Lecture 14 Flashcards
Crick’s three concepts
the role of nucleic acids is to inform the synthesis of protein
the flow of information is unidirectional
the information is a sequence
Cricks central dogma theory
The central dogma of molecular biology is a theory stating that genetic information flows only in one direction, from DNA, to RNA, to protein, or RNA directly to protein
One gene, one enzyme hypothesis
Beadle knew that X-rays induced mutants from his previous work in drosophila and with Tatum they generated 3 new mutants in neurospora which couldn’t grow without certain vitamins
Neurospora can reproduce sexually and Beadle and Tatum showed that the mutants had Mendelian inheritance
this means that genes controlled biochemical reactions later dubbed one gene one enzyme theory
Crick, Barnett, Brenner & Watts-Tobin
used mutagens that either insert or delete, not change, a base pair
they used phage T4 ‘point’ mutants that could not lyse E coli normally and reverted the phenotype with additional rounds of point mutagenesis
they observed that insertions could be reverted with deletions and vice versa but most importantly that 3 insertions would revert a single insertion
Nirenberg and Matthaei
Brenner had shown an RNA intermediate between DNA and protein
Heppel and Singer were able to synthesise polyuridine using the newly described Polynucleotidephosphorylase
they devised a method to lyse E coli but retaining the ribosome activity
they treated their extracts with RNAse and DNAse and then added synthetic poly-U leading to synthesis of poly-Phenylalanine
the first codon was determined
Features of the genetic code
continuous, non-overlapping
degenerate
4 bases can distinguish 20 amino acids in combinations of three
unambiguous- 1codon, one amino acid
universal with a few exceptions
using the genetic code
dsDNA is copied into ssRNA
the coding trend is the non-template
-the RNA sequence is identical to the DNA coding sequence
-codons are read in the 5’-3’ direction
what is the start codon
AUG on RNA
ATG on DNA
Start codon
indicates the first codon of a mRNA to be translated by a ribosome
codes for methionine also in internal positions of the protein
bacteria and organelles incorporated N-formylmethionne at start AUG by modifying the normal tRNA-Met complex to tRNA formylMet
Met are often removed post translationally
What are the stop codons
UAG
UAA
UGA
Stop codons
do not code for any amino acid
have a different efficiency of termination
UAA>UAG>UGA
are usually followed by other accessory sequences
mRNA basic structure
5’UTR (untranslated region)- governs interaction with the ribosome
3’UTR (can be regulatory)
Open reading frame
a portion of a DNA sequence that does not include a stop codon
Wobble base-pairs
only the first two bases of a codon have precise Watson-crick pairing with the corresponding bases of the anticodon of tRNA
four non-standard interactions may take place between the base at the 3’ end of the codon and the first 5’ base of the tRNA
this wobble effect allows different codons to be recognised by the same tRNA
these pairings are essential for secondary structure of many RNAs, some organisms do not have tRNAs for all synonym codons
Codon usage bias
the genetic code is universal but synonym codons allow that each species has favourite codons for certain amino acids
this has implications for translation efficiency of individual genes
Deciphering codon tables
the universality of the genetic code underlies the fact that all life on earth has a common evolutionary history
it also enables scientists to combine genes from different species and express their products in a novel combination and in a new host
Mutation types
Spontaneous
- from replication errors
-very low frequency
Induced
- caused by external agents
chemical: alkylating agents, intercalating agents
physical: ionising radiation
biological: pieces of exogenous DNA inserting into genome
Point mutation types
substitution- change of identity of base pair
insertion- addition of nucleotide pair
deletion- removal of a nucleotide pair
Base substitutions
transition
- Pu to pu or Pyr to Pyr
transversion
- Pu to Pyr
Silent mutations
degeneracy of the code implies that some base changes will lead to synonymous codons
Missense mutations
base changes lead to codons for a different amino acids
Nonsense mutations
base substitutions lead to termination codon
nonsense mutation lead to C-terminal truncations of protein structure
Indels
in coding regions these mutations produce frame shift
Frame-shift mutations
single nucleotide deletion in the squalene epoxidase1 gene leads to a frame shift that changes the sequence of protein
Mutation type by lineage
in multicellular organisms, mutations have different effects depending on which cells harbour them
gremlin mutations are found in oocytes/sperm cells. when transmitted to the offspring all cells will have it .. can cause heriditary diseases
somatic mutations- arise in non-germ cells, they are not passed on to offspring and lead to mosaicism, cancer arises from somatic mutation
