Cell Cycle and Chromosomes

When karyokinesis (nuclear division) is not followed by cytokinesis (cytoplasmic division), the cell ends up with multiple sets of chromosomes, resulting in polyploidy. This condition can occur naturally or be induced by chemicals like colchicine.

The movement of chiasmata (crossing-over points) toward the ends of paired homologous chromosomes during meiosis I is called terminalization. This process occurs during diplotene and continues through diakinesis.

During Interphase  synthesis of protein, DNA and RNA takes place

The centromere is the specific point on chromosomes where spindle fibers attach during cell division. This attachment is crucial for proper chromosome segregation to daughter cells.

In plant cells, the division of cytoplasm takes place by the formation of a cell plate.

Colchicine at 0.1% concentration inhibits spindle fiber formation during cell division. It prevents microtubule polymerization, which stops the formation of the mitotic spindle and thus blocks chromosome segregation.

Zygotene is the phase of meiosis I (prophase I) during which homologous chromosomes come together to form pairs (synapsis). This pairing is essential for crossing over and proper chromosome segregation.

Crossing over is the genetic exchange between non-sister chromatids that creates the chiasmata (X-shaped attachment points). Synapsis pairs the chromosomes first, disjunction separates them later, and terminalization moves chiasmata toward chromosome ends.

The cell cycle or cell-division 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. The correct chronology of the events during cell cycle is G₁, S, G₂, M.

Meiosis I is reductional because homologous chromosomes separate, halving the chromosome number. Meiosis II is equational because sister chromatids separate without further reduction, similar to mitosis.

Meiosis reduces chromosome number by half. Starting with a diploid cell (2n), meiosis produces four haploid daughter cells (n), each with half the original chromosome set.

This is called Tetrad stage with 4 strands

DNA replication occurs exactly once per cell cycle during the S (synthesis) phase of interphase. This ensures each daughter cell receives a complete copy of the genome after cell division.

Crossing over occurs between non-sister chromatids of homologous chromosomes (one maternal, one paternal). Sister chromatids are identical copies and do not exchange genetic material with each other.

Prophase I of meiosis I has five sub-stages: leptotene (chromosomes condense), zygotene (synapsis begins), pachytene (crossing over occurs), diplotene (chiasmata visible), and diakinesis (final condensation).