Sister chromatids

​​​​​​​​​​​​​​Sister chromatids
n., singular: Sister chromatid
[ˈsɪs.tɚ ˈkɹəʊmətɪd]
Definition: The two identical strands joined by a kinetochore

Sister Chromatids Definition

Sister chromatids are defined as the two identical copies of a single replicated chromosome that are joined together by a specialized constricted structure of chromosome called the centromere.

Before cell division, the genetic material as well as the cytoplasmic content of a parent cell needs to be doubled so that daughter cells receive their constituents to become fully functional and independent systems. This highlights the significance of DNA replication that precedes cell division. During the process of DNA replication, the genetic material of a chromosome is duplicated. This results in the formation of two sister chromatids. These chromatids remain closely associated with each other and are considered to be two identical chromatids (genetically) because they contain the same DNA sequence.

So, the key highlighting points of sister chromatids in cell biology are:

• They are exact copies of each other, containing the same genes and alleles of that gene.
• They are formed during DNA replication.
• They are bound to each other through the centromere.
• The protein complex called “cohesin” ensures the cohesion between sister chromatids.
• The cohesion and association of sister chromatids ensure that they don’t separate till they are ready (which is during cell division).
• They play a “crucial role in the accurate distribution of genetic material” to daughter cells during both mitosis as well as meiosis.

​​​​​​​​​​​​​​sister chromatids and homologous chromosomes:

Differences Between Sister Chromatids And Non-Sister Homologous Chromatids

Sister chromatids differ from non-sister chromatids in several ways. Look at the parameters in the table below to gain a comprehensive insight into their peculiarities.

Data Source: Akanksha Saxena of Biology Online

Functions Of Sister Chromatids

Sister chromatids serve several important functions in ensuring the accurate transmission of genetic material during cell division. There is a long list of processes that sister chromatids play a role in. Here, we discuss each one of them:

1. DNA Replication: The formation of sister chromatids happens during the cell cycle’s S phase. As the DNA replicates, each duplicated chromosome gets 2 identical copies also referred to as sister chromatids. This process ensures that each daughter cell receives the same replica copy of the genetic material.
2. Chromosome Stability: The cohesion and tight association of sister chromatids mediated by the action of cohesion proteins (especially at the centromeric region) helps in the maintenance of firmness or stability of the chromosome structure. It also helps in the prevention of any errors in chromosome segregation during cell division.

   

3. Chromosome Alignment: The alignment of sister chromatids along the metaphase plate during mitotic metaphase and meiotic metaphase II ensures equal distribution of genetic material to the daughter cells. Without this chromosomal alignment, it is not possible to carry out accurate segregation of chromosomes during cell division.

   

4. Genetic Recombination: In meiosis, sister chromatids facilitate genetic recombination. During prophase, I, non-sister chromatids of homologous pairs of chromosomes undergo a process called crossing over, wherein segments of DNA are exchanged between them. This genetic recombination results in the shuffling and mixing of genetic information. This thereby contributes to genetic diversity in the resulting gametes.

   

5. Separation and Distribution: During mitotic anaphase or meiotic anaphase II, the cohesin protein complex cleaves leading to the dissolution of the cohesion between sister chromatids. As sister chromatids separate, they start moving towards the cell’s opposite poles, hence ensuring an equal distribution of a complete set of chromosomes to each cell.
6. DNA repair: Sister chromatids have a crucial function in preserving the integrity of the genome through their involvement in DNA repair processes. They typically maintain close spatial proximity to each other, distinguishing them from the homologous chromosome pair. This spatial arrangement is particularly advantageous for accurate proofreading during the G2 phase of the cell cycle, following the duplication of chromosomes.

Structure Of Sister Chromatids At Metaphase

During metaphase, sister chromatids exhibit a distinct structure characterized by their attachment to each other and the formation of the kinetochore complex.

Cohesion between sister chromatids is established by specialized proteins known as cohesins. These cohesins initially distribute along the entire length of the chromosome during prophase, including regions of heterochromatin, resulting in the close association of sister chromatids.

The formation of the centromere, particularly the centromeric heterochromatin, can vary among different organisms. Some examples are:

• In yeast cells, centromeric heterochromatin formation involves RNA interference.
• In other organisms like roundworms and some insects exhibit a diffuse structure along the entire chromosome.
• Human cells possess a specific histone variant called “CENP” that plays a role in centromere formation and the recruitment of specific proteins.

