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23
It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit ... phase to enter an inactive stage called... of cell cycle. Fill in the blanks.
Explanation
It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit $G_1$ phase to enter an inactive stage called quiescent stage ( $G_0$ ) of cell cycle. Muscle cells when reach a level of maturity, no longer divide and just perform their function all through its life. Similar is the case with nerve cells too which are highly specific and lack the ability to divide once they get specialised. They remain in that state until they die.
24

In which phase of meiosis are the following formed? Choose the answers from hint points given below.

(a) Synaptonemal complex ........... .

(b) Recombination nodules ........... .

(c) Appearance/activation of enzyme recombinase ............ .

(d) Termination of chiasmata ......... .

(e) Interkinesis ............ .

(f) Formation of dyad of cells .......... .

Hints (a) Zygotene,

(b) Pachytene,

(c) Pachytene,

(d) Diakinesis,

(e) After Telophase-I/before prophase of meosis-II,

(f) Telophase-I/after meiosis-I.

Explanation

(a) Synaptonemal complex zygotene.

(b) Recombination nodules pachytene.

(c) Appearance/activation of enzyme recombinase telophase-l/after, meiosis-l ... .

(d) Termination of chiasmata diakinesis.

(e) Interkinesis after telophase-I/before prophase of meiosis-II.

(f) Formation of dyad of cells pachytene.

25
State the role of centrioles other than spindle formation.
Explanation
The two centrioles in the centrosome lie perpendicular to each other. Each has organisation like a cart wheel. These form the basal body of cilia and flagella of plant/animal cells besides forming spindle fibre in animal cell division. It also helps in the formation of microtubuless and sperm tail.
26
Mitochondria and plastids have their own DNA (genetic material). What is known about their fate during nuclear division like mitosis?
Explanation
The DNA present in mitochondria and chloroplast is extrachromosomal DNA. It has nothing to do with the nuclear divisions. In mitosis, only nuclear DNA takes part.
27

Label the diagram and also determine the stage at which this structure is visible.

Explanation

The diagram shows the transition stage between prophase and metaphase stage of mitotic cell division.

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