DNA

Replication

DNA

  • Definition: Deoxyribonucleic acids (DNA) are the molecules that carry genetic information.

Central Dogma of Molecular Biology

  • Flow of Information:

    • Replication: DNA is replicated before cell division.

    • Transcription: The process where DNA (Tx) is transcribed into RNA in the nucleus.

    • Translation: The process where RNA (Tsn) is translated into proteins in the cytosol, specifically on ribosomes.

Ribonucleic Acids (RNA)

  • Role: Serves as the intermediary between DNA and proteins in the central dogma.

DNA Structure

Components of DNA

  • Monomer Nucleotide:

    • Sugar: Deoxyribose (Pentose, 5 carbon)

    • Nitrogenous Bases: Adenine (A), Cytosine (C), Guanine (G), Thymine (T)

    • Phosphate Group: 5 carbon phosphate (only 1)

Structure Characteristics

  • Double Helix: DNA structure consisting of two polynucleotide strands twisted into a helix form.

  • Polynucleotides: Chains of nucleotides joined by covalent bonds.

  • Base Composition:

    • Purines: Adenine (A) and Guanine (G)

    • Pyrimidines: Cytosine (C) and Thymine (T)

Arrangement of Nucleotides

  • Phosphate-Sugar Backbone: Characteristic structure where sugars and phosphates alternate, resembling a ladder.

  • Base Pairing:

    • Bases stick out from the sugar-phosphate backbone.

    • Hydrogen bonds can form between complementary nitrogenous bases:

    • A=T (2 hydrogen bonds)

    • G≡C (3 hydrogen bonds)

  • Antiparallel Orientation:

    • Strands run in opposite directions; one runs 5' to 3' and the other 3' to 5'.

DNA Synthesis

  • Directionality: DNA synthesis occurs in the 5' to 3' direction.

  • Nucleotide Addition: New nucleotides are added to the 3'-OH end of the growing strand.

  • Example of Complementarity:

    • Template Strand: 5' ACTAGT 3'

    • Complementary Strand: 3' TTGATCA 5'

Key Scientists in DNA Research

Historical Context

  • Sutton & Boveri (1902) and Morgan (1907):

    • Rediscovered Mendel's paper, linking genes to chromosomes.

Experimental Findings

  1. Griffiths (1928):

    • Demonstrated that dead disease-causing bacteria plus living non-disease-causing bacteria could cause disease in subjects.

  2. Avery, McCarty & MacLeod (1952) and Hershey & Chase (1952):

    • Inferred that DNA (not protein) served as the hereditary material.

  3. Chargaff (1950):

    • Conducted analysis of DNA revealing:

      • Amount of Adenine = Thymine

      • Amount of Cytosine = Guanine

    • This observation is known as Chargaff's Rule.

  4. Wilkins & Franklin (Early 1950s), Watson & Crick (1953):

    • Conducted research in Kings College, London and Cambridge University, leading to the double helix model of DNA.

    • Granted Nobel Prize in 1962 along with Wilkins.

Eukaryotic Chromosome Structure (Chapter 12)

Chromatin

  • Definition: Chromatin is a complex of DNA and proteins that forms chromosomes.

  • Composition of Chromosomes: One chromosome consists of a single DNA molecule wrapped around proteins.

  • Histones:

    • A special group of five positively charged proteins involved in DNA packaging.

    • Form nucleosomes, where DNA wraps around an octamer (8) of four types of histone proteins, resulting in about two wraps of DNA around each histone.

Types of Chromatin

  1. Euchromatin:

    • Less tightly packed structure, making it accessible for transcription.

    • Common during interphase.

  2. Heterochromatin:

    • More tightly packed, not available for transcription.

    • Only type present in metaphase chromosomes.

    • During interphase, includes DNA structures such as Barr bodies, centromeres, and telomeres, as well as certain genes that are turned "off".