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Transcript
Notes on Meiosis and Sexual Life Cycles A ____________________ is the generation-to-generation sequence of stages in the ___________________ history of an organism It starts at the __________________ of an organism and continues until it produces its own ________________ Fertilization and meiosis alternate in sexual life cycles: In humans, each ______________ cell (all cells other than _________ or __________) has _____ chromosomes A _________________ display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same ____________, _______________ position, and _______________ pattern These _________________ chromosome pairs carry genes that control the same __________________ characters An exception to the rule of homologous chromosomes is found in the _________ chromosomes, the _____ and the _____ The pattern of inheritance of these chromosomes determines an individual’s ________ Human females have a _________________ pair of _____ chromosomes (_______) Human males have an _____ and a _____ chromosome (_________) Because only small parts of these have the same ___________, most of their genes have no _________________ on the other chromosome The other 22 pairs are called ____________________ The occurrence of homologous pairs of chromosomes is a consequence of sexual reproduction We inherit _______ chromosome of each __________________ pair from each ______________ The _________ chromosomes in a somatic cell can be viewed as two sets of 23, a __________________ set and a ___________________ set ___________________ or _________ (gametes) have only one set of chromosomes — _________ autosomes and an ______ or a ______ A cell with a single chromosome set is ______________ For humans, the haploid number of chromosomes is __________ (______________) By means of ________________________________, a haploid ____________ reaches and fuses with a haploid ____________ These cells fuse (________________) resulting in __________________ The fertilized egg (______________) now has two _______________ sets of chromosomes bearing genes from the ____________ and ____________ family lines The zygote and all cells with two sets of chromosomes are _____________ cells For humans, the diploid number of chromosomes is ________ (_______________) As an organism develops from a _____________ to a sexually mature ___________, the zygote’s genes are passed on to all ______________ cells by ________________ _____________, which develop in the gonads, are not produced by _____________ Instead, gametes undergo the process of _______________ in which the chromosome number is _____________ Human ____________ or __________ have a ________________ set of 23 different chromosomes, one from each _______________ pair _____________________ restores the ______________ condition by combining two ______________ sets of chromosomes Fertilization and meiosis alternate in sexual life cycles The _____________ of meiosis and fertilization does __________ among species The life cycle of ______________ and other _____________ is typical of one major type _____________, produced by meiosis, are the only _____________ cells Gametes undergo no divisions themselves, but fuse to form a ____________ zygote that divides by ______________ to produce a multicellular organism Steps of Meiosis: Similar to ____________________ However, in meiosis, there are two consecutive cell divisions, __________________ and ___________________, that result in _______ daughter cells Each final daughter cell has only __________ as many chromosomes as the parent cell Meiosis reduces chromosome number by copying the chromosomes ____________, but dividing ______________ The first division, meiosis I, separates ____________________________________ The second, meiosis II, separates _______________________________________ Division in meiosis I occurs in four phases: _________________, ________________, _________________, and ___________________ During the preceding __________________ the chromosomes are ________________ to form _____________________________ These are genetically _________________ and joined at the __________________ The single __________________ is replicated In prophase I, the chromosomes ___________________ and homologous chromosomes _________________ to form ________________ In a process called __________________, special proteins attach homologous chromosomes ______________ together At several sites the chromatids of homologous chromosomes are _______________ (chiasmata) and segments of the chromosomes are _______________ A _______________ forms from each centrosome and spindle fibers attached to ____________________ on the chromosomes begin to move the _________ around At metaphase I, the ________________ are all arranged at the ______________________ _____________________ from one pole are attached to the kinetochore of one ___________________ of each tetrad, while those from the other pole are attached to the other In anaphase I, the homologous chromosomes ________________ and are pulled toward ___________________________ In telophase I, movement of ______________________________________ continues until there is a _______________ set at each pole Each chromosome consists of linked ________________________________ ______________________ by the same mechanisms as mitosis usually occurs __________________________ ________________________ is very similar to _________________ During prophase II a ________________ apparatus forms, attaches to ______________________ of each sister chromatid, and _____________ them around At metaphase II, the sister chromatids are arranged at the _______________________ At anaphase II, the ___________________ of sister chromatids ________________ and the now separate sisters travel toward ____________________________ In telophase II, separated sister chromatids arrive at __________________________ ______________ form around the _________________ ___________________ separates the _________________ At the end of ________________, there are _________haploid daughter cells Mitosis and meiosis have several key differences 1. The chromosome number is _____________ by half in ____________, but not in _______________ 2. Mitosis produces daughter cells that are ______________________________ to the ________________ and to ____________________________ 3. Meiosis produces cells that ______________ from the ______________ and ____________________________ 4. ____________ events, unique to ______________, occur during the ___________ division cycle a. During prophase I, homologous chromosomes pair up in a process called _________________ Later in prophase I, the joined homologous chromosomes are visible as a _______________ At X-shaped regions called chiasmata, sections of _________________ chromatids are ____________________ Chiasmata is the physical manifestation of _______________________, a form of genetic ______________________ b. At metaphase I _____________________________ of chromosomes, not individual chromosomes are _________________ along the metaphase plate In humans, you would see _________ tetrads c. At anaphase I, it is ____________________ chromosomes, not ______________ chromatids, that ________________ and are ____________ to opposite poles of the cell ________________ chromatids remain attached at the _________________ until ________________________ The processes during the _______________ meiotic division are virtually identical to those of _______________ Mitosis produces _____________________________________ daughter cells, but meiosis produces ________________________________ cells Origins of Genetic Variation: The behavior of chromosomes during __________________ and __________________ is responsible for most of the ___________________ that arises each generation during _______________ reproduction Three mechanisms contribute to genetic variation: 1. _______________________________ 2. _______________________________ 3. _______________________________ Independent assortment of chromosomes contributes to genetic variability due to the __________________ orientation of __________________ at the metaphase plate There is a _____________________ chance that a particular daughter cell of meiosis I will get the ____________________ chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the ________________ chromosome Each homologous pair of chromosomes is __________________ independently of the other pairs at ______________________ Therefore, the first meiotic division results in independent assortment of _____________ and __________________ chromosomes into daughter cells The number of combinations possible when chromosomes assort independently into gametes is ____________, where ______ is the haploid number of the organism If ___________, there are ______________ possible combinations For humans with n = 23, there are ____________ or about ___________________ possible combinations of chromosomes Independent assortment alone would find each _____________________ chromosome in a _______________ that would be exclusively maternal or paternal in origin __________________________ produces recombinant chromosomes, which __________________ genes inherited from each parent Crossing over begins very early in _____________________ as homologous chromosomes ___________________ gene by gene In crossing over, homologous portions of two __________________ chromatids trade places (For humans, this occurs ________ to _____________ times per chromosome pair) One sister chromatid may undergo ______________ patterns of crossing over than its _________________ Independent assortment of these ______________________ sister chromatids during _____________________ increases still more the number of genetic types of _________________that can result from meiosis The _____________________ nature of ________________________ adds to the genetic _____________________ arising from meiosis Any __________________ can fuse with any ____________ An ________________ is one of approximately 8 million possible chromosome combinations (actually __________) The successful ________________ represents one of 8 million different possibilities (actually ____________) The resulting _______________ is composed of 1 in ___________________ (______________________) possible combinations of chromosomes ____________________________ adds even more __________________ to this All three mechanisms __________________ the various genes carried by individual members of a ______________________ _____________ and ______________________ are two sources of the continual generation of new genetic ______________________