Page 44 - Biology - XII

2. Zygotene (Zygo—pair). This stage is identifi ed by—
(a) The chromosomes pair up. These pairs are called bivalents.
(b) The chromosomes become thick and short.

(c) Nuclear membrane is present.
(d) Nucleolus is also seen.
3. Pachytene (Pachy—thick). This stage is identifi ed by—
(a) The chromosomes become more thick.
(b) They split lengthwise to form sister chromatids. (This condition of chromosome is called tetrad.)

(c) The chromatids coil at places. (This is called crossing over.) The point of contact is called
chaismata.
(d) Nuclear membrane and nucleolus is present.
4. Diplotene (Diplo—two). This stage is identifi ed by—
(a) The paired chromosomes start separating.

(b) Nuclear membrane starts to disappear.
(c) Nucleolus starts to disappear.
5. Diakinesis (Di—moving between): This stage is identifi ed by—
(a) The chromosomes become more thick.
(b) The chromosomes separate.

(c) Nuclear membrane disappears.
(d) Nucleolus disappears.
Metaphase I. This stage is identifi ed by—

(a) The nuclear membrane disappears.
(b) The nucleolus also disappears.
(c) The centromere of the chromosome is attached to the spindle fi bres. The centromere faces the
centriole/pole of the cell.
(d) The chromosomes are arranged at the equator of the cell.

Anaphase I. This stage is identifi ed by—
(a) The bivalent chromosomes are pulled and pushed apart.
(b) They come and lie at opposite poles of the cell.
(c) Each chromosome has distinct chromatids only attached at centromeres.

(d) The chromosome number is half the original number. (Each group of chromosomes is half (n) of
the original number 2n.)
Telophase I. This stage is identifi ed by—
(a) The chromosomes at the poles uncoil.
(b) The chromosomes are surrounded by a nuclear membrane. Thus, two daughter nuclei are formed.

(c) Nucleolus reappears within the nucleus.
(A cell membrane may be formed/Cytoplasm division may be followed.)

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