Ch 3 Gen Hw

yes yes no

10 Prophase is the first stage of mitosis. It occurs after chromosomes have been replicated in S-phase and before sister chromatids separate during anaphase. The number of chromosomes per cell is equal to the number of functional centromeres. At this stage, each chromosome consists of two sister chromatids (produced by replication during S-phase) connected by the centromere. There are 10 replicated chromosomes.

20 molecules Chromosomes in prophase consist of two identical sister chromatids connected by a centromere. Sister chromatids were produced by replication during S-phase, prior to mitosis. Each sister chromatid is a DNA molecule (double helix). If there are 10 replicated chromosomes, each containing two sister chromatids, then there are 20 DNA molecules in the cell during prophase.

20 chromosomes At metaphase, the 10 replicated chromosomes line up along the midline. During anaphase when centromeres split and sister chromatids separate, each replicated chromosome becomes two unreplicated chromosomes. Because the number of chromosomes is equal to the number of functional centromeres, and each sister chromatid has a centromere, there are 20 chromosomes at the end of anaphase.

20 molecules At the end of anaphase, there are 20 unreplicated chromosomes, each one is a DNA molecule.

10 chromosomes At the end of anaphase, there is one cell with 20 chromosomes - 10 chromosomes at one end of the cell, and 10 chromosomes at the other end. Upon completion of telophase and cytokinesis, the single cell will be split into two cells. Each cell will contain 10 chromosomes, the same amount as the original progenitor cell.

10 molecules At the completion of telophase/cytokinesis, there will be two cells. Each cell will contain the same number of chromosomes as the progenitor cell - 10 unreplicated chromosomes. Each unreplicated chromosome is one DNA molecule. Therefore, at the end of telophase/cytokinesis, each cell will have 10 DNA molecules.

at anaphase in mitosis and anaphase II in meiosis

true

reciprocal exchange of DNA between homologs during prophase I

e. There would be less genetic variation among gametes.

During cell division offspring cells will obtain either 2 copies of acentric chromosomes or daughter cells may not receive any. Acentric chromosomes may shift arbitrarily to only one pole or to the other during cell division. Centromeres are considered as the regions to which sister chromatids bind during chromosomal replication. These centromere components are very crucial on which kinetochore proteins formed and microtubules attached to these proteins during cell division. These are essential components are very important during segregation of the chromosomes. Centromeres are chromosomal regions at which sister chromatids remain attached after replication. The kinetochore is a protein structure that forms on the centromere during mitosis and meiosis, and to which spindle fibers attach. Centromeres are necessary for the correct segregation of chromosomes during anaphase. Chromosomes that lack centromeres do not attach to spindle fibers, and as a result, are randomly distributed to daughter cells or may be lost entirely.

Daughter cells will each receive fragments of dicentric chromosomes. Dicentric chromosomes are pulled in opposite directions during cell division and often break as a result. Chromosomes with two centromeres are pulled in two directions if simultaneously bound to both poles of the mitotic spindle, which often results in chromosome breakage. This may cause daughter cells to receive just a fragment of the chromosome following cell division.

Sister chromatids may drift apart after being synthesized. Cohesin enables sister chromatids to remain attached from the time they are synthesized during S phase until the end of metaphase. If cohesin is defective, sister chromatids may separate before they normally would in anaphase, which would lead to daughter cells that have too few or too many chromosomes.

Sister chromatids cannot be separated during mitosis. Separase is an enzyme that cleaves cohesin, a chromosomal protein complex that joins sister chromatids together. When cohesin is cleaved, anaphase is triggered and spindle fibers pull sister chromatids to opposite ends of the cell. If separase is defective, sister chromatids may remain paired instead of separating in anaphase, which would lead to daughter cells that have too few or too many chromosomes.

During spermatogenesis, germ cells are produced daily, while during oogenesis, germ cells are produced prenatally. The difference in the "age" of the gametes at the time of fertilization may contribute to the observed effect. During ovum production, primary oocytes are arrested for years in meiosis I, increasing the likelihood of components involved in chromosome segregation to break down. Although scientists do not know with certainty why nondisjunction increases with advanced maternal age, both the age of the ovum at the time of fertilization and the fact that oocytes are arrested in meiosis I for years are thought to contribute to this effect. Oogenesis begins during fetal development and oocytes are arrested in prophase I by birth. During puberty, ovulation begins and meiosis is reinitiated in one egg during each ovulatory cycle. As a result, each ovum that is released has been arrested in meiosis I for one month longer than the one released during the preceding cycle. Therefore, women in their 30's and 40's are producing ova that are much older than those which were ovulated when puberty began, which may contribute to nondisjunction.

The cell plate consists of the plasma membrane and cell wall that will eventually separate the two daughter cells. Vesicles from the Golgi apparatus move along microtubules, coalesce at the plane of cell division, and form a cell plate. In plant cell division, after chromosome separation, the microtubules of the mitotic spindle reorganize into a network that guides vesicles derived from the Golgi apparatus to the plane of cell division. These vesicles begin to fuse, forming the cell plate. As more vesicles are added to the cell plate, it grows outward, eventually fusing with the parent cell plasma membrane. Membrane from the vesicles forms the new plasma membrane for each daughter cell. At the same time, materials that were enclosed in the vesicles form the new cell wall between the new plasma membranes of the daughter cells.

X-linked dominant If the skin condition is caused by an X-linked dominant allele, a father would pass the allele on to all of his daughters, who would all have the skin condition. In contrast, the father would not pass the allele on to any of his sons because the sons would receive the father's Y chromosome, not his X chromosome. As a result, none of the sons would inherit the skin condition.