Centrosomes , CentrioleCilia Questions in English

(B) Basal bodies are short,cylindrical structures composed of microtubules. They are derived from centrioles and are responsible for the formation of cilia and flagella. Located at the base of eukaryotic cilia and flagella,they serve as the anchoring site. Structurally,they represent a continuation of the $9$ outer-axonemal microtubule doublets,with the addition of a $C$ tubule to form a centriole-like triplet. Basal bodies act as a nucleating center for axonemal assembly and are anchored in the cytoplasm by rootlets.

(B) Centrioles and centrosomes are specialized cell structures involved in organizing microtubules during cell division.
They are characteristically found in most animal cells.
They are typically absent in higher plants,fungi,and prokaryotes like bacteria and cyanobacteria.
Therefore,the correct option is $(b)$.

(B) Statement $(i)$ is correct as the axoneme of cilia and flagella exhibits a $9+2$ arrangement of microtubules.
Statement $(ii)$ is correct because cilia are small structures that work like oars,moving the cell or the surrounding fluid.
Statement $(iii)$ is correct as flagella are longer than cilia and are primarily responsible for cell movement.
Statement $(iv)$ is correct because both cilia and flagella are extensions of the cell membrane and are covered by the plasma membrane.
Therefore,all the given statements are correct.

(D) Centrosomes or centrioles are specialized organelles found exclusively in animal cells. They play a crucial role in cell division by organizing microtubules to form the spindle apparatus. Plant cells lack centrosomes,although they can still organize microtubules using other mechanisms.

(C) centrosome is an organelle usually containing two cylindrical structures called centrioles. These centrioles are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other and are made up of nine evenly spaced peripheral fibrils of tubulin protein.

(B) The $Centriole$ is a non-membrane-bound organelle found in animal cells that lacks $DNA$.
Despite the absence of $DNA$,it is capable of self-duplication during the $S$ phase of the cell cycle to form a new pair of centrioles for cell division.

(D) Centrioles exhibit a cartwheel organization,consisting of nine evenly spaced peripheral fibrils made of tubulin protein.
Each of these peripheral fibrils is a triplet,meaning it is composed of three microtubules.
Therefore,the total number of microtubules in the peripheral fibrils is $9 \times 3 = 27$.

(A) In a eukaryotic cell,compartmentalization is achieved by the presence of membrane-bound organelles (e.g.,nucleus,mitochondria,endoplasmic reticulum).
Centrioles are non-membranous organelles composed of microtubules arranged in a $9+0$ pattern.
Since centrioles lack a surrounding membrane,they do not contribute to the formation of cellular compartments.
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the Assertion.

(C) The axoneme of a eukaryotic flagellum consists of microtubules arranged in a $9+2$ array.
This means there are $9$ pairs of peripheral doublets and $2$ centrally located microtubules.
The Assertion is correct because the axoneme does contain microtubules running parallel to the long axis.
The Reason is incorrect because it states there is only 'a pair' of peripheral doublets,whereas there are actually $9$ pairs of peripheral doublets.
Therefore,the correct option is $C$.

(B) Centrosome,or cell centre,is situated close to the nuclear envelope and is also called the microtubule-organizing centre $(MTOC)$.
It plays an important role in animal cell division by producing microtubules or bipolar mitotic spindles.
Since nerve cells lack a centrosome,they are not capable of division.

(B) The $Centrosome$ is a non-membrane-bound organelle found in animal cells.
It consists of two cylindrical structures called $Centrioles$.
During cell division, the centrosome organizes the $Microtubules$ to form the $Spindle$ apparatus, which is essential for the separation of chromosomes.
Therefore, the centrosome plays a crucial role in animal cell division.

(A) Cilia are small,hair-like outgrowths of the cell membrane that function like oars,causing the movement of either the cell or the surrounding fluid. In contrast,flagella are longer and responsible for cell movement. Pili and fimbriae are surface structures of bacteria that do not function like oars.

(B) Cilia and flagella are hair-like outgrowths of the cell membrane.
Their core,called the axoneme,possesses a number of microtubules running parallel to the long axis.
The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules.
This arrangement is referred to as the $9+2$ array.

(B) The basal body of cilia and flagella is derived from the centriole.
It possesses a $9+0$ arrangement of microtubule triplets,which is similar to the structure of a centriole.
This structure acts as an anchor for the axoneme (which has a $9+2$ arrangement) to emerge from the cell surface.

(A) The axoneme of cilia and flagella consists of a $9+2$ arrangement of microtubules.
In this structure,there are $9$ peripheral microtubule doublets and $2$ central microtubule singlets.
Each of the $9$ peripheral doublets is connected to the central sheath by a radial spoke.
Therefore,there are $9$ radial spokes in total.

(B) The given diagram represents the cross-section of a cilium or flagellum,which shows a $9+2$ arrangement of microtubules.
In this structure:
- The outer circle represents the Plasma membrane.
- The $9$ peripheral doublets are connected to the central pair by radial spokes.
- $Q$ points to the radial spoke,which connects the peripheral microtubule doublets to the central sheath.
- $P$ points to the central sheath,which encloses the central pair of microtubules.
Therefore,$Q$ represents the radial spoke.

(C) The centriole is a cylindrical structure composed of $9$ microtubule triplets arranged in a peripheral ring.
This specific arrangement of microtubules in the centriole is often described as a 'cartwheel' structure.
Each triplet is connected to the central hub by radial spokes made of protein.
Therefore,the centriole exhibits a cartwheel-like organization.

(C) centrosome consists of two centrioles arranged at right angles to each other.
Each centriole is composed of $9$ sets of microtubule triplets arranged in a ring.
Since there are $3$ microtubules per triplet and $9$ triplets per centriole,one centriole contains $9 \times 3 = 27$ microtubules.
Therefore,the total number of microtubules in a centrosome (containing two centrioles) is $27 \times 2 = 54$. However,in the context of standard biology questions regarding the structure of a single centriole unit within the centrosome,the value $27$ is the standard answer.

(B) The centriole consists of a central proteinaceous rod-like structure known as the $Hub$.
From this $Hub$,radial spokes made of protein extend outwards and connect to the peripheral microtubule triplets.
Therefore,the central part of the centriole is the $Hub$.

(D) The centrosome is an organelle found in animal cells that performs several critical functions:
$1$. During cell division,it organizes microtubules to form the bipolar spindle,which is essential for chromosome segregation.
$2$. It acts as the basal body for cilia and flagella,providing the structural foundation for their formation and movement.
Since all these functions are performed by the centrosome,the correct option is $D$.

(A) Centrioles are cylindrical structures composed mainly of the protein tubulin. Each centriole is made up of $9$ evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibrils is a triplet. The adjacent triplets are linked to each other. The central part of the proximal region of the centriole is also proteinaceous and is called the hub,from which radial spokes made of protein extend and connect to the triplets. The centriole does not have any central microtubules. Therefore,the arrangement of microtubules in a centriole is referred to as the $9+0$ arrangement.