What is Mitosis and Meiosis? MITOSIS - When The Cell Splits Apart What is mitosis? Eventually, cells need to duplicate. There are two main methods of replication: mitosis and meiosis. This tutorial will talk about mitosis. The big idea to remember is that mitosis is the simple duplication of a cell and all its parts. It duplicates its DNA and that the two new cells, daughter cells, have the same pieces and genetic code. Two identical copies come from one original. Beyond the idea that two identical cells are created, there are certain steps in the process. Before a cell divides, it is in resting phase. The cell in resting phase is not actually resting, since it has to maintain its reservoir before it undergoes division. This phase is called Interphase. It prepares the cell for division i.e. protein synthesis, DNA duplication and enzymes. After this phase comes Mitosis, which is further divided into Karyokinesis (division of nucleus) and Cytokinesis (division of cytoplasm). Karyokinesis involves the four basic steps of mitosis that are PROPHASE - METAPHASE - ANAPHASE - TELOPHASE. Here we start with PROPHASE. The chromosomes start coiling and become condensed. Nuclear envelope disintegrates. Gene transcription ceases during prophase and does not resume until late anaphase to early G1 phase. The nucleolus also disappears during early prophase. Centrosome gives rise to mitotic apparatus i.e. generation of spindle tubules. METAPHASE Now all the pieces are aligning themselves for the big split. The DNA lines up along a central axis and the DNA (chromatin) has now condensed into chromosomes. Two strands of chromosome are connected at the centre with centromere. The tubules connect to the centromere, not the DNA. ANAPHASE Here we go! The separation begins. Half of the chromosomes are pulled to one side of the cell; half go the other way. When the chromosomes get to the side of the cell, it's time to move on to telophase. TELOPHASE Now the division is finishing up. This is the time when the nuclear envelope begins to synthesize again along the separated chromosomes. Cytokinesis pinches the cell membrane in the middle. Cell membrane closes in and splits the cell into two. Now, we have two separate cells, each with half of the original DNA. Up till now, we have discussed what is mitosis and its different phases. Now it's important to know about significance of mitosis. SIGNIFICANCE It is an equational division through which identical daughter cells are produced having the same amount and type of genetic constitution as that of the parent cell. It is responsible for growth and development of multicellular organisms from a single-celled zygote. The number of chromosomes remains the same in all the cells produced by this division. Thus, the daughter cells retain the same characters as those of the parent cell. It helps the cell in maintaining proper size. Mitosis helps in restoring wear and tear in body tissues, replacement of damaged or lost part, healing of wounds and regeneration of detached parts as in the tail of lizards. It is a method of multiplication in unicellular organisms. If mitosis remains unchecked, it may result in uncontrolled growth of cells, leading to cancer or tumour. The next important phase of cell cycle is meiosis. Have you ever wondered why you are different from your siblings? The answer lies within the amazing process of meiosis. What is Meiosis? Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females. During meiosis, one cell divides twice to form four daughter cells. These four daughter cells only have half the number of chromosomes than the parent cell; they are haploid. Meiosis takes place in germ or sex cells called gametes; eggs in females and sperm in males. Similar to mitosis, cells also pass through the Interphase, G1, S and G2 phase before they enter Meiosis. Here are the distinct phases of meiosis: INTERPHASE MEIOSIS 1 CYTOKINESIS 1 MEIOSIS 2 CYTOKINESIS 2 First at interphase. In interphase, DNA is copied, resulting in two identical full sets of chromosomes to prepare for division. Outside of the nucleus are two centrosomes, each containing a pair of centrioles. These structures are critical for the process of cell division in next step at meiosis 1. Meiosis 1 takes place in following steps: Prophase 1 Prophase 1 is typically the longest phase of meiosis. During prophase 1, homologous chromosomes pair and exchange DNA (homologous recombination). The new combinations of DNA created during crossover are a significant source of genetic variation, and result in new combinations of alleles, which may be beneficial. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. The process of pairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural), (singular: chiasma). Prophase 1 has historically been divided into a series of substages which are named according to the appearance of chromosomes. Leptotene In this stage of prophase 1, individual chromosomes, each consisting of two sister chromatids, become "individualized" to form visible strands within the nucleus. Leptotene is of very short duration and it's when their progressive condensation and coiling of chromosome fibers takes place. Zygotene Chromosomes approximately line up with each other into homologous chromosome pairs through synaptonemal complex. The paired chromosomes are called bivalent or tetrad chromosomes. Pachytene This is the stage when homologous recombination, including chromosomal crossover (crossing over), occurs. Non-sister chromatids of homologous chromosomes may exchange segments over regions of homology. At the sites where exchange happens, chiasmata formed. Diplotene The synaptonemal complex degrades and homologous chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are at the transition to anaphase 1. Diakinesis It closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. Synchronous Process During these stages, two centrosomes, containing a pair of centrioles in animal cells, migrate to the two poles of the cell. The microtubules invade the nuclear region after the nuclear envelope disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule. Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centrosome. These are called nonkinetochore microtubules or polar microtubules. Metaphase 1 Homologous pairs move together along the metaphase plate: the paired homologous chromosomes align along an equatorial plane that bisects the spindle. Anaphase 1 Kinetochore microtubules shorten, pulling homologous chromosomes, which consist of a pair of sister chromatids, to opposite poles. Non kinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. Telophase 1 The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase 1. Cells may enter a period of rest known as interkinesis or interphase 2. No DNA replication occurs during this stage. Meiosis 2 Meiosis 2 is the second meiotic division and usually involves equational segregation, or separation of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The end result of meiosis 2 is production of four haploid cells, n chromosomes; (23 in humans). The four main steps of meiosis 2 are: prophase 2, metaphase 2, anaphase 2, and telophase 2. In prophase 2, we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division. In metaphase 2, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. This is followed by anaphase 2, in which the remaining centromeric cohesion is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. The process ends with telophase 2, which is similar to telophase 1, and is marked by the disassembly of the spindle and decondensation and lengthening of the chromosomes. Nuclear envelopes reform and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells.