Chromosomal behaviour is just the normal movement of chromosomes during cell division. During mitosis, replicated chromosomes align themselves along the equator of the cell forming a metaphase plate, and then sister chromatids are pulled apart towards opposite poles of the cell. All these complex activities are highly coordinated and controlled by various signals and intra-cellular mitotic checkpoints.
In this article, we will discuss the ultrastructure of chromosomes and the mitotic spindle apparatus. After that, we will discuss the congression of chromosomes, their movement, and segregation. We will also discuss the mechanisms of these processes and the factors affecting them. Lastly, we will have an account of the abnormalities in chromosome movement and segregation of chromosomes.
Overview of Mitosis
Have you ever wondered how a tiny, little seed develops into a massive tree or a small baby, or even smaller, a microscopic zygote change into a 6 feet man? This is the cell division that brings about this change. During growth, the cell divides through mitosis.
By definition, mitosis is a process in which a parent cell divides into two identical daughter cells with the same number of chromosomes as the parent cell. For an overview, in mitosis, a cell’s genetic material is replicated and condensed into mature chromosomes. A mitotic spindle is formed, and chromosomes are aligned along the equator of the cell. Segregation of sister chromatids followed by their movement towards opposite poles takes place. Cell membrane pinches from the center (in animal cells) and ultimately two identical daughter cells are formed.
Structure of Chromosome
One chromosome is a highly condensed DNA molecule. DNA exists in double-helical form. This double first wraps around a core of eight histone proteins forming round structures, nucleosomes. Histones are rich in positively charged amino acid “arginine” which attracts the negatively charged phosphate groups of DNA, thus stabilizes the nucleosome. DNA appears as a “beaded string” having a diameter of 10nm. This beaded string condenses to a 30nm fiber, presenting the genetic material of a non-dividing cell that cannot be seen with LM. In a dividing cell, during prophase, it further condenses into a highly compact structure, chromosomes which now become visible under LM.
At the center of each chromosome, there is a primary constriction, which holds two sister chromatids together, the centromere, which contains a few base pairs as in yeast to millions of base pairs i.e., in metazoans. At the centromere, around 60 different proteins form plate-like aggregations called the kinetochore. Each sister chromatid has its kinetochore. This structure is the actual hook to which k-fibers attach during metaphase.
- Structure of microtubules:
Microtubules are elongated, hollow tubes, having an overall thickness of ~24nm with a lumen diameter of ~15nm. They are polymers of tubulin which is a GTP-binding protein dimer, having two subunits i.e., alpha and beta (each of 55 kDa). Multiple alpha and beta subunits, alternating with each other, binds to form a protofilament. 13 protofilaments, arrange in a circle, form one microtubule. Microtubules show polarity due to the orientation of alpha and beta subunits, one end being plus and the other being minus.
At the plus end, there is always a beta subunit exposed, the rate of accumulation of new subunits (elongation) or dissociation of already present subunits (shortening) is much higher at this end. Contrary to it, the minus-end always has an alpha subunit exposed, with a much slower rate of elongation or shortening.
In animal cells, microtubules originate from the centrosome (having a pair of centrioles), located near the nucleus. In plant cells, as they lack centrioles, the nuclear membrane serves as MTOC (microtubule organizing center).
- Mitotic Spindle:
Having reviewed the chromosomal and microtubular structures, let move towards the mitotic spindle. One of the very important functions of microtubules is the formation of the mitotic spindle during cell division. First, centrioles replicate, and each pair move toward opposite poles determining the polarity of the cell and the mitotic spindle. From centrioles, microtubules elongate sat their plus ends.
Two types of fibers are formed based on whether they bind with kinetochore or not. Those binding with the kinetochore, are called kinetochore fibers or K-fibers. They form parallel bundles extending from the centrosome to the equator of the cell. They play the main role in the segregation and movement of the chromosome during metaphase and anaphase which will be described later. Fibers that do not bind with kinetochore may present as isolated single microtubules or parallel or antiparallel bundles. These are non-kinetochore fibers further divided into two types i.e., astral or interpolar fibers.
Astral fibers extend from the centrosome towards the cell cortex, they may serve as anchoring fibers that keep the centromeres at their place. This provides a fixed end for kinetochore and interpolar fibers.
