(a) Protein synthesis requires the services of all three types of RNAs, namely t-RNA, m-RNA and r-RNA. Explain the role of each of them during the process of protein synthesis in prokaryotes.

or

(b) A homozygous tall pea plant with green seeds is crossed with a homozygous dwarf pea plant with yellow seeds.

i) Write the possible phenotype and genotype of F₁ generation.

ii) State the laws of Mendel that are proved true by the F₁ generation.

iii) Mention the F₂ phenotypic ratio along with their possible phenotypes.

iv) Write the genotypes of the male and female gametes produced by F₁ progeny.

Correct Answer:

Part (a):
• tRNA (transfer RNA): Acts as an adapter molecule that reads the genetic code. It has a specific amino acid-binding site at one end and an anticodon loop at the other, bringing the correct amino acids to the ribosome. Initiator tRNA initiates translation.
• mRNA (messenger RNA): Acts as a template for translation, carrying genetic instructions in the form of codons from DNA. It contains the start codon (AUG) to initiate translation and a stop codon (UAA, UAG, or UGA) to terminate protein synthesis.
• rRNA (ribosomal RNA): Associates with proteins to form ribosomes. The small ribosomal subunit binds to mRNA to initiate translation, while the large subunit provides binding sites for tRNAs and acts as a catalyst (peptidyl transferase) to form peptide bonds between amino acids.

or

Part (b):
i) F₁ Generation: Phenotype is Tall plant with Yellow seeds; Genotype is TtYy.
ii) Mendelian Laws: Law of Dominance and Law of Segregation.
iii) F₂ Phenotypic Ratio: Tall yellow : Tall green : Dwarf yellow : Dwarf green = 9 : 3 : 3 : 1.
iv) Gametes: Both male and female gametes produced by the F₁ progeny are TY, Ty, tY, and ty.

Detailed Step-by-Step Explanation

Part (a): Role of RNAs in Prokaryotic Protein Synthesis

Protein synthesis (translation) is the process by which the genetic information carried by nucleic acids is translated into a sequence of amino acids to form a protein. This process relies on three primary types of RNA molecules:

1. Messenger RNA (mRNA):
• mRNA serves as the direct template for translation. It is synthesized during transcription as a complementary copy of a DNA template strand.
• It carries genetic information in the form of triplets of nucleotides called codons. Each codon specifies a single amino acid.
• Translation is initiated when the ribosome recognizes the initiator/start codon on the mRNA, which is almost always AUG (coding for Formylmethionine in prokaryotes).
• The translation process continues until the ribosome reaches one of the three stop codons: UAA, UAG, or UGA. These stop codons do not code for any amino acid and signal the termination of protein synthesis.

2. Transfer RNA (tRNA):
• tRNA acts as an adapter molecule, bridging the gap between the nucleic acid sequence of mRNA and the amino acid sequence of proteins.
• Each tRNA molecule has a specific amino acid binding site (at its 3' end) that attaches to a specific amino acid, and an anticodon loop containing a triplet of bases complementary to a specific codon on the mRNA.
• tRNA brings the appropriate amino acid to the ribosomal site by matching its anticodon with the codon present on the mRNA template.
• A specialized initiator tRNA carries the first amino acid to start the translation process at the start codon.

3. Ribosomal RNA (rRNA):
• rRNA associates with ribosomal proteins to organize into the structural components of the ribosome, which consist of a small subunit (30S in prokaryotes) and a large subunit (50S in prokaryotes).
• The small subunit binds to the mRNA strand near the start codon to initiate translation.
• The large subunit contains active sites where tRNA molecules bind. The incoming aminoacyl-tRNA binds to the acceptor site, and the peptidyl-tRNA holding the growing polypeptide chain occupies the donor site.
• Importantly, rRNA does not just play a structural role; it also functions catalytically. The 23S rRNA in the large subunit of the prokaryotic ribosome acts as a ribozyme (peptidyl transferase) that catalyzes the formation of peptide bonds between adjacent amino acids.

Part (b): Dihybrid Cross Analysis

Let us define the alleles representing the traits:
• Height: Tall allele (T) is dominant over dwarf allele (t).
• Seed Color: Yellow allele (Y) is dominant over green allele (y).

The parents are:
• Homozygous Tall with Green seeds: TTyy
• Homozygous Dwarf with Yellow seeds: ttYY

i) Phenotype and Genotype of the F₁ Generation:
• The gametes produced by the first parent (TTyy) are all Ty.
• The gametes produced by the second parent (ttYY) are all tY.
• Fertilization results in the F₁ progeny with the genotype: TtYy.
• Since the dominant alleles (T and Y) suppress the expression of the recessive alleles (t and y), the phenotype of all F₁ offspring is Tall plants with Yellow seeds.

ii) Mendelian Laws proved true by the F₁ generation:
• Law of Dominance: When parents with contrasting homozygous traits are crossed, only one of the traits (the dominant trait) is expressed in the F₁ generation, while the other trait (the recessive trait) remains hidden or unexpressed. Here, tall height and yellow seed color dominate over dwarf height and green seed color.
• Law of Segregation: Alleles do not show blending or mixing when present together in a hybrid (TtYy). Although they stay together in the F₁ generation, they segregate (separate) from each other during gamete formation, ensuring that a gamete carries only one allele for each gene.

iii) F₂ Phenotypic Ratio:
When the F₁ progeny (TtYy) is self-pollinated, the independent assortment of alleles produces a dihybrid ratio in the F₂ generation:
• Tall Yellow: 9/16
• Tall Green: 3/16
• Dwarf Yellow: 3/16
• Dwarf Green: 1/16
Thus, the F₂ phenotypic ratio is 9 : 3 : 3 : 1.

iv) Genotypes of Gametes produced by F₁ progeny:
During meiosis in the F₁ plants (TtYy), alleles for plant height segregate independently of alleles for seed color. This results in four possible allele combinations with equal probability in both male and female gametes:
• TY, Ty, tY, and ty.