The Cell Cycle and Mitosis

 The Cell Cycle and Mitosis

In animals, autosomal cells are said to be diploid (2n), which means that they comprise two copies of each chromosome. Germ cells, on the different (other) hand, are haploid (n), containing only one copy of every chromosome. In humans, these figures are 46 and 23, separately; we become heir to 23 chromosomes from every parent. Eukaryotic cells replicate through the cell cycle, a particular series of phases during which a cell grows, synthesizes DNA and divides. Derangements of the cell cycle might lead to unchecked cell division and may be responsible for the formation of cancer.

 The Cell Cycle


The cell cycle, exhibited in Figure 2.1, is a perennial MCAT favorite. For actively dividing cells, the cell cycle incorporates four stages; F1, S, F2, and M. The first three stages (G1, S, and G2) are referred collectively as interphase. Interphase is the longest slice (part) of the cell cycle; even actively separating cells spend about 90 percent of their time in interphase. Cells that do not separate spend all of their time in an offshoot of G₁ called Gₒ. During the Gₒ stage, the cell is merely living and serving its function, without any preparation for division.


During interphase, individual chromosomes are not seeable with light microscopy. Rather, they are in a fewer condensed form named as chromatin. This is because the DNA ought to (must) be available to RNA polymerase so that genes can be transcribed. During mitosis, anyhow, it is preferable to condense the DNA into tightly coiled chromosomes to avoid losing any genetic material during cell division.

G₁ Stage: Presynthetic Gap

During the G₁ stage, cells generate organelles for energy and protein production (mitochondria, ribosomes, and endoplasmic reticulum), while also increasing their size. Also, the passage into the S (synthesis) stage is governed by a restriction point. Certain criteria, such as containing the proper complement of DNA, ought to be met for the cell to pass the restriction point and enter the synthesis stage.

S Stage: Synthesis of DNA

During the S stage, the cell replicates its genetic material so that every daughter cell will have identical copies. After replication, each chromosome comprises of two identical chromatids that are bound together at a specialized region known as the centromere, as shown in Figure 2.2. Note that the ploidy of the cell does not change even despite that the number of chromatids has doubled. In other words, humans in this stage up to this time only have 46 chromosomes, even though 92 chromatids are present. Cells entering G₂ have double as much DNA as cells in G₁.



KEY CONCEPT

Each chromatid is collected from a complete, double-stranded molecule of DNA. Sister chromatids are identical imitates of each other. The term chromosome may be utilized to refer to either a single chromatid before S phase or the pair of chromatids attached at the centromere after S phase.

G Stage: Postsynthetic Gap

During the G₂ stage, the cell passes through another quality control checkpoint. DNA has already been duplicated, and the cell examines to make sure that there are enough organelles and cytoplasm to divide between two daughter cells. Furthermore, the cell checks to make sure that DNA replication proceeded correctly to avoid moving on an error to daughter cells that might further replicate the error in their progeny.

M Stage: Mitosis

The M stage comprises mitosis itself along with cytokinesis. Mitosis is isolated into four phases: prophase, metaphase, anaphase, and telophase. The features of every phase will be debated in the upcoming (next) section. Cytokinesis is the diverging of the cytoplasm and organelles into two daughter cells.

KEY CONCEPT

In autosomal cells, separation results in two genetically identical daughter cells. In germ cells, the daughter cells are not equivalent/equal.

CONTROL OF THE CELL CYCLE

The cell cycle is controlled by checkpoints, most notably between the G₁ and S phase, and the G₂ and M phase. At the G₁/S checkpoint, the cell ascertains if the DNA is in good enough condition for synthesis. As mentioned above, this checkpoint is likewise known as the restriction point. If there has been harm to the DNA, the cell cycle goes into arrest until the DNA has been repaired. The crucial protein in command of this is named as p53.

At the G₂/M checkpoint, the cell is mainly concerned with ensuring that the cell has achieved the adequate size and the organelles have been properly replicated to support two daughter cells. p53 as well plays a role in the G₂/M checkpoint.

