There are a few ways for the creation of new cells. They are growth, asexual reproduction, tissue repair and maintenance. In eukaryotic cells, when the nucleus divides to form two genetically identical nuclei, it is known as mitosis. When the cytoplasm divides to form two cells, it is known as cytokinesis. In prokaryotic cells reproduce by a process known as binary fission. In this process, a single circular chromosome replicates and then the two copies of the chromosome move to opposite sides of the cell and cytokinesis then follows.
The cell cycle is the ordered sequence of events for the life of a cell and refers to the events between one cell division and the next in eukaryotic cells. This cycle can be divided into two parts, interphase and cell division. Interphase is an active period in the cell life when metabolic reactions occur, including protein synthesis, DNA replication, and an increase in mitochondria and/ or chloroplasts. A cell can remain in interphase for a long time so it does not mean that it is the preparation stage for mitosis. Interphase has three phases, the G1 phase, the S phase, and the G2 phase. During the G1 phase, cells grow and DNA trasnscription and protein synthesis take place. During the S phase, the genetic material is copied so that there are two new cells with a complete set of genes. Lastly, during the G2 phase, the cell prepares for division.
Mitosis is the division of a eukaryotic cells’ nucleus into two genetically identical nuclei. Mitosis is needed during the growth of a cell, during embryonic development, when tissues are damaged and have to be repaired, and lastly to reproduce asexually. Before this can occur, there has to be two copies of each chromosome. Each chromosome consists of a single DNA molecule that has to be replicated so that there are two identical DNA molecules called sister chromatids. Though mitosis is a continual process, scientists have split it into four stages; prophase, metaphase, anaphase, and telophase.
In this early stage of this phase, the chromosomes coil and become shorter and fatter. To become short enough they have to coil repeatedly and this is known as supercoiling. At the end of this phase, the nuclear membrane breaks down and each chromosome (consisting of tow identical chromatids created in DNA replication during interphase) are held together by a centromere.
Microtubules grow from the piles of the cell from a structure called the microtubule organizing center (MTOC) to the chromosomes. They form a spindle shape and so the MTOCs together with the microtubules are referred to as the mitotic spindle. In prophase, the spindle microtubules extend from each pole to the equator of the cell.
The spindle microtubules attach to the centromeres. Chromosomes are then moved from the center of the cell to opposite ends, with the spindle microtubules attached to one of the sister chromatics from one pole, and the another spindle microtubules is attached to the other sister chromatid from the other pole.
In anaphase, the centromeres divide and the chromatids are now chromosomes. The pair of sister chromatids separate and the spindle microtubules bring them to the poles of the cell. Up until this point, the centromere has held them together. Because the sister chromatids are pulled to opposite poles, mitosis produces two genetically identical nuclei. To make sure of this, the centromeres of the sister chromatids must be attached in metaphase to spindle microtubules from different poles.
At the early stage of the phase, nuclear membranes are formed around the chromosomes at each pole. Also, the spindle microtubules break down. During the late stage of this phase, the chromosomes uncoil, the cell once again divides, and the two daughter cells enter interphase again and this cell cycle goes on again and again.
Sometimes, certain agents damage the way cell division is regulated, such as ultraviolet light, chemicals and carcinogens or viruses. When this happens, it can cause a change in the genes of a cell and as this cell divides on and on, the other cells inherit these genes. WHen this uncontrolled cell division occurs, it can produce tumors which are a mass of cells. These can grow to large sizes and spread to many areas of the body. This disease is known as cancer.
DBQ: cell size and the cell cycle
1a. DNA most likely occurs at about 2:00 and 3:00. This is because the S phase, which is the phase where DNA replication occurs, is at its peak and more than 50% of the population is in that phase during this time.
1b. Mitosis is most likely to occur when the G2+M stage is at its peak. This is because this occurs right before mitosis does. It will occur at 5:00, 3:00, and 6:00
2. Most of the increase in cell size occurs in the G1 stage of interphase.
3. From the graph, it is evident that during the day when there is the most light, the cell is increasing in size. As it gets darker in the day, the cell decreases in size. For example, at from 12:00 to 18:00, the cell size in increasing and so is the light outside. However as it gets darker outside and the time falls to 6:00, the cell size decreases. This supports the claim that Emiliania hyxleyi take advantage of light resources for the timing of their cycle.