[Music] this video is going to focus on how cells divide both the nucleus and the cytoplasm cells must make copies of themselves if the population of single celled organisms is to be maintained multicellular organisms also need to make copies of their cells to keep growing and to repair themselves cell division is the process of one cell dividing to produce two cells it is important that during cell division the genetic information is passed on to the next generation of cells as well as copies of all organel necessary for proper cell functioning the parent cell is the cell that divides and the two new cells are called the daughter cells remember that this is one tenant of the cell theory all cells come from pre-existing cells in procaryotic cells with a single chromosome of DNA the cell divides by binary fision where DNA is copied and the cell simply divides into two daughter cells in UK carotic cells where there are multiple chromosomes the cell will undergo mitosis for sexually reproducing organisms egg and sperm two divisions are required producing four daughter cells this process is called meiosis when a cell gets to a certain size it needs to divide into two this is done by cyto Kinesis which is division of the cytoplasm and follows the final stage of nuclear division mitosis during division the organel of the cell are distributed evenly between the cells the process is different in animal and plant cells due to the rigid cell wall of plants in animal cells the plasma membrane pinches in forming a cleavage Furrow which is an indentation in the plasma membrane a contractile ring made of actin mein and other proteins forms and that pinches the cell membrane together to divide the cytoplasm because of the cell wall plant cells form a cell plate the cell plate is built by vesicles formed from the Gogi apparatus these vesicles line on the equator where they combine to form new plasma membrane and cell walls between the two new cells when parent cells divide to produce two daughter cells in body tissue the division of cytoplasm in organel is equal so that each new daughter cell is identical and equal in size to the parent cell cells undergoing mitosis have equal division of cytoplasm as does sperm cells in meiosis and example of when this division of cytoplasm is not equal is the production of egg cells U Genesis during this process only one large egg or ovam is produced as well as two to three smaller cells called polar bodies these do not go forward and are not available for fertilization U Genesis is a process that begins in the fetus before birth and the final development is finished during the adult life an uite is an immature egg cell or ovam when the primary uite divides the cytoplasm divides unevenly forming a secondary uite and the first polar body then when the secondary uite divides another polar body is formed polar bodies are holding cells for the genetic material not used and they degenerate their cytoplasm and cellular components except for the genetic material are absorbed by the final ovam you will learn later that the sperm contributes genetic material only it does not provide cytoplasm or organel in the fertilization proc process thus the egg must have everything the growing embryo requires before cells divide they grow cells cannot continue to keep growing because there is a point where the surface area to volume ratio is inadequate to allow transfer of materials into the cell and removal of waste materials and products out of the cell this is when the cell will divide surface area to volume ratio is discussed in more detail in B 2.3 sometimes the difference between cell division and nuclear division can be confusing mitosis is the division of the nucleus while cyto canis is a division of the cytoplasm and thus the cell it is important that nuclear division occur before cell division to avoid producing a cell that does not have a nucleus a cell without a nucleus is called a nucleate there are two processes that divide the nucleus mitosis which occurs in somatic cells and meiosis which occurs in gametes mitosis results in the production of two daughter cells that are genetic gentically identical to the parent especially with respect to genetic material this means that each daughter cell has the same genome as the parent as all genetic information is preserved meiosis is the process that occurs in gamt or sex cells it is very important that each Egen sperm have half of the genetic material in humans that is 23 chromosomes during fertilization the egg will have 23 chromosomes and the sperm will have 23 chromosomes with the resulting zygote having four 6 the defining number of chromosomes in a human diploid cell is a term used to describe a cell that has all the required DNA while haid is a term used to describe a cell that has half the amount of required DNA somatic cells are diploid and during mitosis produce diploid cells while meiosis produces haid cells meiosis is a process that produces four daughter cells each having only half the number of chromosomes and each