BIO CH 15 MITOSIS

Meiosis II separates sister chromatids to create chromosomes

Meiosis I is followed by cytokinesis after telophase I into two haploid cells with 23 pairs of sister chromatids each, 46 sister chromatids all together in each. There were 92 sister chromatids in 46 pairs of sister chromatids in the original diploid cell. Each pair of the 46 pairs represents one of 46 homologous chromosomes that have replicated in the S-Phase of Interphase prior to Meiosis. This DNA replication of the chromosomes does not occur between Meiosis I and II. The sorting events of meiosis II are similar to mitosis but the starting point is different. For a human cell that had 92 chromatids, and split into 2 cells with 46 chromatids each, this is the starting point of meiosis II, the 2 cells with 46 chromatids each. At the starting point of mitosis with human cells, it starts with 92 chromatids. Other than the starting points different with 46 chromatids each cell vs 92 chromatids in a cell, the phases of Meiosis II are the exact same.

15.3 Meiosis and Sexual Reproduction

Meisois takes a single diploid cell which contains 2n, or two sets of homologous chromosomes. A human diploid cell has 46 chromosomes, 23 in each set. Meiosis takes this diploid cell and distributes it from 2 phases into 4 haploid cells that contain exactly half of the chromosomes of the diploid. Before Meiosis, the Chromosomes replicate once again during S-Phase of the cell cycle just like Mitosis. One Meiosis I starts, it separates the diploid cell into 2 haploid cells that have the replicated chromosomes or sister chromatids from the cell cycle/interphase/s-phase. These two cells have only one set of chromosomes, but with 2 replications of the chromosomes, so they have to divide again in Meiosis II. Meiosis II divides the two haploid cells with sister chromatids into 2 haploid cells with individual chromosomes, making 4 haploid cells altogether. The replication of the chromosomes is what makes it possible for 4 haploids to form.

