Cell Cycle and Chromosomes

The kinetochore is the protein structure on chromatids, where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore forms in eukaryotes, assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis.

In prometaphase, once the nuclear envelope has broken down, the spindle microtubules and the chromosomes are no longer separated by a (double) membrane boundary. The microtubules begin to interact with the chromosomes, and the chromosomes undergo what is known as congressional movement, where they ultimately end up with their centromeres all situated in middle of the spindle, at a site known as the metaphase plate. Each kinetochore of the replicated chromosome is pointed towards one side of the spindle; later, in anaphase, each kinetochore moves to one of the two spindle pole regions as the daughter chromosome. The congression of chromosomes and the alignment of centromeres on the metaphase plate represent essential prerequisites for the orderly separation of the replicated genome into two equal parts. The mechanisms underlying congression are under intense scrutiny.

The phragmoplast is a plant cell specific structure that forms during late cytokinesis. It serves as a scaffold for cell plate assembly and subsequent formation of a new cell wall, separating the two daughter cells.

When a dividing cell is treated with hydrogen peroxide, the spindle fibres precipitate due to formation of disulphide bridges. This reaction indicates that the spindle fibres and the microtubules are proteinaceous structures.

A somatic cell is diploid (2n). Without cytokinesis, each nuclear division doubles the chromosome number within the cell. After two successive nuclear divisions (endoreduplication), the cell contains 4 copies of each chromosome. Since the original cell was diploid and no meiosis occurred, all four nuclei remain diploid, not haploid.

Crossing over (genetic recombination) occurs specifically between non-sister chromatids of homologous chromosomes during prophase I of meiosis. This reciprocal exchange of genetic material creates new combinations of alleles. Sister chromatids are identical copies and do not undergo crossing over with each other.

Interphase is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for cell division. In preparation for cell division, it increases its size and makes a copy of its DNA, which is made during the S-phase. Interphase is also considered to be the 'living' phase of the cell, in which the cell obtains nutrients, grows, reads its DNA, and conducts other "normal" cell functions. The majority of eukaryotic cells spend most of their time in interphase. Interphase does not describe a cell that is merely resting but is rather an active preparation for cell division.

Non-disjunction is the failure of chromosome pairs to separate properly during meiosis stage 1 or stage 2, specifically in the anaphase. This could arise from a failure of homologous chromosomes to separate in meiosis I or the failure of sister chromatids to separate during meiosis II or mitosis. The result of this error is a cell with an imbalance of chromosomes. Such a cell is said to be aneuploidy.

In a somatic cell cycle, DNA replication takes place in S-phase. G₂ phase follows interphase. Mitotic phase is a relatively short period of the cell cycle and in interphase, DNA content is doubled. 

Nuclear envelope reformation during telophase requires decondensation of chromosomes and reassembly of the nuclear lamina. Transcription from chromosomes resumes after envelope reformation. The contractile ring and phragmoplast are involved in cytokinesis, which occurs after nuclear envelope reformation in animal and plant cells respectively.

Cyclin-dependent kinases (CdKs) are essential regulators of the cell cycle. CdKs bind to cyclins to form active complexes that drive cell cycle progression. The other options (CCk, CKc, CKd) are not standard abbreviations for known cell cycle regulators. CdK phosphorylates target proteins to advance the cell through its phases.

1. Anaphase  -Chromosomes are moved to spindle equator.
   1. Metaphase -Centromere splits and chromatids separate.
   2. Zygotene -Pairing between homologous chromosomes takes place.
   3. Pachytene -Crossing over between homologous chromosomes takes place.

In an 18-hour mitotic cycle, G1 is typically the longest phase as the cell grows and prepares for DNA synthesis. G1 occupies ~50% (9 hours), S phase ~28% (5 hours) for DNA replication, G2 ~17% (3 hours) for preparation, and M phase ~5% (1 hour) for mitosis itself. This distribution reflects the biological priority of growth preparation.

The interphase nuclei of female mammals differ from those of males in that a heterochromatic body is usually present on the inner face of the nuclear envelop in females. This structure, the sex chromatin or Barr body, protrudes from the surface of polymorphic nuclei (as a drumstick).