Core Concepts in DNA, RNA, Replication, Genetic Code, and Membranes

DNA & RNA

Structure and Properties of DNA and RNA

DNA and RNA are nucleic acids that store and transmit genetic information in cells. Their structures and chemical properties are fundamental to understanding molecular biology.

• Phosphodiester Bonds: The sugar-phosphate backbone of DNA is held together by phosphodiester bonds, which connect the 5' phosphate group of one nucleotide to the 3' hydroxyl group of the next.

• Antiparallel Strands: DNA consists of two strands running in opposite directions: one 5' → 3', the other 3' → 5'.

• ATP as a Nucleotide: ATP (adenosine triphosphate) is a nucleotide used for energy transfer in cells.

• Chargaff's Rule: In DNA, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C).

• Deoxyribose vs. Ribose: DNA contains deoxyribose sugar (lacking an -OH group at the 2' carbon), while RNA contains ribose (with an -OH group at the 2' carbon).

• Base Differences: DNA uses thymine (T), while RNA uses uracil (U) instead.

• Stability: RNA is less stable and more reactive than DNA due to the extra -OH group on ribose.

Example: The double helix structure of DNA is stabilized by hydrogen bonds between complementary base pairs (A-T and G-C).

DNA Replication

Mechanism and Enzymes of DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. It ensures genetic continuity across generations.

• dNTPs: Deoxynucleotide triphosphates (dNTPs) are the building blocks for DNA synthesis.

• Semi-Conservative Replication: Each new DNA double helix contains one old strand and one newly synthesized strand.

• Origin of Replication: Replication begins at specific sequences called origins of replication.

• Replication Fork: The Y-shaped region where DNA is unwound and new strands are synthesized.

• Helicase: Unwinds the DNA double helix.

• Topoisomerase: Relieves supercoiling ahead of the replication fork.

• Primase: Synthesizes short RNA primers to provide a starting point for DNA polymerase.

• DNA Polymerase: Adds nucleotides to the growing DNA strand; requires a primer.

• Ligase: Seals gaps between fragments (e.g., Okazaki fragments on the lagging strand).

• Leading vs. Lagging Strand: Leading strand is synthesized continuously; lagging strand is synthesized in short fragments.

• Proofreading: DNA polymerases have proofreading activity to minimize errors.

• Telomerase: Extends telomeres at chromosome ends, active in stem cells and absent in most somatic cells.

Example: In eukaryotes, multiple origins of replication allow rapid duplication of large genomes.

Transcription

Synthesis of RNA from DNA Template

Transcription is the process by which RNA is synthesized from a DNA template, transferring genetic information for protein synthesis.

• RNA Polymerase: Enzyme that synthesizes RNA from DNA.

• Template Strand: The DNA strand used as a template for RNA synthesis.

• Direction: RNA is synthesized in the 5' → 3' direction.

• Base Pairing: RNA bases pair with DNA: A-U, T-A, G-C, C-G.

• mRNA: Messenger RNA carries genetic information from DNA to ribosomes for translation.

• Initiation: Transcription begins at promoter regions.

• RNA Polymerase vs. DNA Polymerase: RNA polymerase does not require a primer; DNA polymerase does.

• Transcription Product: The RNA sequence is complementary to the template strand and nearly identical to the coding strand (except T is replaced by U).

Example: The sequence 3'-CTA GGA TAC-5' on DNA produces the mRNA sequence 5'-GAU CCU AUG-3'.

Translation & The Genetic Code

Decoding mRNA into Protein

Translation is the process by which ribosomes synthesize proteins using the genetic code carried by mRNA.

• Codon: A sequence of three nucleotides on mRNA that codes for a specific amino acid.

• Start Codon: AUG codes for methionine and signals the start of translation.

• Stop Codons: UAA, UAG, UGA signal termination of translation.

• tRNA: Transfer RNA carries amino acids to the ribosome; its anticodon pairs with the mRNA codon.

• Ribosome: Composed of rRNA and proteins; site of protein synthesis.

• Reading Frame: The sequence of codons is read in sets of three nucleotides.

• Point Mutations: Substitution, insertion, or deletion of nucleotides can alter the protein product.

• Types of Mutations:

  • Silent Mutation: No change in amino acid sequence.

  • Missense Mutation: Changes one amino acid.

  • Nonsense Mutation: Introduces a premature stop codon.

Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene (glutamic acid to valine).

Guidelines for Transfer of Sequence Information

Transcription, Translation, and Mutations

Understanding the flow of genetic information is essential for predicting the effects of mutations and for interpreting genetic sequences.

• Transcription:

  1. Identify the template strand (DNA).

  2. RNA is built in the 5' → 3' direction.

  3. Pair DNA bases with RNA nucleotides: A-U, T-A, G-C, C-G.

• Translation:

  1. Locate the start codon (AUG) on mRNA.

  2. AUG codes for methionine, the first amino acid.

  3. Use the codon table to determine the amino acid sequence.

  4. Read in-frame (three nucleotides at a time).

  5. Translation proceeds from the N-terminus to the C-terminus of the protein.

• Mutations:

  1. Compare wild type vs. mutated DNA sequence.

  2. Types of mutation:

     • Translocation: Segment of DNA moves to a new location.

     • Transition: Purine replaces purine or pyrimidine replaces pyrimidine.

     • Substitution: Nucleotide replaced by another; can be silent, missense, or nonsense.

  3. Substitution affects protein only if it occurs within the coding region.

• Reverse Translation (Protein → mRNA):

  1. Start with the protein sequence (N-terminus → C-terminus).

  2. Assign possible codons for each amino acid.

  3. Do not forget to include stop codons at the end.

Lipids & Membranes

Structure and Function of Lipids and Biological Membranes

Lipids are a diverse group of hydrophobic molecules that form the structural basis of cell membranes and serve as energy storage.

• Main Types of Lipids: Fatty acids, triglycerides, phospholipids, steroids.

• Phospholipids: Amphipathic molecules with polar heads and nonpolar tails; form bilayers in water.

• Saturated vs. Unsaturated Fatty Acids: Saturated fatty acids have no double bonds; unsaturated have one or more double bonds, making them more likely to be liquid at room temperature.

• Hydrogenation: Adds hydrogen to unsaturated fats, converting them to saturated fats and changing their physical properties.

• Steroids: Have a four-ring structure and are amphiphilic.

• Cholesterol: Modulates membrane fluidity and permeability.

• Triglycerides: Formed from three fatty acids and glycerol; main function is energy storage.

• Artificial Bilayers: Liposomes and planar bilayers are used to study membrane properties and drug delivery.

• Selective Permeability: Membranes allow some molecules to cross more easily than others.

• Diffusion Rates: Based on size and polarity, molecules diffuse across membranes at different rates.

Molecule	Relative Rate of Diffusion (Fastest to Slowest)
H2O	Fast
Na+	Slow
Glucose	Intermediate

Example: Phospholipid bilayers form the basic structure of cell membranes, providing selective permeability and compartmentalization.