Key Enzymes and Processes in DNA Replication

Enzymes

• Helicase: Unwinds the DNA double helix by breaking hydrogen bonds between complementary bases.
• DNA Polymerase: Adds complementary nucleotides to the growing DNA strand with high accuracy.
• Primase: Lays down an RNA primer to initiate DNA replication.
• Ligase: Seals gaps between Okazaki fragments on the lagging strand.
• Topoisomerase: Relieves tension in the DNA ahead of the replication fork.

Steps of DNA Replication

1. Initiation: Helicase unwinds the DNA, creating a "replication fork."
2. Elongation: DNA polymerase adds nucleotides (A, T, C, G) in the 5' to 3' direction.
   • Leading Strand: Replicated continuously towards the replication fork.
   • Lagging Strand: Replicated in fragments (Okazaki fragments), later joined by ligase.
3. Termination: DNA replication concludes, enzymes dissociate, and DNA strands rewind.

Transcription Process

• What Happens: DNA is used as a template to create messenger RNA (mRNA).
• Key Process:
  • RNA Polymerase unzips DNA and creates a complementary mRNA strand.
  • mRNA exits the nucleus to the cytoplasm.

Translation Process

• mRNA is read by ribosomes to form an amino acid chain.
• tRNA (Transfer RNA): Brings the correct amino acids to the ribosome.
• Start Codon: mRNA binds to ribosome at start codon (AUG) to begin protein synthesis.
• Codons & tRNA:
  • Each 3-base mRNA codon codes for an amino acid.
  • tRNA matches anticodons to mRNA codons, adding corresponding amino acids.
• Amino Acid Chain: Linked to form a protein.
• Stop Codon: Signals completion of the protein.

tRNA Function

• Carries amino acids to the ribosome based on mRNA's codon sequence.
• Each tRNA has an anticodon matching a codon on mRNA to ensure correct protein growth.

Central Dogma of Molecular Biology

• Describes the flow of genetic information: DNA → RNA → Protein
• Key Steps:
  1. DNA is transcribed into mRNA (Transcription).
  2. mRNA is translated into a protein (Translation).
• Highlights the expression of genetic information in organisms.

Genetic Mutations

Sickle Cell Disease

• Caused by a substitution mutation in the hemoglobin gene.
• Adenine (A) is replaced with thymine (T), changing the 6th amino acid from glutamic acid to valine in hemoglobin.
• Results in sickle-shaped red blood cells, reducing oxygen efficiency.

Types of Point Mutations

• Silent: No change in amino acid sequence.
• Missense: One amino acid changes, potentially altering protein function.
• Nonsense: Premature stop codon, resulting in a shorter, often nonfunctional protein.

Frameshift Mutation

• Occurs when insertions or deletions alter the reading frame.
• Results in a different amino acid sequence, often creating nonfunctional protein.

Chromosomal Mutations

• Definition: Changes in chromosome structure or number affecting large DNA segments.
• Types:
  • Deletion: Loss of chromosome section.
  • Duplication: Multiple copies of a section.
  • Inversion: Section is reversed.
  • Translocation: Section breaks and reattaches to another chromosome.
• Effects: Can cause diseases or genetic disorders.
• Example: Down Syndrome (trisomy 21).

Mutagens

• Agents causing mutations (chemicals, radiation, viruses).
• Potential Outcomes:
  • Neutral: No effect.
  • Beneficial: Advantageous, possibly becoming part of genetic pool.
  • Harmful: May cause diseases or reduce survival.
• Some mutations can be inherited, driving evolutionary changes.