Interpolar fibers are those which extend towards the cell’s equator but do not bind with kinetochores, instead, they overlap and interdigitate with the same fibers coming from the opposite pole. These fibers play a very important role during karyokinesis. They act as bridges between two opposite kinetochore fibers. This bridging action balances the force applied by the K-fibers and hence, it prevents non-disjunction which may lead to serious consequences such as trisomy or monosomy syndromes.
- Regulation of spindle formation:
The growth, dynamics, and movement of microtubules are regulated by microtubules-associated proteins (MAPs). These MAPs determine the time of formation, location, orientation, and length of microtubule fibers. They also define the shape of the spindle and the polarity of the cell. There are thousands of proteins of this kind, hundreds of them are yet unidentified, but the most recent studies suggest that out of them ~200 microtubules associated proteins are considered essential for spindle formation.
The alignment of chromosomes at the spindle equator is known as the chromosomal congression. The spindle equator is a position that is in the center of both spindle poles. This process continues for more than 15 minutes (in human cells). The result is the formation of a metaphase plate at the center.
Chromosomes then enter the metaphase of mitosis and remain connected to both spindle poles until anaphase. The process of congression involves two things. First, the establishment of bipolar kinetochore attachments (sometimes referred to as amphitelic attachments), and second, the alignment of chromosomes at the spindle equator. It is still a matter of research whether the bipolar attachment is necessary for congression or if the chromosome congression is necessary for bi-oriented chromosome attachment.
Research has shown that not all chromosomes use the same mechanism for congression. Moreover, the mechanisms that are involved in the maintenance of this alignment are somewhat different than the mechanisms involved in congression.
However, a very important question is that how monotelic and mono-oriented chromosomes become amphitelic and bi-oriented. Several scientists have proposed different models to explain this phenomenon. Let us discuss some of these models.
- Search and capture model
According to this model, the monotelic chromosomes are found near one spindle pole. A micro-tubule arises from the opposite spindle pole and attaches to the opposite sister kinetochore and the mono-oriented chromosomes become bi-oriented. After that, microtubule depolymerization results in the movement of these chromosomes to the spindle equator.
However, this model is not widely accepted. It is because according to this model, the time required for congression is several hours, but the experiments show that congression is completed in 15 to 20 minutes. That implies that there are some other pathways that are also involved in the chromosome congression.
- Kinetochore-mediated k-fiber formation
This model proposes that a special type of fibers called the k-fibers, originate directly at kinetochores. Most probably, the k-fiber formation takes place when a kinetochore captures short and non-centrosomal microtubules. When these microtubules are captured, the polymerization of plus ends of microtubules starts inside the kinetochore. The process pushes the minus ends outwards. The mechanism of poleward transport of minus ends is called a dynein-mediated mechanism. This results in the repositioning of the k-fiber at the spindle equator. This model looks more universal than the search and capture model.
- Traction fiber model
The traction fiber model is one of the earliest models proposed for chromosome congression. According to this model, the k-fibers generate a poleward force. This force is proportional to the length of k-fibers and when the chromosome is at the equator, a balance is established between sister k-fibers. However, there is not enough experimental data available in the support of the traction fiber model.
- Kinetochore motor model
This model proposes that motor-mediated gliding of unattached kinetochores happen alongside microtubule bundles. According to this model, plus end- and minus end-directed motors associate with kinetochores. This process suggests that motors such as CENPE and cytoplasmic dynein could drive the movement of chromosomes. This chromosomal movement may be either towards or away from the spindle equator.
Recent studies have shown that CENPE is directly involved in the movement of mono-oriented chromosomes. The leading kinetochore is attached laterally instead of attaching in an end-to-end manner. Another key feature of this model is that chromosome congression can precede the formation of bi-polar of amphitelic attachment.
- The polar wind model
This model proposes that polar wind is involved in the positioning of chromosomes at the equator. The polar wind may be generated when the plus ends of microtubules push against the chromosome. This polar wind creates spatial cues that help the chromosomes to find the spindle equator.
When we talk about chromosome movement on the spindle, we generally discuss two things; velocity of movement & direction of movement.
Research has shown that the velocity of both poleward and anti-poleward chromosome movements remains constant in the case of attached chromosomes. The attached chromosomes have a velocity of approximately 2 um per minute. The rates of polymerization and depolymerization of microtubules in the spindle are much higher than the rates of chromosome velocity. Microtubules depolymerize at approximately 17 um per minute.
