In-Depth Notes on the Central Dogma of Molecular Biology, Gene Regulation, Mutations, and Repair

Definition: A conceptual framework positing the flow of genetic information in cells from DNA to RNA to protein. This process is fundamental to the processes of genetic expression and the functioning of cells.
Molecules involved: DNA (deoxyribonucleic acid) is the hereditary material in almost all organisms, while RNA (ribonucleic acid) acts as an intermediary that translates the genetic code into proteins.

Storage of Information: All somatic cells contain the same DNA, which functions as the storage molecule for genetic information.
- Example: Although skin cells and liver cells contain identical DNA sequences, they express different genes through selective gene expression, leading to their distinct functions and behaviors.
Gene Expression: Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically protein. It can vary significantly between different cell types and environmental conditions.
Measurement of Genetic Expression: The presence and quantity of mRNA is a common method for measuring gene expression, as mRNA levels correlate with active gene transcription.

Blueprint Analogy: DNA is analogous to a long blueprint that contains all the instructions needed to build and maintain an organism. Rather than using the entire blueprint, cells utilize RNA as a usable copy to construct specific types of proteins and, consequently, cells.

Structure of DNA and RNA: Both nucleic acids are linear polymers made up of monomer units called nucleotides. Each nucleotide comprises a phosphate group, a sugar (deoxyribose for DNA and ribose for RNA), and a nitrogenous base.
Nitrogen Bases: The nitrogen bases include Adenine (A), Guanine (G), Thymine (T) in DNA, Cytosine (C), and Uracil (U) in RNA. The arrangement of these bases encodes genetic information.
Base Pairing: A pairs with T (or U in RNA), and C pairs with G, forming the base pairs that stabilize the nucleic acid structure.
Stability: RNA is less stable than DNA because of the presence of a hydroxyl group (OH) at the 2' position on the ribose sugar, which increases susceptibility to hydrolysis. This leads to RNA being more reactive compared to the more stable double-stranded structure of DNA.

Polarity: DNA has a defined polarity, as it is read from the 5’ end to the 3’ end during replication and transcription. Additionally, the two strands of double-stranded DNA are oriented in opposite directions (anti-parallel), which is crucial for replication.

Semi-conservative: DNA replication is described as semi-conservative because each newly synthesized DNA molecule consists of one parental (original) strand and one complementary strand. This mechanism ensures that the genetic information is accurately passed on.
Transcription: Involves synthesizing single-stranded mRNA from a double-stranded DNA template. Transcription is a critical step in gene expression, enabling the production of proteins.
Involvement of RNA: mRNA serves as the messenger that carries genetic information from DNA in the nucleus to the ribosomes in the cytoplasm for protein synthesis, as DNA remains protected within the nucleus.

Translation is the process of converting the sequence of mRNA into a sequence of amino acids to form a protein. It involves recognizing codons, which are sets of three nucleotides that correspond to specific amino acids.
Start Codon: The translation process typically begins at the start codon, which is usually Methionine (AUG), signaling the initiation of protein synthesis.
Stop Codons: Translation terminates at stop codons (UAA, UAG, UGA), which signal the end of the protein synthesis, ensuring that the complete protein is produced.

DNA replication occurs during the S-phase of the cell cycle and involves three main steps: initiation, elongation, and termination. This ensures that each daughter cell receives an identical set of DNA.

This involves several critical processes that occur after transcription, including capping (adding a 5' cap), polyadenylation (adding a poly-A tail), and splicing (removing introns and joining exons). These modifications enhance mRNA stability, regulate its translation, and protect it from degradation.

Codons: Triplets of nucleotides (codons) in mRNA code for specific amino acids, dictating the structure of proteins.
Degeneracy: The genetic code is degenerate; multiple codons can code for the same amino acid, providing a level of protection against genetic mutations that may occur.

Definition: Mutations are permanent changes in the DNA sequence that can occur spontaneously during DNA replication or can be induced by environmental factors.
Types of Mutations: Common types include point mutations (substitutions), insertions (addition of bases), deletions (removal of bases), inversions (reversal of bases), and translocations (rearrangement of parts between non-homologous chromosomes).

General Mechanisms: Cells utilize various mechanisms to repair mutations, including Mismatch Repair (MMR) for correcting base-pair mismatches, Base Excision Repair (BER) for repairing single base lesions, Nucleotide Excision Repair (NER) for removing bulky DNA adducts, and Double-strand Break Repair (DSBR) for repairing severe DNA damage.
Role of p53: The p53 protein plays a crucial role in maintaining genome integrity by regulating the cell cycle, coordinating DNA repair processes, and inducing apoptosis in response to irreparable DNA damage.

These are heritable changes in gene expression that do not involve changes to the DNA sequence itself. Mechanisms like histone modifications (which affect chromatin structure) and DNA methylation (which can silence genes) play key roles in cellular differentiation, development, and response to environmental stimuli, enabling cells to adapt without altering their genetic blueprint.