Overview
This lecture introduces and defines key terms related to the processes of gene expression, transcription, and translation in molecular biology.
Key Concepts in Gene Expression
- The central dogma describes the flow of genetic information from DNA to RNA to protein.
- Transcription is the synthesis of mRNA from a DNA template strand.
- Translation is the process by which ribosomes synthesize proteins using mRNA as a template.
Transcription Components and Regulation
- Promoters are DNA sequences where RNA polymerase and factors bind to initiate transcription.
- The TATA box is a conserved eukaryotic and prokaryotic promoter sequence that helps start transcription.
- CAAT box, GC-rich box, and Octamer box are promoter elements enhancing transcription efficiency in eukaryotes.
- The initiation site is the nucleotide where mRNA synthesis begins, labeled +1.
- The transcription bubble is the region of unwound DNA during mRNA synthesis.
- Upstream and downstream refer to sequences before and after the initiation site, respectively.
mRNA Processing and Structure
- Exons are sequences retained in mature mRNA after splicing; introns are removed.
- Splicing reconnects exons and removes introns from pre-mRNA.
- 7-methylguanosine cap is added to the 5’ end of pre-mRNA for protection and translation aid.
- Poly-A tail is added to the 3’ end of pre-mRNA for stability and export from the nucleus.
- RNA editing directly alters nucleotides in synthesized mRNA.
Translation and Genetic Code
- A codon consists of three mRNA nucleotides coding for a specific amino acid or stop signal.
- Degeneracy means several codons can encode the same amino acid; the code is not ambiguous.
- Start codon (usually AUG) signals the beginning of translation and codes for methionine.
- Stop codons (nonsense codons) signal termination of translation.
- Reading frame refers to how nucleotides are grouped into codons during translation.
tRNA and Ribosomes
- Anticodon is a sequence on tRNA matching the mRNA codon.
- Aminoacyl tRNA synthetase links tRNAs with their corresponding amino acids.
- Initiator tRNA is the first tRNA, binding to the start codon in the ribosome P site.
- Peptidyl transferase is the ribozyme that forms peptide bonds in the ribosome.
- Shine-Dalgarno sequence helps start translation in prokaryotes.
- Polysomes are mRNA molecules being simultaneously translated by multiple ribosomes.
Other Genetic Elements and Terms
- Template strand is the DNA strand used to make mRNA; nontemplate strand is its match (except T/U).
- FACT complex aids chromatin transcription by handling nucleosomes.
- Plasmids are extrachromosomal, circular DNA common in prokaryotes.
- Signal sequences direct proteins to correct cellular locations.
Key Terms & Definitions
- Central dogma — flow of information from DNA to RNA to protein.
- Transcription — process of copying DNA into mRNA.
- Translation — process of building a protein from mRNA.
- Promoter — DNA region initiating transcription.
- Exon — coding region retained in mRNA.
- Intron — non-coding region removed from mRNA.
- Codon — set of three mRNA bases coding for an amino acid.
- Anticodon — tRNA sequence complementary to mRNA codon.
- Start codon — first mRNA codon (usually AUG) for translation.
- Nonsense codon — mRNA codon that signals translation stop.
- Shine-Dalgarno sequence — ribosome-binding site in prokaryotic mRNA.
Action Items / Next Steps
- Review each term and its definition for mastery.
- Study the role of these elements in transcription and translation processes.
- Prepare examples for each term to reinforce understanding.