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Chapter 2: Problem 19

During the first meiotic prophase, (a) when does crossing over occur; (b) when does synapsis occur; (c) during which stage are the chromosomes least condensed; and (d) when are chiasmata first visible?

Short Answer

Expert verified
Answer: Crossing over occurs during the Pachytene stage, synapsis takes place during the Zygotene stage, chromosomes are least condensed during the Leptotene stage, and chiasmata are first visible during the Diplotene stage.

Step by step solution

01

(a) Crossing Over Occurs during Pachytene Stage

During the Pachytene stage of the first meiotic prophase, crossing over or genetic recombination takes place. Homologous chromosomes exchange genetic materials through the process of crossing over, which increases the genetic variation within the offspring.
02

(b) Synapsis Occurs during Zygotene Stage

Synapsis occurs during the Zygotene stage of the first meiotic prophase. During synapsis, the homologous chromosomes come together and pair up to form bivalents. Each bivalent consists of two homologous chromosomes that are held together by the synaptonemal complex, a protein structure.
03

(c) Chromosomes Are Least Condensed During Leptotene Stage

The chromosomes are least condensed during the Leptotene stage of the first meiotic prophase. In this stage, chromosomes begin to condense, and the chromatin starts to look more like distinct threads, which are the individual chromosomes.
04

(d) Chiasmata Are First Visible during Diplotene Stage

Chiasmata are the physical sites of crossing over visible under the microscope as X-shaped structures. These are first visible during the Diplotene stage of the first meiotic prophase. This stage is marked by the dissolution of the synaptonemal complex, which holds the homologous chromosomes together. As the synaptonemal complex disappears, the homologous chromosomes start to separate, and the chiasmata can be observed.

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Most popular questions from this chapter

In this chapter, we focused on how chromosomes are distributed during cell division, both in dividing somatic cells (mitosis) and in gamete- and spore- forming cells (meiosis). We found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, answer the following questions. (a) How do we know that chromosomes exist in homologous pairs? (b) How do we know that DNA replication occurs during interphase, not early in mitosis? (c) How do we know that mitotic chromosomes are derived from chromatin?

If two chromosomes of a species are the same length and have similar centromere placements and yet are not homologous, what is different about them?

A species of cereal rye (Secale cereale) has a chromosome number of \(14,\) while a species of Canadian wild rye (Elymus canadensis) has a chromosome number of \(28 .\) Sterile hybrids can be produced by crossing Secale with Elymus. (a) What would be the expected chromosome number in the somatic cells of the hybrids? (b) Given that none of the chromosomes pair at meiosis I in the sterile hybrid (Hang and Franckowlak, 1984 ), speculate on the anaphase I separation patterns of these chromosomes.

In mitosis, what chromatid combination(s) will be present during metaphase? What combination(s) will be present at each pole at the completion of anaphase?

Given the end results of the two types of division, why is it necessary for homologs to pair during meiosis and not desirable for them to pair during mitosis?
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