Unit 4: Genetics - DNA Structure

DNA Structure

• Introduction to Unit 4: Genetics

  • Excitement about studying genetics.
  • Focus on DNA structure as the foundation, progressing to DNA replication.

• Review of Prior Knowledge

  • Content was previously covered in year 10.
  • Recap of key components:
    • Nucleus: Location of DNA storage.
    • Chromosome: Coiled structure for DNA storage.
    • DNA: Deoxyribonucleic acid.
    • Nitrogenous base (A, T, C, G).

Macromolecules and Nucleic Acids

• Revisiting Unit 1 concepts
  • Macromolecules, monomers, and polymers.
• Focus on nucleic acids (nucleotides) in Unit 4.
• Nucleotides: Monomers of DNA.
  • Importance of understanding their structure and function in DNA.

Nucleotide Structure and Types of Nucleic Acids

• Basic Nucleotide Structure
  • Phosphate group.
  • Pentose sugar (pentagon shape).
  • Nitrogenous base.
  • Recognition and labeling of the structure are essential.
• Two Main Types of Nucleic Acids
  • DNA (deoxyribonucleic acid).
  • RNA (ribonucleic acid).

Differences Between DNA and RNA

• Structural Differences
  • Sugars: DNA has deoxyribose sugar; RNA has ribose sugar.
  • Slight chemical structure variation.
• Sugar-Phosphate Backbone
  • Formation: Bonds between sugars and phosphates.
  • DNA: Two chains with sugar-phosphate backbones.
  • Hydrogen bonds between bases hold the chains together.
  • Double helix shape: Twisted ladder structure.
• RNA Structure
  • Single-stranded: Bases are exposed.
  • Enables reading of the genetic code.

Nitrogenous Bases and Base Pairing

• Common Bases
  • Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
• Base Pairing Rules
  • DNA: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G).
  • RNA: Adenine (A) pairs with Uracil (U).
• Uracil (U)
  • Replaces Thymine (T) in RNA.
• Purines and Pyrimidines
  • Purines: Double-ringed (Adenine, Guanine).
  • Pyrimidines: Single-ringed (Thymine, Cytosine, Uracil).
  • Purines always pair with pyrimidines.

Comparison Table: DNA vs. RNA

• Key Differences summarized.
  • Sugars: Deoxyribose (DNA) vs. Ribose (RNA).
  • Bases: Thymine (DNA) vs. Uracil (RNA).
  • Strands: Double-stranded (DNA) vs. Single-stranded (RNA).

Detailed Explanation of DNA Structure

• Phosphodiester Bonds
  • Link nucleotides.
  • Forms one half of the double-stranded DNA.
• Complementary Base Pairing
  • A with T, C with G.
  • Held together by weak hydrogen bonds.
• Twisting of DNA
  • Hydrogen bonding between phosphate groups causes the DNA to twist into a double helix.

Predicting Base Proportions

• Using Base Pairing Rules
  • If a person has 20% Adenine (A), they also have 20% Thymine (T) because A pairs with T.
  • Remaining 60% is split equally between Cytosine (C) and Guanine (G), so 30% each.

Three Prime and Five Prime Ends

• Numbering of Carbons in Sugar
  • Sugar has five carbons, numbered 1' to 5' from the oxygen.
  • Phosphate attached to the 5' carbon, base to the 1' carbon in a single nucleotide.
• DNA Replication
  • Occurs in the 5' to 3' direction.
  • Strands run antiparallel, ensuring the same genetic information is read.

DNA Packaging and Histones

• Amount of DNA
  • Each person has enough DNA to reach the sun and back over 300 times.
• Histones
  • Proteins around which DNA is wrapped.
  • Histone proteins are used in evolutionary trees.
• Nucleosomes
  • Formed when DNA is wrapped around eight histones.
• Chromatin Fiber
  • Compacted nucleosomes.
  • Formed by electromagnetic charges between negative DNA and positive histones.
• Chromosomes
  • Further compacted chromatin.
• Role in Gene Regulation
  • Histones regulate chromosome movement and protein synthesis.

DNA Locations

• Four Main Locations
  • Nucleus of eukaryotes.
  • Chromosomes and plasmids in prokaryotes.
  • Mitochondria.
  • Chloroplasts.
• Eukaryotes
  • DNA is found in chromosomes in the nucleus.
• Mitochondria and Chloroplasts
  • Have their own circular DNA, not bound by histones.
  • Supports endosymbiotic theory: Complex eukaryotic cells evolved from simpler prokaryotic cells.

Prokaryotic Cells

• Bacterial DNA
  • Found in one large circular chromosome, not bound to histones (naked DNA).
  • Also found in small chromosomal structures called plasmids.
• Plasmids
  • Replicate independently.
  • Can be exchanged between bacteria via conjugation.
  • Carry genes, such as antibiotic resistance.
  • Conjugation process: Transfer of valuable genes between bacteria.

Further Exploration

• Atomy quiz to test understanding.
• TED Ed video on the discovery of DNA, focusing on Rosalind Franklin's contribution.
• Videos on DNA replication.
• Practical activity: DNA extraction from strawberries.
  • Simple kitchen experiment.