Cell and Molecular Biology Notes

Overview of Nucleic Acids

  • Types of Nucleic Acids:
    • DNA (Deoxyribonucleic Acid)
    • RNA (Ribonucleic Acid)
  • Central Dogma of Molecular Biology:
    • DNA -> RNA -> Protein
    • DNA replicates to pass information,
    • Transcription: DNA to RNA
    • Translation: RNA to Protein
  • DNA Function:
    • Provides structure and facilitates biological activities.

Structure of DNA

  • Discovery:
    • Watson and Crick (1953) proposed the double-helical structure.
  • **Components:
    • Nucleotides composed of:**
    • Nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
    • Deoxyribose sugar
    • Phosphate group
  • Chargaff's Rule:
    • A = T, C = G (The ratios between bases).
  • Dimensions:
    • 2 nm diameter, with stack spacing of 0.34 nm between bases.

Evidence of DNA Structure

  • Biological Evidence:
    • Base pairing rules and their contributions to stability.
  • Physical Evidence:
    • X-ray crystallography by Rosalind Franklin revealed the helical structure of DNA.

Double Helix Structure

  • Strands:
    • Antiparallel strands with complementary base pairing (A:T and C:G).
  • Backbone:
    • Sugar-phosphate backbone connected by phosphodiester bonds.

DNA Replication

  • Mechanism: The Semi-conservative Model
    • Each strand serves as a template for the new strand.
  • Process:
    1. Helicase enzyme unwinds DNA double helix.
    2. Binding Proteins stabilize single-strand DNA during replication.
    3. DNA Polymerase: synthesizes new DNA strands.
    4. Leading strand synthesized continuously while Lagging strand synthesized in fragments (Okazaki fragments).

Key Enzymes in DNA Replication

  • Topoisomerase: Prevents supercoiling ahead of the replication fork.
  • Primase: Synthesizes RNA primers for DNA polymerases to initiate synthesis.
  • DNA Ligase: Joins Okazaki fragments and seals nicks in the sugar-phosphate backbone.

Conversation of Genetic Information

  • Transcription Process:
    • Template Strand: Used for synthesizing mRNA.
    • Promoter: Initiates transcription by RNA polymerase binding.
    • Stages:
    1. Initiation
    2. Elongation
    3. Termination
  • Post-transcriptional Modifications (Eukaryotes):
    • Addition of 5' cap and poly-A tail, splicing of introns.
    • Exons are expressed sequences that encode proteins.

Translation Process

  • Occurs in Ribosomes:
    • Involves messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
  • Stages of Translation:
    1. Initiation of the polypeptide chain at the start codon (AUG).
    2. Elongation where amino acids are added sequentially.
    3. Termination at a stop codon, releasing the completed polypeptide.
  • Aminoacyl-tRNA synthetase: Enzyme that attaches amino acids to tRNA.

Mutations and Their Impact

  • Types of Mutations:
    • Point Mutations (e.g., silent, missense, and nonsense)
    • Insertions and Deletions (frameshift mutations)
    • Sickle-cell disease is an example of a point mutation affecting protein structure.
  • Consequences:
    • Can lead to significant changes in protein function or structure affecting organism phenotype.