Transcription and Translation Notes

The Central Dogma

• DNA \rightarrow RNA \rightarrow Protein; this is protein synthesis.
• DNA directs protein synthesis through gene expression, involving transcription and translation.

Transcription and Translation Overview

• Transcription: Synthesis of RNA under DNA direction; produces mRNA.
• Translation: Synthesis of a polypeptide under mRNA direction; occurs on ribosomes.
• In eukaryotes, the nuclear envelope separates transcription from translation.

Transcription Details

• DNA-directed synthesis of RNA.
• RNA polymerase catalyzes RNA synthesis, separating DNA strands and hooking RNA nucleotides together.
• Base-pairing rules similar to DNA, but uracil substitutes for thymine in RNA.

Differences Between RNA and DNA

1. Sugar: RNA has ribose, DNA has deoxyribose.
2. Strand: RNA is single-stranded and shorter.
3. Base: RNA has uracil, DNA has thymine.
4. RNA is disposable.
5. RNA can be outside the nucleus, DNA can’t

Types of RNA

• mRNA: Messenger RNA carries instruction copies.
• rRNA: Ribosomal RNA, makes up ribosomes with proteins.
• tRNA: Transfer RNA, brings amino acids to the ribosome.

RNA Transcript Synthesis (Elongation)

• RNA polymerase synthesizes a single strand of RNA against the DNA template, adding nucleotides to the 3’ end.
• RNA polymerase untwists the DNA double helix, exposing 10-20 bases at a time.
• Specific DNA sequences signal termination; RNA transcript is released, and the DNA double helix reforms.

Transcription Overview

• DNA \rightarrow mRNA
• Requires a promoter (where the gene begins), RNA polymerase, RNA nucleotides, and a template strand of DNA.
• Ends at a terminator (where the gene ends).

Post-Transcription RNA Processing

• Eukaryotic mRNAs require processing before translation.
• A protective cap is added to one end.
• Non-coding regions (introns) are removed; may have regulatory purposes or allow for alternative RNA splicing.
• Original transcript (pre-mRNA) contains introns and exons; introns are removed by splicing to produce mRNA.

Translation

• RNA-directed synthesis of a polypeptide.
• Requires mRNA, ribosomes (rRNA), and tRNA.
• Involves genetic coding using codons.

The Genetic Code

• Genetic information is encoded as non-overlapping base triplets (codons).
• Codons are a 3 base code for a specific amino acid.
• With 4 bases and 3 positions, there are 64 possible codons, but only 20 amino acids.
• 3 codons are STOP signals; 1 codon is a START signal.

Transfer RNA (tRNA)

• Single RNA strand about 80 nucleotides long.
• Carries a specific amino acid on one end and has an anticodon on the other end.
• Enzymes pair tRNA molecules with corresponding amino acids.
• The anticodon matches the codon on the mRNA molecule.

Building a Polypeptide

-The AUG start codon is recognized by methionyl-tRNA or Met.

• Ribosomes incorporate amino acids into a polypeptide chain.
• RNA is decoded by tRNA, which transports specific amino acids to the growing chain.
• Translation ends at a stop codon (UAA, UAG, UGA).

Post-Translation

• Polypeptide folds spontaneously into its active configuration and may join with other polypeptides.
• Other molecules (sugars, lipids, phosphates) may attach for specific functions.

Analogy

• DNA is the master plan.
• mRNA is the everyday blueprint.
• rRNA is the builder.
• tRNA is the gopher.
• Amino Acids are the wood.
• Proteins are the building.