General Science 8th Class Chapter 3 Variations, Heredity and Cell Division Solved Exercise

General Science

8th Class

Chapter 3

Variations, Heredity and Cell Division

Questions

1. Encircle the correct option.

(i) Which of these is NOT a genetic variation?

a. Mark of a cut on skin        b. Eye colour        c. Attached earlobes        d. Blood group

(ii) You are different from your brother and sisters. This is called.

a. variation        b. adaptation        c. hereditary        d. mitosis

(iii) In humans, the eye colour is developed due to the effects of:

a. diet        b. environment        c. genes        d. both 'a' and 'b'

(iv) The section of DNA which has information for making a specific protein is called:

a. DNA strand        b. nucleotide        c. chromosome        d. gene

(v) Chromosomes are made of:

a. DNA only        b. protein only        c. DNA, proteins and fats        d. DNA and proteins

(vi) The characters which are passed from parents to offspring:

a. hereditary characters        b. non-inheritable characters

c. environmental characters        d. none of these

(vii) An event that occurs during interphase:

a. division of nucleus        b. division of cytoplasm

c. duplication of chromosomes        d. formation of cell wall

(viii) Reduction of chromosomes take place during:

a. mitosis        b. meiotic-I        c. meiotic-II        d. both mitosis and meiotic-II

(ix) In humans, a sperm has 23 chromosomes. Egg cell has:

a. 23 chromosomes        b. 46 chromosomes        c. no chromosomes        d. 69 chromosomes

(x) Zygote is formed by the fusion of:

a. two sperm cells        b. two egg cells        c. two somatic cells        d. sperm cell and egg cell

2 Write short answers.

(i) What is the primary source of genetic variation?

Ans.

The primary source of genetic variation is mutation. Mutation is a change in the genetic material (gene).

(ii) Plants are adapted to their environment. Give examples.

Ans.

Plants are adapted to their environment in many ways. This is because they have special features that help them to survive.

Examples:

1. The best example is cactus plant to survive in the desert. Its stem can store water.

2. Trees in rainforests  grow very tall to get sunlight.

3. Floating plants like water hyacinth, duckweed and water lily have floating leaves and submerged roots.

(iii) What is difference between variations and adaptations?

Ans.

Variations:

The differences shown by the individuals of one kind of organisms (one species) are called variations.

Examples:

(i) Skin color, blood type, eye color, tongue rolling etc. are genetic variations.

(ii) Language, scar on skin etc. are environmental variations.

Adaptations:

The characteristics of an organism that help it to survive in a given environment are called adaptations.

Examples:

(i) Animals that live in cold areas have adaptations to keep them warm.

(ii) Plants that live in dry areas have adaptations to help them conserve water.

(iv) What is gene?

Ans.

The basic physical and functional unit of heredity is called gene.

(v) Define heredity and describe its importance in transferring of characteristics from parents to offspring.

Ans.

Heredity:

The transmission of characteristics from parents to offspring is called heredity.

Importance of Heredity:

Importance of heredity is stated as:

(i) During reproduction, living things pass on their characteristics to their offspring. This is the reason that babies look like their parents.

(ii) Plants grown from seeds resemble their parent plants.

(vi) Differentiate between mitosis and meiosis.

Ans.

Mitosis:

• During mitosis, two daughter cells are formed from parent cell.
• Mitosis occurs in somatic cells.
• The number of chromosomes in the daughter cells remain the same as in parent cell.

Meiosis:

• During meiosis, four daughter cells are formed from the parent cell.
• The number of chromosomes in the daughter cells is reduced to half as compared to that in the parent cell.
• Meiosis occurs to produce gametes in animals or spores in plants.

(vii) What is the structure of DNA?

Ans.

According to Watson and Crick's model, DNA consists of two strands of nucleotides. The two strands are held together by hydrogen bonds between nitrogenous basis of nucleotides of opposite strands. The nitrogenous base Adenine of one strand makes bonds with Thymine of the opposite strand. Similarly, Guanine of one strand makes bonds with Cytosine of the opposite strand.