Lecture 20: Central Dogma Flashcards
Beadle and Tatum’s Experiments
Beadle and Tatum grew nutritional mutants (auxotrophs) of Neurospora on a minimal medium supplemented with a single amino acid such as arginine (arg)
They hypothesized that each auxotrophic strain had a defect in a gene that codes for an enzyme needed to synthesize a particular amino acid (one gene–one enzyme hypothesis)
•Their hypothesis was later updated to the one gene–one polypeptide hypothesis
•Their hypothesis was later updated to the one gene–one polypeptide hypothesis
How to grow a mould
Provide all the required nutrients: complete medium
Provide only the simplest most basic nutrients needed for growth: minimal medium
Beadle & Tatum’s Question:
•If some mutant moulds cannot grow on minimal medium then do their mutant genes fail to specify the enzymes needed to produce the nutrient?
DNA to RNA to Protein
Beadle and Tatum proposed the one-gene-one-enzyme hypothesis
- One- gene-one –polypeptide hypothesis: The majority of genes code for cellular proteins. Each gene encodes one polypeptide (the simplest building block of a protein).
- Updated because one enzyme (or protein) can have more than one polypeptide component each encoded by a different gene
Pathway from Gene to Polypeptide
In 1956, Francis Crick gave the name central dogma to the flow of information from DNA → RNA → protein
- Transcription is the mechanism by which information encoded in DNA is made into a complementary RNA copy
- Translation uses the information encoded in the RNA copy to assemble amino acids into a polypeptide
Transcription and Translation
In transcription, RNA polymerase copies the DNA sequence of a gene into an RNA sequence
•A protein-coding gene is transcribed into messenger RNA (mRNA)
•In translation, an mRNA associates with a ribosome, on which amino acids specified by the mRNA are joined one by one to form the polypeptide
•Some genes do not encode a polypeptide – they encode various molecules that function in transcription, translation, and other processes
Reading the DNA sequence
Reading the DNA sequence: •The DNA alphabet: A, T, C and G. •The RNA alphabet: A, U, C and G. •Text p. 206 •The genetic code: •The four mRNA bases in combinations of 3 code for 20 amino acid “words”
Codons
Start codon = AUG
Stop Codon = UAA, UAG, UGA
RNA polymerase
Prokaryotes
Binds the promoter sequence of the gene directly
Eukaryotes
Transcription factors facilitate the binding of RNA polymerase
Gene expression regulation
PRO - At transcription
EUK - Transcriptional and
Translational regulation
Transcription termination
PROK - Specific terminator sequences are involved
EUK - There is a polyadenylation signal at the 3’ end and poly (A) polymerase adds a poly (A) tail
Translation
PROK - Occurs throughout cell and may begin while mRNA is still being made
EUK - Occurs in the cytoplasm and begins only after processing the mRNA
Features of the Genetic Code
Sense codons
61 codons specify amino acids
•Most amino acids specified by several codons (degeneracy or redundancy)
•Ex: CCU, CCC, CCA, CCG all specify proline
Start codon or initiator codon
•First amino acid recognized during translation
•AUG; specifies amino acid methionine
Stop codons or termination codons
•End of a polypeptide-encoding mRNA sequence
•UAA, UAG, UGA
Wobble hypothesis
The pairing of the anticodon with the first two nucleotides is specific but the third nucleotide of the codon may be less precise
Know this…
Every protein is assembled on ribosomes according to instructions which are specified by genes and coded in DNA.
Polysomes
Multiple ribosomes can simultaneously translate a single mRNA
Simultaneous Transcription and Translation
Can occur in prokaryotes (no nuclear envelope)
Fig.
Genetic Changes That Affect Protein Structure and Function
Two types of genetic change can alter protein structure and produce an altered phenotype (function):
•Mutation of a base pair in the DNA (a change from one base pair to another)
•Movement of transposable elements (TEs) from one location to another in the genome
Four types of base-pair-substitution mutations affect protein-coding genes: missense mutation, nonsense mutation, silent mutation, and frameshift mutation
Missense Mutation
A sense codon is changed to a different sense codon that specifies a different amino acid
•Whether the function of a polypeptide is altered significantly depends on the amino acid change that occurs
Genetic diseases caused by missense mutations include sickle-cell disease, albinism, hemophilia, and achondroplasia
Nonsense Mutation
A sense codon is changed to a nonsense (stop) codon
•Translation of an mRNA containing a nonsense mutation results in a shorter than normal polypeptide – in many cases, this polypeptide will be only partially functional
i.e. Cystic fibrosis
Silent Mutation
A sense codon is changed to a different sense codon but that codon specifies the same amino acid as in the normal polypeptide
The function of the polypeptide is unchanged
Frameshift Mutation
A single base pair deletion or insertion in the coding region of a gene alters the reading frame of the resulting mRNA
•After the point of mutation, the ribosome reads codons that are not the same as for the normal mRNA, producing a different amino acid sequence in the polypeptide
•Resulting polypeptide typically is nonfunctional because of the significantly altered amino acid sequence