The Cell Cycle Control system Flashcards
The cell control system
is regulated by both internal and external control. It has specific checkpoints where the cell cycle stops until a go-ahead signal is received
G1 check point
the integrity if the DNA is assessed
G2 check point
proper chromosome duplication is assessed
M check point
attachment of each kinetochore to a spindle fiber is assessed
If the cell does not receive the go-ahead signal
it will exit the cycle, switching into a nondividing state called G0 phase
A chromosome consist of
a DNA molecule packed together with proteins
The bacterial chromosome is
a double-stranded, circular DNA molecule associated with a small amount of protein
Eukaryotic chromosomes have
linear DNA molecules associated with a large amount of protein
In bacterium
the DNA is “supercoiled” and found in a region of the cell called the nucleotide
supercoiled
Chromatin
DNA precisely combined with proteins
histones
are proteins that are responsible for the first level of packing in a chromatin
unfolded chromatin resembles
beads on a string, with each “bead” being a nucleosome, the basic unit of DNA packaging
Nuclosomes are composed of
two each of the four basic histone types, with DNA wrapped twice around the core of the eight histones
the N-termini (tails) of the histones protrude from
the nucleosome
Nucleosomes and histone tails are
involved in the regulation of gene expression
Chromosome
double helix of DNA
Chromatin fiber
DNA wrapped around histones
“Beads on a string” DNA wound on nucleosomes
nucleosomes coiled into a chromatin fiber
Double helix
further condensation of chromatin
duplicated chromosome
Chromatin undergoes changes in
packing during the cell cycle
at interphase some chromatin seems to be
organized into a 10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping
Interphase chromosomes occupy
specific restricted regions in the nucleus, and the fibers of different chromosomes do not become entangled
most chromatin is loosely packed in the nucleus during
interphase and condenses prior to mitosis
euchromatin
- transcriptionally active
- DNA is loosely packed
- actively present in prokaryotic and eukaryotic genome
- generally active
- present at inner side of the nucleus
- stained lighter
- early replicative
- aren’t stickey
- allow gene to form a protein
- low genetic density
- consist of 2-3% part of the genome
heterochromatin is when
during interphase a few regions of chromatin (centromeres and telomeres) are highly condensed
dense packing of the heterochromatin
makes it difficult for the cell to express genetic information coded in these regions
heterochromatin
- transcriptionally inactive
- DNA is highly packed
- only present in eukaryotic genome
- generally inactive
- present at nucleus periphery
- stained dark
- late replicative
- are usually sticky
- regulates genetic intergrity, and control gene expression
- high genetic density
- consist 97-98% of the genome
Histones can undergo
chemical modifications that result in changes in chromatin condensation. these changes can also have multiple effects on gene expression
