Q3 of 33 Page 18

The diagram given below represents a certain phenomenon which occurs during meiosis. Name and explain the phenomenon by using the terms - homologous chromosomes, chromatids, crossing-over.

In the first meiotic division, homologous chromosomes come close together to form a pair called a bivalent. A bivalent has two chromosomes, one paternal and one maternal. The pairing of homologous chromosomes is known as synapsis. During pairing, some chromatids of homologous chromosomes break and rejoin at points of intersection, called chiasmata. This causes crossing over with exchange of chromatid segments between the two chromosomes of a homologous pair. It leads to the formation of new gene combination in the resulting chromosomes.


More from this chapter

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1

The diagram below represents a stage during cell division. Study the same and then answer the questions that follow:


(a) Name the parts labelled 1, 2 and 3.


(b) Identify the above stage and give a reason to support your answer.


(c) Mention where in the body this type of cell division occurs.


(d) Name the stage prior to this stage and draw a diagram to represent the same.

 2

Draw a labelled diagram to show the metaphase stage of mitosis in an animal cell having '6' chromosomes.

 4

Given below is a diagram representing a stage during mitotic cell division in an animal cell.

Examine it carefully and answer the questions which follow.



(a) Identify the stage. Give one reason in support of your answer.


(b) Name the cell organelle that forms the 'aster'.


(c) Name the parts labelled 1, 2 and 3.


(d) Name the stage that follows the one shown here.


How is that stage identified?


(e) Mention two differences between mitosis and meiosis with regard to:


(i) The number of daughter cells produced.


(ii) The chromosome number of the daughter cells.

 5

Given ahead are three diagrammatic sketches (A, B and C) of one and the same particular phase during the mitotic type of cell division.

(a) Identify the phase....................


(b) What is the diploid number of chromosomes shown in them? ....................



(c) Identify whether these are animal cells or plant cells?


A ...............................................,


B ...............................................,


C ...............................................,


(d) Which of these is/are shown in the correct direction? (Tick mark the correct answer).


(i) Only A (ii) Only B


(iii) Only A and C (iv) All the three