one of those daughter cells are genetically different from their parent and each other this ensures there is genetic diversity and Offspring while in the diagram shows four genetically different daughter cells remember our earlier discussion that in U Genesis only one true egg is formed with two to three polar bodies this unequal division of cytoplasm does not occur in the production of sperm before cell divides the DNA must be copied to ensure that each daughter cell has the genetic material necessary this occurs in the S phase or synthesis phase of the cell cycle the actual process of DNA replication is covered in another video DNA replication produces double stranded chromosomes and each strand is called a cister chromatid and are genetically identical held together by A centrom Mir a centromere is a DNA sequence that can be seen as a constricted area of a chromosome during division the two sister chromatids are pulled apart with one for each daughter cell do not be confused by the terminology a chromosome is one doubl stranded DNA molecule humans have 46 or 23 pairs during the S phase of the cell cycle the chromosomes replicate whenever you see the familiar X shape of a chromosome remember that occurs during DNA replication and you are seeing the two sister chromatids connecting at the center mirror the total length of all a human's DNA all 46 chromosomes is about 2 m during most of the life of a cell the DNA is spread out in long chains so that DNA is not broken and Tangled it must be condensed in a process that wraps DNA around proteins called histones This is similar to how thread is wrapped around a spool to keep the thread from breaking and tangling the histone proteins are essential in organizing the DNA within the nucleus and are positively charged with a high concentration of amino acids such as lysine and Arginine a nucleosome is the name given to the DNA wrapped around the histone proteins the nucleosome is a unit of eukaryotic chromosome structure and consists of eight histone proteins wrapped by two coils of DNA that consists of approximately 220 base pairs DNA is wrapped around the histone proteins and wrapped again in a process called super coiling this places the coils on top of each other which forms a very compact pair of chromatids the term chromatin is used to refer to DNA that is associated with hisone proteins after supercoiling is finished the chromosome takes that familiar X shape this packaging of DNA is very important as it allows DNA to be moved as one rather than being spread out throughout the nucleus an organel in the cell called a center Zone produces spindal fibers made of microtubules that are used to guide the chromosomes to the correct place before they can divide microtubules are not permanent and can be assembled and disassembled as needed one end of the microtubule is negative and the other end is positive which gives them directionality motor proteins within the cell are used to push and pull items around the cell the microtubules are the tracks on which the motor proteins travel motor proteins can also attach to two microtubules and slide past each other this is an active process and utilizes ATP to provide a confirmational change needed to move the microtubules there are three types of microtubules the Asal microtubules reach out from the center Zone while the kinetic cor microtubules attached to the central mirror of the chromosome the third type the overlapping microtubules are not attached to any chromosomes but rather pass beside them motor proteins are between the overlap proteins and move along the microtubules in a way that pushes the microtubules in opposite directions during nuclear division the cyto chromatids are pulled apart each one being transported to one half of the cell interphase is the term used to describe the cell when not undergoing cellular division during interphase the cell is undergoing normal cell functions such as cellular respiration and protein synthesis as well as growing and preparing to divide mitosis is the process that will produce two genetically identical daughter cells and consists of four phases prophase metaphase anaphase telophase think of prophase as cells preparing to divide the chromatin becomes visible as they become more tightly coiled to form chromosomes the spindal fibers form and the centrosomes move to the poles of the cell the nuclear membrane begins to disappear as does the nucleis think of metaphase as middle as the chromosomes move to the equator of the cell called the metaphase plate the center mirrors line up on the plate and the spindle fibers are attached to the kinetic ores the centrosomes are at opposite po of the cell during anaphase think up heart chromatids have separated into chromos zones they move to the opposite poles of the cell by the motor proteins pushing the microtubules in opposite directions at the end of anaphase each pole has a chromosomes needed for a new cell telophase is the last phase think to nuclei and finds a set of chromosomes at each pole the nuclear membrane