Cell division.pptx

Cell Division P R E S E N T E D B Y: R. A B I E L A C H I N E E B S c. P s y c h o l o g y, M S c. P s y c h o l o g i c a l Counselling  Introduction Every day, every hour, every second one of the most important events in life is going on in your body—cells ar...

Cell Division P R E S E N T E D B Y: R. A B I E L A C H I N E E B S c. P s y c h o l o g y, M S c. P s y c h o l o g i c a l Counselling  Introduction Every day, every hour, every second one of the most important events in life is going on in your body—cells are dividing. When cells divide, they make new cells. A single cell divides to make two cells and these two cells then divide to make four cells, and so on. We call this process "cell division" and "cell reproduction," because new cells are formed when old cells divide. The ability of cells to divide is unique for living organisms.  What is cell division? Cell division is the process cells go through to divide. Why Do Cells Divide? Cells divide for many reasons. For example, when you skin your knee, cells divide to replace old, dead, or damaged cells. Cells also divide so living things can grow. When organisms grow, it isn't because cells are getting larger. Organisms grow because cells are dividing to produce more and more cells. In human bodies, nearly two trillion cells divide every day.  How Many Cells Are in Your Body? You and I began as a single cell, or what you would call an egg. By the time you are an adult, you will have trillions of cells. That number depends on the size of the person, but biologists put that number around 37 trillion cells. Yes, that is trillion with a "T." How Do Cells Know When to Divide? In cell division, the cell that is dividing is called the "parent" cell. The parent cell divides into two "daughter" cells. The process then repeats in what is called the cell cycle. Cells regulate their division by communicating with each other using chemical signals from special proteins called cyclins. These signals act like switches to tell cells when to start dividing and later when to stop dividing. It is important for cells to divide so you can grow and so your cuts heal. It is also important for cells to stop dividing at the right time. If a cell can not stop dividing when it is supposed to stop, this can lead to a disease called cancer. Cont... Some cells, like skin cells, are constantly dividing. We need to continuously make new skin cells to replace the skin cells we lose. Did you know we lose 30,000 to 40,000 dead skin cells every minute? That means we lose around 50 million cells every day. This is a lot of skin cells to replace, making cell division in skin cells is so important. Other cells, like nerve and brain cells, divide much less often.  How Cells Divide? Depending on the type of cell, there are two ways cells divide—mitosis and meiosis. Each of these methods of cell division has special characteristics. One of the key differences in mitosis is a single cell divides into two cells that are replicas of each other and have the same number of chromosomes.  This type of cell division is good for basic growth, repair, and maintenance. In meiosis a cell divides into four cells that have half the number of chromosomes. Reducing the number of chromosomes by half is important for sexual reproduction and provides for genetic diversity  02 types of cell division Mitosis-Mitosis is how somatic—or non- reproductive cells—divide. Somatic cells make up most of your body's tissues and organs, including skin, muscles, lungs, gut, and hair cells. Reproductive cells (like eggs) are not somatic cells Meiosis-The purpose of meiosis is to produce gametes, or sex cells.  Mitosis In mitosis, the important thing to remember is that the daughter cells each have the same chromosomes and DNA as the parent cell. The daughter cells from mitosis are called diploid cells. Diploid cells have two complete sets of chromosomes. Since the daughter cells have exact copies of their parent cell's DNA, no genetic diversity is created through mitosis in normal healthy cells. Mitosis cell division creates two genetically identical daughter diploid cells The Mitosis Cell Cycle Before a cell starts dividing, it is in the "Interphase." It seems that cells must be constantly dividing (remember there are 2 trillion cell divisions in your body every day), but each cell actually spends most of its time in the interphase. Interphase is the period when a cell is getting ready to divide and start the cell cycle.  During this time, cells are gathering nutrients and energy. The parent cell is also making a copy of its DNA to share equally between the two daughter cells.  Stages of Mitosis The mitosis division process has several steps or phases of the cell cycle—interphase, prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis—to successfully make the new diploid cells. Right before prophase, the cell spends most of its life in the interphase, where preparations are made before the beginning of mitosis (the DNA is copied). However, since the actual process involves the division of the nucleus, the prophase is technically the first stage of this process. The different stages of mitosis occurring during cell division are given as follows-  Interphase Before entering mitosis, a cell spends a period of its growth under interphase. It undergoes the following phases when in interphase: G1 Phase: This is the period before the synthesis of DNA. S Phase: This is the phase during which DNA synthesis takes place. G2 Phase: This is the phase between the end of DNA synthesis and the beginning of the prophase.  