Matric Notes Biology Class 9th Ch 5 Cell Cycle Extra Short & Long Questions

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Short Question

Q.1) Define cell cycle and in how many phases it is divided.

Answer: Cell cycle:

 “Cell cycle is the series of events that take place in a cell leading to its division and duplication of its DNA (DNA replication) to produce two daughter cells.”

Phases of the cell cycle:

  The cell cycle consists of two main phases:

• Interphase or resting stage
• Division phase (Mitosis or Meiosis)

Q.2) In which type of cells, meiosis takes place and why it is important?

Answer: Occurrence of Meiosis:

  Meiosis is also called ‘reduction division’, takes place in germ cells (egg and sperm) during sexual reproduction.

Importance of Meiosis:

1-Formation of Gametes for sexual reproduction:

  Meiosis is essential for sexual reproduction and it ensures the correct number of chromosomes in offsprings.

2- Genetic Variations during sexual reproduction:

Meiosis helps to create genetic variations among the offspring. During meiosis, the chromosomes of each parent undergo genetic recombination. It produces gametes with different genetic combinations. Gametes fuse to form a zygote with a different genetic combination. Thus meiosis allows a species to bring variations to handle the changes in the environment.

Q.3) How does normal mitosis ensure normal life?

Answer:  Normal mitosis ensures normal life. The process of mitosis ensures the equal distribution of genetic material/ information between the newly formed daughter cells, so that life process goes on normally. Equal distribution of the genetic material is necessary for each cell to perform its normal function. Any abnormality in the process of mitosis can be harmful to the cell and eventually life.

Example: 

 Unequal distribution of the chromosome can lead to the chromosomal abnormalities which can cause diseases like Down’s syndrome.

Q.4) Give at least four differences between meiosis and mitosis.

Answer:

Factors	Mitosis	Meiosis
Prophase	In this phase, homologous chromosomes do not form pairs. There is no crossing over.	Homologous chromosomes pair up. Crossing over takes place between these homologous chromosomes.
Metaphase	Single chromosomes align to form metaphase plate.	Homologous pairs align to form metaphase plate.
Anaphase	Chromosomes break and individual chromatids are pulled towards poles.	Individual chromosomes are pulled towards poles.
Daughter cells	Daughter nuclei contain a diploid number of chromosomes. Each chromosome has a single chromatid.	Daughter nuclei contain a haploid number of chromosomes. Each chromosome has two chromatids.
Outcome	Chromosome number remains the same (2n), and yields two diploid cells.	Chromosome number reduced to half (n), and yield four haploid cells.
Occurrence	Occurs in somatic cells.	Occurs in germ cells i.e. egg and sperms.

Q.5) What is chiasma and what is its role in crossing over?

Answer: Chiasma and its role in crossing over:

  During Prophase-I of meiosis, the two non-sister chromatids of homologous chromosomes become zipped together, forming complexes known as chiasmata (singular chiasma). At chiasma, the non-sister chromatids of homologous chromosomes exchange their parts. This process is known as crossing over. So, chiasma helps in crossing over and crossing over is essential in producing genetic varieties in germ cells (egg and sperm).

Long Question

Q.1) Describe various phases of interphase of the cell cycle with diagrams.

Answer: Interphase of the cell cycle:

“Interphase is the period between the end of one cell division and the start of next division”

Events of interphase:

   Interphase may be divided into three sub-phases. The following events occur during interphase:

i. G1-phase

ii. S-phase

iii. G2-phase

i. G1-phase:

    G1-phase (G stands for gap) is the first and the longest sub-phase of the interphase. In this phase, newly produced cells grow in size and undergo internal chemical changes, which prepare the daughter cells for DNA replication. During G1 phase RNA, ribosomes and several enzymes are synthesized.

ii. S-phase (synthesis phase):

  In this phase, the cell duplicates its chromosomes. For this purpose, the DNA molecule of each chromosome is copied (replication of DNA) and new protein molecules are attached. Each Chromosome consists of two sister chromatids. Cell growth continues throughout S phase. At the same time, synthesis of proteins and enzymes take place which is involved in DNA synthesis.

iii. G2-phase: 

      G2-phaseis the period between the completion of DNA synthesis and the beginning of active cell division phase. During G2-phase, cell prepares proteins e.g. microtubule framework (spindle fibers) required for cell division.

  A final event in interphase occurs with the replication of the centrioles and their movement to either end of the nucleus. At the same time, microtubules appear radiating from the area of centriole at each end.

