 This video will cover the following objective from the reproductive system, define in contrast mitosis and meiosis. The major function of the reproductive system is to produce the next generation, to produce offspring. The process of reproduction involves two different kinds of cell division. Miosis or meiotic cell division produces the sperm and the egg. Sperm are also known as spermatozoa and the egg is also known as an ovum. The sperm are the male gametes and the ovum or eggs are the female gametes. Gametes are haploid cells meaning they only have 23 chromosomes, only one copy of each of the 23 chromosomes. When fertilization occurs, 23 chromosomes from the sperm join with 23 chromosomes in the egg to produce a zygote. The zygote is an embryo, a single celled embryo, but the zygote is a diploid cell. It has 23 pairs of homologous chromosomes, 23 from the sperm and 23 from the egg. Then the zygote will divide by mitosis or mitotic cell division. So while the gametes are produced by meiotic cell division, the majority of the cells in the body are produced by mitotic cell division or mitosis. So mitosis produces two genetically identical daughter cells from a diploid cell. And as the zygote divides to produce two cells and then four cells, then eight and so on, the embryo will grow during prenatal development to produce all of the tissues and organs and organ systems of the infant. And so the process of meiosis produces four genetically unique haploid gametes and the process of mitosis produces two genetically identical diploid cells. Chromosomes contain the genetic instructions in the form of DNA wrapped around proteins known as histones. Chromosomes are linear strands of DNA wrapped around protein. There are 23 pairs of homologous chromosomes in a diploid cell. The chromosomes we're looking at here, the chromosomes were taken from a leukocyte from a female human. So a leukocyte is a somatic diploid cell, one of the many cells in the body that has 23 pairs of homologous chromosomes. And so we can see all the chromosomes are lined up here from the longest chromosome, which was number one, all the way to the shortest chromosomes, number 21 and 22. And then the 23rd pair of chromosomes are the sex chromosomes. And because these are the chromosomes from a female cell, both of the sex chromosomes are X chromosomes. In contrast, in a male cell, there would be one X chromosome and one Y chromosome forming the 23rd pair. This illustration shows us the phases of the cell cycle. The cell cycle starts with a newborn cell entering into G1 or the first cell growth phase. Then the cell will move into S phase when DNA synthesis occurs to make a new copy of the genetic instructions. And so after S phase, the cell has replicated chromosomes where there are two sister chromatids held together by a centromere within each chromosome. Following S phase, the cell goes into G2 phase, a second cell growth phase, which prepares for mitosis. And the mitotic phase, mitosis is the division of the nucleus. Each new nucleus that's formed during mitosis will contain a complete set of the chromosomes. And then cytokinesis ends mitosis. Cytokinesis is the division of the cytoplasm to form two new daughter cells. And one nucleus will be inside of each of the new daughter cells that's formed during cytokinesis. Then following cytokinesis, the two daughter cells enter into G1 phase to start a new cell cycle. There are five phases to mitosis. Prophase is when the chromosomes condense to become visible under the microscope. So before prophase, the DNA wrapped around proteins was uncondensed in the form of chromatin. Chromatin then condenses during prophase. And at the same time, the centrosomes will start to produce microtubules that grow towards the nucleus to become what's known as the mitotic spindle. So the chromosomes are found within the nucleus surrounded by the nuclear envelope. That nuclear envelope starts to break down during prophase. So then in prometaphase, now that the nuclear envelope has broken down and the mitotic spindle has formed, the mitotic spindle produced microtubules that will bind to the chromosomes. Then metaphase is when the mitotic spindle pulls the chromosomes until they line up right at the midline. And so metaphase is distinguished by all the chromosomes lined up right at the midline of the cell, also known as the metaphase plate, where all the chromosomes are lined up on the midline. Then during anaphase, the chromosomes are pulled apart. And so the replicated chromosomes that were lined up during metaphase, there were two copies of each chromosome known as sister chromatids. And those sister chromatids separate during anaphase. Then telephase is when the nuclear envelope forms around the chromosomes, and those chromosomes start to uncoil, decondense to return to the uncondensed state as chromatids spread through the nucleus. And so telephase is the last phase of mitosis. Following mitosis, which is technically the division of the nucleus, the step of cytokinesis divides the cytoplasm to produce two new daughter cells. During cytokinesis, the plasma membrane starts to constrict at the midline forming a cleavage furrow, and that cleavage furrow eventually pinches off to separate the cytoplasm into two new daughter cells with a nucleus in each cell. And so the two daughter cells produced during mitosis are genetically identical cells. Each