RNA and Gene Expression

• RNA is DNA's more flexible, single-stranded sibling.
• RNA differs from DNA by being single-stranded, containing ribose sugar (instead of deoxyribose), and using uracil (U) instead of thymine (T).
• mRNA (messenger RNA) carries genetic instructions from DNA to the ribosome.
• tRNA (transfer RNA) delivers amino acids to the ribosome.
• rRNA (ribosomal RNA) helps form the ribosome.
• RNA acts as the middleman between DNA and protein production.

Gene Expression

• Gene expression occurs in two main stages: transcription and translation.
• Transcription turns DNA's instructions are copied into an mRNA molecule.
• Translation is where proteins are actually made

Transcription

• Transcription occurs inside the nucleus.
• RNA polymerase is the key player, building mRNA based on the DNA template.
• RNA polymerase binds to the promoter (the "start here" signal on a gene).
• RNA polymerase unzips the DNA and reads one strand, building a complementary RNA strand (replacing T's with U's).
• Transcription ends when RNA polymerase reaches a termination sequence.
• In eukaryotic cells, introns (useless sequences) are removed from mRNA through RNA splicing, leaving only exons (coding sequences).
• The fully processed mRNA leaves the nucleus and heads to a ribosome.

Translation

• Translation occurs in the ribosome.
• The ribosome reads the mRNA in sets of three bases, each set called a codon.
• tRNA molecules bring amino acids, matching their anticodon to the codon on the mRNA.
• The ribosome links the amino acids together, forming a polypeptide chain.
• The polypeptide folds into a functional protein once complete.
• The start codon (AUG) signals the ribosome to begin translation.
• Stop codons (UAA, UAG, UGA) signal the ribosome to finish translation.
• Transcription copies DNA into mRNA, while translation turns that mRNA into a protein.

Mutations

• A mutation is a change in the DNA sequence, either random or caused by external factors.
• Silent mutations do not change the final protein.
• Missense mutations swap one amino acid for another, potentially altering protein function.
• Nonsense mutations create a premature stop codon, cutting the protein short.
• Frameshift mutations occur when a base is inserted or deleted, disrupting the entire reading frame.
• Frameshift mutations can seriously disrupt protein function, because proteins rely on precise sequences

Importance of Gene Expression and Mutations

• Gene expression determines every single trait, including hair color, eye color, and how the body fights infections.
• Mutations can cause diseases but also drive evolution.
• Gene expression is an organized assembly line.
• Mutations are glitches in the assembly line, that can be harmless or game-changing.