Chromatin is the material that chromosomes are made of. During interphase, or the period between cell divisions, chromosomes are not visible in a cell.
Chromosomes are composed of two identical copies of DNA wrapped around specialized protein molecules called histones. The DNA-histone combination is called chromatin.
During mitosis, the process that cells go through to divide into two new cells, chromosomes must be unrolled and organized so that the new cells can be equal in content and function. This process is facilitated by enzymes known as topoisomerases.
The topoisomerases work by breaking and reforming chemical bonds in the chromosome’s DNA and histones. This allows for adequate uncoiling of the chromosomes without damaging the genetic information stored in the DNA.
Depending on the stage of mitosis, different topoisomerases are active to ensure adequate chromosome organization for division into two new cells.
Metaphase
The next phase of mitosis is metaphase. During metaphase, the centrosome organelle stacks the paired chromosomes together in a very specific order.
This is done by attaching a chromosome to a microtubule, which is part of the centrosome. The other end of the microtubule is attached to another chromosome, in what is called a bivalent configuration.
These bi-chromosomes are now lined up in an orderly fashion on the metaphase plate, or spindle. This occurs due to movement of the centrosome alongside the nucleus. Once here, chromosomes are said to be arrested.
During this stage, the nucleolus disappears and the nuclear envelope re-forms around the nucleus. These events occur as a result of cell cycle regulators like Cyclin and Cyclin Dependent Kinase (CDK) Inhibitors like p21cip1.
Telophase
Once cytokinesis is complete, the cell enters into telophase. This is the last phase of mitosis, and it takes place when the chromosomes are in their normal position, when the nuclear envelope reforms, and when the cell returns to a state of normal activity.
Telophase is divided into two parts: restitution and transition. In restitution, proteins and structures that were moved to opposite sides of the cell in an earlier phase are returned to their normal positions.
In transition, the cell starts to prepare for cytokinesis by starting to kink up the cytoplice membrane. This makes room for new nuclei or cells that will divide later on.
After these stages are completed, the cell enters into a stage called interphase. Interphase consists of different phases based on what type of cell it is; this depends on what kind of activity it will have later on.
Interphase
The phase of mitosis called interphase separates prophase, metaphase, and anaphase. During interphase, the cell does not make any changes in size or shape.
During this phase, the chromosomes uncoil and are visible in the Nucleus as chromatin. This is when the chromosomes are counted and checked for any mutations or errors in DNA structure.
Interphase can be long or short depending on what kind of cell it is. For example, a skin cell will have a very long interphase because it does not divide very often. A blood cell will have a shorter interphase since it divides more frequently.
Intensive research is being done on how to intervene in interphase to prevent cancer formation. By identifying cells with mutations and disrupting their interphase, scientists are trying to prevent cancer formation.
Prophase
Once the nuclear envelope has broken down and the chromatin has been organized into chromosomes, the cell is ready to move onto the next stage of mitosis: prophase. Prophase is defined by the loosening of the chromosome organization, the synthesis of new RNA and protein molecules, and the movement of chromosomes to opposite poles.
Prophase I occurs when chromosomes uncoat, or reveal their DNA. This happens due to enzymes called histone deacetylases (HDACs), which loosen the chromatin structure. New proteins are also produced at this stage, called minichromosome maintenance (MCM) complexes. They help with chromosome segregation during anaphase.
Prophase II occurs when spindle fibers form between the chromosomes. These help pull them apart so that each daughter cell will have a separate set of chromosomes after anaphase.
Prometaphase
Chromosomes are visible in prometaphase, the first phase of mitosis. During this phase, the nuclear membrane breaks down and spindle fibers form.
The chromosomes are now free to move to either pole of the cell. This is due to the kinetochores forming on the chromosome arms, which have specialized proteins that attach to the spindle fibers.
Kinetochores form when chromatin folds and interacts with specific proteins. These proteins include Suzhou filaments and crescentin. They help create a structure that holds the chromosome together while allowing it to separate during mitosis.
During prometaphase, chromosomes can be seen as distinct structures inside the cell nucleus. They are easily identified by their length and structure.[/bullet point]
