Unit 3 – Molecular Genetics

Unit 3 – Molecular Genetics

1. Structure of DNA
DNA is a double helix composed of two antiparallel strands of nucleotides.

• Each nucleotide has:

  1. A phosphate group

  2. A deoxyribose sugar

  3. A nitrogenous base (A, T, C, G)

• Base pairing: A–T and C–G (held by hydrogen bonds)

• Backbone: Sugar-phosphate linked by covalent bonds

🧬 Diagram:

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A—T | | T—A | | C—G | | G—C

2. DNA vs RNA

3. DNA Replication Stages

a. Initiation

• Helicase unwinds the DNA

• Single-strand binding proteins (SSBs) stabilize the open strands

• Topoisomerase relieves tension ahead of the fork

• Replication begins at origin

b. Elongation

• Primase adds RNA primers

• DNA Polymerase III builds new DNA strand (5'→3')

• Leading strand synthesized continuously

• Lagging strand made in short Okazaki fragments

c. Termination

• DNA Polymerase I replaces RNA primers with DNA

• Ligase seals gaps between fragments

• Replication ends when full strand is copied

4. Leading vs Lagging Strand

• Leading strand: synthesized continuously toward replication fork

• Lagging strand: synthesized in fragments away from fork (needs multiple primers)

5. Central Dogma
DNA → RNA → Protein

• Describes the flow of genetic information:

  1. Transcription (DNA → mRNA)

  2. Translation (mRNA → protein)

6. DNA Replication vs RNA Transcription

7. Structure & Function

• mRNA: Carries genetic code from DNA to ribosome

• tRNA: Brings amino acids to ribosome (has anticodon)

• Ribosome: Site of protein synthesis (reads mRNA, forms peptide bonds)

8. Triplet Hypothesis

• States that 3 nucleotides (codon) in mRNA specify 1 amino acid

• Example: AUG = methionine (start codon)

Unit 4 – Homeostasis

1. Homeostasis
Maintaining a stable internal environment (e.g., temperature, pH, glucose levels)

2. Feedback Responses

• Negative feedback: reverses change to return to normal (e.g., body temp control)

• Positive feedback: amplifies change (e.g., childbirth contractions via oxytocin)

3. Reflex Arc Function

• Pathway: Stimulus → Receptor → Sensory neuron → Interneuron (spinal cord) → Motor neuron → Effector (muscle)

• Example: pulling hand from hot surface

4. Action Potential Events

1. Resting potential: -70 mV

2. Depolarization: Na⁺ channels open → inside becomes +

3. Repolarization: K⁺ channels open → inside becomes – again

4. Refractory period: Na⁺/K⁺ pump restores balance

📈 Use a membrane potential graph with a peak at +30 mV for clarity.

5. White vs Grey Matter

6. Brain Functions

• Hypothalamus: Regulates hunger, thirst, body temp, hormones

• Medulla oblongata: Controls heart rate, breathing

• Cerebrum: Thought, memory, senses, voluntary movement

• Cerebellum: Coordinates balance, movement

• Pons: Links parts of brain, helps in breathing

7. Autonomic Nervous System Branches

8. Endocrine System & Connection
Includes glands: hypothalamus, pituitary, thyroid, adrenal, pancreas, etc.

• Hypothalamus links nervous and endocrine systems by controlling the pituitary gland.

9. Nervous vs Endocrine System

10. Epinephrine

• As hormone: released by adrenal gland during stress

• As neurotransmitter: used by neurons to send messages in the brain

11. hGH Over/Underproduction

• Too much: gigantism (children), acromegaly (adults)

• Too little: dwarfism or stunted growth

12. Thyroid Function Errors

• Hypothyroidism: slowed growth, fatigue, weight gain

• Hyperthyroidism: rapid development, weight loss, anxiety

13. Thyroid Negative Feedback

• Low T3/T4 → hypothalamus releases TRH → pituitary releases TSH → thyroid releases T3/T4

• High T3/T4 stops TRH and TSH (prevents overproduction)

14. Water Balance Feedback Loop

• Low water → hypothalamus triggers pituitary → releases ADH

• ADH causes kidneys to reabsorb more water → urine becomes concentrated

• Water levels rise → ADH decreases