begins to reform around each set of chromosomes and then nucleolus reappears the spindle apparatus is gone and the cell elongates getting ready to undergo cyto canis looking at this micrograph of mitosis you are able to see all phases of mitosis prophase showing the condensed chromosomes metaphase showing where the chromosomes are lined up on the metaphase plate you can see the chromosomes pulling apart during anaphase and in telophase there is a cell forming a cell plate showing the two nuclei and the cell beginning to undergo cyto canis mitosis produces two genetically identical diploid cells while meiosis produces four genetically different hloy cells meiosis is nuclear division in sex cells and consists of two stages of division meiosis 1 and myosis 2 meiosis is the process that reduces the number of chromosomes from diploid to hpid the first stage of meiosis meiosis 1 is when the chromosome number is haved the names of the phases are the same as mitosis only we add either a one or two to identify which phase many of the processes that happen in meiosis may sound similar to mitosis in prophase one chromosomes become visible and more compact homologous chromosomes are attached to each other and undergo crossing over crossing over only occurs in prophase 1 and parts of the homologous pairs can be exchanged changed the homologous chromosomes together are called a balent bindal fibs begin to form and the nuclear membrane begins to disappear as does the nucleolus in metaphase 1 the homologous chromosomes line up on the metaphase plate and they are randomly oriented which means that either of the chromosomes from the pair is equally likely to be pulled to either pole during anaphase 1 the spindle fibers pull the chromosomes apart to the poles during telophase 1 the spindle fibers disappear and two new nuclear membranes form cyto canis occurs after telophase 1 the cells are now haploid as they only contain one chromosome of each pair but the sister chromatids are still attached so there is not an S phase where chromosomes are duplicated it is during myosis 2 that the cyto chromatids will separate in prophase 2 the DNA condense again but there is no crossing over new spindle fibers form in metaphase 2 the nuclear membrane is gone and the individual chromosome line up randomly along the metaphase plate bindle fibers attach to the cyto chromatids the cyto chromatids are pulled apart during anaphase 2 nuclear membranes form around each of the four new cells a cell plate forms in plant cells and in animal cells the cell membrane pinches in cyto canis can now occur there are some instances when mistakes can be made during meiosis which can result in The Offspring receiving an incorrect number of chromosomes which can include extra or missing chromosomes in humans an example is Down syndrome or triem 21 in this case the child has three copies of chromosome 21 non-disjunction is the term used when chromosomes do not separate and can occur in meiosis one when the B veence fail to separate or during meiosis 2 when the cyto chromatids fail to sep separate if this happens during formation of an egg the egg will have two chromosomes of 21 and if that egg is fertilized The Offspring will now have three chromosomes of 21 two from the mother and one from the father non-disjunction can occur in other chromosomes as well three chromosomes of 18 called Edward syndrome and three chromosomes of 13 called patau syndrome Tricom of other chromosomes are usually not viable and are a frequent cause of miscarriage meiosis generate genetic diversity by crossing over and random orientation during interphase when DNA is copied the cell makes a complete copy of all his genetic information this results in each chromatid having an identical copy or sister chromatid attached to it because meiosis produces four genetically different cells the cells must divide twice meiosis one produces two cells and then each of these cells divides during meiosis 2 producing four cells a major difference between mitosis and meiosis is that there is no no crossing over in mitosis crossing over only occurs in prophase 1 and happens when two homologous chromosomes but not cister chromatids twist around each other break at the same place and connect to the other the place where the two chromosomes cross is called a kayma plural is a kayata virtually all pairs of chromosomes undergo crossing over and form at least one kayma with most having more than one place where crossing over occurs crossing over increases genetic variability due to DNA being exchanged between maternal and paternal DNA random orientation is how the chromosomes line up on the metaphase plate in both metaphase 1 and two each pair lines up independent of how the other chromosomes line up how they line up will determine what chromosomes are in each resulting cell the chance of lining up the same twice can be calculated by two superscript n for humans that is 2 to the 23rd or over 8 million ways the chromosomes can line up this creates a tremendous amount of variability [Music]