Molecular Biology: RNA and Protein Synthesis

Protein Synthesis

• Protein synthesis is the cellular process of creating proteins, the building blocks of life, from DNA instructions.
• It's divided into two main stages: transcription and translation.
• DNA (Deoxyribonucleic Acid): A double-helix molecule carrying the genetic blueprint, composed of nucleotides with a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, cytosine, guanine).
• RNA (Ribonucleic Acid): A single-stranded molecule involved in translating DNA's genetic code into proteins. RNA is similar to DNA, except it uses ribose sugar instead of deoxyribose and uracil instead of thymine.

Transcription

• Transcription is the process of copying a DNA segment into messenger RNA (mRNA).
• It occurs in the nucleus.
• Initiation: RNA polymerase binds to the promoter region of the gene triggering the start of transcription. DNA unwinds.
• Elongation: RNA polymerase moves along the DNA strand, adding complementary RNA nucleotides to the DNA template, building the mRNA. The mRNA synthesis proceeds from a 5' to 3' direction.
• Termination: RNA polymerase encounters a termination sequence, releasing the newly created mRNA. The mRNA, once processed, is ready for translation.

Translation

• Translation is the process of decoding mRNA into a sequence of amino acids, forming a polypeptide chain that will fold into a functional protein.
• It occurs in the cytoplasm at ribosomes.
• Initiation: The small ribosomal subunit attaches to the mRNA near the start codon (AUG), where the initiator tRNA carrying methionine binds. The large subunit joins the complex.
• Elongation: tRNAs carrying specific amino acids enter the ribosome at the A site. A peptide bond forms between the amino acids, and the ribosome shifts. The empty tRNA moves to the E site, ready to exit, and the growing peptide chain moves to the P site, ready for the next tRNA.
• Termination: The ribosome reaches a stop codon (UAA, UAG, or UGA). The polypeptide chain is released, and the ribosomal subunits detach.

mRNA Processing

• Before mRNA can be translated, it undergoes modifications:
• 5' Capping: A modified guanine nucleotide is added to the 5' end for ribosome binding and stability.
• Polyadenylation: A poly-A tail (adenine nucleotides) is added to the 3' end for stability.
• Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together.

Cell Division

• Cell division involves producing new cells from an existing one.
• Two main types: mitosis (somatic cell division) and meiosis (reproductive cell division).
• DNA Replication: is a critical part of cell division - DNA must be duplicated prior to cell division for each daughter cell to receive a complete copy. Key steps include initiation, elongation and termination.
• Mitosis: results in identical daughter cells with the same chromosome number as the parent cell. Phases: Prophase, Metaphase, Anaphase, Telophase, Cytokinesis.
• Cell cycle control: checkpoints at the G1, G2 and M phases monitor DNA integrity and ensure conditions are favorable for division.
• Apoptosis: controlled cell death is important for development and preventing the spread of damaged cells.
• Necrosis: uncontrolled cell death caused by injury or infection, resulting in inflammation.
• Meiosis: reduces the chromosome number by half to produce gametes.

Multiple Choice Questions (MCQs)

• DNA Helicase: Enzyme responsible for unwinding DNA during replication.
• Metaphase: Mitotic phase where chromosomes line up at the metaphase plate.
• mRNA: The type of RNA that carries genetic information from DNA to ribosomes.
• Elongation: Peptide bond formation stage in translation.
• G1 Checkpoint: Ensures damaged DNA is not replicated and prepares the cell for further division.

Clinical Cases (Examples)

• Case 1: Apoptosis and Cancer: Loss of p53 function can lead to uncontrolled cellular division and cancer development through apoptosis evasion.
• Case 2: Meiosis and Genetic Disorders: Chromosomal translocations during meiosis can cause gamete abnormalities and contribute to miscarriages or genetic disorders.