ch 6 Flashcards
(32 cards)
molecular evolution?
the study of how proteins, nucleic acids, and other molecules have changed through time
what does it mean if two molecules are said to be homologous?
they are derivied from a common ancestor and later diverggd from this ancestral sequence
what are the 2 types of homology?
paralogs: homologs that are present within on speciies
2. orthologs: homologs that are present within different species and have very similar functions
how can homlogy be detected?
significant sequence similarity resulting in a common 3d structure
example of orthologs?
bovine and human ribonucleaase
example of paralogs?
human ribonuclease and angiogenin
why is knowing homology useful?
it can be used to infer function. large scale genome seq has resulted in discovery of many new genes whos functionscan be compared with genes of known function
how is sequence similarity determined?
sequence allignment
how are sequence allignments performed?
aligned in regions of siliarity either in the specific amino acid squence or in the physical character of the amino acid
squences are slid past eachother to find windows of greatest similarity
why are gaps sometimes added while performing sequence alignments/
allows all regions of similarity to be included in the allignment. it is needed because one protein has evolved to either gane or lose an amino acid(S)
how are sequence alignments scored to see how likely it is that the similarities occured due to chance
each sequence identity is given +10 points whereas each gap is given a penalty of -25 points.
what does shuffling do?
the amino acid in one of the proteinsfrom the alignemnt is shuffled (bar graph showing likeliness of each shuffle) compared to the alignment score of the 2 proteins
what are conservative subsitutions?
substitutions that result in the replacement of an amino acid with one thath as similar physical or chemical properties / size
what does a substitution matrix tell you?
it shows you the frequency of a substitution: large positive score means the substiution occurs frequently while large negative score indicates a rare substitution
why is substitution matrix a better scoring system for homology?
because it takes into account protein structure
what does blast stand for
basic local alignment search tool
what should the expected value be from blast to be considered significant? what does the expected value mean?
less than 10^-5. expected value is the number of sequences with this level of similarity expected to be in the data base by chance
is 3d structure more closely associated with function or primary sequence?
function
the globins have very similar structures, are they sequentially similar?
no human mioglobin and lupin leghemoglobin is only about 15.6% similar which is not statistically significant
can structural hology be found for proteins having unrelated biochemical function?
yes. an example of actin (component of cytoskeleton) and heat shock protein (hsp) which assists in the folding of proteins inside cells.
suggests they are paralogs and that they descended fro ma common ancestor and adopted different roles
in a family of proteins, which residues are conserved?
residues critical for function
if sequence alignments are not statistically significant does this mean proteins are not related at all?
no, conservation of residues critical for function can be used as a signal in the detection of similar proteins despite not statistically significant alignment..
what is it called if 2 unrelated proteins from a different ancestor evolve to form the same structure or function?
convergent evolution. active sites are almost identical but overll 3d structures are very different
how do repeat motifs occur?
result of gene duplication event
