DNA STRUCTURE

Page 1: DNA Structure

  • Title: DNA STRUCTURE Dr CAMPOS

Page 2: Components of DNA

  • DNA is composed of four nucleotides.

  • Each nucleotide consists of three parts:

    • A phosphate group

    • A sugar (deoxyribose)

    • One of four nitrogenous bases:

      • Adenine (A)

      • Guanine (G)

      • Thymine (T)

      • Cytosine (C)

Page 3: Structure of DNA Nucleotides

  • The Sugar:

    • A 5-carbon sugar called deoxyribose, labeled from 1' to 5'.

  • The Phosphate:

    • Attached to the 5' carbon of the deoxyribose.

  • The Base:

    • Attached to the 1' carbon of the deoxyribose.

    • Contains at least two hydrogen atoms, called nitrogenous bases.

    • Two classes of bases:

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

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

Page 4: Purines

  • Two types of purines:

    • Adenine (A)

    • Guanine (G)

  • Characteristics:

    • Composed of 2 chemical rings of carbon and nitrogen.

    • Larger and heavier than pyrimidines.

    • Present in both DNA and RNA.

Page 5: Pyrimidines

  • Three types of pyrimidines:

    • Thymine (T)

    • Cytosine (C)

    • Uracil (U)

  • Characteristics:

    • Composed of a single carbon-nitrogen ring.

    • Smaller than purines.

    • Cytosine is found in both DNA and RNA.

    • Thymine is exclusive to DNA, Uracil is exclusive to RNA.

Page 6: Base Pairs in DNA

  • Purines and pyrimidines form base pairs, which create the double-stranded structure of DNA:

    • Adenine pairs with Thymine (A-T)

    • Guanine pairs with Cytosine (G-C)

  • In RNA, Adenine pairs with Uracil (A-U).

Page 7: Historical Insights - Chargaff's Rule

  • In the 1940s, Erwin Chargaff analyzed DNA from various organisms.

  • Found consistency:

    • Each species contains equal amounts of Adenine (A) and Thymine (T), and Guanine (G) and Cytosine (C).

  • This finding is known as 'Chargaff's Rule.'

Page 8: X-ray Diffraction Studies

  • Maurice Wilkins and Rosalind Franklin used X-ray diffraction:

    • Concluded DNA is long, thin, and has a uniform diameter (~2 nanometers).

    • DNA structure is helical and twisted.

    • DNA consists of repeating units.

Page 9: Watson and Crick Model

  • Proposed in the 1950s by James Watson and Francis Crick:

  • Structure:

    • DNA consists of two linked nucleotide polymers called strands.

    • Backbone formed by alternating sugar and phosphate groups linked by covalent bonds.

    • Nucleotide bases project outward from the backbone.

Page 10: Strand Orientation in DNA

  • All nucleotides in a strand have the same orientation:

    • One end has a 'free' sugar, and the other has a 'free' phosphate.

Page 11: Hydrogen Bonding and Structure

  • DNA strands held together by hydrogen bonds between bases:

    • Leads to ladder-like structure:

      • Sugar-phosphate columns (vertical sides) and bases (rungs).

    • Twists to form a double helix, oriented in antiparallel directions.

Page 12: Complementary Base Pairs

  • Bases in DNA are complementary:

    • If one strand is A-T-T-C-C-A-G-G-C-T, the opposite strand is T-A-A-G-G-T-C-C-C-G-A.

  • This explains the equality in base pairing.

Page 13: Base Pairing and Helix Diameter

  • Bases:

    • A and G (purines) are larger than T and C (pyrimidines).

    • A-T and C-G pairs ensure consistent width of the DNA ladder, maintaining a constant diameter of the double helix.

Page 14: DNA Duplication Overview

  • Introduction to DNA structure and replication process:

    • Further insights available in video link.

Page 15: Inheritance and DNA

  • DNA transmits species characteristics through generations:

    • DNA considered the chemical basis of inheritance.

    • Located in the chromosomes of the cell nucleus.

Page 16: Chromosome Information

  • Chromosome number varies by species:

    • Bacteria: single chromosome.

    • Humans: 46 chromosomes (23 from each parent).

  • DNA organized in chromosomes by winding around proteins (histones), forming nucleosomes.

Page 17: DNA Characteristics

  • DNA strands differ in length and base sequence, essential for species characteristics.

  • Genetic information must replicate exactly each time a cell divides.

  • Process: DNA replication.

  • Goal: Produce two identical strands from one DNA strand.

Page 18: Key Characteristics of Duplication

  • Semiconservative nature: Each strand serves as a template for new strand synthesis.

  • Simultaneous execution: Occurs on both strands.

  • Bidirectional manner: Progresses in both directions.

  • Monofocal origin (prokaryotes) or multifocal origin (eukaryotes).

Page 19: Semiconservative Replication

  • Each strand serves as a template:

    • Produces two new DNA molecules (one old strand, one new daughter strand).

  • Confirmed by Meselson and Stahl's work on E. coli.

Page 20: Bidirectional Duplication

  • Replication separation begins at replication origins:

    • Progresses in both directions.

  • Replication forks are where double strands separate into single strands.

Page 21: Monofocal Initiation in Prokaryotes

  • Replication starts at a specific point (origin) on the circular chromosome:

    • Forms two replication forks.

Page 22: Multifocal Initiation in Eukaryotes

  • Each chromosome has multiple origins of replication:

    • Leads to multiple replication forks.

    • Allows for timely completion of chromosome replication.

  • Circular chromosome replication begins at a specific point and occurs simultaneously.

Page 23: Semidiscontinuous Nature of Duplication

  • Antiparallel strands: one synthesized continuously, the other discontinuously (Okazaki fragments).

  • Okazaki fragment lengths vary based on cell type.