Unit 4: Cell Cycle — Long Questions

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Definition It is the series of events that take place in a eukaryotic cell from its formation to its division into two daughter cells.

Phases of Cell Cycle The cell cycle can be divided in two main phases i.e., interphase and the mitosis phase.

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Introduction This phase lasts for about 90% of the total time of cell cycle. During interphase, the cell performs the life functions according to its specialty and prepares itself for next division.

Stage of Interphase Interphase consists of the following three phases:

G1 Phase (First Gap Phase): It starts from the end of the Mitosis phase. It is also called the growth phase. During this phase cell makes proteins and organelles and so grows in size. Cell also makes enzymes that are required in S phase for the replication of DNA.

S Phase (Synthesis Phase) During this phase, the DNA of each chromosome is replicated (copied). It results in the duplication of chromosomes (each chromosome consists of two sister chromatids). The total number of chromosomes in cell remains the same.

G2 Phase (Second Gap Phase): In this phase, the cell continues to grow and produces proteins necessary for cell division. The cell checks for any DNA damage that may have occurred during replication and makes necessary repairs. It also begins to reorganize its contents in preparation for mitosis.

G0 Phase 1. Many cells stop dividing and start performing their specific functions. This phase is called G0 phase. It is a so called non-dividing phase.
2. Many cells (e.g. neurons) remain in G0 for indefinite periods.
3. Some cells (e.g. cells of liver and kidney) remain in G0 phase temporarily.
4. Other cells (e.g. epithelial cells) do not enter G0 and continue to divide throughout life.

Division Phase After interphase, the cell enters the division phase and divides into the two daughter cells. The events of cell cycle are controlled by special genes. All phases occur in a sequence.

Table 4.1 Main Phases in Eukaryotic Cell Cycle:

• Interphase: The cell prepares for division and goes through growth in size and DNA replication.
• Gap (G1) Phase: The cell grows and carries out normal functions, preparing for DNA replication.
• Synthesis (S) Phase: The cell replicates its DNA, making an exact copy of its genetic material.
• Gap 2 (G2) Phase: The cell grows further, ensuring all preparations are complete for division.
• Gap 0 (G0) Phase: The cell exits the cycle and stops dividing, often to carry out specialized functions (not all cells enter this phase).
• M phase: The cell divides its genetic material equally into two new, identical cells.

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Definition Mitosis is the type of cell division in which a cell divides into two daughter cells, each with the same number of chromosomes as were present in the parent cell.

Occurrence Mitosis occurs in the somatic cells of eukaryotes. Prokaryotes also divide to make identical cells. But the events of their division are different from mitosis. That is why we call it binary fission.

Phases of Mitosis The German biologist, Walther Flemming discovered the events of mitosis in 1880s. There are 2 major phases of mitosis i.e. karyokinesis (division of nucleus) and cytokinesis (division of cytoplasm).

A. Karyokinesis Karyokinesis means the division of the nucleus. It is further divided into four phases.

i. Prophase
a) Condensation of Chromatin
During prophase, the thread-like chromatin material condenses and makes thick chromosomes. Each chromosome consists of 2 sister chromatids with a single centromere.

b) Disappearance of Nucleolus and Nuclear Membrane
The nuclear envelope and nucleolus break down during prophase.

c) Duplication of Centrosome and Spindle Fibres
The centrosome of cell duplicates into two. The two centrosomes migrate to opposite side of the nucleus. When they are migrating, they make a network of microtubules called spindle fibres (complete set is called mitotic spindle). In plant cells, there is no centrosome. Their mitotic spindle is formed by the aggregation of spindle fibres present in cytoplasm.

ii. Metaphase
i. Binding of Spindle Fibres with Chromosomes
During this phase, some spindle fibres bind with chromosomes. They attach at the point of centromere where special kinetochore proteins are present.

ii. Formation of Interphase Plate (Metaphase plate)
Two spindle fibres from both sides bind with one chromosome. The chromosomes attached with spindle fibres arrange themselves along the equator of the cell. In this way a plate is formed called metaphase plate.

iii. Anaphase
The spindle fibres attached with chromosomes pull toward the poles. Due to this pulling, the chromosome's sister chromatids separate. In this way, there are two similar sets of chromatids, which move towards the poles of the cell.

iv. Telophase
In this phase, new nuclear envelope forms around each set of separated chromosomes. Both sets of chromosomes unfold back into chromatin.

