Nucleic Acids

Nucleic Acids and Protein Synthesis

Nucleotides: Basic units of DNA and RNA, linked to form polynucleotides.
- Related to the structure of nucleic acids, nucleotides consist of a phosphate group, a five-carbon sugar (deoxyribose in DNA and ribose in RNA), and a nitrogenous base.
Purines: Larger, double-ringed molecules (e.g., adenine, guanine).
Pyrimidines: Smaller, single-ringed molecules (e.g., thymine, cytosine, uracil).
Covalent Bonds: Nucleotides in DNA and RNA are linked by phosphodiester bonds, which create a sugar-phosphate backbone and establish the overall structural integrity.
Location: DNA synthesis occurs in the nucleus during interphase, ensuring genetic information is replicated before cell division.

DNA Ladder:
- Sides: Alternating phosphate and deoxyribose molecules create the backbone of the DNA molecule.
- Rungs: Nitrogenous bases linked by hydrogen bonds form pairs (A-T and C-G).
- Strands: DNA is composed of 2 anti-parallel polynucleotide strands held together by hydrogen bonds, exhibiting a double helical structure that is crucial for its stability and function.

Unwinding DNA:
- Enzyme helicase unwinds the double helix by splitting hydrogen bonds, creating a replication fork, allowing access to both strands for synthesis.
Primer Synthesis:
- Enzyme primase synthesizes short RNA primer to start new strand synthesis, necessary for initiation due to DNA polymerase's requirement for a primer.
Strand Synthesis:
- Enzyme DNA polymerase adds nucleotides in a 5' to 3' direction. Continuous synthesis occurs on the leading strand; the lagging strand is synthesized in Okazaki fragments, requiring multiple primers and synthesis events.
Sealing Fragments:
- DNA ligase seals Okazaki fragments to create a continuous double-stranded helix, ensuring the integrity and fidelity of the genetic material.

Polypeptides: Coded for by genes, which are sequences of nucleotides in DNA that encode specific proteins.
Codon: Sequence of 3 nucleotide bases coding for one amino acid, establishing the sequence of the polypeptide chain.
Gene Mutation: Change in nucleotide sequence, resulting in altered polypeptides. Types include theft of base pairs through:
- Substitution: Replace one base with another.
- Deletion: Remove a base.
- Insertion: Add a base.
- Inversion: Reverse the sequence.
- Frameshift: Change the reading frame, affecting the entire sequence downstream.
Alleles: Variants of genes caused by mutations, contributing to genetic diversity. Example: Sickle cell anemia caused by substitution mutation: adenine replaces thymine in the triplet CTT (CAT instead), resulting in valine (GTG) rather than glutamic acid (GAG).

Folding Process:
- Newly synthesized polypeptides undergo folding to achieve their final three-dimensional conformation, which is critical for their function.
- Chaperone Proteins: Assist in the correct folding of polypeptides by preventing aggregation and facilitating the proper conformation, which is essential for functional active sites.
- Levels of Organization: Folding occurs in several stages:
  - Primary Structure: Sequence of amino acids in a polypeptide chain.
  - Secondary Structure: Local folding into alpha-helices or beta-pleated sheets due to hydrogen bonding.
  - Tertiary Structure: The overall three-dimensional shape formed by further folding and interactions between side chains.
  - Quaternary Structure: Assembly of multiple polypeptide chains into a functional protein complex.

a) Transcription

Process of creating mRNA from a DNA template.
- Binding: RNA polymerase binds to the promoter region, signaling DNA unwinding and initiating transcription.
- Reading Direction: RNA polymerase reads the sense strand in a 3' to 5' direction, synthesizing mRNA in a 5' to 3' direction.
- Termination: Occurs at the terminator sequence; RNA polymerase detaches, and the DNA rewinds.
- Stop Codons: Specific codons signal the end of transcription, including ATT, ATC, ACT in DNA (UAA, UGA, UAG in RNA).

b) Translation