24. What is ecological succession? How hydrarch succession is different from that of xerarch succession?

Ans: The sequential, gradual, and predictable changes in the species composition in an area are called succession or ecological succession.

Hydrarch succession: The plant succession which takes place in a wet area or water, leading to successional series progress from hydric to the mesic conditions.

Xerarch succession: The plant succession which takes place in a dry area, leading to a successional series from xeric to mesic conditions.

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25. Alien species invasion caused the decline or extinction of indigenous species. Justify the statement by giving three examples.

Ans: ​Some alien (exotic) species when introduced unintentionally or deliberately, become invasive and cause harmful impact, resulting in the extinction of the indigenous species.

Examples:

1. Nile perch, a large predator fish when introduced to Lake Victoria (East Africa) caused the extinction of an ecologically unique species of Cichlid fish in the lake.

2. Invasive weed species like Parthenium (carrot grass), Lantana, and Eichhornia (water hyacinth) caused environmental damage and posed threat to our native species.

3. The introduction of African catfish (Clarias gariepinus) for aquaculture purposes is posing a threat to the indigenous catfishes of Indian rivers.

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26. Define autogamy. Write the two different kinds of flowers that exhibit autogamy.

Ans: ​It is a type of pollination in which pollens are transferred to the stigma of the same flower.

E.g., wheat, rice, pea, etc…

hasmogamous and Cleistogamous flowers exhibit autogamy.

PART - D

Section - I

Answer any four of the following questions in about 200 to 250 words each wherever applicable;

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27. Explain a mature embryo sac with a neat labelled diagram.

Ans: 

1. The embryo sac is referred to as a female gametophyte.

2. The formation of a female gametophyte (embryo sac) is called megagametogenesis.

3. The megaspore increases in size and its nucleus divide mitotically into two nuclei which move apart to opposite poles. Thus, a 2-nucleate embryo sac is formed.

4. The two daughter nuclei undergo another mitotic division giving rise to the 4-nucleate stage.

5. The third mitotic division gives rise to an 8-nucleate 7-celled embryo sac.

6. The central cell contains 2 nuclei known as polar nuclei.

7. The three nuclei at the micropylar region form the egg apparatus.

8. In the egg apparatus, the middle cell is the largest and is called oosphere/egg/ovum, while the other two naked cells adjoining the egg cell are called synergids.

9. The three nuclei at the chalazal end are surrounded by cytoplasm and cellular wall. These are called antipodal cells.

28. Schematically represent the inheritance of two genes in pea plants with reference to seed colour and shape.

Ans:

29. Mention five salient features of Human Genome Project.

Ans:  Salient features of Human Genome Project (HGP):

1. The Human Genome contains 3164.7 million bases pairs.

2. The average gene consists of 3000 bases; the largest known human gene being dystrophin at 2.4 million bases.

3. The total number of genes is estimated to be 30,000 and 99.9% nucleotide bases are exactly the same in all people.

4. The functions are unknown for over 50% of the discovered genes.

5. Less than 2% of the genome codes for proteins.

6. The Human genome contains large repeated sequences.

7. The repeated sequence is thought to have no direct coding functions, but they throw light on chromosome structures, dynamics and evolution.

8. Chromosome 1 has most genes (2968) and the Y has the fewest genes (231).

9. Scientists have identified about 1.4 million locations where single base DNA sequence differences called single nucleotide polymorphism or SNPs occur in humans.

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