Biology Chapter 5 "Cell Cycle" Short and Conceptual Answers

Important Board Examination brief answers of Biology course For Class 9 BISE, Federal Boards, Competitive and Entrance Exams

    Welcome to our blog post on the topic of the Cell Cycle, an essential topic for students preparing for board examinations such as BISE and Federal Boards, as well as competitive and entry tests like NMDCAT, NEET, EATA, Medical Colleges Admission Tests, Cadet Colleges, and more. 

    In this article, we will delve into the important sub-topics of cell division and its need, the cell cycle and its phases, the interphase of the cell cycle, types of cell division, the significance of mitosis and meiosis, mitosis and its phases, cytokinesis in animals and plants, meiosis and its phases, non-disjunction and its consequences, apoptosis and cell necrosis. Our goal is to help students master the basics of the Cell Cycle in order to achieve higher marks in board exams and success in competitive exams. Get ready to take your understanding of this topic to the next level with our concise and conceptual questions.

Conceptual, Short, and Consie Answers on Cell Cycle
From Biology Library of H.E.S (Health, Education, and Skills)

What do you mean by cell division?  

Cell division

A process by which mature parent cells divide into two daughter cells and get the same number of chromosomes to that of parent cells is called cell division.

Explanation

    As we know that all living organisms are made up of one or more cells, so the cell is the basic unit of structure and function of all living organisms. Cells may be

a. Parent (Mature) cells

The cells that reach their maximum possible limit of its size and are ready to divide are termed Parent cells or mature cells.

b. Daughter cells

The cells that are born just after the division of mature cells and just start growing are termed daughter cells.

    In cell division, the mature cells produce the exact copies in the form of daughter cells. These daughter cells necessarily have the same number of chromosomes as were present in their parent cells. Therefore, the number of chromosomes is unique in number to all species.

Why do we need cell division?  (OR) What is the Importance of Cell Division?

Need for cell division

    The process of cell division is needed in all living organisms for the following reasons.

• All living things retain their existence only due to cell division.
• Cell division is also necessary for the growth and repair of tissues, reproduction, and the maintenance of genetic diversity. 
• In multicellular organisms, cell division allows for the production of new cells that can differentiate into specialized cell types. 
• In single-celled organisms, cell division enables reproduction and the continuation of the species. 
• Cell division plays a crucial role in the healing of wounds and the replacement of damaged or old cells. 

What do you mean by cell cycle?

Cell cycle

The series of events that take place in a eukaryotic cell leading to its replication is called the cell cycle.

Explanation

    The Cell cycle is also known as the Cell Division Cycle. The Cell cycle is important because it is only due to the cell cycle that a single-celled fertilized egg develops in a mature organism and our hair, skin, blood cells, and some internal organs are renewed.

• In prokaryotes, the cell cycle known as “Binary fission” occurs in the following steps.

1. DNA replication: In prokaryotes, the DNA molecules first replicate (make their copy) and then attach each copy to different parts of the cell.
2. Chromosome separation: Chromosome separation occur when the cell begins to pull apart.
3. Identical genetic composition: When a cell divides each cell receives an exact number of chromosomes so have the same genetic composition.

What are the Phases of the Cell Cycle?

Phases of cell cycle

    As the word cycle indicates that it is a series of events, these events can be divided in following two broad periods or phases.

• Cell’s growth phase

    This period or phase is known as “Interphase” in which the cell grows, accumulates nutrients for division, and duplicates its DNA.

• Cell’s division phase

    This period or phase is known as the “Reproductive phase” of a cell in which the cell splits into distinct cells called daughter cells.

Write down the significance of mitosis.

Significance of mitosis

The process of mitosis is involved in the following life-needed activities

• Growth: In multicellular organisms, the increase in the number of cells and thus in growth is only due to mitosis.
• Cells replacement: The process of cell and body repair continues throughout our life. It means that old cells when reaching their maturity die and are then replaced by new cells. These new cells come after the mitotic type of cell division.
• Regeneration: Some animals like lizards etc regenerate (reform) parts of their body and produce new cells only by the process of mitosis.
• Vegetative reproduction: In some plants through cutting, drafting, budding, etc new offspring can be produced through a process called “Vegetative reproduction”. Vegetative reproduction occurs only due to mitosis.

