During Which Phase Of Mitosis Do The Chromatids Become Chromosomes

During mitosis, a crucial process in cell division, the transformation of chromatids into chromosomes occurs during anaphase. This phase is characterized by the separation of sister chromatids, each then recognized as an individual chromosome as they move towards opposite poles of the cell. Understanding this transition requires a closer look at the entire process of mitosis, the structure of chromosomes, and the molecular mechanisms driving anaphase.

Mitosis: An Overview

Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. It is a fundamental process for growth, repair, and asexual reproduction in eukaryotic organisms. Mitosis consists of several distinct phases: prophase, prometaphase, metaphase, anaphase, and telophase, followed by cytokinesis.

1. Prophase:
   • The chromatin condenses into visible chromosomes.
   • The nuclear envelope breaks down.
   • The mitotic spindle begins to form.
2. Prometaphase:
   • The nuclear envelope completely disappears.
   • Spindle fibers attach to the kinetochores of chromosomes.
3. Metaphase:
   • Chromosomes align along the metaphase plate, an imaginary plane in the middle of the cell.
   • Each chromosome is attached to spindle fibers from opposite poles.
4. Anaphase:
   • Sister chromatids separate and move to opposite poles of the cell.
   • Each chromatid is now considered an individual chromosome.
5. Telophase:
   • Chromosomes arrive at the poles and begin to decondense.
   • The nuclear envelope reforms around each set of chromosomes.
6. Cytokinesis:
   • The cytoplasm divides, resulting in two separate daughter cells.

Chromosome Structure and Composition

To fully appreciate the significance of chromatids becoming chromosomes in anaphase, it is essential to understand the structure and composition of chromosomes.

• Chromatin: This is the complex of DNA and proteins (primarily histones) that makes up chromosomes. During interphase, chromatin is less condensed, allowing access for DNA replication and transcription.
• Chromosomes: These are highly condensed structures of DNA that become visible during cell division. Each chromosome consists of two identical sister chromatids during the early stages of mitosis.
• Sister Chromatids: These are identical copies of a chromosome, connected at the centromere. They are produced during the S phase of the cell cycle when DNA replication occurs.
• Centromere: This is the region where sister chromatids are most closely attached. It plays a crucial role in chromosome segregation during cell division.
• Kinetochore: This is a protein structure on the centromere where spindle fibers attach. It is essential for the movement of chromosomes during mitosis.

Detailed Look at Anaphase

Anaphase is a critical phase in mitosis during which the sister chromatids separate and move to opposite poles of the cell. This separation marks the transition from chromatids to individual chromosomes. Anaphase is divided into two distinct sub-phases: anaphase A and anaphase B.

1. Anaphase A:
   • Sister chromatids separate and move towards the poles.
   • This movement is driven by the shortening of kinetochore microtubules.
   • The force for movement is generated by motor proteins associated with the kinetochore.
2. Anaphase B:
   • The poles move further apart, elongating the cell.
   • This movement is driven by the lengthening of polar microtubules.
   • Motor proteins associated with polar microtubules slide them past each other, pushing the poles apart.

The separation of sister chromatids is controlled by the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase that targets specific proteins for degradation. One key target is securin, an inhibitor of separase. When securin is degraded by the APC/C, separase is activated, which then cleaves cohesin.

Cohesin is a protein complex that holds sister chromatids together. Cleavage of cohesin allows the sister chromatids to separate, marking the beginning of anaphase.

The Role of the Spindle Apparatus

The spindle apparatus, consisting of microtubules and associated proteins, plays a crucial role in chromosome segregation during mitosis. Microtubules are dynamic structures that can polymerize (grow) and depolymerize (shrink), allowing for the precise movement of chromosomes.

• Kinetochore Microtubules: These attach to the kinetochores of chromosomes and are responsible for pulling the sister chromatids apart during anaphase A.
• Polar Microtubules: These extend from the poles and overlap in the middle of the cell, interacting with motor proteins to push the poles apart during anaphase B.
• Astral Microtubules: These radiate from the poles and interact with the cell cortex, helping to position the spindle apparatus and contribute to cytokinesis.

The dynamic instability of microtubules is essential for the proper attachment of spindle fibers to chromosomes. Microtubules undergo cycles of growth and shrinkage until they successfully attach to the kinetochores of both sister chromatids. This attachment ensures that each daughter cell receives the correct number of chromosomes.

Molecular Mechanisms Driving Anaphase

The transition from metaphase to anaphase is tightly regulated by several molecular mechanisms, ensuring accurate chromosome segregation.

1. Spindle Assembly Checkpoint (SAC):
   • The SAC is a surveillance mechanism that monitors the attachment of spindle fibers to kinetochores.
   • It prevents the onset of anaphase until all chromosomes are correctly attached to the spindle.
   • Unattached kinetochores generate a signal that inhibits the APC/C, preventing the degradation of securin and the activation of separase.
2. Anaphase-Promoting Complex/Cyclosome (APC/C):
   • The APC/C is a ubiquitin ligase that targets specific proteins for degradation.
   • It is activated by the SAC when all chromosomes are correctly attached to the spindle.
   • The APC/C ubiquitinates securin, leading to its degradation and the activation of separase.
   • The APC/C also ubiquitinates cyclin B, leading to its degradation and the inactivation of Cdk1, a kinase that promotes mitosis.
3. Separase and Cohesin:
   • Separase is a protease that cleaves cohesin, the protein complex that holds sister chromatids together.
   • Separase is inhibited by securin until the APC/C is activated.
   • Cleavage of cohesin allows the sister chromatids to separate and move to opposite poles of the cell.

