Study Guide: Molecular Biology and Genetics Essentials

Molecular Biology: DNA, RNA, and Protein Synthesis

DNA Replication

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

• Key Enzymes:

  • Helicase: Unwinds the DNA double helix.

  • SSB Protein (Single-Strand Binding Protein): Stabilizes unwound DNA strands.

  • Topoisomerase: Relieves supercoiling tension ahead of the replication fork.

  • Primase: Synthesizes RNA primers needed to start DNA synthesis.

  • DNA Polymerase III: Main enzyme that adds nucleotides to the growing DNA strand.

  • DNA Polymerase I: Removes RNA primers and replaces them with DNA.

  • DNA Ligase: Seals nicks between Okazaki fragments on the lagging strand.

• Leading vs. Lagging Strand:

  • Leading Strand: Synthesized continuously in the 5' to 3' direction.

  • Lagging Strand: Synthesized discontinuously as Okazaki fragments, later joined by DNA ligase.

• Semi-Conservative Replication: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

Example: DNA replication in E. coli involves all the above enzymes working in a coordinated manner.

Telomeres and Cancer

Telomeres are repetitive nucleotide sequences at the ends of chromosomes that protect them from deterioration.

• Function: Prevent loss of genetic information during replication.

• Shortened Telomeres: Can lead to genomic instability and increased risk of cancer due to loss of cell cycle control.

• Telomerase: Enzyme that extends telomeres, often active in cancer cells.

Example: Cancer cells often have upregulated telomerase, allowing them to divide indefinitely.

Transcription and RNA Processing

Transcription is the synthesis of RNA from a DNA template. The resulting mRNA is processed before translation.

• Key Steps:

  • Initiation: RNA polymerase binds to the promoter region, often with help from transcription factors and sigma factors.

  • Elongation: RNA polymerase synthesizes the RNA strand.

  • Termination: RNA synthesis ends at a terminator sequence.

• Primer Requirement: Unlike DNA polymerase, RNA polymerase does not require a primer.

• RNA Processing:

  • 5' Cap: Added to the 5' end for stability and ribosome recognition.

  • Poly-A Tail: Added to the 3' end for stability and export from the nucleus.

  • Splicing: Removal of introns and joining of exons to form mature mRNA.

Example: Mature eukaryotic mRNA contains only exons, with a 5' cap and poly-A tail.

Translation and the Genetic Code

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• tRNA: Transfer RNA molecules bring amino acids to the ribosome and match them to the mRNA codon via their anticodon.

• Codon: A sequence of three nucleotides in mRNA that specifies an amino acid.

• Genetic Code: Universal code that relates mRNA codons to amino acids. Each amino acid is coded by one or more codons.

• Ribosome Sites:

  • A site: Entry site for aminoacyl-tRNA.

  • P site: Holds the tRNA with the growing polypeptide chain.

  • E site: Exit site for tRNA after its amino acid has been added.

• Number of Nucleotides per Codon: 3 nucleotides code for one amino acid.

Example: The codon AUG codes for methionine and serves as the start codon.

Ribozymes and Molecular Chaperones

Ribozymes are RNA molecules with enzymatic activity, while molecular chaperones assist in protein folding.

• Ribozyme: RNA molecule that catalyzes chemical reactions, such as self-splicing or peptide bond formation in the ribosome.

• Molecular Chaperones: Proteins that help other proteins fold correctly and transport them to proper cellular locations.

Example: The ribosome's peptidyl transferase activity is due to ribosomal RNA acting as a ribozyme.

Mutations and Genetic Variation

Mutations are changes in the DNA sequence that can affect gene function and phenotype.

• Types of Mutations:

  • Deletion: Removal of nucleotides.

  • Insertion: Addition of nucleotides.

  • Substitution: Replacement of one nucleotide with another.

  • Frameshift Mutation: Caused by insertions or deletions that change the reading frame of the gene.

• Mutagen: An agent that causes mutations, such as chemicals, radiation, or viruses.

Example: UV light is a mutagen that can cause thymine dimers in DNA.

Genotype vs. Phenotype

Genotype refers to the genetic makeup of an organism, while phenotype is the observable traits.

• Genotype: The set of genes an organism carries.

• Phenotype: The physical expression of the genotype, influenced by environmental factors.

• Relationship: Changes in genotype can lead to changes in phenotype, but not all genetic changes are expressed.

Example: A mutation in the gene for melanin production can result in albinism (phenotype).

Biochemistry of Membranes and Lipids

Membrane Structure and Function

Biological membranes are composed of a phospholipid bilayer with embedded proteins and cholesterol.

• Phospholipid Bilayer: Hydrophilic heads face outward, hydrophobic tails face inward, creating a semi-permeable barrier.

• Water Location: Water is found on both sides of the membrane and interacts with the hydrophilic heads.

• Cholesterol: Modulates membrane fluidity and stability.

Example: Cholesterol prevents membranes from becoming too rigid or too fluid.

Lipid Types and Solubility

Lipids are a diverse group of hydrophobic molecules, including fats, oils, and phospholipids.

• Fatty Acids: Long hydrocarbon chains with a carboxyl group.

• Solubility Across Membranes:

  • Most soluble: H2O

  • Less soluble: Sucrose

  • Least soluble: N2, K+

Example: The lipid shown in the image is a fatty acid (likely a saturated or unsaturated fatty acid).

Genetic Code Table

The genetic code is often represented by a codon wheel, which shows which mRNA codons correspond to which amino acids.

Codon	Amino Acid
AUG	Methionine (Start)
UUU, UUC	Phenylalanine
UUA, UUG, CUU, CUC, CUA, CUG	Leucine
UAA, UAG, UGA	Stop codons
Additional info: See codon wheel for full mapping.	Additional info: All 20 amino acids are encoded by 61 sense codons.

Key Definitions and Formulas

• Central Dogma of Molecular Biology: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Genetic Code: $\text{Amino Acid} = \text{Codon} = 3 \text{Nucleotides}$

• Semi-Conservative Replication: $\text{New DNA} = \text{1 Parental Strand} + \text{1 New Strand}$

Summary Table: DNA, RNA, and Protein Synthesis

Process	Main Enzyme	Template	Product
Replication	DNA Polymerase	DNA	DNA
Transcription	RNA Polymerase	DNA	RNA (mRNA)
Translation	Ribosome	mRNA	Protein

Additional info:

• Some questions and images were interpreted and expanded for clarity and completeness.

• Codon wheel and ribosome diagrams are referenced for genetic code and translation site labeling.