Biology 1002B Study Guide - Final Guide: Programmed Cell Death, Antitoxin, Missense Mutation

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18 Apr 2017
Cycle 9: Development and Death
Different types of mutations in protein coding genes
Silent mutation: the codon still codes for the same amino acid
oThere is a difference in DNA sequence, but not in the protein
oDue to the redundancy in genetic code
Missense mutation: codon codes for another amino acid
oImpact is hard to predict (could have no effect or could be devastating)
Nonsense mutation: codon doesn’t code for anything
oCauses a premature ending of translation -> much more severe
Frameshift mutation: inserted/deleted base in the sequence
oEntire sequence is read in the wrong frame which probably produces all the wrong amino acids
Consider a single base pair insertion into the DNA coding for the first exon of a gene.
oSplicing is not affected b/c splicing is caused by snRNAs
oAll codons downstream will be in the wrong frame and code for the wrong amino acids
oWhether the protein is too long or too short depends (we don’t actually know)
Find a stop codon somewhere along the sequence -> too short
No stop codon -> too long
Characteristics that make Drosophila an attractive model system
oGenome codes for 2 organisms – larva and fly
oMutations are easily identifiable phenotypically
Can knockout a gene using RNA interference and see its effect on development
oShort embryo period
oWe can follow a gene at different stages and see if its expression changes
Is it on at some times and off at others?
Notice when the gene products are present
Main stages in Drosophila embryonic development
1. Fertilization
2. Division of the nucleus through mitosis -> blastoderm
No cytokinesis
3. 10th nuclear division -> nuclei migrate to periphery of
4. 3 more divisions -> ~6000 nuclei are organized into
separate cells -> cellular blastoderm
5. 10 hours after fertilization: maternal effect genes create
protein gradients -> segmented embryo
6. 24 hours after fertilization: egg hatches into larva
7. 3 moults -> pupa
8. Homeotic genes turn on
9. 10-12 days after fertilization -> metamorphosis -> adult fly
oWhen mothers make eggs, they pack them full of mRNA
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Main role of maternal-effect, segmentation, and homeotic genes in Drosophila
Maternal-effect genes: transcribed in mothers during oogenesis, mRNAs are
deposited in the egg and then translated in babies
oControl egg/embryo polarity by forming axis
oBicoid – development of the head and thorax (anterior/posterior)
Forms a gradient throughout the zygote (picture)
High concentration at anterior
BICOID: a transcription factor that activates some genes and
represses others throughout the zygote
Mutations: lack thoracic structures and have posterior structures
at each end (b/c those develop in areas of no/low BICOID)
oNanos – development of posterior structures
Mutations: lack abdominal segments, but anterior/posterior
oGurken – establishes top and bottom (dorsal and ventral)
Also a transcription factor
Consider… Drosophila that are heterozygous for a loss of function bicoid.
oNone of their offspring would have no heads b/c mom can still pack
bicoid into baby cells
oHowever, ¼ of the female offspring cannot make bicoid -> their kids
won’t have heads
Males don’t make eggs so it doesn’t matter
oGenotype of mothers matter, not babies
Segmentation genes: subdivides the embryo into regions which determines
segments of embryo/adult
oOnly turned on after protein gradients are in place
oMaternal-effect genes regulate expression of segmentation genes
oEach segmentation gene is expressed at a particular time and location
Homeotic genes: specify what each segment will become after metamorphosis
oControls the development of structures such as eyes, antennae, legs, etc.
by producing transcription factors
Certain homeotic genes are activated in specific segments to
create segment specific structures
oAntennapedia – legs develop instead of antennae
oBithorax – 2 sets of wings (doesn’t have musculature/neurology to work
but structures are there)
Which of the following classes of genes would be the first to be expressed in nuclei of zygotes?
oSegmentation genes!
oMaternal effect genes are expressed in mothers (first one expressed in the embryo, but not in
the nucleus)
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