Molecular Biology: Cells and Central Dogma

Cells: Universal Features and Differences

• All cells are enclosed in a plasma membrane.
• All cells store genetic information as a linear chemical code.
• All cells replicate genetic information through template polymerization.
• All cells transcribe genetic information into RNA.
• All cells translate RNA into proteins through the same mechanism.
• All cells use proteins as catalysts.

Cell Differences

• Cells differ in shape, size, movement, lability, metabolism, and lifestyle.
• Cell classification is based on rRNA sequences.

Molecular Biology: Introduction

• Molecular Biology focuses on the formation, structure, and function of macromolecules essential for life.
• It emphasizes the role of nucleic acids and proteins in cell replication and genetic information transmission.
• It emerged from the intersection of genetics and biochemistry.

Central Dogma

• Genetic information flows in one direction: DNA to RNA to protein.
• RNA can directly code for protein as well.

Nucleic Acids: Chemical Composition

• Nucleic acids are polymers of nucleotides.
• Nucleotides consist of a pentose sugar, a nitrogenous base, and a phosphate group.

Nucleotides: Components

• Phosphate group: Inorganic phosphate (Pi) or phosphoric acid (H3PO4), negatively charged in cells.
• Sugar: Pentose with five carbon atoms (1'-5').
  • Ribose: Present in RNA molecules.
  • Deoxyribose: Present in DNA molecules, lacks a hydroxyl group at the 2' carbon.
• Nitrogenous bases: Aromatic molecules classified into:
  • Purines: Contain a purine ring (pyrimidine and imidazole rings) - Adenine (A) and Guanine (G).
  • Pyrimidines: Contain a pyrimidine ring (four carbons and two nitrogens) - Thymine (T), Uracil (U), and Cytosine (C).

Nucleosides: Nomenclature

• A nucleoside is formed by the binding of a sugar to a nitrogenous base via a N-glycosidic bond.
• Names are formed by adding "idine" to pyrimidines or "iosine" to purines.
• Deoxyribose nucleosides: Prefix "deoxy" added.
• DNA nucleosides: 2' deoxyadenosine (dA), 2' deoxyguanosine (dG), 2' deoxycytidine (dC), and 2' deoxythymidine (dT).

Nucleotides: Formation and Nomenclature

• Nucleotides are formed by esterification of a phosphate group to the 5' hydroxyl group of a nucleoside.
• Nucleotides are called nucleoside monophosphates.
• DNA components: dAMP, dGMP, dTMP, and dCMP.
• RNA components: AMP, GMP, UMP, and CMP.

Palindromes and Polypyrimidine/Polypurine Sequences

• Sites recognized by DNA-binding proteins in DNA are often arranged as palindromes.
• Polypyrimidine or polypurine sequences can form triple helices or H-DNA, playing a role in gene regulation.

Unusual DNA Tertiary Structures: H-DNA

• Alternating T and C sequences can form a mirror repeat centered on a T or C.
• The pyrimidine strand forms a triple helix with the other half of the repeat.
• The purine strand remains unpaired, resulting in a sharp bend in the DNA.

DNA Structures

• Double-stranded helix with two antiparallel strands.
• Sugar-phosphate backbone on the outside, bases inside.
• Bases are paired by hydrogen bonds: A with T, G with C.

Supercoiling of DNA

• Negative supercoiling:
  • Energetically favored.
  • Facilitates unwinding of the double helix for replication, recombination, and transcription.
• Positive supercoiling:
  • Makes opening the helix more difficult.

Topoisomerases

• Enzymes with nuclease and ligase activity.
• Change the amount of supercoiling in DNA.

mRNA: Prokaryotic vs. Eukaryotic

• Prokaryotic mRNA: Polycistronic, encoding multiple polypeptides.
• Eukaryotic mRNA: Monocistronic, encoding one protein.
  • Synthesized in the nucleus as hnRNA (heterogeneous nuclear RNA).
  • Contains noncoding introns and coding exons.
  • Introns are spliced out before translation.
  • Poly (A) tail of 100-200 adenylic acid residues at the 3' end, promoting stability.

rRNA

• Ribosomal RNA, about 65% of ribosomes.
• Ribosomes are supramolecular assemblies where protein synthesis occurs.

Explore the universal features of cells and their differences in morphology and function. This quiz delves into the fundamental principles of molecular biology, including the central dogma of molecular biology, mechanisms of genetic information transmission, and the role of macromolecules. Test your knowledge on how cells replicate and utilize genetic information!