Prophase Prophase immediately follows the S and G2 phases of the cycle and is marked by condensation of the genetic material to form compact mitotic chromosomes composed of two chromatids attached at the centromere. The completion of the prophase is characterised by the initiation of the assembly of the mitotic spindle, the microtubules and the proteinaceous components of the cytoplasm that help in the process. The nuclear envelope starts disintegrating. Prophase  Prometaphase and Metaphase Prometaphase In the prometaphase, the nuclear envelop disintegrates. Now the microtubules are allowed to extend from the centromere to the chromosome. The microtubules attach to the kinetochores(centromere) which allow the cell to move the chromosome around. Metaphase At this stage, the microtubules start pulling the chromosomes with equal force and the chromosome ends up in the middle of the cell. This region is known as the metaphase plate. Thus, each cell gets an entire functioning genome. Metaphase  Anaphase The splitting of the sister chromatids marks the onset of anaphase. These sister chromatids become the chromosome of the daughter nuclei. The chromosomes are then pulled towards the pole by the fibres attached to the kinetochores of each chromosome. The centromere of each chromosome leads at the edge while the arms trail behind it. Sister Chromatids-two identical copies of the same chromosome formed by DNA replication,attached to each other by a structure called the centromere. Anaphase  Telophase The chromosomes that cluster at the two poles start coalescing(come together) into an undifferentiated mass, as the nuclear envelope starts forming around it. The nucleolus, Golgi bodies and ER complex, which had disappeared after prophase start to reappear.  Telophase Telophase is followed by cytokinesis, which denotes the division of the cytoplasm to form two daughter cells. Thus, it marks the completion of cell division.  Meiosis Phases of Meiosis Before meiosis, the DNA is replicated, as in mitosis. Meiosis then consists of two cell divisions, known as meiosis I and meiosis II. In the first division, which consists of different phases, the duplicated DNA is separated into daughter cells. In the next division, which immediately follows the first, the two alleles of each gene are separated into individual cells.  Prophase I Prophase I, the first step in meiosis I, is similar to prophase in mitosis in that the chromosomes condense and move towards the middle of the cell. The nuclear envelope degrades, which allows the microtubules originating from the centrioles on either side of the cell to attach to the kinetochores in the centromeres of each chromosome. Cont.... Unlike in mitosis, the chromosomes pair with their homologous partner. This step does not take place in mitosis. At the end of prophase I and the beginning of metaphase I, homologous chromosomes are primed for crossing-over. Homologous chromosome-Chromosomes that share the same structural features(ex; same size,same banding patterns,same centromere positions). Cont... Between prophase I and metaphase I, homologous chromosomes can swap parts of themselves that house the same genes. This is called crossing-over. Metaphase 1 In metaphase I of meiosis I, the homologous pairs of chromosomes line up on the metaphase plate, near the center of the cell.  Anaphase I Much like anaphase of mitosis, the chromosomes are now pulled towards the centrioles at each side of the cell. However, the centrosomes holding the sister chromatids together do not dissolve in anaphase I of meiosis, meaning that only homologous chromosomes are separated, not sister chromatids.  Telophase 1 In telophase I, the chromosomes are pulled completely apart and new nuclear envelopes form. The plasma membrane is separated by cytokinesis and two new cells are effectively formed.  Results of Meiosis I Two new cells, each haploid in their DNA, but with 2 copies, are the result of meiosis I. Again, although there are 2 alleles for each gene, they are on sister chromatid copies of each other. These are therefore considered haploid cells. These cells take a short rest before entering the second division of meiosis, meiosis II.  Phases of Meiosis II Prophase II Prophase II resembles prophase I. The nuclear envelopes disappear and centrioles are formed. Microtubules extend across the cell to connect to the kinetochores of individual chromatids, connected by centromeres. The chromosomes begin to get pulled toward the metaphase plate.  Metaphase II Now resembling mitosis, the chromosomes line up with their centromeres on the metaphase plate. One sister chromatid is on each side of the metaphase plate. At this stage, the centromeres are still attached by the protein cohesin.  Anaphase II The sister chromatids separate. They are now called sister chromosomes and are pulled toward the centrioles. This separation marks the final division of the DNA. Unlike the first division, this division is known as an equational division, because each cell ends up with the same quantity of chromosomes as when the division started, but with no copies.  Telophase II As in the previous telophase I, the cell is now divided into two and the chromosomes are on opposite ends of the cell. Cytokinesis or plasma division occurs, and new nuclear envelopes are formed around the chromosomes.  Results of Meiosis II At the end of meiosis II, there are 4 cells, each haploid, and each with only 1 copy of the genome. These cells can now be developed into gametes, eggs in females and sperm in males.