B. Cytokinesis / Describe cytokinesis in animal and plant cells.
a) Cytokinesis in Animal Cells
It is the division of cytoplasm. In animal cells, a furrow develops at the equator. At this furrow, the cytoplasm has a ring of microfilaments. The ring contracts and the furrow moves inward. In this way parent cell is pinched into two.

b) Cytokinesis in Plant Cell
In plant cells, Golgi apparatus makes vesicles. These vesicles move to the middle and fuse to form a plate called phragmoplast. The plate grows outward and its membranes fuse with the cell membrane. The result is two daughter cells.

Ans. Following is the significance of mistosis:

i. Growth
Growth in organisms means an increase in size and the number of cells. Mitosis plays a crucial role in growth by producing new cells that are identical to the original cell.

ii. Cell Replacement
Many cells are constantly dying in our bodies. For example, the red blood cells and the cells of the walls of intestine and skin etc. These are replaced by new ones which are exact copies of the older cells. The new cells are formed by mitosis.

iii. Regeneration
Some animals can regenerate parts of the body. For this purpose, they form new cells by carrying out mitosis in the cells of remaining parts.
Example:
Star fish can regenerate its lost arms.

iv. Asexual Reproduction
Mitosis is a means for asexual reproduction.
Example
Hydra reproduces asexually by budding. During this process mitosis forms a mass of cells called bud on the surface of Hydra. Mitosis continues in the cells of the bud and it grows into a new individual.

v. Formation of body
Human body consists of about 200 trillion cells. All these cells were formed from a single cell (zygote) at the start of your life. Millions of cell divisions occurred while your body was reaching its present form. In each of these divisions the genetic material was equally distributed between the daughter cells. It happened through mitosis.

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i. Failure in Separation of Sister Chromatids
Sometime the process of mitosis goes wrong. For example, during the anaphase of mitosis, the sister chromatids of a chromosome may fail to separate. As a result, one daughter cell receives both sister chromosomes and the other will receive none.

ii. Tumor Formation
Introduction:
Chromosomes may also be damaged during mitosis. If the genes that regulate mitosis are mutated (changed), the cells continue to divide. Due to this uncontrolled division, masses of cells are formed. These masses are called tumors.

Types a) Benign Tumor: If the tumors remain in their original location, they are called benign.
b) Malignant Tumor: If they migrate and invade other tissues, they are called malignant tumors (cancer). It is called metastasis (spreading of disease).

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Introduction It is the type of cell division in which each daughter cell recieves half the number of chromosomes as compared to the parent cell. In meiosis, a diploid parent cell divides to produce four haploid daughter cells. Diploid means the cells in which chromosomes are in pairs (homologous pairs) while haploid means the cell with no pairs of chromosomes. It occurs in germline cells during gamete formation.

Discovery Meiosis was discovered in 1876 by a German biologist Oscar Hertwig.

Phases of Meiosis Meiosis consists of two divisions.
1. Meiosis-I
2. Meiosis-II

1. Meiosis-I
In meiosis-I the homologous chromosomes in a diploid cell separate and so two haploid daughter cells are produced. It is subdivided into prophase-I, metaphase-I, anaphase-I and telophase-I.

i. Prophase-I
a) Condensation of Chromatin
During this stage, chromatin condenses and takes the shape of chromosomes. Each chromosome consists of two sister chromatids, because the DNA has already replicated before meiosis.

b) Synapses and Tetrad Formation
Homologous chromosomes move close together. They pair up in a process called synapsis. Each pair of homologous chromosomes is referred called tetrad.

Chiasmata and Crossing Over Non-sister chromatids of homologous chromosomes become "zipped" together, forming X-shaped structures called chiasmata. Each chiasma is the site for crossing over i.e., exchange of portions of chromosomes between non-sister chromatids. Crossing over leads to recombination of genetic material.

Other Events Other events of prophase-I are similar to prophase of mitosis. The nucleoli disappear and nuclear envelope breaks. Centrioles migrate to opposite poles and make spindle fibres to which chromosomes attach.

ii. Metaphase-I
(i.) The tetrads attached with spindle fibres align along the equator.
(ii.) In this way, they form metaphase plate.
(iii.) Two spindle fibres from both poles attach with one chromosome of the pair.

iii. Anaphase-I
a) Separation of Paired Chromosome
Each spindle fiber attached with kinetochores of a single chromosome pulls towards the pole. In this way the paired chromosomes are separated.

b) Formation of Two Haploid Sets
One chromosome is pulled towards one pole and other towards opposite pole. So, two haploid sets of chromosomes are formed. Each chromosome still contains a pair of sister chromatids.

iv. Telophase-I
Spindles disappear and a new nuclear envelope is made around each haploid set. The chromosomes uncoil into chromatin.
Cytokinesis occurs and two daughter cells are made.

