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Crossing overIndependent assortmentRandom fertilizationMutation1. Crossing overCrossing over is the exchange of corresponding segments of non-sister chromatids of homologous chromosomes. During prophase I (of meisosis I), homologous chromosomes synapse to form aggregates called either bivalents (bi = 2, and there are two homologous chromosomes in the aggregate) or tetrads (tetra = 4, and there are 4 chromatids in the aggregate). While the chromosomes are synapsed in prophase I, crossing over occurs. As a result, a chromatid that originally consisted of 100% maternal alleles and a chromatid that originally consisted of 100% paternal alleles become chromatids with a mixture of both maternal and paternal alleles. This shuffles genetic information and increases variation.2. Independent assortmentIn metaphase I (of meiosis I), the tetrads (bivalents) line up at the center of the cell. Which pole the maternal and paternal chromosomes face is a matter of chance; and the direction the maternal and paternal chromosomes face in one tetrad has no bearing on which way the maternal and paternal chromosomes of another tetrad face. Thus, when the homologous chromosomes separate and move to opposite poles, each pole will receive a mix of maternal and paternal chromosomes. This shuffles genetic information and increases variation.3. Random fertilizationBecause of the above processes, male animals can potentially produce millions or hundreds of millions of genetically unique sperm. And, male animals typically release millions of sperm during sex. Which one of the millions of genetically unique sperm happens to end up fertilizing the egg is largely a matter of chance. Thus, except foridentical twins, siblings never have exactly the same genetic information.4 Mutation takes place during replication of DNA which result in new varieties .
Non-disjunction is the failure of a pair of chromosomes or chromatids to separate. Since this happens in both metaphase I and II, non-disjunction can occur in either stage.In metaphase I it is the failure of the two homologous chromosomes in the pair to separate. In this case all of the gametes will be affected.In metaphase II it is the failure of the two sister chromatids to separate. In this case only half of the gametes are affected.Non-disjunction is the cause of a number of genetic conditions eg Down syndrome, Triple-X syndrome.See http://en.wikipedia.org/wiki/Nondisjunction
Mitosis: Prophase, Metaphase, Anaphase, and Telephase (in that order)
metaphase.
Metaphase
Sister chromatids are those replicated from the same chromosome whereas non-sister chromatids may be found in meiosis (particularly metaphase II) where paternal and maternal chromatids line up and eventually separate at the metaphase plate.
Going down to the cellular level, it occurs in meiosis when chromosomes fail to separate (nondisjuction). We get a set of chromosomes from our paternal side and the other set from out maternal side. For example, one of the gamete that may have came from our maternal side contains no chromosomes , O, when it should contain that chromosome x while from our paternal side we get a gamete containing the x chromosomes. This results in a XO, a sterile female when the normal sex chromosomes are XX or XY. -or- nondisjunction
It is most commonly caused by increased maternal age.Add: It is the result of nondisjunction of the 21st chromosome pair, in which the pair fails to separate, so that one cell gets an extra copy of chromosome 21.
Colchicine is a drug that prevents tubulin polymerisation into cytoskeleton structures called microtubules. Microtubules are essential for many functions of the cell, but importantly are needed for cell division. Specifically, they help align homologous chromosomes on the metaphase plate during metaphase and then separate the chromosome pairs during anaphase. So by inhibiting the production of microtubules, the chromosomes never align on the metaphase plate - they will be fully condensed but spread throughout the cell, a situation called a metaphase spread
Crossing overIndependent assortmentRandom fertilizationMutation1. Crossing overCrossing over is the exchange of corresponding segments of non-sister chromatids of homologous chromosomes. During prophase I (of meisosis I), homologous chromosomes synapse to form aggregates called either bivalents (bi = 2, and there are two homologous chromosomes in the aggregate) or tetrads (tetra = 4, and there are 4 chromatids in the aggregate). While the chromosomes are synapsed in prophase I, crossing over occurs. As a result, a chromatid that originally consisted of 100% maternal alleles and a chromatid that originally consisted of 100% paternal alleles become chromatids with a mixture of both maternal and paternal alleles. This shuffles genetic information and increases variation.2. Independent assortmentIn metaphase I (of meiosis I), the tetrads (bivalents) line up at the center of the cell. Which pole the maternal and paternal chromosomes face is a matter of chance; and the direction the maternal and paternal chromosomes face in one tetrad has no bearing on which way the maternal and paternal chromosomes of another tetrad face. Thus, when the homologous chromosomes separate and move to opposite poles, each pole will receive a mix of maternal and paternal chromosomes. This shuffles genetic information and increases variation.3. Random fertilizationBecause of the above processes, male animals can potentially produce millions or hundreds of millions of genetically unique sperm. And, male animals typically release millions of sperm during sex. Which one of the millions of genetically unique sperm happens to end up fertilizing the egg is largely a matter of chance. Thus, except foridentical twins, siblings never have exactly the same genetic information.4 Mutation takes place during replication of DNA which result in new varieties .
This aspect of the Swedish language is surprisingly confusing. Swedish has four words for great-grandfather, all referring to separate people. These are: - farfars far (paternal grandfather's father) - dad's dad's dad - farmors far (paternal grandmother's father) - dad's mom's dad - morsfars far (maternal grandfather's father) - mom's dad's dad - morsmors far (maternal grandmother's father) - mom's mom's dad
Non-disjunction is the failure of a pair of chromosomes or chromatids to separate. Since this happens in both metaphase I and II, non-disjunction can occur in either stage.In metaphase I it is the failure of the two homologous chromosomes in the pair to separate. In this case all of the gametes will be affected.In metaphase II it is the failure of the two sister chromatids to separate. In this case only half of the gametes are affected.Non-disjunction is the cause of a number of genetic conditions eg Down syndrome, Triple-X syndrome.See http://en.wikipedia.org/wiki/Nondisjunction
metaphase
Metaphase
Mitosis: Prophase, Metaphase, Anaphase, and Telephase (in that order)
metaphase.
In metaphase chromosomes alighn along the cell nusleus and in anaphase the chromosomes separate at the kinetochores and move to opposite sides of the cell