The transmission of chromosomes requires a sorting process known as mitosis

Mitosis is the sorting process for dividing one nucleus into two nuclei. The duplicated chromosomes that were replicated in S-phase of the eukaryotic cell cycle are distributed in mitosis so that each daughter cell receives an exact copy of the genetic material. Mitosis: Prophase Prometaphase (not covered) Metaphase Anaphase Telophase Cytokinesis 6 phases all together Before Prophase is Interphase Interphase is made up of G1, S-phase, and G2. It is prior to mitosis. The chromosomes are currently not condensed in Interphase and won't condense until Prophase. They will then de-condense in Telophase at the end of the M-Phase before Cytokinesis. The Chromosomes and the Nucleolus are all in the Nucleus during the Interphase. The Chromosomes won't leave the Interphase until the Nuclear envelope will diminish into vesicles during Prophase and Prometaphase together. (The nucleolus is the site where ribosomal subunits, what are used to create ribosomes, are found.) It diminishes in Prophase. AFTER INTERPHASE, aka Pre-M-Phase and Pre-Mitosis. PROPHASE: During Prophase, the nuclear envelope diminishes and the chromosomes condense. There are already 12 sister chromatids, 6 belonging as pairs to each set appear. The mitotic spindle, which is what will ultimately organize and sort the chromosomes during division, begins to form from the two centrosomes located outside of the nucleus. The nucleolus is no longer visible in Prophase. Prophase extends into a second half called prometaphase, which is ultimately pre-metaphase. PROMETAPHASE: The nuclear envelope has completely diminished into vesicles. The mitotic spindle has completely formed BC sister chromatids attach to the mitotic spindle via kinetochore microtubules. As Prometaphase progresses, the centrosomes move far apart and create 2 poles. Once the nuclear envelope has dissociated, the spindle fibers growing from the centrosomes can grab the sister chromatids via their kinetochores. How do sister chromatids become attached to the spindle apparatus? Initially, microtubules are rapidly formed and can be seen under a microscope growing out from the two poles. The grow by Centrosomes polymerizing tubulin. As it grows, if a microtubule happens to make contact with a kineto chore, and touch it, it is said to be captured and remains firmly attached and stuck to the kinetochore. This is the microtubule that begins to be called the kinetochore microtubule. Alternatively, if a microtububle does not come in contact with a kinetochore belonging to a sister chromatid, then the microtuble eventually depolymerizes and retracts back to the centrosome. This is what makes up polar microtubules. And, this random process is how sister chromatids become attached to the kinetochore microtubules; the act of microtubules randomly capturing kinetochores belonging to sister chromatids. As the end prometaphase nears, the two kinetochores attached to each pair of sister chromatids are attached to kinetochore microtubules from opposite poles. The poles are began by the centrosomes retracting from each other. METAPHASE: EVENTUALLY, the pairs of sister chromatids are aligned in a single row along the metaphase plate. The metaphase plate is a plane or region halfway between the poles extending from the centrosomes. When this alignment is complete of the sister chromatids by the kinetochore microtubules attached to the kinetochore structure of the chromatid, the cell is said to be in metaphase of Mitosis. Before Metaphase, the nuclear envelope had to diminish in Prophase and Prometaphase, the centrosomes had to establish the mitotic spindle in Prometaphase by attaching the kinetochores to kinetochore microtubules extending from each pole, and the sister chromatids had to condense in prophase at the beginning of mitosis. This is why it is easier to visualize in microscopy because of the condension that prophase causes. ANAPHASE: During Anaphase, the connection between the pairs of sister chromatids are broken from the middle of the mitotic spindle AKA the metaphase plate. Each chromatid now becomes a chromosome, each linked to only one pole or one centromere of the mitotic spindle. As anaphase proceeds, the kinetochore microtubles shorten and pull the newfound chromosome from the middle of the mitotic spindle towards their pole that they are attached. Remember, a pole is the centrosome and its two centromere. In addition, the two poles move farther away from each other preparing for cell division. The two poles move farther away because the overlapping polar microtubules push instead of pull like the kinetochore microtubules. TELOPHASE: During telophase, the newfound chromosomes have reached their new poles and they then decondense. Remember that they condensed in Prophase. Now that they have decondensed, the nuclear envelope can re-develop from the vesicles creating from prophase and prometaphase. This creates 2 new nuclear envelopes and 2 new nuclei, each serving the chromosomes that had separates from sister chromatids in Anaphase. The nuclear envelopes develop because the chromosomes have now decondensed again. From 12 sister chromatids, 6 chromatids went to each pole and became chromosomes surrounded by a nuclear envelope. So, 6 chromosomes are in each nuclei; 2 sets that have 3 chromosomes in each and their homologous chromosomes in their respective set. CYTOKINESIS: In most cases of Mitosis, it is followed by Cytokinesis, which separates the two new nuclei into two different cells with newfound chromosomes deriving from separated sister chromatids in Anaphase.. Cytokinesis separates the two newfound nuclei from Telophase into two different daughter cells. The phases of mitosis in plant and animal cells are similar. But, in cytokinesis, they are different. In animal cells, the cell constricts like a backpack drawstring from the cleavage furrow. In animal cells, cytokinesis involves the creation of a cleavage furrow, which a cleavage furrow constricts like a drawstring to separate the cells. In plants, cytokinesis occurs when vesicles from the Golgi Apparatus move along microtubules to the center of the cell and come together and merge to form a cell plate. This cell plate forms a cell wall to separate the two new daughter cells. In plants, cytokinesis occurs when vesicles derived from the Golgi Apparatus, which is where vesicles become created and packaged, move along microtubules in the cytoskeleton towards the center of the cell and merge together to form a cell plate which becomes a cell wall. In animal cells, cytokinesis occurs with the creation of a cleavage furrow, which contracts like a backpack string, and pulls the two daughter nuclei apart into daughter cells.

Sexually Reproducing species produce haploid and diploid cells at different times in their life cycles

