chapter 10 part 2 Flashcards
changes to chromosome structure can be caused by _____________ _________ or other events
chromosome breakage
what can chromosome structure mutations lead to
loss of gain of chromosome segments or partial deletion duplication
when a chromosome breaks, where are both DNA strands severed at?
a location called the chromosome break point
what can the broken ends do at the chromosome break point?
adhere to each other, to other broken ends, or the termini of other intact chromosomes
what can detachment of all or part of one chromosome arm lead to
a terminal deletion
-the broken fragment contains a telomere & some genetic material
what may be lost in terminal chromosome deletion
acentric since it lacks a centromere and the spindle fibers cant attach to it
what are organisms with one normal & one terminally deleted chromosome called?
partial deletion heterozygotes
cru-du-chat syndrome is caused by what
loss of 5p15.2-5p15.3
partial deletion heterozygotes
what is an intersitial deletion
the loss of an internal portion of a chromosome, & results from two chromosome breaks
are interstitial deletions observed in humans
yes
example of interstitial deletions
WAGR syndrome
-series of conditions caused by deletion of multiple genes on chromosome
what can unequal crossover result in
partial duplication on one homolog & partial deletion on the other
what does unequal crossover take place between
2 homologs
partial duplication heterozygote
organism with one normal & on duplicated homolog
partial deletion heterozygore
organism with one deleted & one normal heterozygote
is unequal crossover common?
no, it does not occur often
how does unequal crossover most commonly occur?
when repetitive regions of homologs misalign
ex of unequal crossover
williams-beuren syndrome
partial deletion heterozygotes on chromosome 7
what can show regions of chromosome duplication or deletion
microscopic observation during prophase 1 when homologs synapse
what creates an unpaired loop & what is it?
-when a large deletion or duplication creates an area of mismatch between the altered chromosome & its normal homolog
-the part of one homolog missing on the pairing partner
what can large deletion/duplications be detected by?
microscopy that reveals altered chromosome banding patterns
what can micro-deletions/duplications be detected by?
FISH
-used to detect the presence or absence of a particular DNA sequence
chromosome inversion
reattachment in the wrong orientation
chromosome translocation
reattachment to a nonhomologous chromosome
paracentric inversion
the centromere is outside of the inverted region
pericentric inversion
the centromere is within the inverted region
inversion heterozygotes
have one normal & one inverted homolog
what can crossing over within a paracentric inversion result in?
-dicentric chromosomes (2 centromeres)
-acentric fragment (no centromere)
-dicentric chromosome is pulled toward both poles of cell & breaks
-acentric fragment is lost
what can crossing over within a paricentric inversion result in?
both duplicated & deleted regions in both of the recombinant products
what do both pericentric & paracentric inversion recombination result in?
two normal gametes & 2 abnormal gametes
what are the three types of translocations
-nonreciprocal translocations
-reciprocal balanced translocations
-robertsonian translocations
nonreciprocal translocations
(unbalanced) arise when a piece of one chromosome is translocated to a non-homolog and there is no reciprocal event
-one-way transfer
reciprocal balanced translocations
-occur when pieces of two non-homologs switch places
-two-way transfers
robertsonian translocations
-(chromosome fusions) involve fusion of two non-homologs
-reduction in total chromosome number
in heterozygotes for reciprocal balances translocations, what occurs?
non of the four chromosomes has a fully homologous partner
so an unusual cross-like structure is formed at metaphase 1 of meiosis
are translocation heterozygotes sterile? why?
they are partially sterile, only alternate segregation leads to normal gametes (& even the only 1/2 are normal)
if two pairs of chromosomes fuse by robertsonian translocation, what happens)
the number of chromosomes drops to 2n-2
what is responsible for familial down syndrome
one type of robertsonian translocation between chromosome 21 & (usually) chromosome 14
what is DNA & associated proteins of a chromosome called?
chromatin
what is chromatin made of
1/2 dna & 1/2 protein
(1/2 of protein is made of histone proteins)
what are histone proteins
small basic proteins that tightly bind DNA
what are the remaining proteins in chromatin
non-histone proteins
-very diverse & preform a variety of functions
what are the five major histone proteins
H1, H2a, H2b, H3, & H4
what are nucleosome core particles?
fundamental units of histone protein organization with two molecules of each of the histones except H1
-forms an octamer
core DNA
a span of DNA about 146 bp long that wraps around each nucleosome
nucleosome assembly
-H2A & H2B - dimers
-H3 & H4 - dimers
-H3-H4 dimers form a tetramer
-H2A-H2B dimer associate with the H3-H4 tetramer & form an octamer
first level of DNA condensation
the wrapping of DNA around the nucleosome
-compacts the DNA about 7x
linker DNA
the variable-length “string” between nucleosomes
-10 nm-fiber
is the 10-nm fiber observed under normal cellular conditions?
no. a 30-nm fiber (6 times more condensed) is
when does the 30-nm fiber form
when the 10-nm fiber supercoils into a solenoid structure
-6-8 nucleosomes/turn & histone H1 stabilizing the solenoid
second level of DNA condensation
30nm/solenoid
when does chromatin become maximally condensed
in metaphase of mitosis
-interphase chromosomes have variably sized loops of 30-nm fibers that form a 300-nm fiber
what does chromosome shape depend on?
the chromosome scaffold composed of non-histone proteins
what are chromatin loops of 20 to 100 kb anchored to the chromosome scaffold by? at sites called what?
by nonhistone proteins at sites called MARs (matrix attachment regions)
-metaphase chromatin is compacted 250-fold compared to the 300-nm fiber
what does chromosome compaction allow for?
efficient separation of chromosomes at anaphase
what do the chromatin loops formed during condensation play a role in?
regulation of gene expression
where does active transcription tale place in?
segments of loops away from MARs
what must happen as the replication fork passes
nucleosomes must break down into component parts & release the DNA
what happens after replication
the nucleosomes are reassembled, but we need 2X as many now
Nucleosomes present after replication typically contain:
-some old histone proteins (epigientic marks)
-some new histone proteins
-H3 & J4 tetrameres reassociate randomly with one of the sister chromatids
-H2A & H2B dimers disassemble & are reassembled from both old & new histones
