Revise Structure of nucleic acids and replication of DNA for Cambridge 9700 (6.1)

Both DNA and RNA are polymers, meaning they are large molecules made up of repeating smaller units called monomers. For nucleic acids, these monomers are called nucleotides. Each nucleotide is composed of three essential parts:

1. A pentose sugar: A five-carbon sugar. In DNA, this is deoxyribose; in RNA, it is ribose.
2. A phosphate group: Always the same in both DNA and RNA.
3. A nitrogenous base: A base containing nitrogen. There are five types, categorised into two groups:
   • Purines: Adenine (A) and Guanine (G) – double-ring structures.
   • Pyrimidines: Cytosine (C), Thymine (T) in DNA, and Uracil (U) in RNA – single-ring structures.

DNA is a polynucleotide – a long chain of nucleotides joined together. In DNA, two such polynucleotide strands twist around each other to form the famous double helix structure.

Key features of DNA's structure:

• Sugar-phosphate backbone: The phosphate group of one nucleotide forms a phosphodiester bond with the sugar of the next nucleotide, creating a strong, stable backbone for each strand.
• Antiparallel strands: The two strands run in opposite directions. One strand runs 5' to 3' (five prime to three prime), while the other runs 3' to 5'. This is crucial for replication.
• Complementary base pairing: The nitrogenous bases face inwards, forming 'rungs' between the two backbones. Specific bases always pair with each other:
  • Adenine (A) always pairs with Thymine (T), forming two hydrogen bonds.
  • Guanine (G) always pairs with Cytosine (C), forming three hydrogen bonds.
• Hydrogen bonds: These weak bonds hold the two strands of the double helix together. Their weakness is vital, as they need to break easily during replication.

RNA shares many similarities with DNA but also has distinct differences that suit its diverse roles in gene expression.

Key differences between DNA and RNA:

• Sugar: RNA contains ribose sugar instead of deoxyribose.
• Bases: RNA contains uracil (U) instead of thymine (T). So, in RNA, A pairs with U.
• Strands: RNA is typically a single-stranded polynucleotide, though it can fold into complex 3D shapes (e.g., tRNA, rRNA).
• Length: RNA molecules are generally much shorter than DNA molecules.

Before a cell divides, its DNA must be accurately copied so that each new daughter cell receives a complete set of genetic instructions. This process is called DNA replication, and it occurs in a semi-conservative manner.

Semi-conservative replication means that each new DNA molecule consists of one original (parental) strand and one newly synthesised strand. This mechanism ensures high fidelity in copying.

Steps of DNA replication:

1. Unwinding: The enzyme DNA helicase unwinds the double helix by breaking the hydrogen bonds between complementary base pairs, separating the two DNA strands. This creates a replication fork.
2. New strand synthesis: Each separated parental strand acts as a template for the synthesis of a new complementary strand.
   • Free DNA nucleotides present in the nucleoplasm are attracted to their complementary bases on the template strands (A with T, C with G).
   • The enzyme DNA polymerase then catalyses the formation of phosphodiester bonds between these new nucleotides, forming the sugar-phosphate backbone of the new strand. DNA polymerase moves along the template strand in a 3' to 5' direction, synthesising the new strand in a 5' to 3' direction.
3. Proofreading: DNA polymerase also has a proofreading function to correct errors, maintaining the accuracy of replication.
4. Re-forming helices: As replication proceeds, two new identical DNA double helices are formed, each containing one original and one newly synthesised strand.