Botany: Previous Year Question Paper for   NCERT/ CBSE  Class 12th Botany

Botany: Previous Year Question Paper for   NCERT/ CBSE  Class 12th Botany

Question: Explain with the help diagram the development of a mature embryo sac from a megaspore mother cell in an angiosperm. 

Ans: The diagram below shows how an angiosperm’s megaspore mother cell develops into a mature embryo sac.

1. Meiosis occurs in the megaspore mother cell, producing four haploid megaspores.

2. Only one functioning megaspore is left after three of the megaspores break apart.

3. A multicellular structure with seven cells is produced as a result of the functioning megaspore going through mitosis.

4. Two nuclei, known as the female and male gametes, are developed in the structure’s central cell.

5. The synergids, antipodal cells, and egg apparatus all grow from the surrounding cells of the gametes.

6. The fully formed, fertile embryo sac has now formed.

Megaspore mother cell

Question: A homozygous tall pea plant with green seeds is crossed with a dwarf pea plant with yellow seeds. 

(a) What would the phenotype and genotype be of F1? 

(b) Work out the phenotypic and genotypic ratio of F2 generation with the help of the checkerboard. 

Ans: a)Tall plants with yellow seeds would be the phenotype of the F1 generation. TTyy would be the genotype.

(b) Three tall plants with yellow seeds would be the F2 generation’s phenotypic ratio: One tall plant with green seeds, one dwarf plant with green seeds, and three dwarf plants, or 3:1:1:1. In other words, the genotypic ratio of the F2 generation is 9:3:3:1, or  9 TTyy: 3 Ttyy: 3 ttyy: 1 ttYY

Question: Discuss the characteristics of the genetic code.

Ans: The genetic code is a set of instructions that guide living cells in translating data encoded in genetic material (DNA or RNA sequences) into proteins (amino acid sequences). All species share a genetic code that is highly conserved, which means that all organisms always use the same codons (groups of three nucleotides) to code for the same amino acids. In biotechnology and medical research, this universal genetic code has been extensively researched and utilised to great advantage.

There are 64 codons in the genetic code, and 61 of them are employed to code for amino acids. The final three codons are stop codons, which indicate that protein production has been completed.

Multiple codons can code for the same amino acid because of the genetic code’s high degree of redundancy. This gives some flexibility in the kinds of gene mutations that can happen without dramatically altering the protein that results.

Degeneracy in the genetic code allows for the possibility of numerous codons encoding the same amino acid. This gives some flexibility in the kinds of gene mutations that can happen without dramatically altering the protein that results.

Additionally, the genetic code displays a phenomenon called wobble that enables some codons to bind with several amino acids. This occurrence

 Characteristics of the genetic code

1. There are triplets of nucleotides in the genetic code. It is non-overlapping in the genetic code.

2. Commas are absent from the genetic code.

3. Degenerate genetic code.

4. Order exists in the genetic code. (5’ to 3’)

5. Codons called start and stop can be found in the genetic code.

6. An almost universal genetic code exists.

Key Points

The 20 amino acids that makeup proteins are each designated by one or more nucleotide triplets in mRNA. The amount of amino acids bound to the tRNAs is 20! Given that mRNA has four bases, there are 64 potential triplets; 61 of them identify amino acids and three signal chain terminations. possess no tRNAs!

Question: With the help of an example, describe co-dominance.

Ans: When both alleles of a gene are expressed in the phenotype of the offspring, this type of inheritance is known as co-dominance. The ABO blood type system in humans is an illustration of co-dominance. There are three alleles in the ABO blood group system: IA, IB, and i. I is recessive and creates the O blood type, whereas IA and IB are co-dominant and produce A and B blood types, respectively. A person is said to have an AB blood type when they have both IA and IB alleles, which means they carry both A and B antigens.

Question: Explain the terms: 

(i) Explant 

(ii) Callus 

Ans:  (i) Explant: A piece of tissue called an explant has been taken from a living thing for in vitro culture. To examine the growth and development of the tissue or cells in a lab setting, it is typically necessary to remove a small piece of tissue or cells from the organism’s surface or interior.

(ii) Callus: Undifferentiated cells make up the callus, a type of plant tissue that forms a protective layer over a wound or other harm to a plant. As a protective barrier that hinders further harm and aids in promoting healing and new development, it is created by the plant in reaction to stress or injury. By injecting hormones or other substances into an explant tissue sample, callus can also be generated in a laboratory setting.

(iii) Micropropagation

Ans: A laboratory technique called micropropagation is used to quickly produce several plants that are identical or similar to small tissue samples. It entails using plant explants or cultivated cells in a sterile environment together with specialised growth media, hormones, and other additions. With this innovation, plants can be bred.

Question: Write a short note on Bt. Cotton

Ans: Bt Cotton is a variety of cotton that has undergone genetic modification by inserting genes from the bacterium Bacillus thuringiensis. This variety of cotton is more profitable and environmentally friendly than traditional cotton since it uses fewer pesticides and is resistant to pests like the cotton bollworm. The most frequently used genetically modified crop worldwide is Bt Cotton, which is widely farmed in China, India, and the US.

Question: Explain the concept of the pyramid of numbers. 

Ans: The number of creatures at each trophic level of a food chain or food web is graphically represented by the pyramid of numbers. The amount of people at each trophic level serves as an illustration of the biomass or energy stored in the environment. The number of organisms at each higher level of the pyramid symbolises the number of organisms in the food chain. The number of organisms declines as you move up the pyramid because energy is expended at each level. This is because only a small portion of the energy is transported to the next level and is instead used up in metabolic processes.

 (Very Short Answer Type Questions) 

Question: Name the interaction in each of the following: 

i)Cuscuta growing on a shoe flower plant. 

Ans: Parasitism

(ii) Mycorrhizae living on the roots of higher plants. 

Ans: Symbiosis

Question: What is Ecological Niche? 

Ans: An ecological niche is a species’ place and function in its environment, including how it satisfies its nutritional and habitational demands, stays alive, and reproduces. In ecology, it serves as the species’ “job.” This involves its interaction with other plants and animals as well as the physical environment, which includes things like soil, water, temperature, and light.

Question: Distinguish between, Endangered, Vulnerable and rare species. 

Ans: The term “endangered species” refers to species that are more vulnerable to extinction because of declining population size or habitat degradation. Examples include the blue whale, black rhinoceros, and enormous panda.

Vulnerable: Species that are at a high risk of going extinct soon because of declining population sizes or habitat degradation are referred to as vulnerable species. Sea turtles, polar bears, and mountain gorillas are a few examples.

Rare species are those that exist in very small numbers in the wild and are at risk of going extinct because of their small populations or constrained habitats. The Amur leopard, Sumatran rhinoceros, and manatee are other examples.

Question: Which is the major air pollutant? Give its source. 

Ans: Carbon dioxide is the main pollutant of the air (CO2). It is a byproduct of burning fossil fuels including coal, oil, and natural gas and is a naturally occurring gas. Additionally, it is emitted during various industrial operations and changes in land use, such as deforestation.

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