What are sister chromatids and when do they form?

Sister chromatids are identical copies of a chromosome created during DNA replication in the S phase of the cell cycle. Each chromatid contains the same genetic information and remains attached at a region called the centromere until cell division occurs, ensuring each daughter cell receives identical genetic material.

Where do sister chromatids attach to each other?

Sister chromatids attach at the centromere, a specialized chromosomal region containing repetitive DNA sequences and associated proteins. This attachment point holds the two chromatids together throughout interphase and early stages of cell division until they separate during anaphase, maintaining chromosomal integrity.

What proteins hold sister chromatids together at the centromere?

Cohesin proteins form a protein complex that holds sister chromatids together at the centromere and along chromosome arms. These cohesins maintain the attachment until anaphase, when they are cleaved by separase, allowing the chromatids to separate and move to opposite poles during mitosis and cytokinesis.

Why is sister chromatid attachment important for cell division?

Sister chromatid attachment ensures that identical copies of genetic material are distributed equally to daughter cells during division. Proper attachment and subsequent separation prevent aneuploidy and maintain genetic stability across generations of cells, which is essential for organism growth and function.

How do spindle fibers interact with attached sister chromatids?

Spindle fibers attach to sister chromatids at the kinetochore, a protein structure at the centromere. These fibers pull the attached chromatids toward opposite poles of the cell, and when cohesins are cleaved, the sister chromatids separate and move independently to ensure proper chromosome segregation.

What happens if sister chromatids fail to attach properly?

Improper sister chromatid attachment can lead to unequal chromosome distribution during cell division, resulting in aneuploidy. This chromosomal imbalance can cause cell dysfunction, apoptosis, or in some cases contribute to cancer development, highlighting the critical importance of proper attachment mechanisms.

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Attachment of Sister Chromatids

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During mitosis, following the assembly of a bipolar microtubule array, the kinetochore microtubules attach to the sister-chromatids.

Microtubules attach to each chromatid via the kinetochore located at the centromere. Multiple microtubules from the same spindle pole can bind to a single kinetochore to form the kinetochore fiber.

The kinetochore is a multi-layered protein complex. The inner-kinetochore layer secures the kinetochore complex with the sister chromatid. The outer-kinetochore layer includes specialized, rod-shaped, protein complexes called Ndc80 that link microtubules with the kinetochore.

Multiple copies of the Ndc80 complex bind to the plus ends of microtubules. Binding of Ndc80 permits microtubule polymerization and depolymerization to occur at the plus-end while being linked to the kinetochore.

Kinetochores on a sister-chromatid pair attach to microtubules emanating from opposite poles of the mitotic spindle, resulting in the bi-orientation of sister chromatids.

Bi-orientation generates high levels of tension within the kinetochores of sister chromatids. Strong poleward forces exerted by the microtubules pull the kinetochores towards opposite spindle poles. An opposing force resulting from sister-chromatid cohesion resists the poleward force. Tension is used to sense the correct bi-orientation of chromosomes.

A tension-sensing mechanism involves a protein kinase, Aurora B, tethered to the inner-kinetochore layer. During minimal tension or absence of tension, the Aurora B kinase rests in physical proximity to the outer-kinetochore layer, where it can phosphorylate Ndc80 complexes. This phosphorylation reduces the affinity for microtubule binding.

Once the sister chromatids have bioriented, the opposing effects of opposite poleward forces pull the outer-kinetochore layer away from the inner-kinetochore layer, physically distancing and preventing Aurora B kinase from phosphorylating Ndc80 complexes.

Ndc80 proteins, in an unphosphorylated state, strengthen existing microtubule attachment and exhibit increased affinity for additional microtubules. These changes enable microtubules to form stable attachments with the kinetochore.

Successful attachment of sister chromatids to the opposite spindle poles tugs the chromosomes back and forth to assume a position equidistant from the two poles, at the metaphase plate.

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As cells progress into mitosis, the nuclear envelope breaks down, and the condensed chromosomes are exposed to the array of bipolar microtubules of th…