All questions of Cell Cycle and Cell Division for NEET Exam

Prophase and telophase are stages involved in mitosis or meiosis. During G2 phase division of centrioles, mitochondria and chloroplasts occurs.

Most organisms that undergo sexual reproduction contain two types of cells in their body – haploid and diploid cells. Diploid cells are seen in most parts of the body and contain two copies of every chromosome. Therefore, they contain two genes to determine the sequence of nearly every RNA or protein. Haploid cells are usually male or female gametes formed as a result of meiosis and are seen in sexual organs. They contain only one complete set of chromosomes and are designed to fuse with another haploid gamete to produce a diploid zygote – with one paternal and one maternal set of chromosomes. Though they code for the same genes, their DNA sequences can vary slightly from one another. For instance, the paternal chromosome may contain the genetic information to result in antigen A being present on red blood cells, while the maternal chromosome may code for antigen B. These two antigens are similar in function, but differ from each other marginally in their amino acid sequence.

Same mitosis is also called equavational division becz. before and after mitosis ploidy is same..

Anaphase -The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions.

The crossing over takes place during the pachytene stage of prophase I of meiosis.

Explanation:
Prophase I is the first stage of meiosis I, which is further divided into five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis. During the prophase I, homologous chromosomes pair up and exchange genetic material through a process known as crossing over. This results in the formation of recombinant chromosomes that carry a combination of genetic material from both parents.

The crossing over occurs during the pachytene stage, which is characterized by the following events:

1. Homologous chromosomes pair up: The homologous chromosomes come together and form a bivalent or a tetrad.

2. Synapsis: The paired homologous chromosomes undergo synapsis, which is the physical association of the homologs.

3. Crossing over: The chromatids of the homologous chromosomes exchange genetic material at specific sites known as chiasmata. This leads to the formation of recombinant chromosomes.

4. Formation of the synaptonemal complex: The synaptonemal complex holds the homologous chromosomes together and facilitates the exchange of genetic material.

5. Nuclear envelope breakdown: The nuclear envelope breaks down, and the spindle fibers attach to the chromosomes.

In conclusion, the crossing over takes place during the pachytene stage of prophase I of meiosis.

Plant cells divide by a cell plate that eventually becomes the cell wall whereas animal cells divide by a cleavage furrow.

Mitosis is the mechanism by which the chromosome content of a somatic cell (haploid or diploid) is kept constant through successive cell divisions. The division of the cell is initiated by division of the nucleus i.e. Karyokinesis followed by division of cytoplasm i.e. Cytokinesis.

The arms of chromosome are known as chromatids. These arms are held together at a point called the centromere (or primary constriction). Centromere occurs any where along the length of chromosome. During ceIl division spindle fibres are attached to centromere and help in the movement of chromosomes towards the poles.

Crossing over occurs between prophase 1 and metaphase 1 and is the process where homologous chromosomes pair up with each other and exchange different segments of their genetic material to form recombinant chromosomes. It can also happen during mitotic division, which may result in loss of heterozygosity.

During metaphase, the chromosomes get arranged in the form of a plate called the equatorial plate or metaphase plate at the equator of the spindle. This plate is at right angles to the axis of the spindle and is formed of the kinetochores, the arms of chromatids trailing away. The centromeres are drawn to the equator by the equal pull of two chromosomal fibres which connect the sister kinetochores to the opposite poles. The process of drawing the chromosomes onto the equator of the spindle is known as congression.

Overview of Cell Cycle Phases
The cell cycle consists of several phases, including G1, S, G2, and M. Each phase has distinct roles in cell growth and division.
Statement I: G1 Phase
- The G1 phase is critical for cell growth.
- During this phase, the cell is metabolically active, producing RNA and synthesizing proteins.
- Importantly, DNA replication does not occur in G1; instead, it prepares the cell for the S phase, where DNA synthesis takes place.
Statement II: G2 Phase
- The G2 phase follows DNA synthesis and focuses on preparing the cell for mitosis.
- During G2, protein synthesis continues, particularly those proteins necessary for cell division, including components of the mitotic spindle.
- Cell growth also continues in this phase, ensuring that the cell is ready for the M phase (mitosis) where actual division occurs.
Conclusion
Both statements accurately describe their respective phases:
- G1 Phase: Metabolic activity and growth occur without DNA replication.
- G2 Phase: Preparations for mitosis take place with ongoing protein synthesis and further cell growth.
Thus, the correct answer is option 'C': Both I and II are correct. Understanding these phases is crucial for grasping cellular processes, especially in the context of the NEET syllabus.

In late prophase, the nuclear envelope, nucleolus, Golgi complex, and endoplasmic reticulum disappear, allowing the chromosomes to become fully condensed and visible. This marks the transition into metaphase, where the mitotic spindle fully forms.