UNIT 3 – Microtubules (Q&A) | MZO-001 MSCZOO | IGNOU
SAQ
Fill in the blanks:
a) The basic unit of microtubules is …………… .
Answer: α β tubulin heterodimers
b) α tubulin occurs at ............. end, and β tubulin is at .............. of microtubules.
Answer: plus, minus
c) GTP-GTP tubulin cap stabilises and promotes the .................. .
Answer: polymerisation of microtubules
d) The plus end-directed motor protein of microtubules is …………. .
Answer: kinesin
e) The hydrolysis of GTP from β-tubulin causes …………. of microtubules.
Answer: Depolymerisation
f) The cellular function of γ-tubulins is. ………..... .
Answer: nucleate the growth of MTs
g) Colchicine binds to ………… .
Answer: free tubulin
SAQ 2
a) Classify the microtubules involved in mitosis.
Microtubules play a central role during mitosis by forming the mitotic spindle, which ensures the correct segregation of chromosomes into daughter cells. These microtubules are dynamically reorganized during cell division and can be classified into three main types, based on their structure, origin and function during mitosis. Each type performs a distinct role in chromosome alignment, movement and spindle organization. There are three types of microtubules involved in mitosis, and their names and functions are as follows:
1. Kinetochore Microtubules:
These microtubules are responsible for attaching to the kinetochores, which are protein complexes assembled on the centromeres of chromosomes. Kinetochore microtubules extend from the spindle poles and anchor directly to the kinetochore of each chromatid. Their primary function is to pull the sister chromatids apart during anaphase by shortening, thereby moving each chromatid toward the opposite pole. This ensures equal chromosome distribution in both daughter cells. Their polymerization and depolymerization at the plus end (kinetochore end) drive chromosome movement.
2. Polar (Interpolar) Microtubules:
Polar microtubules extend from one spindle pole and overlap with the polar microtubules from the opposite pole at the spindle equator. Unlike kinetochore microtubules, these do not attach to chromosomes. Instead, they interdigitate with each other and help in maintaining the structure and length of the mitotic spindle. During anaphase, motor proteins like kinesins push these overlapping polar microtubules apart, which helps elongate the cell and further separates the poles, aiding in the physical separation of the two daughter cells.
3. Astral Microtubules:
Astral microtubules radiate out from the centrosomes (spindle poles) toward the cell cortex but do not contact the chromosomes or the central spindle region. Their main function is to anchor the spindle apparatus to the cell membrane and to assist in proper spindle orientation and positioning. They interact with cortical proteins at the cell membrane and are involved in determining the plane of cell division. Dynein motor proteins attached to the cell cortex pull on the astral microtubules, helping to position the spindle apparatus symmetrically.
b) Differentiate between centrosomes and centrioles.
Before discussing the differences between centrosomes and centrioles, it is important to understand what each of these structures are and their roles in cellular functions. Both centrosomes and centrioles are key components involved in organizing microtubules within a cell and they are essential for proper cell division. Centrosomes serve as the primary microtubule organizing centers (MTOCs) of animal cells, while centrioles are specialized cylindrical structures located within centrosomes that play a critical role in organizing the spindle apparatus during cell division. These two organelles work together to facilitate the alignment and separation of chromosomes during mitosis.
Difference between centrosomes and centrioles based on different categories:
1. Based on Definition and Identity
Centrosome:
The centrosome is the main microtubule-organizing center (MTOC) found in animal cells. It is composed of two centrioles surrounded by a dense protein matrix called the pericentriolar material (PCM). The centrosome is crucial for organizing microtubules that maintain cell shape, enable intracellular transport and are essential for mitotic spindle formation during cell division.
Centriole:
A centriole is a cylindrical structure composed of microtubules arranged in a 9+0 pattern, where nine triplets of microtubules are organized in a ring. Each cell typically has two centrioles that lie perpendicular to each other. Centrioles are part of the centrosome, but they are structurally distinct and do not have the full functionality of the centrosome.
