Problem 138 - Objective Biology for NEET

Question

Assertion: Kinetochore is essential for cell division. Reason: Kinetochore serves as the sites of attachment of spindle fibres to the chromosome.

Step-by-Step Solution

Answer

The assertion and reason are correct. The kinetochore is essential for cell division because it serves as the site of attachment of spindle fibers to the chromosome, enabling the process of cell division.

Step 1: Understand the Assertion

The assertion is saying that the kinetochore is essential for cell division. This means that without the kinetochore, cell division could not occur. To verify this, an understanding of cell division and the role of the kinetochore is required.

Step 2: Analyze the Reason

The reason given is that the kinetochore serves as the sites of attachment of spindle fibers to the chromosome. During cell division, spindle fibres are crucial for the separation of chromosomes. If the kinetochores provide a place for these spindle fibers to attach on to the chromosomes, then it can be inferred that they play a crucial role in cell division.

Step 3: Correlate Assertion and Reason

Comparing the assertion and the reason given, it is found that they are aligned. The reason effectively substantiates the assertion by elucidating the manner in which the kinetochore is instrumental for cell division – by serving as attachment sites for spindle fibres.

Key Concepts

Cell Division

Cell division is a fundamental process in all living organisms that enables growth, repair, and reproduction. During this process, a single cell divides into two or more daughter cells. Cell division occurs in two main stages namely mitosis, where the nucleus divides, and cytokinesis, where the cytoplasm divides. This orchestrated procedure ensures that each daughter cell receives an identical set of chromosomes, which carry the organism’s hereditary information.

The primary goal of cell division is to precisely duplicate a cell's genome and equally distribute it between the daughter cells. This fidelity in genome duplication is crucial for maintaining the integrity and function of the resulting cells, ensuring that they can continue to perform necessary biological processes effectively.

Spindle Fibres

Spindle fibres are essential components of cell division, known for their role in the movement and separation of chromosomes during mitosis and meiosis. These fibrous structures are formed from microtubules, which are protein filaments that emerge from the centrosomes within the cell.

During metaphase of cell division, spindle fibres extend from each pole of the cell to attach to the kinetochores located on the chromosomes. This attachment is critical for aligning the chromosomes along the metaphase plate, preparing them for segregation. In anaphase, the spindle fibres shorten, pulling the sister chromatids apart and ensuring that each new cell receives an accurate set of chromosomes.

Chromosome Attachment

Chromosome attachment to spindle fibres is a vital step in ensuring accurate chromosome segregation during cell division. This connection occurs mainly through a structure called the kinetochore. The kinetochore is a protein complex that assembles on the centromere of each chromosome.

The kinetochore not only facilitates the attachment of chromosomes to the spindle fibres but also generates mechanical forces needed to move chromosomes during division. The precise attachment of spindle fibres to kinetochores helps in equal distribution of genetic material, ensuring the accurate formation of daughter cells. Improper attachment can lead to aneuploidy, where cells have an abnormal number of chromosomes, which can contribute to diseases like cancer.

Cell Cycle Regulation

Cell cycle regulation encompasses a series of checks and balances that monitor and coordinate the different phases of the cell cycle, ensuring orderly progression and division of cells. The cell cycle is divided into four main phases: G1 (growth), S (synthesis), G2 (preparation for mitosis), and M (mitosis).

Complex networks of signals and checkpoints monitor the cell’s internal and external environments, confirming that each phase has been properly completed before progressing to the next. Key proteins called cyclins and cyclin-dependent kinases (CDKs) drive the cell cycle forward by activating or deactivating target proteins.

• G1 checkpoint ensures the cell is ready for DNA synthesis.
• The G2 checkpoint confirms all DNA has been replicated and intact.
• M checkpoint makes sure all chromosomes are correctly attached to spindle fibres before segregation.

These checkpoints prevent errors such as DNA damage or incomplete chromosome segregation, contributing to the maintenance of genomic stability and prevention of diseases.