Ch 7 Lecture (Chromosomes) Flashcards
Chromosome
A discrete unit of the genome carrying many genes
Nucleoid
The structure in a prokaryotic cell that contains the genome
Chromatin
The state of nuclear DNA and its associated proteins during interphase of the eukaryotic cell cycle
Packing Ratio
The ratio of the length of DNA to the unit length of the fiber containing it
Ex. 1.4 cm packed into 2 um
Packing ratio: 7000
Capsid
The external protein coat of a virus particle
The length of DNA that can be incorporated into a virus is limited by:
The structure of the capsid
Two primary methods of constructing capsid with nucleic acids:
Assemble the capsid around the nucleic acid
Construct the capsid and load nucleic acid into the empty structure
Natural closed bacterial DNA is:
Negatively supercoiled
Ethidium bromide induces:
Positive supercoiling
The nucleoid is composed of individual domains, each of which:
Can retain their own supercoiled status
Bottlebrush Nucleoid Structure
Supercoiled loops emanating from a central core
Nucleoid Associated Proteins (NAPs)
Regulators of nucleoid structure and gene expression
Protein HU
Histone-like structure
Dimer that plays a role in DNA flexibility
NAP
Protein H-NS
Histone-like structure
Preference for AT-rich regions
NAP
how is metaphase DNA arranged
as 60kb loops attached to a proteinaceous scaffold
metaphase scaffold
a proteinaceous structure in the shape of a sister chromatid pair
during interphase, what does the metaphase scaffold do?
fills the entire nucleus and is called the nuclear matrix
how is interphase DNA attached to the nuclear matrix
MARs (matrix attachment regions)
MARs are
AT rich but do not have a consensus sequence
MARs often contain
Cis-acting transcription regulatory sites
5’intron sites
topoisomerase II recognition sites
MARS can also bind to
the metaphase scaffold
proteins regulate association of MARs with matrix to regulate
transcription
individual chromosomes can only be seen in
the M phase
DNA is … times more condensed than chromatin
5-10
during interphase, the general mass of chromatin is in the form of
euchromatin
where is heterochromatin found
nuclear periphery, around nucleolus, and packed as aggregates called chromocenters.
open active chromatin
euchromatin
actively transcribed genes
highly transcribed and weakly transcribed
minority of genome (20%)
closed inactive chromatin
euchromatin and heterochromatin
euchromatin is considered quiescent
heterochromatin is considered facultative or constitutive.
facultative heterochromatin
regulated chromatin
developmentally repressed genes
can be turned on in specific circumstances
barr bodies
constitutive heterochromatin
“permanently” condensed
replicates late in S phase
repeat rich
reduced recombination frequency
reduced gene density with low levels of transcription
chromosome territories
distinct regions of the interphase nucleus associated with chromatin from specific chromosomes
chromosomes interact at the … of territories
periphery
homologues are… in the nucleus
separated
gene dense regions are found in
the center of the nucleus
active genes are often found at
territorial borders
heterogeneity of nucleoplasm is facilitated by
biomolecular condensate formation
transcriptionally active regions of chromatin are actively extended towards the
the center of the nucleus and transcription factories
what is G-banding
when the metaphase spread is digested with trypsin and stained with Giemsa dye
In G-banding, what are the g-bands
a series of striations that are lower in GC content
in G-banding, where are enriched genes loacted
interbands
in humans, G-bands are approximately how big
10^7
centromere
a constricted region of a chromosome that includes the site of spindle attatchment
how are chromosomes attached to the mitotic spindle
the kinetochore
microtubule organizing center
a region from which microtubules emanate
centrosome in animals
cohesins
proteins that hold sister chromatids together
gradually degrade during anaphase to allow separation
how is chromatin structure specified
epigenetically
centromeres are characterized by
centromere specific histone H3 variant
post-translationally modified H2A and H3
often contain satellite DNA rich heterochromatin
kinetochore
protein structure at centromere that facilitates spindle attachment
telomeres are required for
protection of linear chromosome ends
extension of chromosomal ends
pairing of homologous chromosomes and recombination
why were telomeres created
limitations of DNA replication enzymes
what enzyme helps maintain telomere length
telomerase
stability of telomeres is due to
the t-loop and shelterin
the t-loop is catalyzed by
TRF2 protein
how is the t-loop made
the 3’ repeating unit of the GT rich strand forms a loop by displacing its homologue in an upstream region of the telomere
role of telomeres in meiosis
telomeres cluster at nuclear membrane immediately preceding coiling of homologous chromosomes
disruption of telomeres leads to defects in recombination
proteins involved in influencing telomerase binding
Yeast
Rif 1 and 2
Rap 1
Cdc13
Human
TRF 1 and 2
Tin 2
Tpp1
Pot 1
Rap 1
shelterin complex
TRF 1 and 2
Tin 2
Tpp1
Pot 1
Rap 1
how long are telomeres in humans
5-15 kb
loss of telomeres results in
senescence
escape from senescence can occur in yeast if
telomerase is reactivated
chromosomes are circularized
unequal crossing over
cells from multicellular eukaryotes obey the
hayflick limit
what cells do not obey the hayflick limit
stem cells
the majority of cancer cells contain
reactivated telomerase