The attachment of chromosomes to the spindle apparatus during metaphase is mediated by the kinetochore, a multi-layered protein complex. The deepest layer of the kinetochore interacts with centromeric proteins, including CENP histones.

Towards the end of prophase, the outer layer of the kinetochore forms and consists of proteins that possess “anchoring sites for microtubules”. The outermost domain of the kinetochore comprises a dynamic arrangement of proteins involved in regulating mitotic checkpoints and chromosome behavior.

The two kinetochores of sister chromatids face opposite directions. This enables the chromosomes to attach to microtubules emanating from different poles of the cell.

Microtubules originating from one pole undergo periods of growth and shrinkage, facilitating their search for kinetochores.

Once a microtubule attaches to a kinetochore, it stabilizes, leading to the exposure of the kinetochore of the opposing sister chromatid towards the opposite pole.

The tension generated by microtubules from opposite poles counteracts the adhesive property of cohesins along the centromere.

This tension is detected by the spindle assembly checkpoint, ensuring the proper assembly and attachment of the mitotic spindle. Once all chromosomes are aligned on the metaphase plate, the cell progresses into anaphase.

Separation Of Sister Chromatids During Anaphase

During anaphase, sister chromatids undergo “structural changes” that facilitate their separation and movement toward opposite poles of the cell.

At the onset of anaphase, the cohesion between sister chromatids is precisely cleaved, allowing them to separate and become individual chromosomes.

Once the cohesion is dissolved, the separated sister chromatids begin to move towards opposite poles of the cell.

This movement is propelled by the action of microtubules of the mitotic spindle. Microtubules attached to the kinetochores of sister chromatids shorten, generating a pulling force that helps separate the chromatids. As a result, each chromatid is directed toward its respective spindle pole.

Simultaneously, the cell changes to accommodate the separation of sister chromatids.

The spindle poles move apart, elongating the cell. This elongation creates space for the sister chromatids to be physically pulled toward opposite ends.

As anaphase progresses, the separated sister chromatids reach their respective spindle poles. Once they have reached the poles, the spindle apparatus starts to disassemble.

At this stage, new nuclear envelopes begin to form around each set of chromosomes, preparing for the formation of two distinct nuclei in the resulting daughter cells.

NOTE IT!

Different Fates of Sister Chromatids

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Sister chromatids have different fates during mitosis and meiosis, the two types of cell divisions. Let’s see how these fates are contrasting during the different stages of mitosis and meiosis respectively!

Fate of Sister Chromatids During Mitosis

During mitosis, sister chromatids undergo a series of events that ensure their accurate segregation and distribution to the daughter cells.

• Interphase: This is the phase before mitosis (specifically the S phase of interphase) which is marked by the “production of sister chromatids” by the duplication of each chromosome.
• Prophase: Prophase marks the onset of mitosis where one can notice the “tightly regulated association of sister chromatids” by the centromere.
• Prometaphase: During prometaphase, the spindle fibers (which are composed of microtubules) are formed as they extend between the two spindle poles. This also marks the assembly of kinetochores on each of the sister chromatids. Kinetochores are protein structures located at the centromeres of sister chromatids. This ensures that “sister chromatids’ attachment with the spindle apparatus”.
• Metaphase: In metaphase, the “alignment of the sister chromatids” along the metaphase plate is the key feature. The metaphase phase is an imaginary plane at the center of the cell. This alignment ensures equal distribution of genetic material to the daughter cells. NOTE: An important point to note here is that the tension exerted by the spindle fibers from opposite poles of the cell helps maintain the alignment of the sister chromatids at the metaphase plate.
• Anaphase: Anaphase is the stage marked by the “separation of sister chromatids”. This is followed by the “movement of sister chromatids” toward opposite poles of the cell. NOTE: Cohesion proteins (like cohesin) that held the sister chromatids together till this stage of mitosis are cleaved further allowing the chromatids to dissociate. The microtubules attached to the kinetochores shorten. This generates a pulling force that aids the separation of chromatids toward their respective spindle pole. A characteristic cell elongation is noticed as the spindle poles move apart.
• Telophase: In telophase, the “separated sister chromatids reach their respective spindle poles”. The spindle apparatus disassembles, and new nuclear envelopes form around each set of chromosomes.

The result of mitosis is the formation of two daughter cells, each containing an identical set of chromosomes. Each chromosome consists of a single chromatid as the sister chromatids have been separated during anaphase. The accurate segregation and distribution of sister chromatids ensure the preservation of genetic information and the formation of genetically identical daughter cells.