A life cycle is a sequence of events that produces another generation of an organism. Life cycles are either diploid-dominant or haploid-dominant in meaning that their sequence of events usually involve diploid cells more often or haploid cells more often. The animal life cycle is diploid dominant. The fungal life cycle is haploid dominant. The plant life cycle is an alternation of generation - which means that the life cycle has a mixture of diploid and haploid sequences to create another generation. A gamete is always a haploid cell that combines with another gamete to create a diploid zygote. Animal life cycle: (diploid dominant) In an animal multicellular diploid organism, the cells within testes and ovaries undergo meiosis to create haploid gametes. The haploid gametes of two animals undergo fertilization to create a diploid zygote. The diploid zygote undergoes mitotic cell division to create diploid somatic cells. These somatic cells keep mitotically dividing to produce the full multi-cellular organism animal. Fungal life cycle: (haploid dominant) In a fungal life cycle, the fungus is a haploid multicellular organism that has certain haploid cells that act as reproductive cells (or gametes). These haploid reproductive cells unite from two parents to form a diploid zygote. The diploid zygote undergoes meiosis to produce 4 haploid spore cells. (unlike the animal zygote, which undergoes mitosis) The 4 haploid spores undergo mitosis to create 4 full multicellular haploid fungal organisms. Plant life cycle: (alternation of generation dominant) In a plant life cycle, the plant is a diploid sporophyte organism. And in the plant, certain cells within the plant's diploid sporophyte cellular structure undergo meiosis to produce haploid spores. The haploid spores undergo mitosis to create a haploid multicellular gametophyte organism. Certain cells in the gametophyte act as gametes like in the animal life cycle, and the gametes in another parent gametophyte unite to form a diploid zygote. This diploid zygote undergoes mitosis to form a multicellular sporophyte. A gene is a trait. A trait is just what is expressed by the gene. The gene may be homozygous or heterozygous and contain 3 different possible alleles in the chromosome's locus. A variant of a gene or trait is called an allele. 2 different alleles of the same gene exist in homologous chromosomes; they may be the same or different, homozygous or heterozygous. A locus is a spot on a chromosome where a gene is located. A chromosomes contains DNA and associated protein. Genetic information is the DNA. In recessive allele disorders, an affected individual may have unaffected parents who are heterozygous with the gene and the dominant allele supresses the mutative allele. The affected offspring receives two gametes with the mutative allele that create a homozygous recessive affected individual. In dominant genetic diseases, the gene that carries a mutative allele will suppress the normal allele. In this case, all affected individuals who received a gamete that had a mutative dominant allele WILL usually have a parent who is affected as well by the dominant allele in their gene. Huntington disease for example. A trait is what is expressed by a gene. pedigree analysis is when geneticists will study family trees following one trait or gene and determine who was homozygous or heterozygous for the gene, etc, and whether the variant alleles of the gene are dominant, recessive, etc. A recessive mode of inheritance means that all the offspring of affected parents will produce affected offspring, because the parents have homozygous recessive alleles for the gene in their genome and no dominant normal allele to pass off and create normal kids. Also two unaffected parents (that have heterozygous alleles in their gene) can pass off the recessive allele in their gametes to their diploid offspring. So an affected child may have unaffected parents Recessive mode of inheritance - recessive mutant allele, dominant normal allele Dominant mode of inheritance - dominant mutant allele, recessive normal allele A dominant mode of inheritance means that affected parents may not produce all affected offspring, because they can be heterozygous parents that can pass the recessive normal allele to their offspring. (if they're homozygous recessive, they will create all affected offspring because there is no normal allele in their gene at all to create a normal gamete) Also the people who are affected usually have at least one affected parent who had the dominant allele to pass off in their gametes. An X-linked gene is a gene that occurs only on the X chromosome of the X and Y sex chromosomes of humans. An X-linked gene found in males is NOT homozygous OR heterozygous because there isn't a copied pair of the X chromosome available with the specific gene allele. Males have X and Y chromosomes. Females can be homozygous or heterozygous for X-linked chromosomes because they have 2 X chromosomes in their pair, thus 2 copies of the same gene, maybe different alleles or maybe the same alleles. Hemophilia is a recessive X-linked disorder with a recessive X-linked allele that can be passed down in gametes. Males tend to have it more because if they receive a X-linked hemophilic allele, they don't have another X chromosome like females to possibly cross it out with a dominant normal allele. The males are hemizygous in sex chromosomes and have only one copy of a chromosome and their paired chromosome isn't homologous but rather a Y chromosome that determined that male development.

Meiosis I separates Homologous Chromosomes

The sorting that occurs in Meiosis I that results into two haploid cells results in homologous sister chromatids (because they have replicated in Interphase during the s-phase) being separated from each other. The assortment of a respective homolog to a pole is random, and the resulting two haploid cells from human cells may receive a 2^23 random assignment of two different haploid cells. Meiosis I and II are composed of similar phases but with key differences in the haploid production. Prophase I: The replicated chromosomes condense, providing visibility. The homologous chromosomes or homologous sister chromatids form bivalents and then cross over chromosomal segments of DNA. The nuclear envelope around the nucleus begins to become small vesicles and fragmented. Prometaphase I: The nuclear envelope is completely fragmented into vesicles and the spindle apparatus or mitotic spindle apparatus is formed by the centrosomes (often with 2 centrioles) and their grown tubulin protein-replicated microtubules. Kinetochore microtubules attach themselves to the kinetochore region of the sister chromatids. HOWEVER, the kinetochore microtubules cannot reach the other side of every sister chromatid pair like mitosis because the sister chromatid pairs are associated in a bivalent with their homologous sister chromatid pair. So, the key difference of Prometaphase in Mitosis and Meiosis is that in Meiosis the Sister chromatid pair is only attached to one pole that they are respectively close towards. Metaphase I: In metaphase I, the bivalents are organized along the metaphase plate. In mitosis, one pair of sister chromatids and not their homologous bivalent were organized along the metaphase plate in the center. The sister chromatids in meiosis I are aligned in a double row rather than single row. The arrangement of blue and red homologs in this double row is random in their pole assignment. This created genetic diversity, as well as concerning that the homologous chromosomes have crossed over chromosomal segments in the bivalent created by synapsis in Prophase I. The possible number of different alignments in eukaryotic species is 2^n with their diploids meaning that they have two sets. n is the number of chromosomes per set, so a human diploid has the possibility of randomly organized 2^23 or over 8 million possibilities over meiosis metaphase arrangement. Each cromosome or pair of sister chromatids in this case has a homologous chromosome or homologous pair of sister chromatids, and each homologous member of these pairs in the two sets can arrange themselves on either side of the metaphase plate gravitating towards their respective poles. IT IS A MATTER OF CHANCE which daughter cell of the two in meisosi I will get the maternal chromosome of the pair or the paternal chromosome of the pair. Either way, one of the sister chromatids of these respective homologous chromosomes that are replicated will have crossed over genetic information from the other set in prophase I, so the ultimate possibilities are insanely unique. When meiosis is complete, it is very unlikely that any two haploid cells or gametes created will have the same genetic information in their split and crossed over chromosomes. ANAPHASE I: The segregation of homologs or homologous chromatids more specifically homologous sister chromatids that have crossed over one chromatid with the homologous chromatid's chromosomal segements will occur during Anaphase I. The connections between the bivalents break in Anaphase I, but not the connections that hold sister chromatids together. This isn't mitosis.. Each joined pair of chromatids in the broken bivalents will migrate to one pole or centrosomes with microtubules created from it. The homologous pair of sister chromatids will separate from each other pulled by the kinetochore microtubules. TELOPHASE I: At telophase I, the sister chromatids now reach their respective poles and decondense, now becoming less visibile under microscopy. The nuclear envelope now reforms to construct two separate nuclei respectively containing homologous pairs of sister chromatids opposite from each other. The end of meiosis I is called reduction division, each with their own respective pairs of sister chromatids. The end of meiosis I to create a human gamete will result in 92 chromatids with 46 chromosomes split into two separate nucleids with 46 chromatids and 23 chromosomes. Chromosomes that have chromatids have double the number of chromosomes that were replicated.