daughter cell contains 23 pairs of homologist chromosomes or 46 chromosomes in each of those diploid daughter cells produced in mitotic cell division. In contrast to mitosis, meiosis involves two rounds of cell division, and meiosis will produce four genetically unique haploid daughter cells from one diploid cell. The diploid cell will have DNA replication during S phase and then move into G2 phase, and then when it moves into the M phase of the cell cycle, instead of entering into a mitotic division, the cell will enter into meiosis I. So meiosis has the same basic names of its phases as what we saw for mitosis during pro phase I of the first round of myotic cell division. The nuclear envelope, the centrosomes will start to organize the mitotic spindles. During pro metaphase, the mitotic spindles, microtubules will attach to the chromosomes. During metaphase I of meiosis I, the chromosomes will all line up at the midline of the cell. Then during anaphase I of meiosis, when the chromosomes separate and are pulled to opposite ends of the cell by the mitotic spindle, it is the homologous chromosomes that separate, and they're still replicated with two sister chromatids within each of those homologous chromosomes. And so at the end of anaphase I, the homologous chromosomes are separated and pulled to opposite sides of the cell. And then in telophase I, the nuclear envelope will reform around those chromosomes so that each nucleus will contain 23 replicated chromosomes. Then following telophase I is a cytokinesis step to divide the cytoplasm producing two new daughter cells. Each of those daughter cells contains 23 replicated chromosomes. Now we'll go through each of the phases again, but this is the second round of meiosis. So we'll indicate these as that phase number two for the second round of meiosis, such as pro phase two is the beginning of meiosis two. And pro phase two is similar to pro phase one or the pro phase we saw during mitosis. During pro phase two, the nuclear envelope breaks down and the microtubules start to grow from the centrosome to form the mitotic spindle. Then during pro metaphase two, the mitotic spindles, microtubules attach to the chromosomes. During metaphase two, the mitotic spindles pull all of the chromosomes to the midline of the cell to line up at the metaphase plate. Then during anaphase two, what occurs is a little different than what we saw during anaphase one. During anaphase two, sister chromatids separate and are pulled to opposite ends of the cell. And so the telephase two will reform the nuclear envelope around the chromosomes so that there are two nuclei within each cell. And within each of those nuclei, there are 23 chromosomes that have been separated so they're not replicated chromosomes. There's just one copy of each of the 23 chromosomes. And then cytokinesis will divide the cytoplasm to produce two daughter cells from each cell. And you'll notice that meiosis two is happening for each of the daughter cells that exited meiosis one. And so at the end of cytokinesis for each of those daughter cells will have two haploid cells produced. And so from each diploid cell that enters into meiosis, there will be four haploid genetically unique cells produced. So there's two processes that contribute to the unique genetic instructions that are in the haploid cells. During prophase of meiosis, there's an event that occurs called crossing over. Crossing over occurs during prophase one of meiosis. And it is essentially a mechanism that shuffles the genetic instructions between the maternal and paternal chromosomes. So in this illustration, the blue color represents the father's chromosome and the red color represents the mother's chromosome. And while the same genes are on both chromosomes, there are different versions or different alleles represented with uppercase and lowercase letters. And so while the two sister chromatids contain the same versions of each of those genes at the same locations, initially crossing over will trade segments of the maternal and paternal chromosomes. And so this crossing over essentially shuffles the alleles, shuffles genetic instructions so that the chromosomes that are inherited by the gametes are all genetically unique. Gametogenesis is the production of the gametes. In men, gametogenesis is also known as spermatogenesis and produces the sperm. In women, gametogenesis is also known as oogenesis and produces the ova. So both spermatogenesis and oogenesis initially start with mitosis to produce the stem cells. The diploid stem cells known as primary spermatocytes are produced inside of the testes, and then will enter into mitosis going through two rounds of cell division to produce four haploid spermatids, which are the immature cells that will then finish maturation in the process of spermeogenesis to become a sperm, also known as a spermatizoa. Whereas in oogenesis, mitosis produces primary oocytes, which are the diploid cells that will enter into meiosis. And from each primary oocyte, four genetically unique haploid cells can be produced. However, three of the four will be polar bodies that are just small cells that function to remove the excess chromosomes and will then just be recycled as a waste product. And only one will become the mature ovum, and that mature ovum is only produced immediately before fertilization if a sperm is encountered inside of the...