10.7 - EXERCISES

The average cell cycle span of a mammalian cell is approximately 24 hours. This encompasses the time taken for the entire process of cell growth and division, known as the cell cycle, which includes interphase and the M phase (mitosis). During this time, the cell not only prepares for division but also duplicates its DNA and synthesizes proteins needed for cell division.

Cytokinesis and karyokinesis are essential parts of cell division. Karyokinesis refers to the division of the cell's nucleus, where the chromosomes are separated into two new nuclei. Cytokinesis, on the other hand, is the process that divides the cytoplasm of the parent cell into two daughter cells. While karyokinesis ensures each daughter nucleus receives the proper amount of genetic material, cytokinesis physically separates the two resulting cells.

Interphase is the preparation period before a cell divides. It consists of three phases: G1 (Gap 1), S (Synthesis), and G2 (Gap 2). During G1, the cell grows and synthesizes proteins, preparing for DNA synthesis. In S phase, DNA replication occurs, doubling the genetic material. Finally, in G2, the cell continues to grow and prepares for mitosis by producing more organelles and proteins, ensuring all components are ready for division.

The G0 phase, also known as the quiescent phase, is a state where cells are not actively dividing. Cells in this phase may still be metabolically active but do not enter the cell cycle to divide. This phase can occur for various reasons, such as when cells are fully differentiated or when conditions are not favorable for division. Quiescent cells can remain in this phase temporarily or for extended periods.

Mitosis is referred to as equational division because it results in daughter cells that have the same number of chromosomes and genetic content as the parent cell. During mitosis, each chromosome is duplicated and then evenly divided between the two new nuclei, ensuring that each daughter cell maintains the same chromosomal and genetic information.

Each of the events listed occurs during specific stages of the cell cycle:
(i) Chromosomes align at the spindle equator during metaphase.
(ii) The centromere splits during anaphase.
(iii) Pairing of homologous chromosomes occurs during prophase I of meiosis.
(iv) Crossing over between homologous chromosomes also takes place during prophase I of meiosis.

Synapsis is the process in meiosis where homologous chromosomes pair together. A bivalent is the structure formed during synapsis consisting of a pair of homologous chromosomes, each made up of two sister chromatids. Chiasmata are the X-shaped structures that form at the point of crossing over between non-sister chromatids of homologous chromosomes, indicating where genetic material has been exchanged.

Cytokinesis in animal cells occurs through the formation of a cleavage furrow that pinches the cell membrane, ultimately dividing the cell into two daughter cells. In contrast, plant cells form a cell plate that develops in the center of the cell, which eventually becomes the new cell wall, dividing the two daughter cells. This difference is primarily due to the presence of a rigid cell wall in plant cells.

In organisms like certain fungi and algae, meiosis can result in four haploid daughter cells that are equal in size. However, in animals, particularly in females, meiosis often produces one large ovum and three smaller polar bodies, resulting in unequal-sized daughter cells. This occurs because the cytoplasm is unevenly distributed during cytokinesis.

In anaphase of mitosis, sister chromatids separate and move towards opposite poles, resulting in two identical daughter nuclei. In anaphase I of meiosis, homologous chromosomes separate while sister chromatids remain attached at their centromeres, resulting in two cells that are not identical due to the reduction in chromosome number.

Mitosis and meiosis serve different purposes and have different outcomes. Mitosis results in two genetically identical diploid daughter cells, while meiosis produces four genetically diverse haploid daughter cells. Mitosis involves one cell division, whereas meiosis includes two divisions. Mitosis occurs in somatic cells, while meiosis occurs in germ cells during gamete formation.

Meiosis is vital for sexual reproduction as it creates gametes with half the chromosome number needed to maintain the species' chromosome count across generations. It also increases genetic diversity through processes like crossing over and independent assortment, which is crucial for evolution and adaptation.

Haploid insects, such as male honeybees (drones) and certain lower plants can undergo mitosis and continue producing additional haploid cells. In contrast, in certain higher plants, such as ferns, the haploid gametophyte generation does not frequently divide but is involved in the reproduction cycle leading to the diploid sporophyte generation.

No, mitosis requires DNA replication to produce the necessary sister chromatids for division. If DNA has not been replicated during the S phase, the products of mitosis would lack the full set of chromosomes, leading to genetic abnormalities in the daughter cells.

Yes, DNA replication can occur without cell division. This is common in certain types of cells, such as during the preparation for meiosis or in some cancer cells where the DNA is duplicated but the cell does not properly divide, leading to multiple sets of chromosomes in one cell.

As a cell progresses through the cell cycle, the number of chromosomes remains constant during interphase but doubles during the S phase when DNA is replicated. The amount of DNA per cell also changes: it is at 2C (diploid) during G1, doubles to 4C during S phase, and returns to 2C after mitosis. The chromosome number remains constant with a diploid state regardless of the DNA content during the interphase phases.