Theme 4: DNA Replication and Mitosis - Module 1: The Cell Cycle Flashcards
the ability of a pre-existing cell to give rise to another cell is due to what?
regulated process of cel division
for prokaryotes, what is cell division also considered?
reproduction
why is prokaryotic cell division also considered reproduction?
because cell division gives rise to a new organism (made up of one cell)
what are founding prokaryotic cells with regards to reproduction?
all essential elements necessary to reproduce
what are prokaryotic cells capable of?
making exact copies of their genomes and then segregating one copy of each genome to each two daughter cells
what does the process of cell division in prokaryotes require?
identical genetic material distributed amongst the daughter cells
what is the process of cell division in prokaryotes a form of?
asexual reproduction
what is the asexual reproduction of prokaryotes referred to as?
binary fission
when is the process of cell division in prokaryotes initiated?
when the DNA of the bacterial chromosome is attached by proteins to the inside of the plasma membrane
where does DNA replication begin?
along an origin of replication region of the bacterial chromosome
what occurs as the chromosome continue to replicate?
cell begins to elongate and newly synthesized DNA is also anchored to the plasma membrane
how long does the cell continue to elongate for?
until the two DNA attachment sites are at opposite ends of the elongated cell
when DNA replication is complete and the bacterium is around double its size what does the bacterial cell begin to do?
bacterial cell begins to constrict along the midpoint of the cell
what is the constriction of the cell accompanied by?
the synthesis of new cell membrane and wall - leads to complete division of the two identical daughter cells
what does the regulated process in eukaryotes refereed to as ?
mitosis
what does division in eukaryotic cells allow for?
unicellular fertilized egg to develop into a complex multicellular organism
what do early embryos contain?
stem cells
what are stem cells?
unspecialized cells that can both reproduce indefinitely and under appropriate conditions
what are stem cells able to do?
able to differentiate into specialized cells of one of one or more types
what can cell division lead to after an organism is fully grown?
lead to continual renewal and repair of cells that make up various tissues
are there adult stem cells?
yes
what are adult stem cells not able to do?
not able to give rise to all cell types in the organism
what are adult stem cells able to do?
replace non-responding specialized cells
explain the property of the adult skeletal muscle
stable tissue with little cell turnover
what happens when muscle cells undergo injury
quiescent (non-dividing) satellite stem cells that are present in the basement membrane of the muscle tissue are able to become activated and begin diving again to enable muscle regeneration
what does the activation of the satellite cells lead to?
proliferation, differentiation, and fusion of muscle precursor cells
what are the muscle precursor cells called
myoblasts
what to the my blasts eventually become committed to forming?
the mature muscle cells that make up the muscle fibers (myofibers)
what happens when the myofibers are formed?
no longer able to divide
what is one of the main distinctions between prokaryotic and eukaryotic cell division?
- eukaryotic DNA is larger
- organized into linear chromosomes
- highly condensed into the nucleus of the cell
what does the process of cell division in eukaryotes require? why?
- more regulated control
- larger cell cycle
how many distinct stages does the eukaryotic cell cycle consist of?
two
what is one of the stages?
interphase
what does this stage consist of?
- S phase
- 2 gap growth phases G1 and G2
- M phase
what occurs in the S phase?
DNA synthesis
what occurs in the M phase?
mitosis and cytokinesis
what must occur with each mitotic cell division?
the linear chromosome of eukaryotes must be replicated and then separated into daughter cells
what occurs in during the interphase stage?
cells prepare for cell division
how do cells prepare for cell division?
- replication of DNA in nucleus
- overall increase in cell size
where does replication of DNA occur?
in the S (synthesis) phase
what do the G1 and G2 phases prepare the cell for?
DNA synthesis and mitosis
how long does it take for specific cells to pass through the cell cycle?
depends on the type of cells in question
do all cells participate in regular cell cycle that leads to regular divisions?
no
can cells pause in the cycle? if so, in what phase?
yes
G0 phase
what happens with cells in the G0 phase?
they pause somewhere between the M and S phase
how long is the pause?
wide range - can be short or long (days - years)
what are considered non-dividing cells?
cells that enter the G0 phase permanently
what type of cells enter a permanent G0 phase?
- cells that make up the lenses of our eyes
- nerve cells
- mature muscle cells
can stem cells reproduce indefinitely?
yes
do stem cells have periods of quiescence and thus undergo no cell division?
yes
is it true that skeletal muscles have little to no cell division?
yes
what happens when an injury occurs?
the quiescent satellite stem cells are activated from the dormant G0 phase of the cell cycle and reenter the cell cycle
with the cells reentering the cell cycle what occurs as a result?
enables proliferation, differentiation and maturation of new muscle cell precursors that can fuse and repair the muscle tissue with new muscle fibres
what happens once the myofibers are formed?
they exit the cell cycle and enter the quiescent G0 phase
how many stages does mitosis consist of?
five
how can these stages characterized?
can be morphometrically characterized based on the distinctive changes that occur to the chromosomes that are involved in the cell division process
what did Walther Flemming discover?
distinct stages of mitosis could be staged based on chromosomal position and features
due to Flemmings work on salamander embryos, we know the five stages of mitosis, what are they?
