Transcription and Translation of DNA & mRNA

Transcribing a Strand of DNA

• Transcription uses a DNA strand as a template to create an mRNA strand.
• RNA polymerase adds complementary RNA nucleotides to the DNA template strand (3’ → 5’) to create the mRNA strand (5’ → 3’).
• Base-pairing rules include Adenine to Uracil, Thymine to Adenine, Cytosine to Guanine, and Guanine to Cytosine.
• For example, a DNA sequence of 3’ TAC GGA TCC 5’ is transcribed into an mRNA sequence of 5’ AUG CCU AGG 3’.

Translating mRNA Using a Codon Chart

• Translation occurs in the ribosome, where mRNA codons are matched with amino acids.
• Codons are groups of three mRNA bases that correspond to specific amino acids.
• A codon chart is used to determine which amino acid each codon codes for.
• For example, the mRNA sequence AUG CCU AGG translates to the amino acid sequence Methionine - Proline - Arginine.
• AUG is the start codon (methionine), CCU codes for proline, and AGG codes for arginine.

Three Parts of Transcription and Translation

Transcription Stages

• During initiation, RNA polymerase binds to the promoter region (TATA box in eukaryotes).
• During elongation, RNA polymerase adds nucleotides to synthesize pre-mRNA.
• During termination, RNA polymerase reaches a termination signal and releases mRNA.

Translation Stages

• During initiation, the ribosome binds to mRNA, and tRNA brings the first amino acid (AUG – methionine).
• During elongation, tRNAs bring amino acids, forming peptide bonds in a growing chain.
• During termination, a stop codon (UAA, UAG, UGA) signals the release of the polypeptide.

Comparison in Prokaryotes vs. Eukaryotes

• In prokaryotes, transcription and translation occur simultaneously due to the absence of a nucleus.
• In eukaryotes, transcription occurs in the nucleus, while translation occurs in the cytoplasm, and mRNA processing is required.

mRNA Modifications (pre-mRNA to mature mRNA)

• A modified guanine (G) is added at the 5’ end as a 5’ cap, protecting mRNA from degradation and aiding in ribosome binding.
• A long chain of adenines (A) is added to the 3’ end to create a Poly-A tail, which stabilizes mRNA and regulates its export from the nucleus.
• Splicing involves removing introns (non-coding regions) and joining exons (coding regions), allowing for proper reading of the genetic code.

Alternative Splicing

• Alternative splicing is a process where different exons are included or excluded, leading to multiple protein variants from one gene.
• Alternative splicing increases protein diversity without needing additional genes.

Redundancy in the Genetic Code

• Multiple codons code for the same amino acid, protecting against mutations.
• If a mutation changes a base, the amino acid might remain the same (silent mutation), preventing harmful effects.
• For example, GGU, GGC, and GGA all code for glycine.

Three Types of RNA & Their Roles

mRNA (Messenger RNA)

• mRNA is a single-stranded copy of DNA carrying genetic instructions.
• It provides the template for protein synthesis during translation.

rRNA (Ribosomal RNA)

• rRNA is a structural component of ribosomes.
• rRNA helps form the ribosome and catalyzes peptide bond formation.

tRNA (Transfer RNA)

• tRNA is a small RNA molecule with an anticodon and an amino acid.
• tRNA brings amino acids to the ribosome based on codon-anticodon pairing.