2. Based on Structure
Centrosome:
The centrosome consists of two centrioles placed at right angles to each other, embedded in the pericentriolar material (PCM). The PCM is rich in proteins that help in microtubule nucleation and anchoring. This complex structure enables the centrosome to organize and stabilize the microtubules within the cytoplasm, guiding the cell's overall architecture.
Centriole:
Each centriole is a cylindrical organelle composed of nine sets of triplet microtubules, forming a hollow structure. The microtubules within a centriole are arranged in a very specific, orderly pattern that is crucial for its function in organizing the centrosome and aiding in cell division. However, centrioles themselves do not nucleate microtubules; they serve primarily as a structural scaffold.
3. Based on Function
Centrosome:
Centrosomes are the primary sites of microtubule nucleation and organization in animal cells. They play a critical role in establishing cell polarity, enabling intracellular transport and forming the mitotic spindle during mitosis. During the cell cycle, centrosomes duplicate and migrate to opposite poles, providing the structural framework for chromosome segregation. Centrosomes are also essential for organizing the mitotic spindle during meiosis and mitosis, ensuring that the chromosomes are accurately distributed to the daughter cells.
Centriole:
Centrioles themselves do not directly organize the microtubules involved in mitosis, but they are vital for centrosome duplication and function. They help in the structural organization of the centrosomes and in their absence, the cell struggles to form a normal spindle apparatus during cell division. Additionally, centrioles give rise to basal bodies, which are necessary for the formation of cilia and flagella in ciliated cells.
4. Based on Role in Mitosis
Centrosome:
During mitosis, the centrosome plays a central role in forming the spindle apparatus. It duplicates at the beginning of the cell cycle and the two centrosomes move to opposite poles of the cell during prophase. Microtubules emanate from each centrosome, forming the mitotic spindle, which is responsible for separating chromosomes during anaphase. Without a functional centrosome, proper chromosome segregation and mitosis cannot occur.
Centriole:
Centrioles are essential for organizing the centrosome but do not actively participate in microtubule nucleation. They ensure that the centrosome functions correctly by acting as structural templates. During cell division, they aid in the formation of spindle poles by ensuring that microtubules emanate from the correct location. Without centrioles, cells may still form a spindle, but it is often abnormal and less efficient.
5. Based on Presence in Organisms
Centrosome:
Centrosomes are primarily found in animal cells and some lower eukaryotes like fungi and algae. They are absent in most plant cells and in higher plants, which instead rely on other mechanisms for microtubule organization during mitosis, such as the spindle pole body.
Centriole:
Centrioles are present in almost all animal cells and in some lower plants, but they are notably absent in most plant cells, fungi, and some protists. In plants, microtubule organization is carried out through other mechanisms, such as the microtubule organizing centers (MTOCs) without centrioles.
6. Based on Presence in Cell Cycle
Centrosome:
Centrosomes are visible during all stages of the cell cycle. They duplicate during the S-phase and become active in organizing the mitotic spindle during mitosis. Their number is maintained throughout the cycle and is integral to proper cell division. Centrosomes also help maintain cell integrity during interphase by controlling the arrangement of microtubules.
Centriole:
Centrioles are present in cells throughout the cell cycle and they replicate in the S-phase. Each centriole duplicates and after division, they contribute to the formation of two new centrosomes in the next cycle. Their role becomes critical during the G1 to G2 transition when centrosomes prepare to organize the spindle apparatus for mitosis.
SAQ 3
a) Differentiate between cilia and flagella.
Cilia and flagella are hair-like appendages that extend from the surface of many eukaryotic cells. They are primarily responsible for generating movement either by propelling the cell itself or by moving substances over the cell surface. Structurally, both are made up of microtubules arranged in the 9+2 pattern and are connected to the cell by a basal body. Although both structures arise from the same basic components and show internal similarity, but they differ significantly in their length, number per cell, type of movement, function, and location in organisms. These distinctions make it important to compare cilia and flagella under specific headings to understand their unique biological roles.
Difference between cilia and flagella based on different categories:
1. Based on Definition and Identity
Cilia:
Cilia are small, hair-like projections that cover the surface of some cells. They are short in length and are typically found in large numbers. They help move fluids or particles across the surface of the cell.