Q.2) Discuss different events of meiosis-I with the help of diagrams.

Answer: Meiosis-I:

Different events of Meiosis-I are:

i) Prophase-I

ii) Metaphase-I

iii) Anaphase-I

iv) Telophase-I

i) Prophase-I:

  Prophase-I usually accounts for 90% of the total time spent in meiosis. In this phase, the chromatin material condenses and chromosomes become visible.

Tetrad:

  However, unlike prophase of mitosis, the two homologous chromosomes (chromosome similar in shape and size) pair up or synapse to form tetrads (because a pair of the chromosome has four chromatids).

Chiasmata:

  The two non-sister chromatids of homologous chromosomes become zipped together, forming complexes known as chiasmata (singular chiasma).

Crossing over:

   At chiasma, the non-sister chromatids of homologous chromosomes exchange their parts. This process is known as crossing over.

  As in the prophase of mitosis, here also the nucleoli disappear and nuclear envelope breaks. Centrioles migrate to opposite poles and make spindle fibers

ii) Metaphase-I:

  Homologous chromosomes arrange themselves during the metaphase plate. Spindle fibers from one pole attach to one chromosome of each homologous pair while spindle fibers from the opposite pole attach to the other chromosome of the homologous pair.

iii) Anaphase-I:

  During anaphase-I, spindle fibers pull towards the poles and separate the homologous chromosomes. Each member of the pair is pulled toward opposite poles of the cell. During this phase, the sister chromatids remain attached. In this way, one haploid set of chromosomes is formed at each pole. Each chromosome in both haploid sets still contains two sister chromatids.

  Anaphase-I is a key event in creating new genetic combinations in sexually reproducing organisms. New combinations of genetic material created by any process including crossing-over, is termed as genetic recombination.

iv) Telophase-I:

  After the chromosomes reach their respective poles, the nuclear membrane reappears along with the appearance of nucleoli. Later cytokinesis occurs and two haploid daughter cells are produced.

Q.3) Explain different stage of mitosis with diagrams and at what stage, cytokinesis takes place?

Answer: Mitosis:

“Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent cells.”

Different stages of mitosis:

   The process of mitosis can be divided into two main stages:

I. Karyokinesis

II. Cytokinesis

I. Karyokinesis:

   “The division of nucleus is called karyokinesis.”

Stages of karyokinesis:

   It is further divided into four phases:

1- Prophase

2- Metaphase

3-Anaphase

4- Telophase.

1- Prophase:

   Chromatin threads condense, coil and shorten to become chromosomes. Each chromosome consists of two sister chromatids attached at the centromere. The chromosomes are now visible under a microscope.

• In an animal cell, the two pairs of centrioles reach opposite poles of the cell. They make a network of spindle fibers between the two poles. The complete set of spindle fibers is called the mitotic apparatus.
• ln plant cells, there are no centrioles. Their mitotic spindle is formed by the aggregation of spindle fibers present in the cytoplasm.

   As a part of the changes of prophase, the nucleolus disappears and the nuclear envelope breaks down.

2- Metaphase:

     In metaphase, some spindle fibers attach with chromosomes. Two spindle fibers from both sides attach to one chromosome. The chromosomes attached with spindle fibers arrange themselves at the equator of the cell. In this way, a plate is formed called the metaphase plate.

3- Anaphase:

   The centromere of each chromosome splits. The spindle fibers pull the chromatids apart to opposite poles. Once the chromatids are separated, they are called daughter chromosomes.

  In early anaphase, the centromere split. Half the chromosomes move to one pole and half to the other pole. In late anaphase, the chromosomes have almost reached to their respective poles. The cell membrane begins to move inward at the center.

4- Telophase:

   In telophase, spindle fibers break down. A nuclear envelope forms around the chromosomes at each pole of the cell. Thus two daughter nuclei are formed, each with the same number of chromosomes as was present in the parent nucleus.

II. Cytokinesis:

       “Cytokinesis is the division of the cytoplasm.”

 Cytokinesis is the last stage of cell division. Nucleolus forms and the chromosomes uncoil to become thin chromatin like network. After the division of the nucleus, the final step is the division of the cytoplasm. The division of cytoplasm is called cytokinesis.

• In animal cells, cleavage or furrows appear in the cytoplasm between the two nuclei. The furrows deepen and two identical daughter cells are finally produced.
• In plant cells, a cell plate is formed between the two daughter nuclei. It divides the cell into two daughters cells. The cell plate is formed by the fusion of small, fluid-filled vesicles produced by the Golgi apparatus.