Cell Cycle and Division of a Cancer Cell

Cell Cycle and Division of a Cancer Cell. From Biology, Mc. Dougal Littell, 2008.

Cell Division • In order to reproduce, most cells will divide at some point in their lifespan. The actual process varies, depending on the cell type. • Somatic, or non-reproductive cells, divide through the process of mitosis. • Gametic, or reproductive cells, undergo mitosis then utilize a second process called meiosis.

Cell Cycle and Interphase • For most cells, the majority of their lifespan is actually spent in interphase, the time in between cell divisions when they perform normal functions.

• During interphase, the DNA takes the form of chromatin. – Chromatin consists of long strands of DNA wrapped around a cluster of protein called a histone. • During mitosis, the chromatin condenses into chromosomes.

• Human cells have 46 chromosomes. This includes 23 from the mother, and 23 from the father.

• Cells in interphase can be identified under a microscope by looking for nuclei that contain chromatin, as opposed to the more condensed worm-like chromosomes. – The nucleolus may also be visible as a dark area.

• Interphase consists of three phases. – During the G 1 phase, the cell replicates its organelles and cytosol so there is enough for two functional cells. – During the S phase, the DNA of the cell is completely duplicated. – During the G 2 phase, any additional growth needed by the cell before division takes place.

Mitosis • Mitosis is the part of the cell cycle where the nucleus physically divides into two, each containing an exact copy of the original DNA.

• During prophase, the DNA begins to condense, forming chromosomes. • The nucleolus disappears and spindle fibers formed by centrioles attach to the centromeres. • There are two joined copies of each chromosome before mitosis, called chromatids. – The chromatids are connected by a single point, called the centromere.

• Metaphase begins as the chromosomes move and align themselves in a narrow central zone in the cell.

• Chromosomes cannot move independently. Their migration occurs as a result of a network of protein microtubules called the spindle apparatus produced by the centrioles.

• Anaphase begins when the centromere of each chromatid pair splits. – Now called daughter chromosomes, they are pulled towards opposite ends of the cell by the spindle apparatus.

• With the chromosomes now fully divided, the nuclear membranes form during telophase. – The DNA begins returning to its uncoiled chromatin form. – The nucleolus becomes visible again.

Cytokinesis • The process of mitosis only separated the nucleus. The cytosol, organelles, and cell membrane are physically divided during cytokinesis. – The cells return to interphase at this point.

• Cytokinesis proceeds much differently in animal and plant cells. • In animal cells, cytokinesis causes the formation of a shallow indentation across the cell membrane called a cleavage furrow. – The furrow tightens, eventually pinching shut as the cells separate.

• In plant cells, vesicles containing cell wall material will collect at the middle of the dividing cell, forming a cell plate.

Cell Division • This is a time-lapse microscopy of an epithelial cell with fluorescent markers undergoing cell division. The entire process usually takes about 20 minutes.

Cell Cycle • Once cell division is complete, the daughter cells will re-enter interphase until receiving a signal to divide once again.

Cancer • Most cells of multicellular organisms do not divide constantly, rather, they have signals or conditions needed to initiate the process of mitosis. • Growth factors are proteins secreted by body cells that stimulate other cells to divide.

• Cells may also exhibit density-dependent inhibition, where they stop dividing when conditions become too crowded.

• Cells also exhibit anchorage dependence, meaning they must be in contact with a solid surface before they will divide. • Cancer cells disregard these normal controls and grow and divide uncontrollably, forming masses of cells called tumors. – If the tumor is encapsulated by other tissue and does not spread, it is considered benign. – If part of the tumor is able to break free and spread through the circulatory system of the body to other places to divide, it is considered malignant.

Cell Division and Reproduction • In multicellular organisms, mitosis is primarily used for tissue growth, regeneration, and repair. • In many unicellular organisms, mitosis is the primary form of reproduction.

Asexual Reproduction • Asexual reproduction is reproduction that involves a single parent producing an offspring. – The offspring produced are, in most cases, genetically identical to the single cell that produced them. – Asexual reproduction is a simple, efficient, and effective way for a single organism to produce a large number of offspring by using only mitosis. • Prokaryotic organisms reproduce asexually. • Only a few eukaryotic organisms, like sponges, are able to reproduce asexually.

Sexual Reproduction • In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow. – The offspring produced inherit some genetic information from both parents. – Most animals and plants, and some single-celled organisms, reproduce sexually. • Organisms that reproduce sexually do not produce clones – their offspring have more genetic diversity. – This requires the use of both mitosis and meiosis, a separate cell division process.

Spermatogenesis • The male gametes, called sperm, begin as stem cells in the testes. • During spermatogenesis, the formation of sperm, the stem cells will undergo mitosis produce two exact copies, called daughter cells. – One copy remains behind in the testes to generate future sperm. – One copy begins to migrate towards the epididymis.

• At this point, the gametes are given the diploid (2 n) notation, meaning they contain a full set of 46 chromosomes. • The daughter cell that migrates away will undergo a second type of cell division called meiosis, which will result in the production of four genetically different cells with only half the normal number of chromosomes. – The end products of meiosis are given the haploid (n) notation, since they only have half the normal number of chromosomes.

Meiosis • There are two separate sets of cell divisions that occur in meiosis. These are labeled as Meiosis I and Meiosis II.

• During prophase I, the homologous chromosomes from each of the father’s parents come together to form a tetrad instead of remaining separate. • The tetrads then line up in the middle during metaphase I.

Crossing Over • A unique phenomenon occurs in prophase I of meiosis, called crossing over. • The homologous chromosomes exchange segments, producing combinations of genes unique to those of the original stem cell. In this diagram, blue indicates chromosomes from the father, and red from the mother of the individual producing the games.

• Following anaphase I and telophase I, the diploid (2 n) stem cell has divided into two haploid (n) daughter cells.

• The stages of meiosis II mirror what happens normally during mitosis; the two haploid (n) daughter cells duplicate. – The end result is four haploid (n) genetically different daughter cells.

• Meiosis also occurs during oogenesis as the ovaries produce eggs, but cytoplasm is all concentrated in only one of the four daughter cells. – The rest are called polar bodies and are re-absorbed by the body.