Fate of Sister Chromatids During Meiosis

During meiosis, sister chromatids undergo a unique series of events that lead to their separation and the generation of “genetically diverse” daughter cells which is contrasting to mitosis.

• Interphase: This is the phase before meiosis where DNA replication occurs and results in the “formation of two identical sister chromatids” held together at the centromere just like in mitosis.
• Prophase I: We can say that prophase I am when the “4 chromatids assemble in the form of a tetrad with two sister chromatids from each homologous chromosome”. This pairing facilitates genetic recombination between the chromatids which eventually results in the exchange of genetic material through crossing over. NOTE: One important point that we should note is that “crossing over manifests between non-sister chromatids of homologous chromosomes”. This happens during prophase I (meiosis I). This process does 2 things; “promotes genetic recombination” and “contributes to the genetic diversity observed in offspring”.
• Metaphase I: Unlike in mitosis, the tetrad alignment along the metaphase plate occurs in pairs of homologous chromosomes rather than individual sister chromatids. This alignment is random thus contributing to the genetic diversity.
• Anaphase I: Anaphase I is marked by the separation of homologous chromosomes. We should note that the “cohesion between sister chromatids remains intact” at the centromere during anaphase I of meiosis I. Two sister chromatids in the form of one homologous chromosome are pulled towards one pole while the other two sister chromatids of the second homologous chromosome are drawn towards the opposite poles of the cell. This separation of homologous chromosomes contributes to the reduction of chromosome number in the resulting daughter cells.
• Telophase I: During telophase, I of meiosis, the separated homologous chromosomes with “intact and attached sister chromatids” reach the poles of the cell. NOTE: It is important to note that during telophase I, the sister chromatids remain attached at the centromere. The separation that occurs during telophase I is between the homologous chromosomes, not the sister chromatids. Hence, 2 haploid daughter cells are formed after telophase I, each containing one chromosome from each homologous pair, with each chromosome still consisting of two sister chromatids.
• Prophase II: Meiosis II begins with prophase II, where a new set of spindle fibers forms, and the nuclear envelope breaks down. The two daughter cells from meiosis I are haploid and contain “duplicated sister chromatids”.
• Metaphase II: Unlike metaphase I of meiosis I, now the “duplicated sister chromatids align along the metaphase plate” in metaphase II. This is similar to the metaphase in mitosis. The alignment is individual as the homologous chromosomes are no longer present.
• Anaphase II: In anaphase II, the “cohesion between sister chromatids is dissolved” at the centromere. The separated chromatids which are now considered individual chromosomes are pulled towards opposite poles of the cell.
• Telophase II: Telophase II is the final stage of meiosis II. In this stage, “sister chromatids separation finally takes place” and now each one of them is considered an individual chromosome. As they reach the respective opposite poles, there is the reformation of the nuclear envelopes around each set of chromosomes. Cytokinesis follows, resulting in the formation of four haploid daughter cells, each containing a single chromatid.

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References

1. Jeff Hardin, et al. (2012) Becker’s World of Cell 8th Edition. San Fransisco. Pearson Education, Inc.
2. Baudoin, N. C., & Cimini, D. (2018). A guide to classifying mitotic stages and mitotic defects in fixed cells. Chromosoma, 127(2), 215-227.
3. Collin County Community College District. (2013) The Formation of Sister Chromatids
4. Hou, W., Li, Y., Zhang, J., Xia, Y., Wang, X., Chen, H., & Lou, H. (2022). Cohesin in DNA damage response and double-strand break repair. Critical reviews in biochemistry and molecular biology, 57(3), 333–350. https://doi.org/10.1080/10409238.2022.2027336
5. Cimini D, Mattiuzzo M, Torosantucci L, Degrassi F. (2003) Histone hyperacetylation in mitosis prevents sister chromatid separation and produces chromosome segregation defects.
6. Chittori, S., Hong, J., Saunders, H., Feng, H., Ghirlando, R., Kelly, A. E., … & Subramaniam, S. (2018). Structural mechanisms of centromeric nucleosome recognition by the kinetochore protein CENP-N. Science, 359(6373), 339-343.
7. Alberts B., Johnson A., Lewis J., Raff M., Roberts K., Walter P. (2008) Molecular Biology of The Cell, 5th Edition, New York: Garland Science.(Page 1272)

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