- prophase
- prometaphase
- metaphase
anaphase
telophase
what must occur before entering mitosis?
chromosomes of cells must be duplicated and condensed
why must the chromosomes of cells be duplicated and condensed before entering mitosis?
to allow for the daughter cells to acquire the same amount of genetic information as the parent cell in a relatively short period of time
what form is each chromosome in during most of interphase?
a long, thin chromatin fiber
what occurs prior to mitosis?
exact copies of every chromosome are created
what phase are the copies of the exact chromosome created in? through what process?
- S phase
- process of DNA replication
where are DNA sequences replicated from?
end to end of the DNA molecule
what are the newly synthesized molecules associated with?
histones and other chromosomal proteins that allow for tight compaction
is the centromere fully replicated?
yes
why do the paired centromeres appear fused together?
because they’re so highly compact
when chromosomes are duplicated into two identical copies what are they referred to as?
sister chromatids
what allows us to have 23 distinct chromosome pairs?
since we inherit a paternal and maternal chromosome
how many homologous chromosomes do we have?
22 (one maternal and one paternal in origin)
how many are sex chromosomes?
1
what happens as a cell transitions from G2 to the M-phase?
duplicated chromosomes begin to condense and the individual chromosomes becomes visible even with a light microscope
what is the first stage of mitosis?
prophase
during prophase how will each chromosome appear?
as identical sister chromatids that are joined at their centromeres
what are centromeres?
duplicated cellular microtubule organizing centres
what do centromeres do in prophase?
radiate long microtubules forming a mitotic spindle
where do centromeres become positioned?
at opposite poles of the cell
what are the mitotic spindles crucial for?
separating the chromosomes into two daughter cells
summarize prophase
chromosomes condensed - centrosomes radiate microtubules and migrate to opposite poles
what follows prophase in mitosis?
prometaphase
what is the defining feature of pro metaphase?
fragmentation of he nuclear envelope
what are kinetochores?
specialized protein structures that associate with each one of the two sister chromatids on either side of the centromere
what can occur because the nuclear envelope breaks down?
the microtubules that are extending from each centrosome as part of the mitotic spindle are able to attach to specialized region on the centromeres o the chromosome (referred to as kinetochores)
is it true that some microtubules that radiate from the centrosome attach directly to the kinetochore regions?
yes
what are the kinetochores essential for?
essential to help pull the chromosomes to the poles of the cell
what are other microtubules that also radiate from the centrosome as part of the mitotic spindle?
polar microtubules
what do polar microtubules do?
interact with each other and help push the poles of the cell away from each other
summarize prometaphase?
microtubules of each mitotic spindle attach to chromosomes
- nuclear envelope starts to break down
what is the third stage of mitosis?
metaphase
what is metaphase marked by?
the alignment of chromosomes at the centre of the cell
when the chromosomes are aligned at the centre of the cell what is this region identified as?
the metaphase plate
what facilities the alignment at the metaphase plate?
the kinetochore microtubules of each chromosome are attached at the kinetochores of each sister chromatid
what follows metaphase?
anaphase
what happens to the kinetochore microtubules during anaphase?
begin to shorten
what happens when the kinetochore microtubules begin to shorten?
the sister chromatids separate into individual chromosomes that are pulled towards the opposite spindle poles of the cell
what do the polar microtubules do during anaphase?
they push against each other and help elongate the cell
what is present at the end of anaphase?
the two ends of the cell will have equivalent and complete sets of chromosomes
summarize anaphase
sister chromatids (which become individual chromosomes when the centromere splits) separate and travel to opposite poles
what is the final stage of mitosis?
telophase
what is telophase the stage of?
two new daughter nuclei form in the cell
why does this occur?
because the nuclear envelope reforms around the chromosomes at the opposite poles of the dividing cell
what happens to the chromosomes and spindle microtubules during telophase?
- chromosomes begin to decondense
- spindle microtubules are depolymerized/broken down
what marks the end of mitosis?
the division of one nucleus into two genetically identical nuclei
what must follow the process of mitosis?
the division of the cell into two identical cells
summarize telophase
nuclear envelope re-forms and chromosomes decondense
what does the process of cytokinesis do?
division of cytoplasm and therefore the cell
how does cytokinesis begin in animal cells?
with the formation of a contractile ring made up of motor proteins that contract bundles of actin fibers along the midline of the cell
what does this lead to?
formation of a defined cleavage furrow
what does a cleavage furrow do?
separates the cell into two distinct and seperate daughter cells
are the stages of mitosis similar across all eukaryotic cell types?
yes
where can differences in cytokinesis be observed?
depends on the dividing cell types
is the process of cytokinesis distinct and different in plant and animal cells?
yes
why is there a difference in cytogeneses between plant and animal cells?
plant cells have a cell wall
what happens during cytokinesis of plant cells?
plant cells lay down a newly developed cell wall along a cell plate region in the middle of the diving cell
when is cytogeneses complete in plant cells?
once the forming ell wall fuses with the original cell wall
when is cell division important?
during developmental growth and with regards to maintenance and repair
what did research in the 1970s begin to shed light on?
that there could be a mitosis promoting factor
what would this mitosis promoting factor allow?
the transition from the G2 to M phase of the cell cycle
what was studied in the 1980s?
protein level changes of dividing sea urchin embryos
what did this research team do?
