Biology: Ch. 2 Reproduction

• In animals, autosome ( chromosomes that are not a sex chromosome) cells are diploid (2n) means they contain two copies of each chromosome.
• Germ cells, are haploid (n) containing one cope of each chromosome.
• Eukaryotic cells replicate through the cell cycle
• Cell cycle consists of four stages of G1, S, G2, and M.
• The first three stages of cell cycle of G1, S, and G2 are known as interphase.
• Interphase is the longest part of the cell cycle; even actively dividing cells spend about 90% of their time.
• Cells that do not divide spend all their time in an offshootof G1 called G0.
• During G0 stage, cells simply living and carrying out its function, without being divided.
• During interphase, individual chromosomes are not visible with light microscope bc they are in a less condensed form called chromatin. This is because DNA must be available to RNA polymerase so genes can be transcribed. During mitosis, it is preferable to condense the DNA into tightly coiled chromosome to avoid losing any genetic material during cell division.
• G1 Stage: Presynthetic Gap=>
• During G1 stage cells create organelles for energy and protein production of mitochondria, ribosomes, and endoplasmic reticulum, while also increasing their size. In addition, passing to S of sunthesis stage governed by a restriction point. Certain criteria, such as containing the proper complement of DNA, must be met for the cell to pass the restriction point and enter the synthesis stage.
• S stage is the synthesis of DNA
• During the S stage, the cell replicates its genetic material so that each daughter cell will have identical copies.
• After replication, each chromosome consists of two identical chromatids that are bound together at a specialized region known as the centromere.
• Humans have 46 chromosomes even though 92 chromatids are present.
• Cells entering G2 have twice as much DNA as cell sin G1
• G2 stage is the postsynthetic Gap
• In G2 stage, DNA has already been duplicated and the cell checks to ensure that there are enough organelles and cytoplasm for two daughter cells.
• In G2 stage, the cells make sure that DNA replication proceeded correctly to avoid passing on an error to daughter cells that may further pass on the error to their progeny.
• M stage is the mitosis and cytokinesis
• Mitosis is divided into four phases of prophase, metaphase, anaphase, and telophase
• Cytokinesis is the splitting of the cytoplasm and organelles btw the two daughter cells
• The cell cycle is controlled by check points, most notably btw G1 and S phase and the G2 and M phase

  
• At G1/S checkpoint, the cell determines if the condition of the DNA is good enough for synthesis. This check point is known as restriction point. If there is a damage to the DNA, the cell cycle goes into arrest until the DNA has been repaired. The main protein in control of this is p53.
• At the G2/M checkpoint, the cell is mainly concerned with ensuring that it has achieved adequate size and the organelles have been properly replicated to support the two daughter cells. P53 also plays a role in the G2/M checkpoint.
• The molecule responsible for the cell cycle are known as cyclins and cyclin-dependent kinases (CDK)
• In order to be activated, CDKs require the presence the right cyclins. These cyclins bind to CDKs, creating an activated CDK-cyclin complex. This complex can then phosphorylate transcription factors. Transcription factors can promote transcription of genes required for the next stage of the cell cycle.
• Cell cycle control is essential to ensure that cells that are damaged or inadequately sized do not divide.
• When cell cycle control is deranged and damage cells are allowed to undergo mitosis, cancer may result.
• One of the most common mutations found in cancer is mutation of the gene that produces p53, called Tp53
• When there is a mutation of the gene that produces p53, Tp53, the cell cycle is not stopped to repair the damaged DNA
• Cancer cell divide continuously w/o regard to the quality and quantity of the new cells produced.
• Often cancer cells undergo rapid cell division, creating tumors and if they start making right factors such as protease the damage cells are then able to reach other tissues reaching local invasion or distant spread of cancerous cells though bloodstream or lymphatic syste. the latter results known as metastasis.
• Mitosis is the process by which two identical daughter cells are created from a single cell.
• Mitosis consists of four phases of prophase, metaphase, anaphase, and telophase- occurs in somatic cells,, or cells that aren't involved in sexual reproduction
• Prophase involves in condensation of the chromatin into chromosomes. Also, the centriole pairs separate and move toward opposite poles of the cell.
• These centriole pairs of paired cylindrical organelles are located outside of nucleus in a region called centrosome and are responsible for the correct division of ZDNA
• WHen centrioles migrate to opposite sites of the pole, they begin forming spindle fibers, which are made of microtubules.
• This is kinetochores serve as attachment points for specific fibers of spindle apparatus or kinetochore fibers

  
• In metaphase, the centriole pairs are now at the opposite ends of the cel. The kinetochore fiber interacts with the fiber of spindle apparatus to align the chromosome at the metaphase plate ( equatorial plate), which is equidistant from the two poles of the cells.