Cell Division - Genetics

Failure to meet the checkpoint requirements may lead to delays from DNA damage, improper nutrients, or cell size. it may also lead to exiting the cell cycle and apoptosis (cell death). Necrosis is unprogrammed cell death, usually caused by inflammation, disease, or lack of oxygen/blood.

Telomeres are repetitive DNA sequences at the end of chromosomes and they shorten which each division. Poly-a-tails are added to RNA at the end of transcription. 5' caps are nucleotides added to mRNA aiding in translation. The kinetichore is a protein structure on chromatids that allow spindle fiber attachment.

Mutations can lead to lack of regulation, which overall, leads to genomic instability providing opportunities for uncontrolled growth — for example, cancer from the loss of cell cycle control. Proto oncogenes are normal genes that promote and regulate cell growth. Mutations to the proto oncogene itself could lead to oncogenes, which are cancer promoting.

Proto oncogenes promote proteins via proliferation with regulation. Oncogenes promote cell proliferation without regulation, leading to genomic instability, which can lead to cancer. A mutagen is a physical or chemical substance that can increase the frequency of mutations. A carcinogen is a substance that is directly involved in causing cancer.

p53 is a famous tumor suppressor, which blocks cell cycle progression preventing damaged/mutated DNA from being duplicated. Proto oncogenes are normal genes that regulate cell growth and proliferation. Oncogenes are mutated proto oncogenes that lead to unregulated cell proliferation. The checkpoints are in the S, G1, G2/M phases.

If the genes were linked, there would be an incredibly small recombination frequency. If the genes were on opposite ends of the chromosome or on separate chromosomes, the recombination frequency would approach the maximum of 50%.

Because the recombination frequency is relatively intermediate, we can conclude that the distance between the genes does not fall at either extreme. The genes are neither very close, nor very far apart.

The larger the recombination frequency, the larger the distance between two genes. By looking at the data, we know that genes W and X are close to one another. Also, genes X and Y are close to one another. Gene Z, however, seems to be far away from both W and Y (but closer to W). We can represent these distances relatively in a picture:

W - - - X (3)

X - - Y (2)

Y - - - - - - - - - - - - - Z (13)

Z - - - - - - - - W (8)

The most likely explanation is that W, X, and Y are close to one another and Z is located slightly farther away on whichever side W is closest. A spatial map would look something like this:

Z - - - - - - - - - - W - - - X - - Y

The S phase is the "synthesis" phase, where DNA is duplicated. Cells that are not dividing are considered to be in the G0 phase, however, they can enter the cell cycle if they receive the proper signals. The S phase occurs before the M phase (mitosis) as the cell begins to prepare for division.

Apoptosis is the term used to describe programmed cellular death. Apoptosis is useful because it prevents the replication of damaged/mutated cells. p53 is an important signal that when activated, instructs a cell to commit apoptosis. Cancer is partially caused by a failure of the p53 signal, thus allowing for uninterrupted cellular growth.

In metaphase, spindle fibers align chromosomes along the middle of the cell nucleus. A good way to remember this is "M" in metaphase stands for middle. In this portion of mitosis, the cell ensures that each daughter cell will receive the appropriate genetic material. Metaphase occurs right before Anaphase.

The correct order is as follows:

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Cytokinesis signals the end of mitosis. In this phase, a fiber ring contracts around each daughter cell (thus making two complete daughter cells).

The nucleolus disappearing is an important part of the cell cycle. This step takes place in prophase. In prophase, chromosomes become visible under the light microscope. They are visible as the chromatin become condensed. There are important facts to remember about prophase.

Diploid means 2_n_, where n is the number of chromosomes in a particular species. If there are 10 chromosomes, there will be 20 chromatids. For each chromosome there are two chromatids. Since this is before the cells have fully divided, there will be 2 times the number of chromosomes during prophase. Therefore, the answer is 20.

The G0 phase is for cell cycle arrest. Cells in this phase are not actively dividing and are not preparing to divide. Being in G0 is technically "outside" of the cell cycle. A cell must receive certain and specific signals to exit the G0 phase and enter the G1 phase (to prepare for replication and division). Cells within the cell cycle might receive certain signals telling them to stop dividing. in this case, a cell will exit the cell cycle and enter the G0 phase.

When the cell moves past the restriction point, it has committed to dividing to form a daughter cell. S phase follows immediately after this point, and the cell begins to duplicate all of the chromosomes comprising its genetic material.

Prophase and metaphase happen before anaphase, and the duplicated chromosomes (sister chromatids) condense and align. During anaphase they are pulled apart and start migrating to opposite cell poles. Telophase and cytokinesis follow anaphase to complete the division of the cell into two new cells.

The mitotic spindle is a long cytoskeletal structure that physically pulls the chromatids apart so that each new cell has a complete set of chromosomes. Centromeres are the location that spindles attach to the chromosomes, and centrioles anchor them in the cell poles. The nucleus and endoplasmic reticulum do not participate in this process.

Cytokinesis is the last processes of cell replication and division and consists of the cell membrane dividing into two.

During Meiosis, the cell starts with 2n in interphase, then duplicates its DNA to have 4n throughout prophase I, metaphase I, and anaphase I. The cell then splits in two during telophase/cytokinesis, and there are 2n during prophase II, metaphase II, and anaphase II. The end product after telophase II is 1n.