Exam 2: Ch 6 Organization of Eu Chromo and Functional Elements Flashcards
the packing of DNA is crucial
to cell architecture
to prevent tangles in long DNA during cell division
histones
abundant nuclear proteins that compact and organize chromosomal DNA
chromatin
complex of histones and DNA (1/2ea)
dispersed in interphase
further folding and compaction of chromatin during mitosis produces…
visible metaphase chromosomes
the structural organization of _______ allows vast DNA length to be compacted into the microscopic constraints of the nucleus
chromatin
5 major types of histone proteins
H1, H2A, H2B, H3, H4
rich in pos charged aa that interact with neg charged phosphate on DNA
extracted chromatin appearance depends on
salt concentration
low = beads on a string
physiological = condensed fiberlike form 30nm in diameter
beads on a string
nucleosomes connected by linker DNA
nucleosomes
primary structural units of chromatin
about 10nm in diameter
less susceptible to nuclease digestion than linker DNA
structure of a histone
DNA wrapped around octamer histone protein core
2 copies of each histone (not H1) into a disk like structure
when is DNA assembled into nucleosomes
during cell replication after the replication fork passes
uses histone chaperones
structure of 30nm fiber
chromatin zig-zag ribbon
two strands of stacked nucleosomes wound into a left handed double helix
contains H1
30nm fiber purpose
chromatin in chromosomal regions not being transcribed or replicated
modifications of _____ ______ control chromatin condensation and function
histone tails
histone tail
flexible n-terminus and C-terminus in H2A and B extending from histone octamer core
required for chromatin to condense from beads on a string to 30nm fiber
histone tail modification
post-translational modifications like acetylation, methylation, phosphorylation, and ubiquitination
histone code
the different combinations of tail modification
alter binding sites on chromatin-associated proteins
histone acetylation
tail lysines undergo reversible acetylation by enzymes acting on N-terminus
acetylation neutralizes lysine’s positive charge –> beads on a string (increases transcription and replication)
correlated with increased chromatin digestion by nucleases
histone methylation
prevents acetylation to maintain positive charge on lysine
makes chromatin less available
heterochromatin
dark staining condensed regions of chromatin
remains compacted after mitosis
examples of heterochomatin
centromeres, telomeres, transcriptionally inactive genes
euchromatin
light staining loose chromatin
most transcribed regions of DNA
histone code of modified aas in tails is “read” b proteins that…
bind to modified tails and promote condensation or decondensation
forms open or closed chromatin structures as judged by their sensitivity to DNase I digestion
proteins that read histone code have a _______
chromodomain that binds histone tails when they’re methylated at specific lysines
example of a histone code reading protein
HP1 heterochromatin protein I
has a 2nd domain called chromoshadow domain that helps spread methylation into heterochromatin
bromodomain
binds to acetylated histone tails in euchromatin
TFIID
a transcription factor with a bromodomain
histone acetylase activity
x-chromosome inactivation in females
epigenetic gene control through repression by heterochromatin
half of cells have inactive Xmom and inactive Xdad
dosage compensation
process that ensures cells of females express same level of proteins encoded on x-chromosomes as cells as males who only have 1
Polycomb in dosage compensation
binds methylated histone tails and contains histone methyl transferase activity
epigenetic process
affects expression of specific genes and is inherited by daughter cells w/o being result of a change in DNA sequence
nonhistone proteins organize….
long chromatin loops
scaffold associated regions (SARs) or matrix attachment regions (MARs)
SARs/MARs
located at bases of DNA loops
found between transcription units
insulate neighboring genes
insulators
DNA sequences that separate transcription units from each other
can proteins regulating transcription of one gene influence the transcription of a neighboring gene separated by an insulator?
no
structural maintenance of chromosome proteins (SMCs)
hold bases of chromatin loops in place in interphase
nonhistone proteins that a crucial for maintaining structure of condensed chromatin in mitosis
chromonema fiber
100-130nm fiber condenses during prophase
middle prophase chromatid
200-250nm structure made from a chromonema fiber
folds into 500-750nm chromatids observed in metaphase
interphase and metaphase chromosomes contain…
proteins other than histones that regulate transcription and replication
transcription factors in interphase
high mobility group proteins (HMG) assist binding of TFs
each metaphase chromosome consists of two sister _____ that are linked at the centromere
chromatids
karyotype
number, sizes and shapes of the metaphase chromosomes
different for each species
human: 22 autosomes 1 pair of sex chromosomes
FISH
chromosomes painting that differentiates chromosomes os similar size and shape
probes specific for sites scattered along length of chromosome
how does FISH work
probes labeled with different fluorescent dyes are hybridized to chromosomes
observed with fluorescence microscope
what is FISh used for
detect abnormal number of chromosomes, translocations
finer structure analysis to reveal deletions or duplications or chromosomal regions
reveal evolution of chromosomes
conserved synteny
genes occurring in the same order on a chromosome in 2 different species
polytene chromosome
enlarged interphase chromosomes with a large number of reproducible well-demarcated bands
salivary glands of fruit flies
precisely locate specific DNA sequences, translocations, and inversions (immunostaining)
3 functional elements required for replication and correct segregation
replication origins where DNA polymerase + other proteins initiate synthesis
centromere (proper segregation)
telomeres
kinetochore
complex protein structure that assembles at centromeres and uses mitotic spindle fibers
telomere
repeat sequence TTAGGG at end of chromosomes to prevent shortening
lagging strand not replicated by DNA polymerase in its entirety so lose end of chromo
telomerase
specialized form of reverse transcriptase that carries an internal RNA template to direct DNA synthesis
adds telomeres to ends of chromosomes
telomerase is active/inactive in…
active: cancer, germ/stem cells
inactive: cells of adult tissues that replicate a limited number of times or never again (postmitotic)