Why Chromatids Become Chromosomes in Anaphase

The transition of chromatids into chromosomes during anaphase is not merely a change in name but reflects a fundamental shift in their status and function. Before anaphase, sister chromatids are physically connected and share a common centromere. They are essentially two halves of the same replicated chromosome, ensuring that each daughter cell receives an identical copy of the genetic material.

However, once the sister chromatids separate, each becomes an independent chromosome with its own centromere. This separation signifies that each chromatid is now a complete and autonomous unit capable of being segregated into separate daughter cells. The term "chromosome" is used to denote this independent status, emphasizing the genetic individuality of each entity.

Furthermore, the separation of sister chromatids is crucial for maintaining genomic stability. By ensuring that each daughter cell receives a complete and accurate set of chromosomes, mitosis prevents aneuploidy (an abnormal number of chromosomes), which can lead to developmental defects, cancer, and other disorders.

Implications of Errors in Anaphase

Errors during anaphase can have severe consequences for the resulting daughter cells. Non-disjunction, the failure of sister chromatids to separate properly, can lead to aneuploidy. Aneuploidy can result in genetic disorders such as Down syndrome (trisomy 21), where an individual has an extra copy of chromosome 21.

Other errors, such as lagging chromosomes (chromosomes that fail to move properly to the poles), can also lead to aneuploidy or chromosome loss. These errors can arise from defects in the spindle apparatus, the kinetochores, or the regulatory mechanisms that control anaphase.

Cancer cells often exhibit defects in mitosis, leading to genomic instability and uncontrolled cell proliferation. Understanding the mechanisms that regulate anaphase is therefore crucial for developing new cancer therapies that target mitotic defects.

Conclusion

In summary, the transformation of chromatids into chromosomes occurs during anaphase, a critical phase of mitosis characterized by the separation of sister chromatids. This separation is driven by the shortening of kinetochore microtubules and the activity of motor proteins, and it is tightly regulated by the spindle assembly checkpoint and the anaphase-promoting complex/cyclosome. Each separated chromatid is now considered an individual chromosome, ensuring that each daughter cell receives a complete and accurate set of genetic material. Understanding this process is essential for comprehending the fundamental mechanisms of cell division and its implications for growth, development, and disease.

FAQ

1. What is the difference between a chromatid and a chromosome?
   A chromatid is one half of a duplicated chromosome, connected to its sister chromatid at the centromere. A chromosome is the structure formed when DNA is tightly packed during cell division. After the sister chromatids separate during anaphase, each chromatid is considered an individual chromosome.

2. What happens if anaphase goes wrong?
   If anaphase goes wrong, it can lead to non-disjunction, where chromosomes fail to separate properly. This can result in daughter cells with an abnormal number of chromosomes (aneuploidy), which can cause genetic disorders or cancer.

3. What is the role of the spindle assembly checkpoint (SAC)?
   The SAC is a surveillance mechanism that ensures all chromosomes are correctly attached to the spindle before anaphase begins. It prevents the activation of the APC/C until all kinetochores are properly attached, preventing premature separation of sister chromatids.

4. What is the anaphase-promoting complex/cyclosome (APC/C)?
   The APC/C is a ubiquitin ligase that targets specific proteins for degradation, including securin and cyclin B. Degradation of securin activates separase, which cleaves cohesin and allows sister chromatids to separate. Degradation of cyclin B inactivates Cdk1, promoting the exit from mitosis.

5. Why is it important that each daughter cell receives the correct number of chromosomes?
   It is crucial that each daughter cell receives the correct number of chromosomes to maintain genomic stability. Aneuploidy (an abnormal number of chromosomes) can lead to developmental defects, genetic disorders, and cancer. Accurate chromosome segregation during mitosis is essential for the health and proper functioning of organisms.

6. What are the different types of microtubules involved in mitosis?
   There are three types of microtubules involved in mitosis:

   • Kinetochore microtubules: attach to the kinetochores of chromosomes.
   • Polar microtubules: extend from the poles and overlap in the middle of the cell.
   • Astral microtubules: radiate from the poles and interact with the cell cortex.

7. How does the separation of sister chromatids occur during anaphase?
   The separation of sister chromatids during anaphase is initiated by the activation of separase, which cleaves cohesin, the protein complex that holds sister chromatids together. This allows the kinetochore microtubules to pull the sister chromatids apart and move them to opposite poles of the cell.

8. What is the role of motor proteins in anaphase?
   Motor proteins play a crucial role in anaphase by generating the force required for chromosome movement. Motor proteins associated with kinetochore microtubules pull the chromosomes towards the poles, while motor proteins associated with polar microtubules slide them past each other, pushing the poles apart.

9. Can errors in mitosis be targeted for cancer therapy? Yes, errors in mitosis are often exploited in cancer therapy. Since cancer cells frequently exhibit mitotic defects, targeting these defects can selectively kill cancer cells while sparing normal cells. Many chemotherapeutic drugs disrupt microtubule dynamics, interfere with spindle assembly, or target key regulatory proteins involved in mitosis.

10. How does cytokinesis relate to mitosis? Cytokinesis is the final stage of cell division, following mitosis. While mitosis involves the division of the nucleus and segregation of chromosomes, cytokinesis involves the division of the cytoplasm, resulting in two separate daughter cells. Cytokinesis typically begins during anaphase or telophase and ensures that each daughter cell receives its own nucleus and complement of cellular organelles.