Transkripcio en Procariote

Gene Function and Mechanisms

• Human Genome Size: Approximately 3 x 10⁹ nucleotide pairs (3 billion base pairs).
• Coding Genes: Roughly 4 x 10⁴ genes responsible for synthesizing proteins, ribosomal, and transfer RNA.
• Genome Composition (Proportions): Coding genes (exons) account for about 1% of total DNA; non-coding regions (introns) comprise 24%; remaining DNA (intergenic regions) is about 75%. This "extra" DNA, also known as "non-functional DNA" or "silent DNA," is a crucial feature of eukaryotic cells.

Primary Genetic Code

• Three-base Codons: Genetic instructions represented by three nucleotides (triplets). This allows for 64 (4³) combinations from four nitrogenous bases (adenine, thymine, guanine, and cytosine), sufficient to code for 20 amino acids.
• Francis Crick's Validation (Experiment): Confirmed the existence of the code using proflavin-induced mutations in phage T4.

Secondary & Tertiary Genetic Code: Chromatin Codes

• Secondary Code (Chromatin Code): Adenine nucleotide pairs have a periodic occurrence in DNA sequences, interacting with histone octamer complexes.
• Tertiary Code (Chromatin Condensation Code): Eukaryotic DNA structure and wrapping around histone octamer complexes is regulated by non-histone proteins in response to calcium ions (Ca++) and phosphorylation, affecting localized DNA coiling and chromatin structure in the nucleus. This is crucial for controlling gene activity by affecting accessibility to different regions of the genome. This code influences the functionality and expression of genes in specific areas.

Genetic Code Properties

• Universality: The genetic code is the same across all organisms, from viruses to humans. Each amino acid is coded by the same codon across species.
• Non-overlapping: Each nucleotide is part of only one codon.
• Redundancy/Degeneracy: Most amino acids are coded for by more than one codon. This allows some variations in the third base of a codon without altering the encoded amino acid.

Gene Organization in Prokaryotes

• Operons: Genes grouped together in clusters that are transcribed into a single mRNA molecule, called polycistronic mRNA. This allows for coordinated expression of related genes. This organization allows bacteria to quickly switch their metabolism between different substrates. A key example is the lac operon in E. coli, which controls the uptake of lactose. The operon structure includes regulatory and structural genes.
• Cis-regulation: Transcription regulation occurs when a regulatory protein is located close to the regulated genes.
• Trans-regulation: A regulatory protein free in the cellular environment regulates distant genes within the cell.

Gene Organization in Eukaryotes

• Complex Structure: Eukaryotic genomes are complex; many genes are composed of exons and introns, separating coding regions (exons) from non-coding regions (introns).
• Introns/Exons: Non-coding introns interrupt the coding sequences (exons).
• Transcription: More complex in eukaryotes, with three types of RNA polymerase involved in transcription. The process is significantly regulated through a multitude of factors

Gene Structure in Eukaryotes

• Promoter: The promoter region of a gene is crucial for coordinating RNA polymerase binding and initiating transcription. It often contains several key elements.
• TATA Box: A short sequence of DNA found near the start of a gene that is important for transcription initiation.
• Transcription Start Site: The precise location where mRNA synthesis begins.
• Exons/Introns: Genes contain coding exons and non-coding introns.
• Intron Removal (Splicing): During processing, introns are removed, and the remaining exons are joined to form mature mRNA. Alternative splicing of exons is an important feature of eukaryotic genes where a single gene can give rise to multiple different mRNA molecules and proteins.

Ribosomes and Translation

• Ribosomes: Cellular structures responsible for protein synthesis, composed of proteins and ribosomal RNA (rRNA). Their Svedberg value (S) indicated the sedimentation rate in a solution. (70S (30S + 50S) for prokaryotes and 80S (40S + 60S) for eukaryotes).
• Translation Steps (General overview): Activation of amino acids, initiation, elongation, termination.
• Transfer RNA (tRNA): Adapters that bring amino acids to the ribosome, matching them to their corresponding codons on mRNA.

Other Important Factors

• Gene Families: Multiple copies of related genes. These often arise from duplication events. Globin genes are an example of a gene family where slightly different versions of a protein are needed.
• Pseudogenes: Nonfunctional copies of genes within a family, commonly arising from duplicated genes and mutations that disrupt functionality.
• Tandem Arrays: Repeated copies of a gene arranged in a cluster. This is often seen in genes coding for ribosomal RNA.
• Amplification: Some genes can increase their copy number within the genome via repeated replication, resulting in increased levels of their product.

Gene Therapy and Genetic Diagnosis

• Gene Therapy: Inserting normal genes into cells to treat inherited diseases.
• Genetic Diagnosis: Employing methods like polymerase chain reaction (PCR) to identify genetic variations associated with diseases or family relationships.