Molecular Biology

Step 1: Understand the genetic code
The genetic code is degenerate, meaning multiple codons can code for the same amino acid. However, not all synonymous codons are used equally.
Step 2: Define codon bias
Codon bias refers to the non-random usage of synonymous codons in coding sequences. For a particular amino acid, certain codons are preferred over others in the genome of an organism.
Step 3: Biological relevance
Codon bias is linked to translational efficiency, accuracy, and the availability of corresponding tRNAs. Highly expressed genes often use codons that match abundant tRNAs, improving protein synthesis.
Step 4: Evaluate the options
Option (1) accurately describes the phenomenon.
Option (2) is incorrect — codon choice is evolutionary, not human.
Option (3) is false — plants do exhibit codon bias.
Option (4) is misleading — codon usage varies across species or expression levels, not per protein.

Step 1: Translation initiation in prokaryotes
In prokaryotes, translation begins when the ribosome assembles at the start codon (AUG) on the mRNA. However, efficient recognition of the start codon requires a special upstream sequence known as the Shine-Dalgarno sequence.
Step 2: Role of the Shine-Dalgarno sequence
The Shine-Dalgarno sequence is a purine-rich region located about 6--10 nucleotides upstream of the start codon on mRNA. It base-pairs with a complementary sequence on the 3 end of the 16S rRNA of the small (30S) ribosomal subunit.
Step 3: Function of 16S rRNA in initiation
This base-pairing helps align the ribosome correctly with the start codon and is essential for the initiation of protein synthesis in prokaryotes. Therefore, the correct answer is: 16S rRNA.

Step 1: Understand Assertion (A)
Adeno-associated viruses (AAVs) are commonly used vectors in gene therapy and vaccine platforms.
AAVs deliver single-stranded DNA (ssDNA) into host cells.
These vectors are non-pathogenic and have low immunogenicity.
AAV-based vaccines can generate long-lasting immune responses, thereby reducing the need for multiple booster doses.
Hence, Assertion (A) is correct.
Step 2: Understand Reason (R)
AAVs often require the presence of helper viruses like adenovirus or herpesvirus to complete their life cycle and enhance the transgene expression.
These helper viruses provide essential helper functions (e.g., transcriptional activation, replication enzymes) that increase the efficiency of AAV transduction.
By enhancing gene expression, they also enhance the immune response, potentially eliminating the need for additional booster doses.
Hence, Reason (R) is also correct.
Step 3: Relationship between A and R
The co-administration of adenovirus or herpesvirus is a critical helper mechanism that boosts the effectiveness of AAV-based vectors.
This mechanistic help is exactly why AAV-based vaccines often do not require boosters.
Therefore, R correctly explains A.

Step 1: Understand the Central Dogma.
The Central Dogma of molecular biology, proposed by Francis Crick in 1958, describes the unidirectional flow of genetic information: Step 2: Processes originally included.
The processes covered were:
Replication: DNA makes copies of itself.
Transcription: DNA to RNA.
Translation: RNA to protein.
Step 3: Reverse transcription not included.
Reverse transcription (RNA to DNA) was not part of the original Central Dogma and was only discovered later with the study of retroviruses (e.g., HIV).
Conclusion: Reverse transcription was not included in the original Central Dogma.