Flagella:
Flagella are longer, whip-like structures found on some cells. Usually, there are only one or two flagella per cell. Flagella help the cell move through its environment by propelling it forward.
2. Based on Structure
Cilia:
Cilia are shorter, generally ranging from 5 to 10 micrometers long. They cover the surface of cells in large numbers and are made of microtubules arranged in a 9+2 pattern in most eukaryotic cells.
Flagella:
Flagella are longer, ranging from 10 to 200 micrometers in length. Unlike cilia, flagella are found in smaller numbers, typically one or two per cell. They also have a 9+2 microtubule arrangement in eukaryotic cells.
3. Based on Movement Pattern
Cilia:
Cilia move in a coordinated back-and-forth motion. This movement is like a rowing action, helping to move fluid or particles across the cell's surface. The movement of cilia is usually very rhythmic and can be observed in organisms like Paramecium.
Flagella:
Flagella move in a more fluid, undulating manner. They usually move in a whip-like motion or a spiral movement, which helps propel the entire cell forward. This type of movement can be seen in sperm cells and certain types of protozoans.
4. Based on Function
Cilia:
Cilia have a variety of roles. In humans, they help move mucus and trapped particles out of the respiratory system. In other organisms, they help with the movement of the whole cell or help circulate fluid across the cell's surface.
Flagella:
Flagella are mainly responsible for cell movement. In many unicellular organisms like Euglena, flagella help the organism swim in water. In animals, sperm cells use flagella to swim towards the egg.
5. Based on Number per Cell
Cilia:
Cilia are usually present in large numbers on the surface of a cell. A single cell can have hundreds of cilia.
Flagella:
Flagella are usually present in smaller numbers. Most cells have just one or two flagella.
6. Based on Occurrence in Organisms
Cilia:
Cilia are found in many eukaryotic cells, including cells in the lungs, fallopian tubes and certain protists. They are also present in some microorganisms to aid in movement.
Flagella:
Flagella are found in certain eukaryotic cells, like sperm cells and some algae. Flagella are also common in prokaryotic organisms, like bacteria, where they help in movement.
7. Based on Occurrence in Prokaryotes and Eukaryotes
Cilia:
Cilia are found only in eukaryotic cells. They are completely absent in prokaryotic organisms. In eukaryotes, they are especially present in certain protozoans, epithelial cells of the respiratory tract, and fallopian tubes of vertebrates.
Flagella:
Flagella occur in both prokaryotic and eukaryotic cells. In eukaryotes, they have the typical 9+2 microtubule structure, while in prokaryotes (especially bacteria), they are composed of flagellin protein and operate through a rotary mechanism rather than the bending motion seen in eukaryotes.
TERMINAL QUESTIONS
1. Define the microtubules.
Microtubules are hollow, cylindrical structures present in the cytoplasm of all eukaryotic cells. They are one of the three main components of the cytoskeleton, along with microfilaments and intermediate filaments. Microtubules are dynamic, meaning they can continuously grow and shrink as per the cell's requirement. They are mainly composed of tubulin proteins and play several essential roles in cellular structure, movement and division. Their constant assembly and disassembly help the cell to change shape, move materials and divide properly.
Origin, Composition and Structure of Microtubules:
Microtubules always arise from a specific region inside the cell known as the Microtubule Organizing Center (MTOC). In animal cells, this center is called the centrosome, which is found close to the nucleus. From this central region, microtubules start growing and extend outward into the cytoplasm. In dividing cells, another temporary organizing structure called the spindle pole also helps in forming microtubules that are used during mitosis. In plant cells, although centrosomes are not present, microtubules still originate from certain regions near the nucleus and around the nuclear envelope. This origin point acts as an anchoring site where the minus ends of microtubules are fixed and the plus ends grow outward.
Now talking about the composition, microtubules are made up of tubulin proteins, which are small, globular proteins. These proteins come in two forms: alpha-tubulin and beta-tubulin. One alpha and one beta tubulin molecule join to form a stable pair called a heterodimer. These heterodimers are the building blocks of microtubules. They keep joining one after another to form a long chain called a protofilament.
