Biology 2581B Chapter Notes - Chapter All: Methyltransferase, Histone H2A, Histone H2B

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24 Apr 2016
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13: Chromosomal Rearrangements (p. 276-294)
Syntenic Segments: Identity, order, and transcriptional direction are almost the
same/conserved within the two chromosomes.
Mouse and human genome evolved and reshaped their chromosomes through approximately
300 events
oDifferences cannot be seen through karyotyping, but can be seen by comparing DNA
sequences.
Deletions: Large =identifiable by karyotyping; small = must analyze gene (e.g. PCR).
Homozygotes or heterozygotes for deletions are usually lethal.
oE.g. of exception: heterozygous deletion for short arm of chromosome 5 = cri du chat.
oHeterozygosity will change gene dosage (genetic imbalance)
Sometimes, heterozygous gene dosage of ONE gene is okay, but with more than one gene it’ll be
lethal
oIf human genome is >3% deleted, it will be lethal.
Also, a gene that’s only present in one copy (heterozygous deletion) will be more vulnerable to
mutations
No recombination can occur in the deletion loop of the normal chromosome, so del/+
heterozygotes will always inherit the DNA in the deletion loop as a unit. Also, crossovers
between flanking regions of deletion loop will appear less frequently (i.e. map units appear
smaller) because fewer crossovers can occur between them.
Pseudodominance = Del/”recessive gene” genotype such that the recessive phenotype appears
due to a deletion of the other, dominant allele. This demonstrates a lack of complementation
because neither gene is wildtype.
Polytene chromosomes: Sister chromatids never separate after mitosis and homologous
chromosomes remain tightly paired along their lengths.
oCan use deletions to determine order of genes
oAlso can use probes to determine if a sequence is deleted.
Duplications
Tandem: Reverse/non-reverse
Non-tandem: reverse/Non-reverse
Detectable by karyotyping if large duplication  duplication loop created on the mutated
chromosome.
E.g. Triplolethal gene – 0,1,3 copies all lethal. Del/dup version okay
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If >5% of human genome is duplicated, it will be lethal for heterozygous duplications.
Unequal crossing over can occur to create deletions/duplications.
Inversions
Pericentric vs. paracentric
Usually do not cause phenotypic changes unless moved to a new promoter, or placed near
heterochromatin (called position effect variegation)
Inversion loops will pair up but homologous recombination events will result in faulty
chromosomes:
oPericentric = deletion of a segment of DNA + duplication of the gene from reciprocal
region  this causes abnormal gene dosage = death
oParacentric = generates an acentric (lost because spindles can’t attach) and dicentric
fragment (broken during anaphase by 2 spindles pulling to opposite sides) -> therefore,
no surviving progeny.
oBasically both scenarios result in cell death.
Because crossovers at both types of inversions lead to cell death, inversions are crossover
suppressors, meaning that chromosomes with inversions will never crossovers (maybe they do
but the offspring will always die so appears that there is no crossover).
oBalancer chromosomes can be created (i.e. chromosomes that contain a lot of
inversions) such that recombination with its homologous chromosome (the
chromosome of interest, which we do not want recombined) will not occur. The
chromosome of interest is passed onto the next generation as is, without
recombination.
Translocations
Non-reciprocal intrachromosomal translocation = same chromosome but translocated DNA to
different ends
Non-reciprocal interchromosomal translocation = DNA from a chromosome translocate to
another non-homologous chromosome.
Reciprocal translocation = non-homologous chromosomes swap DNA.
Robertsonian translocation = translocation of a large part such that it contains all the genes and
the non-translocated parts can be lost without consequence. The large part is now a
metacentric chromosome. (Fig. 9.19)
E.g. bcr/c-abl genes can cause myelogenous leukemia due to lack of regulation of c-abl gene.
Caused by a reciprocal translocation such that c-abl is attached to the promoter of bcr.
Translocations usually have no significant consequences.
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oHomozygous – homologous chromosomes pair and segregate normally because both
chromosomes are mutated.
oHeterozygous – creation of genetically unbalanced gametes.
P. 292: “Alternate” = balanced and surviving gametes
Adjacent-1 & adjacent-2 = non-surviving and unbalanced
Semisterellity = only half of gametes survive (only those with “alternate”
segregation pattern.
Translocation of part of chromosome 21 can give rise to Down syndrome (called translation
Down syndrome).
18: RNA Editing (p. 614 & 227-228)
RNA editing done by trypanosomes = uracil  gRNA (guide RNA) encoded by mini circles help
edit maxi-circle transcripts by adding/deleting uracils in mitochondria.
Plants & fungi also do RNA editing in the mitochondria  add/delete cytosines.
Mitochondria does not follow the universal genetic code.
oThis is allowed because mitochondria carry their own tRNA/rRNA genes.
Ciliates use on of the stop codons for glutamine or cysteine in the nucleus.
oPerhaps because the stop/nonsense codon didn’t have much of a function so it was
switched to a “sense” codon.
20: Transposable Elements (p294-311)
Copia from Drosophila: Present in 30-50 copies in polytene chromosomes.
oCan tell the time since Drosophila species were separated geographically because some
copia are in the same position.
LINEs & SINEs are mammalian. LINEs take up more space.
Retrotransposons = copia, LINEs, & SINES
Retrotransposons contain either:
oA long poly-A-tail (Non-LTR) following the reverse transcriptase gene.
oOr two long terminal repeats (LTR) flanking the reverse transcriptase gene.
Note: Integrase makes the sticky ends for insertion; not restriction enzymes
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