Stages of Meiosis Worksheet (Answers)

Meiosis Stages of Meiosis Define meiosis The reduction division of a diploid cell to produce four haploid daughter cells ………………………………………………………………………………………………………………………………………………….......... ………………………………………………………………………………………………………………………………………………….......... Complete the following diagram to show meiosis in a cell containing FOUR chromosomes Identify the main differences between meiosis I and meiosis II Meiosis I: Involves the separation of homologous chromosomes (bivalents / tetrads) ………………………………………………………………………………………………………………………………………………….......... Is a reduction division (diploid to haploid) ………………………………………………………………………………………………………………………………………………….......... Promotes genetic variation (crossing over and random assortment) ………………………………………………………………………………………………………………………………………………….......... Meiosis II: Involves the separation of sister chromatids ………………………………………………………………………………………………………………………………………………….......... Is a mitotic division (haploid to haploid) ………………………………………………………………………………………………………………………………………………….......... Does not promote genetic variation ………………………………………………………………………………………………………………………………………………….......... Differentiate between homologous chromosomes and sister chromatids Homologous chromosomes are the maternal and paternal copies of a given chromosomes ………………………………………………………………………………………………………………………………………………….......... Homologous chromosomes have the same structure and the same genes at the same loci positions ………………………………………………………………………………………………………………………………………………….......... Sister chromatids are the duplicated copies of the chromosome’s DNA (copied during S phase of interphase) ………………………………………………………………………………………………………………………………………………….......... ………………………………………………………………………………………………………………………………………………….......... Compare the processes of meiosis and mitosis Meiosis Type of cell produced Mitosis Sex cells (gametes) Body (somatic) cells Number of cells produced Four Two Number of divisions Two One Haploid Diploid Shows genetic variation Are genetically identical Ploidy of daughter cells Genetics of daughter cells Genetic Variation Outline how crossing over and independent assortment give rise to infinite genetic variety Crossing Over: Crossing over involves the exchange of genetic material between non-sister chromatids of a bivalent ………………………………………………………………………………………………………………………………………………….......... Bivalents are connected at points called chiasma during the process of synapsis (during Prophase I) ………………………………………………………………………………………………………………………………………………….......... It is at these chiasma that recombination occurs ………………………………………………………………………………………………………………………………………………….......... ………………………………………………………………………………………………………………………………………………….......... Independent Assortment: Bivalents will line up at the cell’s equator in a random orientation during Metaphase I ………………………………………………………………………………………………………………………………………………….......... Meaning there is equal probability of a gamete containing the maternal OR paternal copy for any chromosome pair ………………………………………………………………………………………………………………………………………………….......... Because human cells have 23 chromosome pairs, there are 2^23 possible chromosome combinations ………………………………………………………………………………………………………………………………………………….......... (This equates to over 8 million different gamete combinations) ………………………………………………………………………………………………………………………………………………….......... Explain how random gamete fusion promotes variation within a species When two haploid gametes fuse, they form a diploid zygote which can grow into a new organism ………………………………………………………………………………………………………………………………………………….......... Because gamete fusion is random, each successive offspring will be composed of a distinct combination of ………………………………………………………………………………………………………………………………………………….......... maternal and paternal chromosomes ………………………………………………………………………………………………………………………………………………….......... This means every member of a species is unique (promoting biodiversity) ………………………………………………………………………………………………………………………………………………….......... Non-Disjunction Explain, with the aid of the diagrams, how non-disjunction can give rise to aneuploidy Anaphase I Anaphase II Non-disjunction describes the failure of chromosomes to separate during cell division ………………………………………………………………………………………………………………………………………………….......... If non-disjunction occurs in anaphase I, all four gametes will be affected (two = n + 1 ; two = n – 1) ………………………………………………………………………………………………………………………………………………….......... If non-disjunction occurs in anaphase II, only two of the gametes are affected (two = n ; one = n + 1 ; one = n – 1) ………………………………………………………………………………………………………………………………………………….......... If a gamete with one extra chromosome fuses with a normal gamete, the offspring will have trisomy ………………………………………………………………………………………………………………………………………………….......... List the genetic condition that causes Down syndrome and identify a contributing factor Down syndrome is caused by trisomy 21 ………………………………………………………………………………………………………………………………………………….......... Increased maternal age increases the risk of non-disjunction ………………………………………………………………………………………………………………………………………………….......... Describe the method by which cells are obtained for karyotyping Cells are isolated and treated with drugs to promote cell division (makes chromosomes visible to microscope) ………………………………………………………………………………………………………………………………………………….......... Cells are arrested during mitosis and then chromosomes are isolated and visualised ………………………………………………………………………………………………………………………………………………….......... ………………………………………………………………………………………………………………………………………………….......... Compare the benefits and risks associated with amniocentesis and chorionic villi sampling Amniocentesis: Cells are extracted from the amniotic fluid ………………………………………………………………………………………………………………………………………………….......... Occurs later in the pregnancy (~15 weeks) but has a slightly lower risk of miscarriage (~0.5%) ………………………………………………………………………………………………………………………………………………….......... Chorionic Villi Sampling: Cells are extracted from the placenta (chorionic villus) ………………………………………………………………………………………………………………………………………………….......... Occurs earlier in the pregnancy (~11 weeks) but has a slightly higher risk of miscarriage (~1%) …………………………………………………………………………………………………………………………………………………..........