Q.Differentiate between the followings:

During transcription, only one of the two DNA strands acts as a template for RNA synthesis. The choice of which strand serves this purpose defines the template and coding strands.

• Template Strand: This strand has a $3'$ to $5'$ polarity and serves as the actual template for RNA polymerase. The enzyme reads its sequence, and the newly synthesized RNA molecule is complementary to this strand. It is also known as the antisense strand. …

Let's delve into these fundamental distinctions in molecular biology, which are crucial for understanding how genetic information is organized and expressed.

(a) Repetitive DNA and Satellite DNA

Our genome is not just a continuous stretch of protein-coding genes. A significant portion consists of sequences that are repeated many times. This is where the terms repetitive DNA and satellite DNA come into play.

• Repetitive DNA refers to DNA sequences that are present in multiple copies within the genome. These sequences can range from a few base pairs to hundreds of base pairs long and can be repeated thousands or even millions of times. They are often interspersed throughout the genome or clustered in specific regions. A key characteristic is that these regions typically do not code for proteins. Their functions are diverse, including contributing to chromosome structure, gene regulation, and evolutionary processes.

• Satellite DNA is a specific category of repetitive DNA. The term 'satellite' originates from its behavior during density gradient centrifugation. When genomic DNA is subjected to this technique, the bulk of the DNA forms a major peak. However, certain repetitive DNA sequences, due to their distinct base composition (and thus different density), separate out and form smaller, distinct peaks, which are referred to as 'satellite peaks'. This fraction of DNA is called satellite DNA.

  Note

  Satellite DNA is further classified into mini-satellites and micro-satellites based on the length of the repeating unit and the number of repetitions. These variations are highly specific to individuals and form the basis of DNA fingerprinting, where Variable Number Tandem Repeats (VNTRs), a type of mini-satellite, are analyzed.

  While all satellite DNA is repetitive, not all repetitive DNA is classified as satellite DNA in this specific sense of forming a distinct peak during centrifugation. Satellite DNA is typically found in regions like centromeres and telomeres, playing structural roles rather than coding for proteins.

(b) mRNA and tRNA

Both mRNA and tRNA are types of RNA molecules, but they perform vastly different roles in the crucial process of protein synthesis, known as translation.

• mRNA (messenger RNA) acts as the carrier of genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm. Imagine it as a temporary blueprint. When a gene needs to be expressed, its DNA sequence is transcribed into an mRNA molecule. This mRNA molecule then travels to the ribosome, where its sequence of codons (three-nucleotide units) dictates the specific order of amino acids to be assembled into a protein. mRNA molecules are typically linear and vary greatly in length, reflecting the size of the protein they encode. They are generally less stable and are degraded after serving their purpose, ensuring that protein synthesis is regulated.

• tRNA (transfer RNA), on the other hand, functions as an adaptor molecule. It has a unique clover-leaf-like structure (in 2D) and an L-shaped structure (in 3D). Each tRNA molecule has two critical sites:

  • An anticodon loop that contains a sequence of three nucleotides, complementary to a specific codon on the mRNA.
  • An amino acid acceptor end to which a specific amino acid is attached. The role of tRNA is to "read" the codons on the mRNA and bring the corresponding amino acid to the ribosome, ensuring that the correct amino acid is added to the growing polypeptide chain. There are specific tRNAs for each of the 20 amino acids, and sometimes more than one tRNA for a single amino acid. tRNA molecules are relatively small and more stable than mRNA.
  Important

  mRNA carries the message (the genetic code), while tRNA transfers the correct amino acids according to that message.

(c) Template strand and Coding strand …

Instead of writing three separate prose paragraphs, build a single differentiation table with one row per pair and columns for 'defining feature' and 'example/location' -- e.g. Repetitive DNA (any repeated sequence) vs Satellite DNA (a repeated-sequence fraction that specifically separates as a distinct peak on density-grad …