Plant Growth Regulators

Step 1: Understanding chlorophyll types.
Chlorophyll-a is the main pigment involved in oxygenic photosynthesis in plants and algae.

Step 2: Examining photosynthetic bacteria.
Photosynthetic bacteria such as purple and green bacteria perform anoxygenic photosynthesis using bacteriochlorophyll instead of chlorophyll-a.

Step 3: Option analysis.
(A) Vascular plants possess chlorophyll-a.
(B) Algae contain chlorophyll-a.
(C) Photosynthetic bacteria lack chlorophyll-a.
(D) Bryophytes contain chlorophyll-a.

Step 4: Conclusion.
The correct answer is (C) photosynthetic bacteria.

Step 1: Understanding the effect of auxins.
Auxins are plant hormones used to regulate plant growth. 2,4-D is an auxin commonly used as an herbicide to target dicotyledonous plants. It mimics the action of natural auxins but causes abnormal growth that leads to the death of dicotyledonous weeds.
Step 2: Analyzing the options.
(A) 2,4-D: Correct, 2,4-D is an auxin commonly used to control dicot weeds in monocot fields.
(B) IBA: Incorrect, IBA is a natural auxin but is primarily used for rooting, not for killing weeds.
(C) IAA: Incorrect, IAA is a natural auxin but not effective as a herbicide in this context.
(D) NAA: Incorrect, NAA is used for root initiation but not for weed control.
Step 3: Conclusion.
The correct answer is (A) 2,4-D, as it is an effective herbicide for dicotyledonous weeds in monocotyledonous fields.

Step 1: Understanding codon–anticodon pairing.
During translation, mRNA codons pair with complementary anticodons present on tRNA molecules to add specific amino acids to the growing polypeptide chain.
Step 2: Identifying stop codons.
Some codons do not code for any amino acid and hence do not have corresponding tRNA anticodons. These are called stop codons.
The three stop codons are UAA, UAG, and UGA.
Step 3: Analysis of options.
(A) AUU: Codes for isoleucine and pairs with tRNA.
(B) UAG: Correct — it is a stop codon and does not pair with any tRNA anticodon.
(C) UUU: Codes for phenylalanine and pairs with tRNA.
(D) CUC: Codes for leucine and pairs with tRNA.
Step 4: Conclusion.
The codon that does NOT pair with any tRNA anticodon is UAG.

Step 1: Understanding leaf expansion.
Leaf expansion involves both cell elongation and cell enlargement, leading to an increase in leaf size and surface area.
Step 2: Role of gibberellins.
Gibberellins promote cell elongation and expansion, especially in leaves and stems, and are therefore used to enhance leaf growth in leafy vegetables.
Step 3: Conclusion.
Hence, the phytohormone useful for leaf expansion is Gibberellins.

Step 1: In mitotic cell division, the number of cells doubles after each division. Starting with one cell, the pattern of division is as follows:
- After 1 division: 2 cells
- After 2 divisions: 4 cells
- After 3 divisions: 8 cells
- After 4 divisions: 16 cells

Step 2: Since the question asks about the total number of cells that have lost the ability to divide after four mitotic divisions, we need to consider cells that are no longer in the active cell cycle. Typically, after reaching a certain stage in development, some cells may enter a phase like G0, where they stop dividing.
Step 3: Given this, the correct number of cells that have lost the ability to divide is 4, because the cells from the 5th and subsequent stages have entered G0.
Conclusion: After 4 divisions, 4 cells have lost the ability to divide.

Plant growth is regulated by:
Auxins: Promote cell elongation and phototropism.
Gibberellins: Stimulate stem elongation and seed germination.
Cytokinins: Promote cell division and delay senescence.
These hormones act synergistically or antagonistically to regulate overall growth and development.
Final Answer:

Gibberellins (GA) are plant hormones used to promote the elongation and enlargement of fruits, including grapes. Key roles of gibberellins in grapes include:
Stimulating cell elongation, leading to larger and more uniform grape size.
Reducing cluster compactness, making grape clusters more marketable.
Enhancing the overall quality of grapes by improving fruit development.
Farmers often apply gibberellins during fruit development to ensure better yield and quality.
Final Answer:

Apical dominance is the phenomenon where the apical bud inhibits the growth of lateral buds. It is primarily regulated by the hormone auxin, which is produced at the shoot tip. Cytokinin, however, acts in opposition to auxin and promotes the growth of lateral buds, thereby reversing apical dominance. Auxin maintains the dominance of the apical bud by suppressing the lateral bud growth. Cytokinin encourages cell division in the lateral buds, effectively overcoming the inhibitory effect of auxin. Thus, the hormone that reverses apical dominance is cytokinin.

Step 1: Gibberellins are plant hormones that play a critical role in promoting growth, including fruit elongation. They are commonly used to increase the size of fruits like grapes.

Step 2: - (A) Auxins promote cell elongation but are primarily involved in root growth and development.
- (B) Cytokinins are involved in cell division and promote shoot formation.
- (D) Ethylene is associated with fruit ripening rather than elongation.
Conclusion: Gibberellins are the hormone responsible for fruit elongation in grapes.