- added radioactively labelled amino acids to the sea urchin eggs
- research team was sure the radio labelled methionine would be incorporated into any newly synthesized proteins in the embryos
- good way to measure protein changes in developing embryos
when and how did the team observe changes?
- took samples of rapidly dividing embryos every 10 minutes
- visualized any changes in protein levels using gel electrophoresis (allows for distinct separation of different protein types)
what did Hunt and his research team discover?
most protein bands on the gel became darker as cell division and embryonic development progressed
what happened with one protein band?
oscillated in intensity
what was found about the oscillating protein?
protein increased then decreased with each subsequent cell division
what was this protein called?
cyclin (due to its cyclic nature)
what did Hunt and researchers suspect this protein was involved in?
playing some sort of regulatory role on cell cycle progression
what did follow up work by Hunt and his colleagues identify?
that the mitosis promoting factor consists of a cyclin protein and a cyclin-dependent kinase (CDK) protein - together they control progression of the cell cycle
what is kinases?
enzymes that activate or inactive other proteins
how do kinases activate or inactivate other proteins?
by phosphorylating key amino acids on the target proteins
is it true that many kinases that regulate the cell cycle stay at a constant convention in the cell?
yes
for much of the time are kinase active or inactive?
inactive
how do kinase become active?
activated by binding to cyclin proteins
how did kinase acquire the name cyclin-dependent kinases?
because the activity of the kinases is dependent on being bound to cyclins
what can the cyclin-cyclin dependent kinase complex trigger? how is this done specifically?
- the multitude of changes that occur during the various cell cycle events
- by phosphorylation of target proteins that promote cell division
what is the activity of the cyclin0dependent kinase?
it rises and falls with changes in the concentration of its activating cyclin protein
is it true that there are many different types of cyclin-CDK complexes that are involved in the regulation of each stage of the cell cycle?
yes
when is cyclin-CDK regulation important?
during three steps of the eukaryotic cell cycle
what is the G1/S cyclin-CDK complex needed for?
the transition from the G1 to S phase and helps to prepare the cell for DNA replication (i.e. increasing the expression of histone proteins)
what does the S-cyclin-CDK complex help with?
to initiate DNA synthesis
what does the M cyclin-CDK complex initiate?
the process of mitosis
what other key factors play an important role in the regulation of the cell cycle?
presence of multiple check points
what do cell-cycle check points serve as?
form of cellular surveillance
what are cell-cycle check points able to do?
block cyclin-CDK activity should something go wrong during the progression of the cell cycle
what can cell-cycle check points do?
pause cell division
how long can cell-cycle check points pause cell division?
until the preparation for the next stage of the cell cycle is complete
what else can a cell-cycle check point serve as ?
opportunity for damage to be repaired
what are the three major check point of the cell cycle?
- DNA damage checkpoint at the end of G1 phase
- DNA replication checkpoint at the end of the G2 phase
- spindle assembly checkpoint before anaphase during mitosis
due to cellular monitoring only what kind of DNA will be able to enter the S phase or replication due to the G1 checkpoint?
undamaged DNA
due to G2 checkpoint when can a cell enter the mitosis phase?
only when all DNA is replicated
due to the M phase checkpoint when will a cell complete mitosis?
if all chromosomes are attached to a microtubule from the mitotic spindle
looking at an example of the DNA damage checkpoint, are the genes that normally inhibit cell cycle progression turned on or off?
off
what is p53?
protein that can inhibit the cell cycle when turned on
what happens when damage occurs to the structure of DNA (damage that includes double-stranded breaks in the phosphodiester backbone)
specific protein kinases are able to phosphorylate p53
p53 protein is normally present in very high or very low levels in the nucleus? what is most of this protein doing?
- very low levels
- most is being exported out of the nucleus degraded
what happens during phosphorylation?
p53 is able to accumulate within the nucleus and acts as a transcription factor to turn on genes that will inhibit the cell cycle
what does this lead to?
production of a CDK inhibitor protein
what does the inhibitor protein do? what does this give the cell?
- bind to and block the activity of the G1-S cyclin-CDK complex and thus stop/pause the cell cycle in the G1 phase
- gives cell the opportunity to repair the damaged DNA
spindle assembly example, as early as the pro metaphase stage of mitosis, regulatory proteins that are associated with the spindle assembly checkpoint are able to monitor what?
the degree to which the sister chromatids are attached to the microtubules of the mitotic spindle at their kinetochore regions
what type of signal do unattached kinetochores create?
“wait” signal
what does the “wait” signal lead to?
the recruitment of spindle-assembly check point proteins
what are these proteins activated by?
lack of tension in the centromere area
when is progression of metaphase and entry into anaphase allowed?
when each sister chromatid is attached to a kinetochore microtubule
what happens when this occurs?
spindle checkpoint proteins are removed from the centromere region and separase, a specialized enzyme, is able to break sister chromatid attachments