2. Meiosis-II
Phase of Meiosis-II:
Meiosis-II closely resembles mitosis and consists of four phases: prophase-II, metaphase-II, anaphase-II, and telophase-II.

i. Prophase-II
In prophase-II, the nucleoli and nuclear envelope disappear, and the chromatin condenses. Centrioles move to the poles, forming spindle fibres.

ii. Metaphase-II
During metaphase-ll, spindle fibres attach to the kinetochores of chromosomes, aligning them at the cells equator.

iii. Anaphase-II
In anaphase-II, spindle fibres pull sister chromatids apart toward opposite poles.

iv. Telophase-II
Finally, in telophase-II, chromosomes uncoil back into chromatin, nuclear envelopes reform. Cytokinesis occurs which results in the formation of four daughter cells, each with half number of chromosomes.

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Significance of Meiosis.

1. Meiosis Maintains the Number of Chromosomes

(a) In Animals
i. In Animals, special cells in reproductive organs undergo meiosis. The daughter cells, called gametes, have half the number of chromosomes (with no pairs).
ii. During sexual reproduction, male and female gametes join to make the first cell (zygote) of new generation.
iii. The original number of chromosomes is restored in zygote.
iv. It undergoes mitosis many times and develops into the new animal.

(b) In Flowering Plants
i. In flowering plants, specialized cells in flowers undergo meiosis. The daughter cells, called spores, have half number of chromosomes.
ii. These spores grow into new generation inside the flowers.
iii. This generation produces gametes by mitosis.
iv. The gametes join to make zygote with full set of chromosomes. The zygote undergoes mitosis and develops into new plant.

2. Meiosis Brings Genetic Diversity
Crossing over creates new combinations of genes on chromosomes. Each chromosome in the gametes carries a unique set of genes. When diverse gametes from two parents combine, the resulting zygote is genetically different from both parents. In this way, meiosis contributes to genetic diversity in populations.

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Disjunction During meiosis-I, chromosomes separate while during meiosis-II sister chromatids separate. It is called disjunction.

Non-Disjunction Sometimes non-disjunction occurs. Due to it, the daughter cells (gametes) receive more or less than the normal number of chromosomes. If such gametes fuse to form zygote with abnormal number of chromosomes, the resulting baby suffers from severe medical problems.

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Similarities 1. DNA replication occurs during interphase (S phase) before both divisions.
2. Both divisions begin with a parent cell that has chromosomes in pairs.
3. In both divisions, chromatin condenses and chromosomes become visible during prophase.
4. Both mitosis and meiosis involve the formation of a spindle apparatus.
5. Both involve prophase, metaphase, anaphase, and telophase. However, meiosis has two rounds i.e., meiosis-I and meiosis-II.
6. In both divisions, sister chromatids separate. In mitosis, it happens during anaphase. In meiosis, it happens in anaphase II.
7. Cytokinesis occurs at the end of both divisions. During cytokinesis, the cytoplasm divides and two new cells are formed.

Differences Difference between mitosis and meiosis

• Mitosis: A parent cell divides only once; two daughter cells are produced. | Meiosis: A parent cell undergoes two divisions; four daughter cells are produced.
• Mitosis: Chromosome number in daughter cells remains the same as the parent cell. | Meiosis: The chromosome number is reduced by half in daughter cells.
• Mitosis: Variations do not occur. | Meiosis: Variations occur due to crossing-over.
• Mitosis: Occurs in somatic cells. | Meiosis: Occurs in germ line cells.
• Mitosis: Homologous chromosomes do not form pairs. | Meiosis: Homologous chromosomes form pair.
• Mitosis: No crossing over occurs during prophase. | Meiosis: Crossing over occurs during prophase.
• Mitosis: Single chromosome aligns to form a metaphase plate. | Meiosis: Homologous pairs align to form a metaphase plate.
• Mitosis: During anaphase, chromosomes break and individual chromatids are pulled towards poles. | Meiosis: During anaphase-I individual chromosomes are pulled towards poles.
• Mitosis: Occurs for growth, development, and maintenance of multicellular organisms. | Meiosis: Occurs for producing gametes in animals and spores in plants for sexual reproduction.