Interphase

Interphase

Interphase Includes the Synthesis and Gap Phases of the Cell Cycle. During Interphase, Cells Grow and Prepare for Mitosis.

Interphase is the portion of the cell cycle between two consecutive cell divisions. The events that occur during interphase—the G1, S, and G2 phases—involve very important preparations for the successful completion of mitosis. During G1, cells undergo the major portion of their growth.During the S phase, each chromosome replicates to produce two sister chromatids, which remain attached to each other at the centromere. In the G2 phase, the chromosomes coil even more tightly.

Centromere and Kinetochore 

The centromere is a point of constriction on the chromosome containing repeated DNA sequences that bind specific proteins. These proteins make up a disklike structure called the kinetochore. This disk functions as an attachment site for microtubules necessary to separate the chromosomes during cell division (figure below). Each chromosome’s centromere is located at a characteristic site along the length of the chromosome.

Figure:Kinetochores. Separation of sister chromatids during mitosis depends on microtubules attaching to proteins found in the kinetochore. These kinetochore proteins are assembled on the centromere of chromosomes. The centromeres of the two sister chromatids are held together by cohesin proteins.

After the S phase, the newly synthesized sister chromatids appear to share a common centromere, but at the molecular level the DNA of the centromere has already replicated, so there are two complete DNA molecules. This means that you have two chromatids held together by cohesin proteins at the centromere, and each chromatid has its own set of kinetochore proteins.

In multicellular animals, most of the cohesin that holds sister chromatids together after replication is replaced by condensin as the chromosomes are condensed. This leaves the chromosomes still attached tightly at the centromere, but only loosely attached elsewhere.

Cell Growth 

A eukaryotic cell typically grows throughout interphase. The G1 and G2 segments of interphase are periods of active growth, during which proteins are synthesized and cell organelles are produced. However, the cell’s DNA replicates only during the S phase of the cell cycle.

After the chromosomes have replicated in S phase, they remain fully extended and uncoiled, although cohesin proteins are associated with their centromeres at this stage. In G2 phase, they begin the process of condensation, coiling ever more tightly. Special motor proteins are involved in the rapid final condensation of the chromosomes that occurs early in mitosis. 

Also during G2 phase, the cells begin to assemble the machinery they will later use to move the chromosomes to opposite poles of the cell. In animal cells, a pair of barrel-shaped organelles called centrioles replicate, producing one for each pole. These act as microtubule-organizing centers: Surrounding each centriole is pericentriolar material, ring-shaped structures composed of tubulin that can nucleate the assembly of microtubules. Plants and fungi lack centrioles but still contain microtubule-organizing centers. All eukaryotic cells undertake an extensive synthesis of tubulin, the protein that forms microtubules.