BIO Chapter 10
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This 4 page Class Notes was uploaded by Natalie Berry on Sunday October 16, 2016. The Class Notes belongs to BIO 101 at Missouri State University taught by Kyoungtae Kim in Fall 2016. Since its upload, it has received 4 views. For similar materials see Biology in Your World in Science at Missouri State University.
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Date Created: 10/16/16
Key: Light Blue = main idea Purple = important parts of the main idea Green = examples Orange = key terms Pink = the definition of the key term Red = especially important Chapter 10: Meiosis Overview of Meiosis How do chromosomes avoid doubling up with each generation? Chromosome reduction happens before chromosome union in sexual reproduction The reduction come in cells that give rise to sperm and eggs When the cell divides the result is sperm or egg cells with only have the usual somatic number of chromosomes Basically human sperm and eggs have only 23 chromosomes (they aren’t pairs of chromosomes just 23 single) They can then unite to produce 46 chromosome zygote that can turn into a new human being This continues in each generation The kind of cell division that results in the halving of chromosomes is meiosis In animals, cell that are produced in meiosis are always reproductions cells, or gametes These reduced cells are in a haploid state Haploid = single number But when the sperm and egg unite it is diploid (double number) state meaning 46 chromosomes in human beings Meiosis: a process where a single diploid cell divides to produce four haploid reproductive cells N: a living cell being haploid: having a single set of chromosomes 2n: a living cell being diploid: having 2 sets of chromosomes Eggs and sperm are n and the diploid cells that give rise to them are 2n The steps in meiosis Know the two essential differences between meiosis and mitosis First: the formula for mitosis is duplicate once, divide once: duplicate the chromosomes once, then divide the original cell once With meiosis it’s duplicate once, divide twice: meiosis includes a chromosome duplication followed by two cell divisions This makes four cells instead of the two from mitosis Second: meiotic divisions are a separation of homologous chromosomes not of chromatids in mitosis Meiotic division is, however, just like mitotic division: It separates the chromatids that make up homologous chromosomes Remember: homologous chromosomes are chromosomes that are the same in function and size The steps of meiosis are separated into two multistep phases: Meiosis I and II The gist of it in in meiosis I, homologous chromosomes are positioned close together for opposite sides of the cellular equator or metaphase plate Then those chromosomes will move apart to opposite parts of the cell; the parts that will be daughter cells Meiosis I Key: Light Blue = main idea Purple = important parts of the main idea Green = examples Orange = key terms Pink = the definition of the key term Red = especially important Prophase I Meiosis I begins in prophase I with 46 chromosomes Each pair of homologous chromosomes link up as they condense Ex. Maternal chromosome 5 aligns with paternal chromosome 5 and so on This is tetrad: grouping formed by the linkage of two homologous chromosomes in prophase I of meiosis The part swapping takes place between the non-sister chromatids of the pair chromosomes This is called crossing over (recombination) Once that’s done, the homologous chromosomes begin to separate from each other but remain aligned Metaphase I Still as tetrads, the homologous chromosomes are moved by microtubules to the metaphase plate In this step of meiosis the maternal member of a given pair lies on one side of the plate and the paternal pair on the other It’s critical that the alignment adopted by any one pair of chromosomes bear no relation to the alignment of any other It is this chance event that determines what cell a chromosome joins because each side of the plate becomes a separate cell once division is finished Anaphase I Now paired homologous chromosomes begin to move away from each other to their respective poles via microtubules The sister chromatids that make them up are still together Telophase I Now the original cell undergoes cytokinesis, dividing into two daughter cells Now there are two haploid cells when we started with one diploid Why are they haploid? Meiosis I moved 1 set of chromosomes to one cell and the second set to another cell Therefore each cell has only 1 set of chromosomes meaning each is haploid Meiosis II What happens next is another division, or set of divisions since now there are two cells Each cell now has 23 duplicate chromosomes in it After a brief prophase II these chromos line up at a new metaphase plate with sister chromatids on opposite sides of each plate Now the sister chromatids separate going towards opposite poles Now they are full fledged chromosomes (anaphase II) Once separation is complete, cytokinesis starts Key: Light Blue = main idea Purple = important parts of the main idea Green = examples Orange = key terms Pink = the definition of the key term Red = especially important Now we’ve gone from two cells to 4 (telophase II) The difference between this process and mitosis is that each of these cells have 23 chromosomes instead of 46 The significance of Meiosis There are two kinds of diversity meiosis brings out The first is diversity in offspring Genetic diversity through cross over Remember in prophase I the homologous pairs aligned an engaged in crossing over, or recombination In this there is an exchange of parts of chromosomes prior to living up at metaphase plate Crossing over: a process where homologous chromosome exchange reciprocal portions of themselves The ability of reciprocal lengths of DNA to be exchanged between chromosomes provides means where the genetic deck can be reshuffled prior to the formation of gametes while genetic info on any 2 homologous will be similar they will not be identical Genetic diversity through independent assortment This the 2 ndway Takes place in metaphase I right after crossing over it’s the random alignment of chromosomes on the metaphase Remember no chance can have the same alignment This random alignment leads two the separation of them because a chromosomes that likes up on one side will end up on that side when the cell divides Independent assortment: ribosome distribution of homologous chromosomes pairs during meiosis Independent assortment ensures that offspring will not just be diverse, but unique; no other offspring will be exactly alike in genetic terms Exception; identical twins This matters because the chromosomes the offspring gets determines eye color, height, etc. It all depends how our maternal and paternal chromosomes lined up during meiosis Important to note Meiosis makes genetic diversity, mitosis does not Mitosis makes genetically exact copies of cells It retains the qualities that cells have from 1 generation to the next Meiosis mixes genetic elements each time it produces reproductive cells and their bring out genetic variation Meiosis and Sex Outcome In humans there is one exception to the rules that he chromosomes come in homologous pairs Human females have: 23 pairs of matched chromosomes Key: Light Blue = main idea Purple = important parts of the main idea Green = examples Orange = key terms Pink = the definition of the key term Red = especially important 22 pairs of autosomes 1 matched of sex chromosomes Sex chromosomes: meaning the chromosomes that determine what sex an individual will be Sex chromosomes in females are called X chromosomes and each female has two In males: 22 autosome pairs a X sex chromosomes One Y sex chromosomes Y leads to male sex Sex determination is simple Each egg produced by female has 1 X If the sperm that fertilize is an X it’s a girl If it’s a y it’s a boy Gamete formation in humans We know that in meiosis, a single haploid cell gives rise to four haploid gametes What are the diploid cells that go through meiosis? There’s a whole process by which egg and sperm are produced called gamete formation Oognia: the starting female cells in gamete formation Spermatagonia: the starting male cells in gamete formation The diploid cells above give rise to 2 other sets of diploid primary oocytes and primary spermatocyte These are the cells that undergo meiosis, producing haploid egg and sperm cells