The motor proteins such as CENPE and dynein are also involved in the movement of chromosomes towards the spindle equator. However, according to recent research, the movement does not depend entirely on these factors. To cut a long story short, chromosome motility during congression is due to a mixture of interactions between both microtubules and forces related to motor proteins.
According to research, chromosomes exhibit an oscillatory motion at the spindle. This movement is both away and towards the spindle equator and this phenomenon is called the directional instability of chromosomes.
Initially, it was thought that the oscillatory motion is the result of coordinated polymerization on the two sister kinetochores. However, now the research has shown that mono-oriented chromosomes also exhibit oscillatory motion and that a single attached kinetochore can also support these oscillations. Moreover, it is suggested that both microtubule attachments and dynamics at kinetochore are involved in the oscillatory motion.
Another research concludes that oscillations of a mono-oriented chromosome and its position relative to the spindle pole are the results of an imbalance between two forces. These two forces are; the poleward pulling forces and an ejection force generated within the aster.
Segregation of chromosomes during mitosis
After the formation of the metaphase plate during metaphase, the next step of mitosis starts that is known as anaphase. The anaphase is further divided into anaphases A and anaphase B. Two different types of movements occur in these stages.
During anaphase A, the sister chromatids disjoin and move poleward and during anaphase B, the spindle poles move away from each other. In most organisms, anaphase A and anaphase B happen at the same time. However, the research suggests that in some cells these phases occur in a sequential manner.
On the onset of anaphase, the enzyme separase acts on the cohesions (proteins binding the sister chromatids). This action splits the sister chromatids and now the chromatids become the individual chromosomes. This completes the anaphase A.
In anaphase B, chromosomes move towards the opposite poles. There are two mechanisms proposed for this movement. One of them is the Pac-man mechanism, the name of which comes from a video game in which the character moves eating the dots coming in its way. According to the Pac-man mechanism, motor proteins present on the kinetochores walk the chromosomes along the k-fibers. After the motor proteins have passed, the k-fibers depolymerize at the kinetochore end.
Other researchers working with some different species suggest that the chromosomes are “reeled in” by the motor proteins and arrive at the poles. After they arrived at the poles, the k-fiber depolymerizes. With research advancement, there comes a consensus now that both these mechanisms may play their relative roles in different cell types.
As mitosis is a highly controlled process, usually the daughter cell is hundred percent identical to the parent cell. But in rare cases, error may occur either in the number of chromosomes or in the structure of chromosomes of the daughter cell. these are called chromosomal anomalies.
- Chromosomal abrasion
In structural chromosomal anomalies/chromosomal abrasion chromosome may lose some of its genetic material or there may occur addition, deletion, or substitution of one or more base pairs during the replication of chromosomes. These changes affect the genes and thus the phenotype of the individual.
One of the examples is the trait of sickle cell anemia in which substitution of a base pair on chromosome number 11 in the gene of Beta chain of the hemoglobin A. In these patients when hypoxia occurs, the abnormal hemoglobin polymerizes to form elongated fibers that change the normal biconcave disk-shaped red cells into sickle-shaped cells. When these abnormal cells cannot pass through microcapillaries. This results in even more hypoxia leading to more sickling of the cells. A vicious cycle starts that is known as the sickle cell crisis.
- Nondisjunction of chromosomes
Nondisjunction is a type of numerical chromosomal anomalies. Due to any defect in k-fibers or cytoplasmic dynein protein (motor protein associated with k-fibers), error may occur in the segregation of sister chromatids. This is called nondisjunction of chromosomes which results in the passage of chromosomal pair or chromatids to one daughter cell. The other daughter cell, when this occurs, receives no chromosomes. This results in trisomy or monosomy.
In trisomy, three copies of the same chromosome are present. These are the most common numerical chromosomal anomalies. Three main trisomy syndromes are described here.
First is Down syndrome in which trisomy occurs at chromosome number 21. Clinical features of down syndrome are a flattened face, broad forehead, protruding tongue, short stature, upward slanting eyelids, poor muscle tone, and supernumerary teeth.
The second is Edward syndrome having trisomy at chromosome number 18. Some important clinical features are retarded growth, mental deficiency, ventricular septal defect, and flexed digits with hypoplastic nails.
