Mitosis and Meiosis

Mitosis and Meiosis Francisco Estrada Period 2

Differentiation Mitosis Meiosis I • Sexual Reproduction • Meiosis consist of two successive nuclear divisions Meiosis I and Meiosis II • Prophase I: Pairing of homologous chromosomes occurs leading to the formation of tetrads, following by crossing over between homologous chromosomes. • Asexual Reproduction • Prophase: The sister chromatid pair together to form dyads, but homologous chromosomes do not pair together. The events of crossing over do not take place during mitosis. • Metaphase: chromosomal dyads made up of two sister chromatids align at the equatorial plate

Differentiation • Anaphase: Centromeres split and the sister chromatids are seperated in this phase. These sister chromatids are then pulled towards the opposite ends, to be assorted into the daughter cells. • Telophase: Two genetically identical daughter cells are formed marking the end of mitosis. Genetic variation is not introduced due to the lack of crossing over. • Metaphase I: Chromosomal tetrads align at the equatorial plate during metaphase I. • Anaphase I: the centromeres remain intact. Chromosomes separate from their homologous partners, but the pairs of the sister chromatids remain intact during anaphase I. The pairs split up during metaphase II.

Differentiation • Telophase I: Two haploid cells with duplicate copies of chromosomes are formed after telophase I. Telophase II, leads to the formation of four genetically distinct haploid cells.

The Important Roles of Mitosis • Like many things, cells wear out and die. If an organism is to live and grow it must reproduce. Therefore cell division serves an important role in an organism's health and growth. Cell division occurs rapidly in living organisms. • Mitosis refers to the process by which cells multiply. The importance is that the process enables your cells to reproduce and regenerate tissue in the body. Single celled organisms reproduce in this way. • It is important that the cells divide and replace old worn out cells and more importantly be able to replicate the duties of the cells they replace. • It is also important for genetic stability. By duplicating the exact copy of our genetic material it ensures that our genetic material is stable and able to carry out its function correctly

Regenerating Cells • It is also important for cell replacement, regeneration. • An example for cell replacement: a lizard tail is a good example of cells regenerating when the tail is detached from the lizard body. The lizard is able to regenerate another one by the process of mitosis.

The Important Roles of Meiosis • Meiosis performs a key task necessary in a sexual life cycle. • In animal life cycles, the meiotic cell division in the life cycle immediately precedes the development of gametes. • To create gametes that are haploid. • Divides one nucleus into four. • Meiosis reduces the number of chromosomes in half. • Meiosis produces genetically different daughter of nuclei • Meiosis increases genetic diversity, continue evolution, and maintain a species.

Phases of Mitosis • Prophase: During prophase you can see the duplicated chromatids attach to their centromere. Also during prophase the cells starts to build a spindle, a fanlike system of microtubulus that will help separate the duplicated chromosomes. Then the centrioles start to move toward the opposite poles.

Metaphase • During metaphase, the centromeres of the duplicated chromosomes line up across the center of the cell. Spindle fibers connect the centromeres of each chromosomes to the two poles of the spindle.

Anaphase • Anaphase begins when sister chromatids suddenly separate and begin to move apart. • The chromosomes separate and move along spindle fibers to the opposite ends of the cell.

Telophase • The chromosomes begin to spread out into a tangle of chromatin. A nuclear envelope re-forms around each cluster of chromosomes. • The spindle begins to break apart, and a nucleolus becomes visible in each daughter nucleus.

Forming New Cells • Step 1: The cells gets ready for mitosis. The duplicated chromosomes are held together. Fibers made of protein begin to form that will eventually help pull the pairs of chromosomes apart. • Step 2: The membrane that surrounds the cell’s nucleus brakes apart and the chromosome duplicates line up at the middle of the cell. The fibers have become stronger and attach at both ends of the cell as well as to each chromosome.

Step 3: The thick fibers attached to opposite ends of the cell pull the duplicated chromosomes apart into two groups. Step 4: A nucleus membrane forms around both groups of chromosomes and the rest of the cell begin to divide. With the same genetic material, these two cells are just like the one they were made from.

Maintaining Chromosomes • Mitosis – separation of chromosomes into two identical sets of daughter cells • In mitosis chromosomes separates and form into two identical sets of daughter nuclei, and it is followed by cytokinesis (division of cytoplasm). In mitosis the mother cell divides into two daughter cells which are genetically identical to each other and to the parent cell.

Process of Meiosis

Meiosis I • Prophase I: Homologous Chromosomes in the nucleus begin to pair up with one another and then split into chromatids (one half of a chromosome) where crossing over can occur. Crossing offer can increase genetic variation. • Metaphase I: Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up is at random increasing genetic variation by independent assortment.

Anaphase I - The homologous chromosomes move to opposing poles from the equator Telophase I - A new nuclei forms near each pole alongside its new chromosome compliment. At this stage two haploid cells have been created from the original diploid cell of the parent.

Meiosis II • Prophase II: The nuclear membrane disappears and the second meiotic division is initiated. • Metaphase II: Pairs of chromatids line up at the equator. • Anaphase II: Each of these chromatid pairs move away from the equator to poles from spindle fibers. • Telophase II: Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote.

This process of meiosis creates gametes to pass genetic information from parents to offspring.

Crossing-Over • During meiosis, homologous chromosomes are paired together, there are points along the chromosomes that make contact with the other pair. This point of contact is deemed the chiasmata, and can allow the exchange of genetic information between chromosomes.

Independent Assortment • Random distribution of maternal and paternal homologous to the gametes. Random distribution of genes located on different chromosomes

Formation of Haploid Gametes • Meiosis results haploid gametes by crossing-over and independent assortment. • Independent Assortment: During Metaphase I the homologous pairs (consisting of one maternal and one paternal chromosome) are situated at the metaphase plate. Each pair may orient its maternal or paternal homologous closer to either pole. Each of the pairs are positioned independently, each side have a 50% chance of receiving either maternal or paternal chromosomes. • Crossing Over: During Phrophase I homologous chromosomes pair loosely along their lengths and the exchange of two corresponding segments of two non-sister chromatids (one paternal and one maternal) occurs.

Cancer and Mutation • The abnormalities in cancer cells usually result from mutations in protein-encoding genes that regulate cell division. Over time more genes become mutated. This is often because the genes that make the proteins that normally repair DNA damage are themselves not functioning normally because they are also mutated. Consequently, mutations begin to increase in the cell, causing further abnormalities in that cell and the daughter cells. Some of these mutated cells die, but other alterations may give the abnormal cell a selective advantage that allows it to multiply much more rapidly than the normal cells.

Mitosis and Meiosis