The molecules accountable for the cell cycle are called cyclins and cyclin-dependent kinases (CDK) In order to be activated, CDKs require the presence of the right cyclins. During the cell cycle, concentrations of the varying cyclins increase and decrease during specific stages. These cyclins bind to CDKs, generating an activated CDK-cyclin complex. This obsession (complex) can then phosphorylate transcription factors. Transcription factors then upgrade the transcription of genes obligatory for the next stage of the cell cycle.

Cancer

Cell cycle control is essential to make sure that cells that are harmed or inadequately sized do not divide. When cell cycle control becomes deranged, and harmed cells are allowed to undergo mitosis, cancer may result. One of the most familiar/common mutations found in cancer is a mutation of the gene that produces p53, called TP53. When this gene is mutated, the cell cycle is not stopped to repair harmed DNA. This allows for mutations to accumulate, eventually resulting in a cancerous cell that separates continuously and without regard to the quality or quantity of the new cells produced. Frequently, cancer cells undergo rapid cell division, creating tumors. Eventually, if the cell begins to produce the right factors (such as proteases that might digest basement membranes or factors that encourage blood vessel formation), the damaged cells are then able to reach other tissues. This may include both local invasions as well as the distant spread of cancerous cells through the bloodstream or lymphatic systems. This concluding/latter result is known as metastasis.

BRIDGE

Cancer-causing genes can often be classified into oncogenes (genes that, when mutated, actively promote cell division) and tumor suppressor genes (genes that, when mutated, lose their capability to regulate or pause the cell cycle). Different cancer types are often connected with specific mutations in either oncogenes or tumor suppressor genes or both. The biochemistry of these genes is discussed in Chapter no 6 of the MCAT Biochemistry Review.

MITOSIS


Mitosis, shown in Figure 2.3, is the procedure by which two identical daughter cells are created from a single cell. Mitosis includes four distinct phases⎯prophase, metaphase, anaphase, and telophase⎯and occurs in somatic cells or cells that are not involved in sexual reproduction.

Prophase

Prophase is the primary step/phase of mitosis. The first step in prophase includes condensation of the chromatin into chromosomes. Also, the centriole pairs isolate and move toward opposite poles of the cell. These paired cylindrical organelles, shown in Figure 2.4, are placed outside the nucleus in a region known as the centrosome and are responsible for the correct division of DNA. Once the centrioles migrate to opposite poles of the cell, they start to constitute spindle fibers, which are made of microtubules. Every one of the fibers radiates outward from the centrioles. Some microtubules from asters that anchor the centrioles to the cell membrane dissolve during prophase, allowing these spindle fibers to contact the chromosomes. The nucleoli become less clear and may disappear totally. Kinetochores come into sight at the centromere. Kinetochores are protein structures located on the centromeres that act as attachment points for specific fibers of the spindle apparatus appropriately called kinetochore fibers.



KEY CONCEPT

The phases of mitosis:
  • Prophase⎯chromosomes condense, spindle forms
  • Metaphase⎯chromosomes align
  • Anaphase⎯sister chromatids isolate 
  • Telophase⎯new nuclear membranes form

Metaphase

In metaphase, the centriole couples are now at opposite ends of the cell. The kinetochore fibers interact with the fibers of the spindle apparatus to align the chromosomes at the metaphase plate (equatorial plate), which is equidistant amidst the two poles of the cell.

Anaphase

During anaphase, the centromeres split so that every chromatid has its own distinct centromere, thus allowing the sister chromatids to separate. The sister chromatids are dragged in the direction of the opposite poles of the cell by the shortening of the kinetochore fibers.

Telophase and Cytokinesis

Telophase is predominantly the reverse of prophase. The spindle apparatus vanishes. A nuclear membrane reforms around every set of chromosomes, and the nucleoli reappear. The chromosomes uncoil, restarting their interphase form. Reach of the two new nuclei has acquired a complete copy of the genome identical to the original genome and to each other.

At the end of telophase, cytokinesis is the partition of the cytoplasm and organelles so that each daughter cell has sufficient supplies to survive on its own. Each cell undergoes a finite number of separations before programmed death; for human somatic cells, this is ordinarily between 20 and 50. After that, the cell can no longer divide ceaselessly.

MCAT Concept Check 2.1:

Before you proceed on, evaluate your understanding of the material with these given next questions.

1. What are the five stages of the cell cycle? What happens in each stage?

2. What are the four phases of mitosis? What happens in each phase?

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