BIOB11H3 Lecture Notes - Lecture 3: Optical Tweezers, Nucleoside Triphosphate, 18S Ribosomal Rna
Lecture 3
Genetic Expression
Gene: The entire nucleic acid sequence required for synthesis of functional gene product
oRNA
oPolypeptide
Central dogma of molecular biology:
oDNA RNA Protein
George Beadle and Edward Tatum (1940s): One gene - one enzyme hypothesis
The Beadle – Tatum experiment
Looked at growth of a bread mold Neurospora crassa
Hypothesis: if the DNA of this organism were damaged in some way, certain genes may be mutated and
hence would not be able to express specific proteins that are necessary for growth
Concept:
Relationship between Genes and Proteins
Different mutants were isolated
Genetic defect in mutants prevented the mutants from carrying out a particular metabolic reaction
Conclusion: Genes carry the information for the construction of a particular enzyme – One gene-one
enzyme hypothesis (later modified to one gene - one polypeptide)
Mutation in a single gene causes a single substitution in an amino acid sequence of a single protein
Flow of information in a Eukaryotic Cell
find more resources at oneclass.com
find more resources at oneclass.com
Messenger RNA (mRNA): An intermediate between a gene and a polypeptide
Transcription: The process by which RNA is synthesized from a DNA template in the nucleus
Translation: The process by which proteins are synthesized from an mRNA template in the cytoplasm.
Transcription: from DNA to RNA
Carried out by enzymes called DNA-dependent RNA polymerases
Go from information storage (DNA) to information use (mRNA) in the cell large stationary molecule to
a small mobile molecules
omRNA is complementary to the DNA template strand
omRNA has short half-life
omRNA acts as a template for translation into a polypeptide (by ribosomes in cytoplasm)
Amplification of genetic information:
oSeveral mRNA molecules can be synthesized from a single DNA molecule
oEach mRNA molecule can be used for synthesis of several polypeptides
Three major stages of transcription
RNA synthesis from DNA is a cyclic process with three main stages:
1. Initiation: RNA polymerase binds to a specific region of DNA close to the beginning of a gene (promoter
sequence). The enzyme requires help of transcription factors to recognize the promoter.
2. Elongation: Upon binding a conformational change occurs in the polymerase. DNA strands temporarily
unwind and a complementary RNA strand is synthesized by linking ribonucleotides by phosphodiester
bonds
3. Termination: RNA polymerase reaches a specific signal sequence on the DNA that causes an extended
pause in synthesis. The transcript is released from the enzyme, the DNA strands come back together
and the polymerase floats off to find another promoter.
Transcription: Initiation Transcription: Elongation and Termination
Transcription overview
RNA is made complementary to one strand of DNA – Template Strand (antisense strand)
RNA sequence is the same as the coding strand (sense strand) except uracil replaces thymine
Uses ribonucleotides triphosphates (rNTPs) as building blocks
find more resources at oneclass.com
find more resources at oneclass.com
Nucleotides are added in an order dictated by the template strand
Linked in 5’ à 3’ direction by phosphodiester bonds
Bases in RNA
Thymine in DNA
Nucleotides of RNA
Synthesis of RNA
Chain elongation: attack by the 3’ OH of the nucleotide at the growing strand end on the 5’-P of the
incoming nucleoside triphosphate
RNAn + NTP RNAn+1 +PPi
PPi 2 Pi
Transcription overview
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Gene: the entire nucleic acid sequence required for synthesis of functional gene product: rna, polypeptide. Central dogma of molecular biology: dna rna protein. George beadle and edward tatum (1940s): one gene - one enzyme hypothesis. Looked at growth of a bread mold neurospora crassa. Hypothesis: if the dna of this organism were damaged in some way, certain genes may be mutated and hence would not be able to express specific proteins that are necessary for growth. Genetic defect in mutants prevented the mutants from carrying out a particular metabolic reaction. Conclusion: genes carry the information for the construction of a particular enzyme one gene-one enzyme hypothesis (later modified to one gene - one polypeptide) Mutation in a single gene causes a single substitution in an amino acid sequence of a single protein. Messenger rna (mrna): an intermediate between a gene and a polypeptide. Transcription: the process by which rna is synthesized from a dna template in the nucleus.
