Study Notes on DNA Structure and Analysis

DNA Structure and Analysis

Genetic Material

  • DNA and RNA are types of genetic material composed of polymers of nucleotides.
  • Nucleotides (monomers) consist of:
    • A 5-carbon sugar
    • A nitrogenous base
    • A phosphate group

Pentose Sugars

  • In DNA: Contains deoxyribose (lacks an oxygen on the 2' carbon).
  • In RNA: Contains ribose (has a hydroxyl group on the 2' carbon).
  • Structural Formula:
    • 5' end is the phosphate group end.
    • 3' end is the hydroxyl group end.

Nitrogenous Bases

Classification
  • Purines (double-ring structures):
    • Adenine (A)
    • Guanine (G)
  • Pyrimidines (single-ring structures):
    • Thymine (T) (found in DNA)
    • Cytosine (C)
    • Uracil (U) (found in RNA)
Structural Characteristics
  • Purines are characterized by their two-ring structure containing various carbon and nitrogen atoms.
  • Pyrimidines are characterized by their single-ring structure.
  • Example structures illustrate the configurations of these bases, including bonds and functional groups.

Phosphodiester Bond

  • These are stable covalent bonds that link nucleotides into a polynucleotide strand:
    • Formed between the phosphate group of the 5' carbon of one nucleotide and the hydroxyl group of the 3' carbon of a neighboring nucleotide.
  • The backbone of DNA and RNA is composed of alternating phosphate-sugar units:
    • Phosphate-Sugar-Phosphate-Sugar pattern.
  • DNA and RNA are polar molecules with distinct ends:
    • 5' end: Phosphate group
    • 3' end: Hydroxyl group

The Structure of the DNA Molecule

  • Erwin Chargaff (1940s) discovered important nucleotide pairing principles:

    • The number of purines equals the number of pyrimidines: A = T and G = C.
    • Ratio calculation:
    • \((A + G)/(C + T) = 1\)

  • James Watson and Francis Crick (1953) proposed the double-helix model:

    • Consists of two polynucleotide chains in a double helix formation.
    • Chains are arranged in an anti-parallel orientation.
    • The sugar-phosphate backbone resides on the outside, with base pairs stacked internally.
    • Nitrogenous bases pair through hydrogen bonds: A pairs with T, and G with C.
    • Strands are complementary to each other, allowing for precise pairing.
    • Base pairs are spaced 0.34 nm apart, and each complete helical turn measures 3.4 nm in length, accommodating 10 base pairs per turn.
    • Major and minor grooves formed in the structure are significant for protein binding.
Different Forms of DNA Structure
  • A-DNA
  • B-DNA: The most common form of cellular DNA.
  • Z-DNA

RNA Structure

  • RNA consists of a ribose sugar.
  • Instead of thymine, RNA contains uracil, which binds to adenine.
  • Functional RNA within cells is typically single-stranded.
  • Internal base pairing can result in complex secondary structures.
  • Some viruses utilize either single-stranded or double-stranded RNA genomes.

What is a Genome?

  • A genome encompasses all the DNA present in an organism, which might be organized into:
    • Multiple chromosomes or just a single chromosome.
    • Chromosomes may either be circular or linear.
  • Nucleic acids can be RNA or DNA and exist in single-stranded or double-stranded forms.

Euchromatin and Heterochromatin

  • Most DNA remains relaxed (not condensed) during interphase, a form known as Euchromatin:
    • Transcribes actively, does not contain repetitive DNA sequences.
  • Heterochromatin refers to DNA that remains condensed at all times:
    • Not actively transcribed.
    • Examples include centromeres and Barr bodies.

Centromeres and Telomeres

  • Centromeres can vary between chromosomes and between organisms, demonstrating functionality without DNA sequence conservation.
  • Telomeres are essential for DNA replication and stability:
    • They are short, species-specific, and consist of tandemly repeated sequences.

Unique-Sequence DNA

  • Unique-sequence DNA contains sequences that appear only a few times (if at all) in a genome.
  • Prokaryotic organisms predominantly consist of unique-sequence DNA.
  • Most protein-encoding genes are classified as unique-sequence DNA.
  • Approximately 65% of human DNA is categorized as unique-sequence.

Repetitive DNA

  • Repetitive DNA consists of sequences that are repeated across various locations in the genome or clustered together.
  • Eukaryotes exhibit a blend of unique and repetitive sequences.
  • Dispersed Repetitive Sequences:
    • Spread throughout the genome and can consist of transposons (mobile genetic elements).
    • LINEs (Long Interspersed Repeated Sequences): Comprise segments of 1,000 to 7,000 bp or longer.
    • SINEs (Short Interspersed Repeated Sequences): Typically range from 100 to 500 bp.

Repetitive DNA (Continued)

  • Tandemly Repetitive Sequences are prevalent in eukaryotes, comprising various lengths of repeats:
    • Short repeats (e.g., STRs or microsatellites: 1-10 bp).
    • Lengthy repeat sequences, extending to complete genes.
    • Notable examples include Centromeres and Telomeres.