Write down the significance of meiosis.

Significance of meiosis

Following are the significances of meiosis due to which daughter cells differ from parent cells.

• Production of gametes: Due to meiosis that all living organisms produce gamete cells (sperm and egg).
• Reduction of chromosomes: Each daughter cell has haploid (half) the number of chromosomes as were present in their parent cells in diploid (double) form, so the chromosomes are reduced.
• Independent Assortment: Independent assortment during anaphase-I creates daughter cells that contain a different assortment of material and parental chromosomes.

Describe the Cytokinesis in animals and plants.

Cytokinesis

The process by which the cytoplasm of the cell divides is called Cytokinesis.

• Cytokinesis in animals: In animal cells, the plasma membrane in the center of the cell begins to fold inward between the two daughter nuclei. When the infolding is completed the cell divides into two cells.
• Cytokinesis in plants: Beginning in late anaphase same membranous vesicles form which grow outward and form a double-layered membrane. A cell wall begins to form between the two layers and then the cell divides into two. The two daughter cells then pass into the Interphase, during which they grow to the size of the original mother cell ready to repeat the process of mitosis.

What do you mean by non-disjunction? What are some of its consequences?

Non-disjunction

The failure of replicated chromosomes to separate during meiosis and thus one gamete receiving both while the other receives none of the affected chromosomes is termed as non-disjunction.

Explanation

    In the case of non-disjunction, if the defective gametes take part in fertilization the following abnormal number of chromosomes will be found in every cell of the embryo.

1. Trisomy (2n + 1): In this case, a third copy of the homologous chromosome is present in the cell.
2. Monosomy (2n - 1): In this case, the cells lack one chromosome.

Consequences of non-disjunction

    Following are some of the consequences of non-disjunction.

• Down’s syndrome or Mongolism: It is an autosomal non-disjunction of chromosome number 21. It is a case of trisomy. A person with Down’s syndrome will have the following characteristics
• Facial features
• Short stature
• Heart defects
• Mental retardation
• Klinefelter’s syndrome: It is a male-specific disorder, in which males grow to be adults but fail to mature sexually. They have an extra sex chromosome, the sex chromosomes are XXY. Males with two Y chromosomes XYY also occur and studies show that they have criminal tendencies.
• Turner’s syndrome: It is a female-specific disorder. Females with this syndrome have only 45 chromosomes. Instead of XX chromosome, they have only one X chromosome.

Write a short note on 

i. Apoptosis, and ii. Necrosis

1. Apoptosis

• It is also called “programmed cell death”. 
• During apoptosis, unwanted cells are removed. 
• Apoptosis is a crucial process for maintaining tissue homeostasis and preventing diseases such as cancer.
• During apoptosis, the cellular contents are not released and hence there will be no inflammation.

Example: The disappearance of the tadpole tail during metamorphosis.

2. Necrosis

• It is cell death due to unexpected and accidental cell damage. 
• Many toxic chemicals and physical events cause necrosis e.g. toxins, heat, radiation, lack of Oxygen (Hypoxia), etc.
• During necrosis, the cells swell and burst and release intracellular contents in the surroundings.

Extra For Your Information: The Mechanism of Apoptosis

    Apoptosis is a highly regulated process that occurs in cells when triggered by internal or external signals. The mechanism of apoptosis involves the following steps:

1. Activation of apoptotic signals: The initiation of apoptosis can be triggered by various internal signals such as DNA damage, oxidative stress or abnormal cellular proliferation, or external signals like growth factor deprivation or death ligand binding.
2. Activation of caspases: The activation of specific proteins called caspases marks the start of the execution phase of apoptosis. Caspases are a family of cysteine proteases that cleave other proteins and induce morphological changes leading to cell death.
3. Destruction of cellular components: Caspases trigger a series of events that result in the destruction of cellular components, including the nucleus, cytoskeleton, and plasma membrane.
4. Phosphatidylserine exposure: The outer layer of the plasma membrane changes, leading to the exposure of phosphatidylserine on the surface of the dying cell. This acts as a signal to neighboring cells and the immune system that the cell is undergoing apoptosis.
5. Formation of apoptotic bodies: The dying cell shrinks and forms apoptotic bodies, which are then engulfed and eliminated by phagocytes.