Genomes and Proteomes Connection: The Genomes of Diverse Animal Species Encode Approximately 20 Proteins Involved in Cytokinesis.

Researchers often try to identify the genes that within a given species that encode proteins necessary for a process. Cytokinesis has been analyzed by searching for the genes that encode proteins necessary for the process. By comparing the results of this testing of cytokinesis from vertebrates, insects, worms, and other animals, researchers have identified 20 common proteins that are constantly being produced for a number of functions The functions of these 20 proteins in cytokinesis are: 1.Contractile Ring: Seven proteins, including actin (a protein filament involved in the cytoskeleton) myosin (a motor protein) and other proteins that regulate the actual actin and myosin function are necessary for the FORMATION OF THE CONTRACTILE RING which is attached to the plasma membrane and creates the backpack type pull. 2. Signal Transduction: (Transduction- The act of converting a message into another form) Five proteins are components of a signal transduction pathway that initiates the formation of the contractile ring. 3. Central spindle: Eight proteins are known to be components that attach to the central spindle of the mitotic spindle and are necessary for cytokinesis and splitting the cell 4.Cell separation via membrane insertion: Two proteins are needed for the final separaton of the cell into two daughter cells. The four functions of the common 20 proteins of cytokinesis: 1. Contractile ring formation 2. Signal transduction to initiate the formation of contractile ring 3.Central spindle attachment necessary for cytokinesis 4.Cell separation via membrane insertion

Bivalent formation and crossing over occur at the beginning of mitosis

TWO key events occur that occur at the beginning of meiosis that do not happen meiosis. The first is that homologous pairs of sister chromatids associate with each other in a bivalent, also called a tetrad. The PROCESS of forming a bivalent is called synapsis. And, in most eukaryotic species, a protein structure called the synaptonemal complex connects the homologous sister chromatids at the beginning of meiosis. Once its connected, the formation is called a bivalent and synapse is done. The second key event that occurs is crossing over, which involves a physical trade of chromosomal segments between the two homologous sister chromatids bonded by the bivalent done thru synapse. This creates genetic variation in the future 4 haploid cells. And, the arms of the chromatid begin to separate after crossing over happens, staying connected thru the synaptonemal complex at the switched, traded chromosomal segments. This connection of the crossed over chromosomal segments is called a chiasma. The number of crossovers is carefully controlled by cells and depends on the size of the chromosomes and the species. The range of crossovers for eukaryotic species is one or two to a couple dozen. During the formation of sperm in humans for example, an average homologous chromosome undergoes slightly more than two crossovers with its respective homologous chromosome. Plant species can expect 20 or more crossovers on the other hand.

Variations in Inheritance Patterns and Their Molecular Basis

The term Mendelian inheritance describes the inheritance patterns or typical results of genes that segregate or separate and assort independently in their own random assignment. A gene is composed of usually 2 or more alleles. The gene defines a character, and a trait is what is expressed by a gene and its specific alleles. Certain allele patterns in a pair of genes express only one trait of the dominant allele. Allele is a variant of a gene. Simple Mendelian inheritance is when one gene is found in 2 variants or alleles of which one is dominant over the other will ultimately affect the traits of the gene or the gene expression. alleles of a gene = traits of a character