Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi. In the G 2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation.
Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G 2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. Learning Objectives Describe the events that occur during Interphase. Cells spend most of their lives in interphase, specifically in the S phase where genetic material must be copied.
The cell grows and carries out biochemical functions, such as protein synthesis, in the G 1 phase. Related questions Why is mitosis necessary for living organisms? How is metaphase distinguished? How do cytokinesis and telophase differ? How do chromosomes divide during anaphase? How can mitosis be a form of asexual reproduction? How does mitosis conserve the chromosome number? The two new duplicate sister chromosomes are called chromatids.
The histones bind the two matching chromatids together tightly partway down their lengths. The point where they are joined is called the centromere. See Resources for a visual representation of this. To add to the complicated movements happening during DNA replication, many eukaryotic cells are diploid, which means that their chromosomes are normally arranged in pairs. Most human cells are diploid, with the exception of the reproductive cells; these include oocytes eggs and spermatocytes sperm , which are haploid and have 23 chromosomes.
Human somatic cells, which are all of the other cells in the body, have 46 chromosomes, arranged in 23 pairs. The paired chromosomes are called a homologous pair. During mitosis, the nucleus will split into two new nuclei, pulling one of each chromatid from each homologous pair away from its sister.
If the cell passes the S phase checkpoints, which are especially concerned with making sure that the DNA was not damaged, that it replicated correctly and that it replicated only once, then regulatory factors allow the cell to proceed into the next stage of interphase. This is G 2 , which stands for Gap phase 2, like G 1.
It is also a misnomer, as the cell is not waiting, but is very busy during this stage. The cell continues to do its normal work. Recall those examples from G 1 of a leaf cell performing photosynthesis or a white blood cell defending the body against pathogens.
It also prepares to leave interphase and enter mitosis M phase , which is the second and final stage of the cell cycle, before it divides and begins all over again. Another checkpoint during G 2 ensures that the DNA was replicated correctly, and CDK allows it to move forward only if it passes muster.
During G 2 , the cell replicates the centromere that binds the chromatids, forming something called a microtubule. This will become part of the spindle, which is a network of fibers that will guide the sister chromatids away from each other and to their proper places in the newly divided nuclei.
During this phase, mitochondria and chloroplasts also divide, when they are present in the cell. When the cell has surpassed its checkpoints, it is ready for mitosis and has finished the three stages of interphase. During mitosis, the nucleus will divide into two nuclei, and at almost the same time, a process called cytokinesis will divide the cytoplasm, meaning the rest of the cell, into two cells.
By the end of these processes, there will be two new cells, ready to begin the G 1 stage of interphase again. During the synthesis phase , a cell copies its DNA. DNA replication is a massive effort that requires a lot of proteins.
Since DNA does not exist by itself in a cell but is packaged by proteins, more packaging proteins must also be made during the S phase. Histones are proteins around which DNA wraps. The production of new histone proteins starts at the same time as DNA synthesis. Blocking DNA synthesis with a chemical drug also blocks histone synthesis, so the two processes are linked during the S phase. During the G2 phase , the cell prepares to enter mitosis. The DNA has already been duplicated during the S phase, so the G2 phase is when the organelles of the cell need to duplicate.
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