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biologia
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- Nucleotide & DNA Structure:
- Nucleotides are monomers composed of a pentose sugar and a nitrogenous base.
- DNA features 4 bases (A, C, G, T), anti-parallel strands, and stable hydrogen bonds between complementary bases (3 for C-G, 2 for T-A).
- The constant diameter of DNA ensures stability, recognized by repair systems.
- Phosphodiester bonds are strong covalent linkages forming the DNA backbone (5'-3' direction).
- DNA Conformations & Denaturation:
- DNA exists in B, A, and Z forms, differing in shape and accessibility of grooves for protein interaction.
- DNA denaturation (strand separation) is carried out by helicases, breaking hydrogen bonds, and can be induced in labs by heat or ionic changes.
- Histones:
- Found in eukaryotes, histones are proteins that compact DNA into a cylindrical structure, forming nucleosomes. Histone H1 helps maintain DNA wrapping.
- DNA Replication:
- A semi-conservative process, where each parental strand serves as a template for a new identical daughter molecule.
- Requires RNA primers for initiation, DNA polymerases for synthesis (5'-3' direction), and various proteins like topoisomerases (to relieve supercoiling) and SSB proteins (to stabilize single strands).
- In eukaryotes, replication starts at multiple origins, producing Okazaki fragments on the lagging strand.
- Key Replication Proteins (Eukaryotes):
- DNA Polymerase α (primer synthesis), DNA Polymerase δ (DNA synthesis), PCNA (processivity), Replication factors C and A (elongation/SSB), Topoisomerases I and II (strand separation), FEN1 (RNA primer removal), DNA Ligase (nick sealing).
- Telomerase:
- A specialized DNA polymerase that uses an RNA template (reverse transcriptase activity) to synthesize DNA at telomeres, compensating for end replication problems and preventing DNA damage due to shortening.
- Transcription:
- The process of synthesizing RNA from a DNA template, occurring in a 5'-3' direction.
- In eukaryotes, primary transcripts (pre-mRNA) undergo modifications like splicing (removal of non-coding sequences) and the addition of a 5' cap.
- The 5' cap protects mRNA from degradation and aids in ribosome binding.
- Actin:
- A key cytoskeleton protein existing as G-actin (monomer) and F-actin (polymerized filament).
- F-actin forms bundles and networks, providing structural support and enabling cell contraction through interaction with myosin 2.
- ATP hydrolysis to ADP is crucial for G-actin to F-actin conversion.
- Intermediate Filaments:
- Cytoskeletal components found in all cells, providing structural integrity. Nuclear lamins are a specific type of intermediate filament.
- Myosin:
- Motor proteins, like Myosin II, interact with actin to facilitate cell movement and contraction, utilizing ATP for energy (e.g., “colpo di frusta” mechanism).
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