D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 27
GENE EXPRESSION: TRANSCRIPTION
1.
Genes and Proteins:
• Gene expression: the link between genotype and phenotype involves enzymes;
enzymes (proteins) are determined by the cell’s genetic information in the nucleus; the
implementation of the information involves processes of transcription &
translation
• Evidence that genes code for enzymes:
- Beadle & Tatum conducted metabolic investigations in mold (Neurospora)
- identified nutrient-dependent mutants through growth on minimal
medium + selected nutrients, e.g. arginine-requiring mutants included three classes of
mutant: orn-, cit- & arg- (fig. 17 – 2 & ppt. 1)
- concluded each enzyme in the metabolic pathway for arginine synthesis
was determined by a separate gene; articulated one gene - one enzyme (now one
gene - one polypeptide) concept
2.
Importance of mRNA:
• DNA is mostly nuclear; protein synthesis is cytoplasmic; RNA content of a
cell correlates with level of protein synthesis; conclusion: RNA is liaison between
nuclear DNA and site of protein synthesis (fig. 17 - 3 & ppt. 2)
- radioactive tracer studies: RNA synthesized in nucleus moves to
cytoplasm (genetic messenger, a working copy of gene) (fig. 17 – 4 & ppt. 3)
Bio 102, spr. 2013
3.
lec. 27 - p. 2
Transcription: (production of mRNA, rRNA, tRNA)
• RNA polynucleotide: single-stranded; ribose replaces deoxyribose; uracil
substitutes for thymine (fig. 5 - 27 & ppt. 4)
• promoter: DNA base sequence binds RNA polymerase (+ other transcription
factors in eukaryotes)
- start signal: base sequence near promoter activates RNA polymerase II
to produce RNA single strand (5’-->3’ direction) (figs. 17 – 7, 17 – 8 & ppts. 5 & 6);
RNA synthesis follows template copy mechanism
- termination: special sequence in DNA produces termination sequence in
RNA transcript that causes RNA polymerase & transcript to break away from DNA
• eukaryotic transcription: primary transcript includes intervening sequences
(introns) separating expressed sequences (exons)
• post-transcription processing:
- introns excised in nucleus; small nuclear ribonucleoproteins (snRNPs -”snurps”) collaborate with other proteins in a spliceosome to facilitate excision &
splicing process (figs. 17 – 10, 17 – 11 & ppts. 7 & 8)
- special fragments added to protect ends of mRNA (5’ cap & poly-A tail)
(fig. 17 – 9 & ppt. 9)