Archibald Garrod- 1902
- recognizes that alkaptonuria is inherited via a recessive allele
- proposed that patients with the disease lacked a particular enzyme
Beadle and Tatum 1941
Deliberately set out to create mutations in chromosomes and verify that they behaved in a Mendelian fashion in crosses
Beadle and Tatum studied neurospora crassa
- used X rays to damage DNA
- looked for nutritional mutations. Had to have minimal media supplemented to grow
Beadle and Tatum looked for fungal cell’s lacking specific enzymes
- the enzymes were required for the biochemical pathway producing the amino acid arginine
- they identified mutants deficient in each enzyme of the pathway
Beadle and Tatum- one gene/one-enzyme hypothesis has been
Modified to one-gene/one-polypeptide hypothesis
Central dogma
First described by Francis Crick. DNA to RNA to protein.
Transcription=
DNA to RNA
Translation=
RNA to protein
Retroviruses violate this order using
Reverse transcriptase to convert their RNA genome into DNA
Transcription
- DNA directed synthesis of RNA
- Only template strand of DNA used
- U (Uracil) in DNA replaced by T (Thymine) in RNA
- mRNA used to direct synthesis of polypeptides
Translation
- synthesis of polypeptides
- takes place at ribosome
- requires several kinds of RNA
RNA
- all synthesized from DNA template by transcription
- mRNA
- ribosomal RNA (rRNA)
- transfer rna (tRNA)
- small nuclear RNA (snRNA)
- signal recognition particle RNA (SRP RNA)
- micro-rna (miRNA)
Genetic code
- Francia Crick and sydney vrenner determined how the order of Nucleotides in DNA encoded amino acid order
- codon
- introduced single nucleotide insertions or deletions and looked for mutations- frameshift mutations
- indicates importance of reading frame
Codon
Block of 3 DNA nucleotides corresponding to an amino acid
Spaced codon
Codon sequence in a gene punctuated
Unspaced codon
Codons adjacent to each other
Stop codons
- 3 codons (UUA, UGA, UAG) used to terminate translation
Start codon
- codon (AUG) used to signify the start of translation
Code is degenerate, meaning that
Some amino acids are specified by more than one codon
Mitochondria and chloroplasts have
Some differences in “stop” signals
Prokaryotic transcription
- single RNA polymerase
- Initiation of mRNA synthesis does not require a primer
- requires promotor, start site, termination site. (Transcription unit)
Promoter
- forms a recognition and binding site for the RNA polymerase
- found upstream of the start site
- not transcribed
- asymmetrical: indicate site of initiation and direction of transcription
Elongation
- grows in 5’-to-3’ direction as ribonucleotides are added
- transcription bubble: contains RNA polymerase, DNA template, and growing RNA transcript
- after the transcription bubble passes, the now-transcribed DNA is rewound as it leaves the bubble
Termination
Marked by sequence that signals “stop” to polymerase
- causes the formation of phosphodiester bonds to cease
- RNA-DNA hybrid within the transcription bubble dissociates
- RNA polymerase releases the DNA
- DNA rewinds
Prokaryotic transcription is coupled to translation
- mRNA begins to be translated before transcription is finished
- operon: grouping of functionally related genes. Multiple enzymes for a pathway. Can be regulated together
Eukaryotic transcription: 3 different RNA polymerases. Each RNA polymerase recognizes its own promoter
- RNA polymerase I transcribes rRNA
- RNA polymerase II transcribes mRNA and some snRNA
- RNA polymerase III transcribes tRNA and some other small RNAs
Initiation of transcription: requires a series a transcription factors
- necessary to get the RNA polymerase II enzyme to a promoter and to initiate gene expression
- interact with RNA polymerase to form initiation complex at promoter
Termination
Termination sites not as well defined
mRNA modifications
In eukaryotes, the primary transcript must be modified to become mature mRNA
mRNA modifications: addition of a 5’ cap
Protects from degradation; involved in translation initiation
mRNA modifications: addition of a 3’ poly-A tail
Created by poly-A polymerase; protection from degradation
mRNA modifications: removal of non-coding sequences (introns)
Pre-mRNA splicing done by spliceosome