Download When DNA Changes – Chap. 17

When DNA Changes – Chap. 17
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Left: A normal fruit fly (drosophila).
Middle: A fruit fly with its legs jutting from its head; a mutation
induced by radiation.
Right: A disastrous effect of mutations on the human body. The boy
is a Chernobyl nuclear plant accident victim.
DNA is stable
• bases (A,T,C,G) are protected inside of sugarphosphate backbone
• tight wrapping around proteins called histones
(eukaryotes) and coiling do not allow for direct
access to DNA
• complimentary base pairing to fix any
accidental “changes”
DNA is not stable
• mutations can be beneficial, neutral, or
harmful for the organism
• mutations are random
• there are lots of places for mutations to
occur; however, not all mutations matter
for evolution
– somatic mutations occur in
non-reproductive cells and
won’t be passed onto offspring
• only mutations that matter to large-scale
evolution are those that can be passed on to
offspring
– these occur in reproductive cells like eggs and
sperm and are called germline mutations
• chromosomal defects are surprisingly
frequent events
– about 20% of all human conceptions have them
– many, if not most, irregular chromosomes are
the result of a failure of meiosis in the
production of sperm and ova (egg)
• most are so severe as to cause
spontaneous abortions early in
pregnancies
• mutations in the DNA molecules that make
up chromosomes are not usually detected
Frequency of Mutations
• in humans and other mammals, mutations
occur at the rate of about 1/50 million
nucleotides added to the chain
– with 3 billion base pairs in a human cell, that means
that each new cell contains about 120 new
mutations.
• however, not that big of a deal because of
synonomous codons and “junk DNA”
• for eukaryotes this often happens during
interphase where all DNA is replicated into two
copies before mitosis or meiosis
Damaging DNA
mutagens are agents that cause mutations – a
failure to repair DNA
• radiation—forms of energy (gamma rays, xrays, UV) that penetrate the cells and
damage DNA
• chemical—damage DNA through chemical
reactions
Three levels of mutational change
Level 1. Point or Single-Base Mutations
• Involves changes in one or only a few
nucleotides in DNA.
• 3 Types:
I. (Base) Substitutions (Fig. 17.5 in text) –
substitutions of a single base pair. Three
outcomes are...
Silent mutations – has no effect on the
structure of the resulting protein since it
still codes for the same a.a. because of
synonymous codons
 Neutral mutations – changes an a.a. in the
protein but has no effect its performance
 “Drastic” mutations -- has serious effects on
future structure and performance of proteins.
• Ex) Sickle cell – a codon for an amino acid
in hemoglobin mutates from GAA (glutamic
acid) to GUA (valine)
• Ex) Tay Sacs disease
NOVA. One Wrong Letter
II. Chain-Termination Mutations – a new stop
codon is produced or a stop codon is mutated into
a codon other than a stop
III. Frameshift Mutations (Fig. 17.6) result from
the deletions and additions of bases into a
nucleotide sequence. These cause shifts in the
reading of codons during transcription and result in
an abnormal protein
Level 2. Chromosomal Mutations
• major changes in chromosomal structure
rather than simple DNA sequence
changes
• a complete breaking, misalignment, and
rejoining of the double helix in one or
more chromosomes
3 Types:
• Deletions – entire sections of the
chromosome are left out
• Inversions – a middle fragment may flip
over and then rejoin
• Translocations – two nonhomologous
chromosomes break, swap segments, and fuse
together (ex. human #9 and #22 resulting in a type
of lukemia)
Level 3. Transpositions
• certain segments of DNA frequently “jump”
(sometimes called "jumping genes“) to new
locations in the same DNA molecule or a
different one
• not caused by mutagens but are considered
mutagens themselves because they cause the
mutation
• could an organism “purposely” evolve?
• discovered by Barbara Clintock (nobel prize
1983)
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