topic 6 Flashcards
can evolution happen wo variation
NO
key questions in evolution?
• Variation is universal
○ What maintains it
○ How variation is shaped by selections and other evolutionary factors
what makes up our genetic code? stop and start codons?
• 4 bases, 20 amino acids
64 diff types of codon - represent 20 amino acids
start = ATG(methinonine)
stop = TAA,TAG,TGA
two typed of codon changes
• Synonymous - silent- changes/mutations - do not alter amino acids
Nonsynonymous - replacement - changes alter amino acids
how do genetic changes occur in pops? who came up w mutation
○ Proposed by dutch botanist Hugo de Vries in 1886
○ Observed plant species w new characters appearing that had no history in previous generations
○ From latin word mutatio - to change
• Necessary ingredient for change
what are gene mutations? what do they lead to? when do they have evolutionary consequences?
Process of alteration of a gene or chromosome Product of the alteration • Lead to New change in morphology, survivor, behaviour, or some heritable traits Have evolutionary consequences only if they are transmitted to succeeding generations
how do the nucleotides pair?
purine: A and G
pyrimidine: C and T
AT and CG
hydrogen bonds
describe point mutations
Alterations of a single base pair
Change of a single nucleotide in a DNA template strand
can lead to the production of an abnormal protein
two types of point mutations
• Base pair substitutions
— Replacement of one base pair by another at a specific DNA site in
a population or a species
• Frameshifts
— An insertion or deletion of base pairs in a DNA sequence that
alters the reading frame, resulting in multiple changes
types of base pair mutations
• • Transition ○ Substitution ○ A purine for a purine (a-g) ○ A pyrimidine for a pyrimidine (c-t) ○ • Transversions ○ Substitution ○ Purings for pyrimidines or vice versa ○ A or G - C or T ○ Eight possibilities
look at diagram of transitions vs transversions. which is more common? why ?
Transitions are more common than transversions because
DNA repair enzymes can recognize wrong insertion representing a
a transition better than a transversion
how do frame shift mutations work
- Alteration of reading frame = many changes downstream
- Insertions and deletions (indels)
describe indels
A single base pair or many Triplet reading frame shifted by one nucleotide or more-> downstream triplets are read as different codons -> different amino acids frequent compared to base pair substitutions
how does recombination lead to sequence changes. caused by?
• Typically based on precise alignment of
DNA sequences
• Intragenic recombination due to difference
in sequence at two or more base pairs
○ Generates new DNA sequences
Due to gene conversion
what is gene conversion
When a damaged DNA strand of one
chromosome repaired by enzymes inserting
bases complementary to sequence on
undamaged homologue
describe unequal cross over . look at diagram
• Imperfect alignment - unequal exchange btwn two homologous sequences or chromosomes
Results:
Tandem (identical) duplication on one recombination
product & a deletion on the other
Length of affected region may range from a single base
pair to a large block of loci
sickle cell anaemia ex.
• A single base change in DNA results in a change in one RNA codon, causing production of abnormal hemoglobin called HbS •
• Enough to disrupt normal action of 02-carrying globular Hb mols & hence, of the RBCs • Painful episodes • How does it happen? Change of a "T" to "A" results in making of valine instead of normal glutamic acid on ß-subunit of Hb protein
why is sickle cell maintained?
Sickle Hb confers a survival advantage to
individuals living in malaria infested areas
—7% of people of African descent carry an
allele
cysticfibrosis- how does it happwn?
mos \t common in NA, NEUR, SW AFR
appens bc mutations in gene encoding Na- channel protein Two frameshift mutations — one caused by a two-nucleotide insertion and
the other by a one-nucleotide deletion
Mutations predicted to shift the reading frame of the protein and to
introduce a terminal codon
> 500 other base pair substitutions in this gene
Doesn’t allow whole protein to develop - introduces a stop codon
how high are mutation rates
• Generally speaking, low per locus ○ Species with larger genomes tend to have higher mutation rates per base pair per generation Doesn't hold for viruses; ○ mutation rates in RNA viruses (AIDS, Ebola, influenza) 1000's X higher Hence, favours for rapid evolution to evade host defenses
Typi y ranges
10-5 — 10-9
implications of gene mutation rate bacteria vs humans
• Although slow per locus, summed over all genes - has considerable effect
In bacterial in general:
1 mutation/100 million genes/generation
ex. ecoli 10 mil new mutations/day
In humans, in a population of 500,000, there’re
more than 2 million new mutations
A small fraction could have a substantial effect
describe somatic mutations
Occur in non-reproductive cells Cannot be passed on to offspring Do not matter for evolution Rarely noticed, except when there's health impact
describe germline mutations
○ Mutations in reproductive cells such as eggs
and sperms
Effects of germline - range of effects possible
No change in phenotype
Occurred in a stretch of DNA with no function
-Occurred in a protein-coding regions but had no effect on amino acid sequence
Small change in phenotype
Backward curl of cat’s ears: single
mutation
BIG change in phenotype
DDT resistance in insects
Could even be lethal
what are control genes? when are mutations on them worse?
- A control gene regulates expression of other genes
* Bigger the “control” larger the “effect:
homoeotic mutations (on control gene)
May alter 1 or more phenotypic characters
Body size, colouration, action of an enzyme
Effecting survival/reproduction & in turn, fitness
Homeotic genes regulate development of anatomical structures
e.g., mutations in Antennapedia gene complex cause legs to develop in
place of antenna
limitations to mutationa?
• Majority of mutations alter pre-existing
traits
• Can alter developmental processes but NOT
developmental foundations
• Ex, Just because they need one, great apes
cannot be able to develop monkey’s tail!
• We are dictated by our phylogenetic
constraints!
• Mutations occur at random
○ Chance that a particular mutation will occur is
not influenced by whether the organism is in an
environment in which that mutation would be
advantageous