Structurally, a single microtubule is a hollow, tube-like cylinder. It is built from 13 parallel protofilaments arranged in a circle, forming the wall of the hollow tube. The overall diameter of a microtubule is about 25 nanometers. This hollow structure gives both mechanical support and flexibility to the cell. Each protofilament has a specific direction or polarity, with a fast-growing plus (+) end and a slow-growing or disassembling minus (–) end. This polarity is very important for the movement of motor proteins and cellular transport.
Another special feature of microtubules is their dynamic nature, which means they can quickly grow or shrink depending on the needs of the cell. This behavior is called dynamic instability, and it allows the cell to reorganize its internal structure during processes like cell division, growth and repair.
Functions of Microtubules
Microtubules are involved in many crucial cellular functions. These are described under different categories:
1. Maintenance of Cell Shape:
- Microtubules form a supportive framework that helps the cell maintain its three-dimensional shape and resist compression forces.
2. Intracellular Transport:
- Microtubules act like highways for transporting substances within the cell. Motor proteins like kinesin and dynein move along them, carrying vesicles, organelles, and other materials to different parts of the cell.
3. Chromosome Movement in Cell Division:
- During mitosis and meiosis, microtubules form the mitotic spindle. This structure pulls chromosomes apart and ensures proper segregation into daughter cells.
4. Formation of Cilia and Flagella:
- Microtubules form the internal structure of cilia and flagella, which are hair-like structures that help in movement (e.g., sperm cells) or in moving fluids over the cell surface (e.g., in the respiratory tract).
5. Positioning of Organelles:
- Microtubules help in holding organelles like the Golgi apparatus and endoplasmic reticulum in their correct positions within the cytoplasm.
6. Signal Transmission and Structural Reorganization:
- Their dynamic nature allows them to rapidly change in response to external signals. This flexibility helps the cell to reorganize itself when needed during development or repair.
2. Explain the role of microtubules in cell division.
Microtubules are essential components of the cytoskeleton in eukaryotic cells. They are long, hollow tubes made of tubulin protein subunits, and they play crucial roles in various cellular functions, including providing structural support, enabling intracellular transport and maintaining cell shape. In cell division, microtubules are particularly important, as they form the mitotic spindle, which ensures accurate chromosome segregation during mitosis and meiosis. The role of microtubules in cell division is integral for the successful division of the cell into two daughter cells.
Role of Microtubules in Cell Division
Microtubules have a critical role in the process of cell division, particularly during mitosis and meiosis. Their involvement ensures the proper alignment, separation and movement of chromosomes, making them indispensable for accurate cell division. Below are the primary roles of microtubules in cell division:
1. Formation of the Mitotic Spindle
- During cell division, microtubules form the mitotic spindle, a structure that facilitates the movement and segregation of chromosomes into two daughter cells. The spindle is composed of both kinetochore microtubules, which attach to chromosomes and non-kinetochore microtubules, which help in maintaining the structural integrity of the spindle.
2. Chromosome Alignment
- Microtubules assist in aligning chromosomes at the cell's equator during metaphase. They attach to the kinetochores, protein structures on the centromere of chromosomes, aligning them in preparation for anaphase.
3. Chromosome Separation
- During anaphase, the kinetochore microtubules shorten, pulling the sister chromatids apart towards opposite poles of the cell. This ensures that each daughter cell receives an identical set of chromosomes.
4. Cytokinesis
- Microtubules also play a role in cytokinesis, the final stage of cell division, by contributing to the formation of the contractile ring. This ring, which is made of actin filaments, constricts the cell membrane to divide the cytoplasm and separate the two daughter cells.
5. Spindle Checkpoint Regulation
- Microtubules are involved in regulating the spindle checkpoint, a mechanism that ensures chromosomes are correctly attached to the spindle before the cell proceeds from metaphase to anaphase. If any chromosomes are not properly aligned, the checkpoint halts progression, allowing time for corrections.