The third is Patau syndrome with trisomy at chromosome number thirteen. Clinically, it is characterized by severe central nervous system abnormalities, malformed ears, microphthalmia, bilateral cleft lip and palate, and polydactyly. Babies born with Edward or Patau syndrome are severely malformed with mental retardation and die early in infancy.
In monosomy, one chromosome is completely or partially missing. One example of this type of chromosomal abnormality is turner syndrome.
Turner syndrome only affects females, results when there is only one X chromosome due to nondisjunction or loss of chromosome due to chromosomal abrasion. Signs and symptoms of turner syndrome are slight at birth, develop slowly with time, may become significant with age leading to heart defects.
At birth, affected babies may have low-set ears, short fingers and toes, small lower jaw, swelling in different areas of the body i.e., hands and feet. They show poor growth and have a broad chest. With increasing age, cardiac problems such as heart failure due to edema may occur. Affected teenagers show retarded growth, have short stature, webbed neck, and are unable to conceive.
During mitosis, loose chromatin material condenses into highly compact chromosomes. Mature chromosomes move towards the equatorial plate, sister chromatids bind with kinetochore fibers, segregate, and move towards opposite cell poles. One complete chromosome is a highly condensed single DNA molecule, with a centromere having kinetochore, one for each sister chromatid.
The mitotic spindle is composed of fibers called microtubules. Each microtubule has a plus and a minus-end which differs in its rate of elongation or shortening. Microtubules originate from centrioles. Two types of microtubules from the spindle. Those which bind with the kinetochore are kinetochore fibers/K-fibers and others are non-kinetochore fibers. Non-kinetochore fibers are further divided into Interpolar and astral fibers.
Chromosomes align along with the equatorial plate during metaphase. This alignment is called chromosomal congression. Several mechanisms have been proposed for congression. In the “search and capture model”, the microtubule arises from one spindle pole, reaches the chromosome, and binds with it, depolymerization of the microtubule takes place and the chromosome is brought to the equator. This process is time-consuming and is not widely accepted.
The “Kinetochore-mediated K-fiber formation” model suggests that K-fibers originate from kinetochores and them repositioned at the spindle equator. This mechanism has a more universal approach than the previous one.
According to the “Traction fiber model”, K-fibers generate a force that stabilizes the sister K-fibers at the equator, but this idea lacks enough experimental evidence.
Another idea is of “polar wind model”, which says that polar wind may be generated when the plus ends of microtubules push against the chromosome. This creates spatial cues that may help chromosomes to find the spindle equator.
The velocity of chromosomes is approx. 2um per minute but the rate of polymerization and depolymerization of microtubules is much higher i.e., 17 um per minute.
The movement of chromosomes is influenced by interactions between motor proteins such as dynein and forces related to them.
During anaphase, chromosomes segregate into sister chromatin by the action of enzyme separase and then move towards opposite poles. This movement takes place by two mechanisms. “Pac-man” mechanism says that depolymerization of k-fibers follows the movement of chromosomes along with it. However, the “Reeled-in” mechanism suggests that depolymerization takes place after the chromosome has reached the pole.
During chromosomal movements, many errors can occur which may result in numerical or structural chromosomal anomalies leading to serious consequences.
- Poonperm, Rawin; Takata, Hideaki; Hamano, Tohru; Matsuda, Atsushi; Uchiyama, Susumu; Hiraoka, Yasushi; Fukui, Kiichi (1 July 2015). “Chromosome Scaffold is a Double-Stranded Assembly of Scaffold Proteins”. Scientific Reports. 5 (1): 11916. Bibcode:2015NatSR…511916P. doi:10.1038/srep11916. PMC 4487240. PMID 26132639.
- Lodish, U.H.; Lodish, H.; Berk, A.; Kaiser, C.A.; Kaiser, C.; Kaiser, U.C.A.; Krieger, M.; Scott, M.P.; Bretscher, A.; Ploegh, H.; others (2008). Molecular Cell Biology. W. H. Freeman. ISBN 978-0-7167-7601-7.
- “Chromosome Mapping: Idiograms” Nature Education – 13 August 2013
- Naumova N, Imakaev M, Fudenberg G, Zhan Y, Lajoie BR, Mirny LA, Dekker J (November 2013). “Organization of the mitotic chromosome”. Science. 342 (6161): 948–53. Bibcode:2013Sci…342..948N. doi:10.1126/science.1236083. PMC 4040465. PMID 24200812.