stage, as it allows chromatin bridges to respond to mechanical forces and ensures full chromatid resolution.
Functions of Sister Chromatid
- The primary role of sister chromatids is to ensure that each daughter cell formed during cell division receives a complete set of chromosomes.
- In cells with replicated DNA, the sister chromatid acts as a template for repairing DNA double-strand breaks through a homologous recombination mechanism.
- Mutations in proteins required for sister chromatid cohesion can lead to defects in chromatin structure and disruptions in gene regulation.
- A failure in maintaining proper sister chromatid cohesion may result in congenital developmental disorders such as Cornelia de Lange syndrome, Roberts/SC Phocomelia syndrome, and Warsaw Breakage syndrome.
- In human oocytes, defects in sister chromatid cohesion can cause aneuploidy, which is recognized as a major factor contributing to spontaneous abortion.
Sister chromatids in mitosis
- During prophase of mitosis, sister chromatids begin to move toward the center of the cell in preparation for alignment.
- In metaphase, the sister chromatids align precisely along the metaphase plate, positioned at right angles to the cell poles.
- In anaphase, the sister chromatids are separated and pulled toward opposite poles of the cell. After separation, each chromatid is considered a complete single-stranded chromosome.
- During telophase and cytokinesis, the separated sister chromatids are fully divided into two distinct daughter cells. At this stage, the individual chromatids are referred to as daughter chromosomes.
Sister chromatids in meiosis
- Meiosis, like mitosis, is a type of cell division but involves two successive divisions to reduce the chromosome number by half.
- In prophase I and metaphase I, sister chromatids move together and remain attached as homologous chromosomes pair up and align at the metaphase plate.
- During anaphase I, homologous chromosomes (not sister chromatids) are pulled to opposite poles of the cell, while sister chromatids remain connected at their centromeres.
- The actual separation of sister chromatids occurs later, during anaphase II of the second meiotic division.
- Meiosis results in the formation of four daughter cells, each containing half the number of chromosomes as the original diploid cell.
- These resulting cells are sex cells (gametes), such as sperm and egg cells in animals, and they are haploid in nature.
Non-sister Chromatid
- A non-sister chromatid refers to one of the two chromatids belonging to a paired homologous chromosome, distinct from its sister chromatid.
- Non-sister chromatids are genetically non-identical, as they come from different parental origins—one maternal and one paternal.
- During prophase I of meiosis I, non-sister chromatids engage in genetic recombination by forming structures called chiasmata, where segments of DNA are exchanged.
- This crossing over between non-sister chromatids enhances genetic diversity in the resulting gametes.
Sister Chromatid vs. Non-sister Chromatid
Sister Chromatid vs. Non-sister Chromatid
| Basis | Sister Chromatid | Non-sister Chromatid |
|---|---|---|
| Definition | The centromere connects the two chromatids of a replicated chromosome. | The two chromatids are from two different homologous chromosomes. |
| Genes | They are genetically identical since they are produced by DNA replication. | Genetically non-identical since each non-sister chromatid is inherited from each parent. |
| Alleles | Contain the same alleles at the same loci. | Contain different alleles of the same genes at the same loci. |
| Formation | Formed during the S phase of the interphase. | Formed during the metaphase I of meiosis. |
| Location | They are found on the same chromosome. | Found in a homologous chromosome pair. |
| Involvement | Involved in asexual reproduction. | Involved in sexual reproduction. |
References
- Biology Dictionary. (2017). Sister Chromatid. Retrieved from https://biologydictionary.net/sister-chromatids/
- Biology Online. (2022). Non-sister Chromatid. Retrieved from https://www.biologyonline.com/dictionary/non-sister-chromatid
- BioNinja. (2022). Sister Chromatid – Meiosis. Retrieved from https://ib.bioninja.com.au/standard-level/topic-3-genetics/33-meiosis/sister-chromatids.html
- Chu, L., Zhang, Z., Mukhina, M., Zickler, D., & Kleckner, N. (2022). Sister chromatids separate during anaphase in a three-stage program directed by interaxis bridges. Proceedings of the National Academy of Sciences, 119(10). https://doi.org/10.1073/pnas.2123363119
- Khan Academy. (2022). Chromosomes and DNA. Retrieved from https://www.khanacademy.org/science/ap-biology/cell-communication-and-cell-cycle/cell-cycle/a/dna-and-chromosomes-article
- Lakna. (2017). Difference Between Sister and Non-sister Chromatids. Pediaa. Retrieved from https://pediaa.com/difference-between-sister-and-nonsister-chromatids/
- National Human Genome Research Institute (NIH). (2022). Chromatid – Genetics Glossary. Retrieved from https://www.genome.gov/genetics-glossary/Chromatid
- Peters, J. M., & Nishiyama, T. (2012). Sister chromatid cohesion. Cold Spring Harbor Perspectives in Biology, 4(11), a011130. https://doi.org/10.1101/cshperspect.a011130