6. Polarization of the Cell
- During mitosis, microtubules contribute to the polarization of the cell, ensuring that the two daughter cells have distinct poles, which are necessary for their future functionality and orientation.
3. What is motor protein? How do they help in cellular transport? Discuss.
Motor proteins are specialized molecules within cells that convert chemical energy stored in ATP (adenosine triphosphate) into mechanical work, enabling the movement of various cellular components along cytoskeletal tracks. These proteins are essential for numerous cellular processes, including intracellular transport, muscle contraction, cell division and the maintenance of cell structure. In the context of cellular transport, motor proteins help move cargo (such as vesicles, organelles, and proteins) to specific destinations within the cell, ensuring proper cellular function.
Types of Motor Proteins
Motor proteins can be categorized into three main types based on their structure and the type of filament they move along:
1. Kinesins:
- These motor proteins primarily move along microtubules in the anterograde direction, meaning they transport cargo from the center of the cell towards the periphery.
2. Dyneins:
- Dyneins travel in the retrograde direction, from the cell periphery towards the center. They are responsible for transporting materials back to the cell's central regions.
3. Myosins:
- These motor proteins move along actin filaments and are especially important in processes such as muscle contraction, cytokinesis and the transport of vesicles within cells.
How Motor Proteins Help in Cellular Transport
Motor proteins facilitate cellular transport by moving various cellular cargo along the cytoskeletal network, enabling the efficient functioning of the cell. Their movement is driven by the hydrolysis of ATP, which releases energy and drives the mechanical work required for transporting materials. Here is a breakdown of how motor proteins help in cellular transport:
1. ATP Hydrolysis and Movement:
Motor proteins use the energy released from ATP hydrolysis to generate movement. ATP is broken down into ADP and inorganic phosphate (Pi), and this energy is used to drive conformational changes in the motor protein, allowing it to "walk" along the cytoskeletal tracks (microtubules or actin filaments). These conformational changes allow motor proteins to attach, move and detach from the track, facilitating cargo movement.
2. Intracellular Vesicle Transport:
One of the primary roles of motor proteins is to transport vesicles (membrane-bound sacs that carry proteins, lipids, or other molecules) between different cellular compartments. Kinesins move vesicles from the cell center towards the outer parts (anterograde transport), while dyneins move them from the cell periphery back towards the center (retrograde transport). This bidirectional transport is essential for processes like protein sorting and organelle positioning.
3. Organelle Positioning:
Motor proteins are also critical for positioning organelles, such as mitochondria, within the cell. For example, mitochondria are moved to areas of the cell with high energy demands, ensuring proper energy distribution within the cell. This helps maintain cellular function, particularly in cells with high metabolic activity.
4. Cell Division:
During mitosis, motor proteins are essential for the proper alignment and separation of chromosomes. Kinesins and dyneins are involved in organizing the mitotic spindle, ensuring that chromosomes are correctly distributed between the two daughter cells. This is crucial for maintaining genetic stability during cell division.
5. Endocytosis and Exocytosis:
Motor proteins also help in endocytosis (the process of taking materials into the cell) and exocytosis (the process of releasing materials from the cell). Dyneins assist in transporting endocytic vesicles back to the cell’s center, while kinesins help in moving vesicles towards the plasma membrane during exocytosis. These processes are vital for nutrient uptake, waste removal, and communication with the external environment.
4. Explain why MTs have polar structures.
Microtubules have polar structures because of the specific arrangement and asymmetric nature of their building blocks called tubulin heterodimers. Each heterodimer is made of two different protein subunits: α-tubulin (alpha tubulin) and β-tubulin (beta tubulin). These two are chemically and structurally different, so they cannot switch positions. They always stay in the same fixed orientation, with α-tubulin at one end and β-tubulin at the other.
When these α/β-tubulin dimers come together to form microtubules, they arrange in a head-to-tail manner in one direction only. The β-tubulin of one dimer attaches to the α-tubulin of the next dimer and this pattern repeats again and again. Because of this straight, uniform and unidirectional arrangement, the microtubule has a distinct structure with two different ends.
