Crossing over between homologous chromosomes takes place in which sub-stage of prophase I?
- A.Leptotene
- B.Zygotene
- C.Pachytene
- D.Diakinesis
Correct answer
Explanation
The correct answer is C, pachytene. By this sub-stage the homologous chromosomes have already paired in zygotene and are held together as bivalents or tetrads, each made of four chromatids. In pachytene the non-sister chromatids of the paired homologues exchange corresponding segments at structures called recombination nodules, a process known as crossing over, and the enzyme recombinase makes it possible. Crossing over is the chief source of new combinations of genes and therefore of genetic variation. A, leptotene is only the stage in which chromosomes first become visible as slender threads. B, zygotene is the stage of pairing or synapsis, brought about by the synaptonemal complex, but no exchange has yet occurred. D, diakinesis is the last sub-stage, marked by terminalisation of chiasmata and the breakdown of the nuclear envelope.
Read the full article: Cell Division: Mitosis and Meiosis Explained for Exams
Practice Questions
View allIn which phase of the cell cycle does DNA replication take place?
- A.G1 phase
- B.S phase
- C.G2 phase
- D.M phase
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During this phase the DNA of the cell is copied so that the amount of DNA in the nucleus doubles, while the number of chromosomes remains unchanged because each chromosome now simply consists of two identical sister chromatids joined at the centromere.
Read the full explanation →Mitosis is called the equational division because
- A.it produces four daughter cells
- B.the chromosome number of the daughter cells is the same as that of the parent cell
- C.it halves the chromosome number
- D.it occurs only in reproductive cells
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In mitosis each centromere splits in anaphase and one chromatid of every chromosome goes to each pole, so the two daughter cells receive exactly the same number of chromosomes as the parent cell had, and exactly the same genetic information.
Read the full explanation →At which stage of mitosis do the chromosomes line up at the equator of the cell?
- A.Prophase
- B.Metaphase
- C.Anaphase
- D.Telophase
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In this stage the condensed chromosomes, each consisting of two sister chromatids, are moved by the spindle fibres until their centromeres lie in one plane across the middle of the cell, known as the metaphase plate or equatorial plate.
Read the full explanation →The splitting of the centromere and the movement of sister chromatids to opposite poles occurs in
- A.prophase
- B.metaphase
- C.anaphase
- D.telophase
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Once the chromosomes are aligned on the metaphase plate and every kinetochore is attached to a spindle fibre, the centromere holding the two sister chromatids together splits.
Read the full explanation →Cytokinesis in a plant cell differs from that in an animal cell because
- A.a cleavage furrow forms in the plant cell
- B.a cell plate forms in the plant cell
- C.the plant cell does not divide its cytoplasm
- D.the plant cell divides its nucleus twice
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An animal cell divides its cytoplasm by a cleavage furrow, in which the plasma membrane is pulled inward around the middle until the cell is pinched into two.
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