Nucleic Acids

From DNA to Protein: Structure and Function of DNA and the Flow of Information in Cells

Central Dogma of Molecular Biology

  • Definition: The Central Dogma illustrates the flow of genetic information within a biological system and follows the sequence: DNA → RNA → Protein.

Learning Goals

  • All living organisms are composed of cells.
  • Cells arise from pre-existing cells.
  • Cells must replicate and communicate through DNA copying.
    • To replicate, cells synthesize a new DNA copy from an existing DNA template.
    • For communication, cells transcribe DNA into RNA within the nucleus (Transcription).
    • RNA is translated into proteins in the cytoplasm on ribosomes (Translation).
    • Includes processes such as RNA splicing and protein processing.

Cellular Structures

  • Key Organelles:
    • Cytoskeleton
    • Ribosomes
    • Centriole
    • Lysosome
    • Flagellum
    • Nucleus
    • Plasma Membrane
    • Mitochondrion
    • Rough Endoplasmic Reticulum (ER)
    • Golgi Apparatus
    • Smooth Endoplasmic Reticulum (ER)

Nucleus and DNA

  • The nucleus contains the instructions for cell function encoded in DNA.
    • Components:
    • Chromatin: DNA and protein complex.
    • Nucleolus: Site for ribosome subunit assembly.
    • Nuclear Pore: Allow passage of materials in and out of the nucleus.
    • Nuclear Membrane: Protective barrier surrounding the nucleus.

Nucleic Acids

  • Types:
    • Deoxyribonucleic Acid (DNA)
    • Ribonucleic Acid (RNA)

The Nucleic Acid Alphabet

  • Composed of four nucleotides:
    • Adenine (A)
    • Thymine (T) (or Uracil (U) in RNA)
    • Guanine (G)
    • Cytosine (C)

Nucleic Acid Language

  • Consists of three-letter words called base triplets in DNA and codons in RNA.
    • Codon Definition: A sequence of three nucleotides that specifies an amino acid.
    • 1 Codon = 1 Amino Acid (with certain exceptions).

Base Pairing Guidelines

  • Chargaff’s Rule: DNA bases pair complementarily:
    • Adenine (A) pairs with Thymine (T)
    • Cytosine (C) pairs with Guanine (G)

Structure of DNA

  • Watson and Crick's Discovery:
    • Discovery of the DNA double helix structure.
    • Rosalind Franklin: Generated X-ray diffraction images of DNA that contributed to this finding.

DNA Replication

  • Process: Occurs during cell division, ensuring genetic information is faithfully passed to daughter cells.
    • Involves the enzyme DNA Polymerase which synthesizes a new daughter strand by copying the parental DNA strand.

The Flow of Genetic Information

  • Transcription and Translation:
    • Transcription (DNA to RNA):
    • Occurs in the nucleus where RNA nucleotides are assembled from a DNA template by RNA Polymerase.
    • Translation (RNA to Protein):
    • Occurs in cytoplasm on ribosomes where mRNA is decoded into a polypeptide chain.

Steps of Transcription

  1. Initiation:
    • RNA Polymerase binds to a specific sequence called a promoter.
  2. Elongation:
    • As RNA polymerase moves along the DNA, it elongates the RNA strand by adding RNA nucleotides complementary to the DNA template.
  3. Termination:
    • Transcription stops when RNA polymerase reaches a terminator signal on the DNA
    • The RNA strand detaches, and DNA strands rejoin.

RNA Processing in Eukaryotes

  • Post-transcriptional Modifications:
    • Capping: Addition of a 5′ cap for stability.
    • Tailing: Addition of a poly-A tail to the 3′ end.
    • Intron Removal: Introns are non-coding segments that are spliced out.
    • Exon Splicing: Exons, which are coding regions, are joined together to form mature mRNA.

Translation Requirements

  • Key Components:
    • mRNA, ATP, Enzymes, Ribosomes, Transfer RNA (tRNA)
  • tRNA Function: Carries specific amino acids and matches them with the corresponding codons on mRNA using anticodons.

Ribosome Structure

  • Composed of two subunits that facilitate translation by coordinating mRNA and tRNA interactions.
    • Holds tRNA and mRNA for efficient translation during protein synthesis.

Alternative Splicing

  • Definition: A mechanism allowing a single gene to produce multiple proteins by joining different combinations of exons.

Protein Processing and Secretion

  • Protein Functionality: Requires proper folding into secondary and tertiary structures.
  • Chaperone Proteins: Assist in protein folding and maintaining the correct structure.
  • Proteins synthesized on free ribosomes are destined for the cytosol, whereas proteins for secretion are synthesized on the Rough ER and processed through the Golgi apparatus.

Summary of Learning Goals

  • All living things are made of cells.
  • Cells come from other cells.
  • The process of DNA replication and RNA transcription is vital for cellular communication, ensuring the synthesis of proteins essential for life.