nps391E.tmp - 2 3 A similar cartoon of meiosis is in your...

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1 Glossary for lecture 2a Bivalent : Paired homologous chromosomes during meiosis. Crossing over (crossover) : The site where homologous chromatids break and exchange segments, so that a piece of the paternal chromosome is attached to maternal chromosome and vice versa . Crossing over is always reciprocal. Chiasma : The physical point at which crossing over between homologues took place is the cytological structure called chiasma, which is the point where the homologues are held together until the onset of anaphase I. Disjunction : Separation of homologous chromosomes at anaphase I (AI), which is the physical basis of gene segregation. Synaptomenal complex (SC) : A zipper-like structure consisting of two lateral elements and the central element that is built between pairing homologues in zygotene and holds them together. SC falls off the bivalent at diplotene except in the place where a chiasma is. Synapsis : Pairing of homologous chromosomes during zygotene and pachytene.
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Unformatted text preview: 2 3 A similar cartoon of meiosis is in your book. Beware! The chromosomes in leptotene and the later stages had been replicated in the preceding interphase and consist of two chromatids, not just one as the cartoon shows. One of many differences between mitosis and meiosis is that in mitosis chromosome line-up on the equator by their centromeres whereas in meiosis I they line up by their chiasmata while centromeres point to the opposite poles. 4 Synapsis starts at the termini of chromosomes. Recognition of homologues is facilitated by the attachment of the telomeric sequences to the nuclear envelope. There the telomeres are attached to cytoplasmic microtubules. The nucleus is then moved from side to side at the opposite pole by cytoplasmic streaming, which results in telomeres sliding to one pole of the nucleus, forming the bouquet . Once homologues bump into each other, they will begin to synapse by SC....
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nps391E.tmp - 2 3 A similar cartoon of meiosis is in your...

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