CHAP 9 What Genes Are - Key Concepts

CRISPR-Cas9

  • Purpose: A genome-editing tool used to add or delete genes from organisms.

  • Components: A combination of a protein (Cas9) and two single-stranded RNA molecules that guide Cas9 to precise DNA sites.

  • Mechanism: Cas9 efficiently cuts both strands of the targeted DNA, allowing for inactivation (e.g., PERV DNA in pigs) or insertion/deletion of genes.

  • Advantages: Inexpensive, allows researchers to alter DNA in nearly any organism.

  • Applications: Generating transplant organs from pigs (xenotransplantation) by inactivating pig endogenous retroviruses (PERVs) or growing human organs in pigs.

DNA Structure

  • Composition: DNA (deoxyribonucleic acid) is built from two parallel strands of repeating nucleotide units.

  • Nucleotide Structure: Each nucleotide contains the sugar deoxyribose, a phosphate group, and one of four bases: Adenine (A), Cytosine (C), Guanine (G), or Thymine (T).

  • Bonds: Strong covalent bonds connect nucleotides within a single strand. Weak hydrogen bonds connect bases between the two strands, allowing for separation during replication.

  • Base-Pairing Rules: Adenine (A) pairs only with Thymine (T); Cytosine (C) pairs only with Guanine (G).

  • Complementarity: The sequence of one DNA strand dictates the sequence of the complementary strand, allowing strands to serve as templates.

  • Shape: The DNA ladder twists into a double helix.

  • Genome Size: The human genome has approximately 3.23.2 billion base pairs.

DNA Compaction

  • Levels of Compaction (Least to Most Compact):

    • Naked DNA

    • Nucleosomes: Short lengths of double-stranded DNA wound around histone proteins (known as "beads-on-a-string").

    • Chromatin fiber: Nucleosomes are compressed and coiled.

    • Chromosome: Chromatin fiber is further looped, coiled, and condensed.

DNA Replication

  • Purpose: The duplication of a DNA molecule, occurring before cell division (mitosis) to ensure each new cell receives a copy.

  • Process:

    1. Special proteins bind at origins of replication, unwind DNA, and break hydrogen bonds between strands.

    2. DNA polymerase uses each original strand as a template to construct new strands, starting at short RNA primers.

    3. Two identical DNA molecules are formed.

  • Semiconservative Replication: Each new DNA double helix contains one old (template) strand and one newly synthesized strand.

Polymerase Chain Reaction (PCR)

  • Purpose: A technique to rapidly produce millions of copies of a specific DNA sequence from a small initial amount (amplification).

  • Mechanism (differs from natural replication):

    1. Heat separates the double strands of DNA (instead of special proteins).

    2. As the mixture cools, targeted primers (determined by scientists) pair with the original DNA strands (instead of natural origins).

    3. DNA polymerase synthesizes new strands.

  • Relates to DNA Replication: It is essentially fast, artificial DNA replication.

Mutations & DNA Repair

  • Mutation: A change to the sequence of nucleotides in an organism's DNA.

  • Types of Mutations:

    • Point Mutation: Change of one nucleotide in a single base pair.

      • Substitution: One nucleotide replaced by another.

      • Insertion: One nucleotide added to a sequence.

      • Deletion: One nucleotide removed from a sequence.

    • Chromosomal Abnormality: Addition or deletion of one or more whole chromosomes.

  • DNA Repair: DNA polymerase