These two ends are called the plus end and the minus end, and they behave differently:
- Plus end (β-tubulin is exposed):
- This end is more active and dynamic. Tubulin dimers can be added or removed very fast at this end. So, it grows and shrinks quickly. It is the main site for microtubule elongation.
- Minus end (α-tubulin is exposed):
- This end is less dynamic and mostly stays stable. It usually remains anchored to a special site inside the cell called the microtubule organizing center (MTOC), like the centrosome. Very little growth or shrinkage happens at this end.
Because the two ends behave differently and have different exposed subunits (β at the plus end and α at the minus end), microtubules are said to be polar. This polarity is not just structural but also functional. It helps motor proteins like kinesin and dynein move in specific directions. It also helps in proper organization of the cytoskeleton, cell division and transport of materials inside the cell.
So, the reason microtubules have polar structure is because of the fixed head-to-tail arrangement of asymmetric α/β-tubulin dimers during polymerization, which makes the two ends different in both structure and function.
5. Describe the assembly and disassembly process of microtubules.
Microtubules are essential components of the cytoskeleton in eukaryotic cells. They are involved in critical processes like maintaining cell shape, enabling intracellular transport and supporting cell division. The assembly and disassembly of microtubules are dynamic processes regulated by the polymerization and depolymerization of tubulin dimers, which are the building blocks of microtubules.
These processes can be broken down into specific steps that are crucial for the cell's functions. The assembly process involves the formation of new microtubules from tubulin dimers, while the disassembly process involves the breakdown of existing microtubules.
Steps in Microtubule Assembly
Microtubule assembly is the process through which tubulin dimers polymerize to form long, stable microtubules. This process involves four key steps:
1. Nucleation at the Microtubule-Organizing Center (MTOC)
The process begins at the microtubule-organizing center (MTOC), where small clusters of tubulin dimers (α-tubulin and β-tubulin) form. These clusters serve as the initial seed or nucleus for the microtubule. The MTOC acts as a base for the microtubule's growth and this step is essential because it provides the starting point for polymerization.
2. Polymerization of Tubulin Dimers
Once nucleation occurs, the tubulin dimers begin to add to the growing microtubule. The dimers add predominantly to the plus end, which is the more dynamic and growing end. As tubulin dimers (bound to GTP) add to the microtubule, the structure elongates.
3. GTP Hydrolysis and Microtubule Stabilization
As new tubulin dimers are added to the microtubule, GTP bound to the β-tubulin is hydrolyzed to GDP. This process happens quickly, and the GTP-bound tubulin dimers remain at the plus end, which keeps the microtubule stable. However, once GTP is hydrolyzed to GDP, the microtubule becomes less stable, making the microtubule more prone to disassembly if the GTP cap is lost.
4. Elongation of the Microtubule
The microtubule continues to grow as additional tubulin dimers are added to the plus end. This process allows the microtubule to increase in length. The minus end of the microtubule (which is anchored at the MTOC) remains more stable, providing a fixed base for the structure.
Steps in Microtubule Disassembly
The disassembly of microtubules allows the cell to regulate the length and stability of its cytoskeleton. This process is just as dynamic and essential as the assembly process and can be broken down into three key steps:
1. Loss of the GTP Cap at the Plus End
Disassembly begins when the GTP cap (the GTP-bound tubulin dimers) at the plus end of the microtubule is lost. The GTP cap stabilizes the microtubule, and its loss causes the microtubule to become unstable. This instability leads to the shrinking of the microtubule from the plus end.
2. Depolymerization and Catastrophe
Once the GTP cap is lost, depolymerization occurs, a rapid process where tubulin dimers dissociate from the microtubule. This phase, known as catastrophe, causes the microtubule to shrink rapidly. This process is often abrupt and can result in the complete breakdown of the microtubule if it is not rescued.
3. Rescue and Re-Polymerization
In some cases, before the microtubule completely disassembles, new GTP-bound tubulin dimers may reattach to the plus end. This stabilizes the microtubule and prevents its complete disassembly. This phase is called rescue, where the microtubule recovers from catastrophe and resumes polymerization.
6. Draw the labelled diagram of microtubules.
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