Rucete ✏ AP Biology In a Nutshell
14. Transcription and Translation
This chapter explores the central dogma of molecular biology: DNA → RNA → Protein. It explains the processes of transcription and translation in prokaryotes and eukaryotes and describes how genetic information flows from the nucleus to the cell membrane.
Transcription
• Transcription converts a DNA sequence into an RNA molecule using the enzyme RNA polymerase.
• RNA types: – mRNA: carries codons to the ribosome. – tRNA: carries amino acids; has anticodon to match mRNA codons. – rRNA: makes up ribosomes; catalyzes peptide bond formation.
• Initiation begins when RNA polymerase binds to the promoter region (e.g., TATA box).
• RNA is synthesized 5′ → 3′ using the 3′ → 5′ DNA template strand.
• In prokaryotes: mRNA is ready for translation immediately.
• In eukaryotes: pre-mRNA is modified before leaving the nucleus.
RNA Processing in Eukaryotes
1. Splicing: introns are removed and exons are joined by spliceosomes (snRNA + snRNPs). – Alternative splicing allows one gene to produce multiple mRNAs.
2. 5′ GTP Cap: added for protection and recognition by ribosomes.
3. 3′ Poly-A Tail: adenine tail added to protect RNA and control how long it remains in cytosol.
Translation
• Occurs at ribosomes in the cytoplasm or on rough ER.
• In prokaryotes, transcription and translation occur simultaneously.
• Three stages:
1. Initiation: – Ribosome binds to start codon (AUG). – tRNA with anticodon (UAC) binds and brings first amino acid (methionine).
2. Elongation: – Ribosome moves along mRNA, reading codons. – New tRNAs bring amino acids, which are linked by peptide bonds.
3. Termination: – Ribosome reaches stop codon. – Release factors trigger ribosome disassembly and release of polypeptide.
• The genetic code is redundant: multiple codons can specify the same amino acid.
• Codon charts are provided on exams; memorization of codons is not required.
Flow of Genetic Information
• Eukaryotic gene expression pathway: DNA → mRNA → Protein at ribosome → Protein to Golgi → Protein to membrane → Protein released
• Ribosomes on rough ER → Golgi → vesicles → secretion through membrane.
Retroviruses and Reverse Transcriptase
• Some viruses (e.g., HIV) use RNA as their genetic material.
• Use reverse transcriptase to make DNA copy from RNA, which integrates into host genome.
• Reverse transcriptase is error-prone → high mutation rate in retroviruses.
In a Nutshell
Genetic information flows from DNA to RNA to protein. Transcription produces RNA using a DNA template, and in eukaryotes, this RNA is processed before translation. Translation converts mRNA codons into amino acid chains using ribosomes and tRNAs. In viruses, RNA can be the starting point, and reverse transcriptase allows RNA to be copied into DNA.