Human Genome Overview and DNA Structure & Replication Concepts

Overview of the Human Genome

  • The human genome comprises approximately 3.2 billion nucleotides.
  • Contains around 20,000 protein-coding genes.
  • Structurally, the genome is organized into:
    • 2 sex chromosomes (XX in females and XY in males).
    • 44 autosomes (22 pairs of non-sex chromosomes).
    • 37 genes located on circular mitochondrial chromosomes.

Structure of DNA

  • Nucleotides:
    • Each nucleotide is comprised of:
    • A nitrogenous base:
      • Purines: Adenine (A), Guanine (G)
      • Pyrimidines: Thymine (T), Cytosine (C)
    • A phosphate group.
    • A 5-carbon sugar (deoxyribose).
  • DNA Double Helix:
    • Formed by the arrangement of nucleotides.
    • Backbone: sugar-phosphate backbone held by covalent phosphodiester bonds.
    • Base Pairs:
    • Adenine pairs with Thymine (2 hydrogen bonds).
    • Guanine pairs with Cytosine (3 hydrogen bonds).

Packaging of DNA in the Nucleus

  • DNA is negatively charged and is wrapped around positively charged histone proteins forming nucleosomes (10 nm thick).
  • These nucleosomes further coil to form chromatin fibers (30 nm thick) which condense to form chromosomes.

Coding vs Non-coding DNA

  • Coding DNA:
    • Represents a small fraction of the genome.
    • Codes for proteins.
  • Non-coding DNA:
    • Includes:
    • Genes for other types of RNAs.
    • Promoter regions.
    • Introns and tandem repeats.
    • Telomeres, pseudogenes, and viral DNA (more viral DNA than coding DNA).
    • Often referred to as "junk" DNA but may have unknown functions.

Key Differences Between DNA and RNA

  • DNA (Deoxyribonucleic Acid):

    • Sugar: Deoxyribose.
    • Bases: Thymine.
    • Generally double-stranded.
    • Primary role: stores genetic information.

  • RNA (Ribonucleic Acid):

    • Sugar: Ribose.
    • Bases: Uracil replaces Thymine.
    • Typically single-stranded.
    • Functions in coding, decoding, regulation, and expression of genes.

Steps of DNA Replication

  1. Unzipping the Double Helix:
    • DNA Helicase breaks hydrogen bonds to separate the strands.
    • Single-stranded DNA-binding proteins prevent strands from rejoining.
    • Topoisomerase alleviates supercoiling.
  2. Attaching a Primer:
    • Primase synthesizes an RNA primer that serves as a starting point for nucleotides.
  3. Adding Complementary Nucleotides:
    • DNA Polymerase III adds nucleotides in a 5' to 3' direction starting from the primer.
    • Leading Strand is synthesized continuously.
    • Lagging Strand is synthesized discontinuously, forming Okazaki fragments due to RNA primers.
  4. Proofreading:
    • DNA polymerase has proofreading abilities to correct errors during nucleotide addition.
  5. Finalizing Replication:
    • RNA primers are replaced with DNA by DNA polymerase I.
    • DNA ligase stitches together Okazaki fragments to create a continuous strand.

Polymerase Chain Reaction (PCR)

  • PCR is a laboratory technique used to amplify DNA samples for testing.
  • Principle Steps:
    1. Add ingredients (DNA sample, nucleotides, buffer, primers, DNA polymerase).
    2. Cycle through three temperature stages:
    • Denaturation (94-95°C): DNA strands separate.
    • Annealing (50-56°C): Primers attach to target DNA.
    • Extension/Elongation (72°C): DNA polymerase synthesizes new DNA strands.
  • Each cycle doubles the DNA, and this can be repeated about 30 times depending on the amount needed.