Class 8 General Science Notes: - Unit-3: VARIATIONS, HEREDITY AND CELL DIVISION: Solved Exercise Part- D (Structured Questions and Answers)

(National Book Foundation - New Book/New Syllabus Based on National Curriculum Pakistan 2023-2024 and Onwards)

(Class 8 General Science Notes for 8th Centralized Exam 2023-2024 and onward, FDE, Islamabad)

Click Here for Solved MCQs Unit-3

D. Structured questions

1. Look at the coloured spiral structure of DNA

i) Show units of DNA molecule in different colours.

Adenine_____________Red_________________(colour)

Guanine____________Blue________________(colour)

Cytosine__________White____________________(colour)

Thymine____________Green__________________(colour)

ii) Explain how these units pair to form DNA molecule.

Ans:
DNA is composed of two strands that are held together by complementary base pairing. The four nucleotide bases—adenine (A), thymine (T), guanine (G), and cytosine (C) pair in a specific manner:



1. Adenine (A) pairs with Thymine (T):

Adenine always forms a pair with thymine while cytosine forms a pair with Guanine. A double hydrogen bond is present between Adenine and Thymine.

2. Guanine (G) pairs with Cytosine (C):

Guanine and Cytosine form another complementary base pair. A trippe hydrogen bond is present between Cytosine and Guanine.

i) Explain the difference in the appearance of chromosomes at prophase and telophase stage.

Ans:

At prophase, chromosomes appear condensed and tightly coiled, with individual chromatids visible. The nuclear membrane begins to break down, and spindle fibers form inside the cell.

in the last stage i.e. telophase, chromosomes appear less condensed as they de-condense back into chromatin. Nuclear membrane reform around the separated sets of chromosomes, spindle fibers disappear, and cytokinesis may begin.

ii) Why chromosomes are not visible in non-dividing cell?

Ans:

Chromosomes are not visible in non-dividing cells because the DNA is in a less condensed form called chromatin, which is spread out within the nucleus.

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