Question

Given below is a diagram depicting the cell cycle. Study the same and then answer the questions that follow:


(i) What is meant by DNA replication which occurs during the interphase stage?

(ii) Mention any three identifying features of the prophase stage.

(iii) Draw a neat labelled diagram of the metaphase stage of an animal cell having six chromosomes.

(iv) Mention two events occurring during the Anaphase stage.

Answer 1

(i) DNA replication occurs during the S phase of interphase. S phase is preceded by the G 1 phase. During the G 1 phase, the building blocks for DNA replication, its associated proteins, and energy are produced in the cell. After passing the G checkpoint, the cell enters into the S phase.

(ii) In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite poles of the cell. Prophase (versus interphase) is the first true step of the mitotic process. During prophase, a number of important changes occur: (any Three)

• Chromatin fibers become coiled into chromosomes, with each chromosome having two chromatids joined at a centromere.
• The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm.
• The two pairs of centrioles (formed from the replication of one pair in Interphase) move away from one another toward opposite ends of the cell due to the lengthening of the ​microtubules that form between them.
• Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell's equator.
• Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers.
• The kinetochore fibers "interact" with the spindle polar fibers connecting the kinetochores to the polar fibers.
• The chromosomes begin to migrate toward the cell center.

(iii)


(iv) In anaphase, the paired chromosomes (sister chromatids) separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell. At the end of anaphase, each pole contains a complete compilation of chromosomes. During anaphase, the following key changes occur: (any Two)

• The paired centromeres in each distinct chromosome begin to move apart.​
• Once the paired sister chromatids separate from one another, each is considered a "full" chromosome. They are referred to as daughter chromosomes.​
• Through the spindle apparatus, the daughter chromosomes move to the poles at opposite ends of the cell.​
• The daughter chromosomes migrate centromere first and the kinetochore fibers become shorter as the chromosomes near a pole.​
• In preparation for telophase, the two cell poles also move further apart during the course of anaphase. At the end of anaphase, each pole contains a complete compilation of chromosomes.