Cell Cycle, Division, and Replication Flashcards
what is the basic function of the cell cycle?
to duplicate the DNA in the chromosomes
segregate the DNA into genetically identical daughter cells
what does each genetically identical daughter cell receive?
a complete copy of the entire genome
what are the three general parts of the cell cycle?
- cell growth and chromosome duplication
- chromosome segregation
- cell division
what does the duration of the cell cycle depend on?
- the availability of energy sources
- the way the cell is differentiated
- its surroundings
- whether the cell passed the internal tests for readiness to divide (checkpoints)
what happens during cell division of frog embryos?
embryos divide in synchrony even though they aren’t connected
what are the four phases of the eukaryotic cell cycle?
M phase (mitosis/cytokinesis)
G1 phase (growth)
S phase (synthesis)
G2 phase (checks completion)
what is M phase of the cell cycle?
consists of mitosis (nuclear division) and cytokinesis (cytoplasmic division)
the most intense, dynamic phase of the cell cycle
what is the G1 phase of the cell cycle?
growth phase
cells recover and repair
what occurs during the S phase of the cell cycle?
synthesis
duplication of genome through DNA replication
what occurs during the G2 phase of the cell cycle?
checks completion of divided cells
full replication of chromosomes
which phase of the cell cycle is the most important?
G1 phase
which phase of the cell cycle is the most tightly controlled and demanding?
S phase
when does S phase occur?
between the gap phases (G1 and G2)
what is interphase?
a combination of G1, S, and G2 phase (the entire cell cycle minus M phase)
which phase of the cell cycle is the shortest?
M phase
why is M phase the shortest?
because the cell tends to minimize the time it spends with completely inactive (condensed) genome without the ability to transcribe it
how are chromosomes represented before replication?
each chromosome has a homologous pair (each chromosome has 2 homologous chromosomes)
diploid
2n
what does n represent in chromosome karyotype?
the number of homologous chromosomes
what happens to chromosomes in each cell after S phase?
they become tetraploid (4n)
which phase is the most crucial decision-making point in the cell cycle?
G phase
which phase of the cell cycle is the longest?
G1 phase
what is the duration of cell cycle stages proportional to?
the amount of cells at certain stage in unsynchronized population
what is the order of cell cycle phases by their duration from longest to shortest?
G1, G2, S, M
where would S phase or M phase be located in the following graph?
in the dip between A and B
what kind of cells would be found at site A on the graph?
diploid
what kind of cells would be found at site B on the graph?
Tetraploid
what phase are the majority of cells found in?
G1 phase
where were the genes responsible for the cell cycle control first discovered?
in yeast
what role does cyclin and cyclin-dependent protein kinases (Cdks) play in the cell cycle control system?
they appear and disappear in a cyclic manner
Cdks play a major role in moving cells between stages
cyclin needs the Cdks to activate it
what happens to M cyclin concentration during the cell cycle?
it goes up and down depending on the stage but is always present
how was the cell cycle control system in mammals deciphered?
using frog eggs as a model system
what were the results of the experiment using the frog egg model system to decipher the cell cycle control system?
extracts taken from the frog egg at different stages can trigger or stop normal cell cycle add in trans
what happened to the frog egg when cytoplasm from M-phase was injected into the cell?
spindle easily detected
oocyte was driven into M phase
what happened when cytoplasm from interphase was injected into the frog egg cell?
blocks cell division
oocyte does not enter M phase
what does each phase have to help it through the cycle?
each phase has its own specific cyclin and Cdk (cyclin-Cdk protein complex) that regulate a different set of target proteins
when does the S cyclin begin to rise?
from the middle of G1 phase through S phase to end of G2
when does M cyclin begin to rise/end?
rises in middle of G2 phase to end of M phase
why are cyclins and Cdks important?
Cyclins drive the events of the cell cycle by partnering with a family of enzymes called the cyclin-dependent kinases (Cdks)
what heavily influences the formation of cyclin-Cdk complexes?
environmental conditions
extracellular signals that permit cells to divide.
what triggers the degradation of the previous Cdk?
transition to the next stage
what causes the destruction of cyclins?
proteasomal pathways
ubiquitylation
what does the destruction of cyclins do?
drives transition from one phase to the next
what does the Anaphase-promoting complex (APC) do?
marks M-cyclin for degradation and takes the cell out of mitosis
destroys securin and triggers segregation during anaphase
how is a cyclin degraded?
a ubiquitin chain attaches to the cyclin, separating it from the active Cdk.
This causes the cyclin to degrade and the Cdk to become inactive
how do inhibitory kinases work? What is happening in the following pathway?
Cdks protein concentrations in the cells increase gradually.
Cdk comes in inactive, phosphorylated form to trigger the abrupt activation of Cdk activity.
In this case, M-cyclin is phosphorylated by inhibitory kinase Wee1.
The removal of phosphate by phosphatase Cdc 25 activates the complex.
what happens to M-Cdk while it is phosphorylated?
it is inactive
how is M-Cdk activated?
when phosphatase Cdc 25 removes the inhibitory phosphates from it
what is the purpose of inhibitory subunits?
The activity of the complex can be transiently delayed (paused) allowing the cells to make a decision whether or not to move to the next phase, for example, from G1 to S.
how do inhibitory subunits delay the activity of a cyclin-Cdk complex?
an inhibitory subunit, like p27, binds to the active cyclin-Cdk complex, inactivating it
how does a cell exit each stage of the cell cycle?
cell only exits a phase if everything is set up properly
what happens if a cell has errors?
it gets frozen in M phase until everything is corrected
if corrections cannot be made/fixed apoptosis occurs
what happens if DNA replication is incomplete or DNA is damaged during transition from G2 phase to M phase?
inhibition of activating phosphatase cdc25 blocks entry of cell to Mitosis
what happens if chromosomes are not properly attached to spindles at the end of M phase?
inhibition of APC/C activation delays exit from mitosis
nondisjunction
what happens if the environment is not favorable during transition from G1 phase to S phase?
Cdk inhibitors block entry of cell to S phase
why is G1 phase the most important decision making point in the cell cycle?
Staying G1 in phase allows the cell to monitor the environment for stimulatory factors and to make a decision whether to proceed to S phase, or to pause, or to enter which could be permanent (either senescent or terminally differentiated-neuronal or aging cells) or temporary non-dividing (quiescent- hepatocytes) state (called G0)
what happens if you remove senescent/terminally differentiated cells? how was this determined?
you will live longer
studied in mice
what triggers cell division in mammals?
external protein activators called mitogens
what would happen if a cell is deprived of mitogens?
the cell would remain in G1 phase
what are mitogens?
they are transcription regulators that trigger cell division
what do many transcription regulators also act as?
tumor suppressors
why are transcription regulators able to function as tumor suppressors as well?
they are absent in various tumors
tumor cells tend to divide uncontrollably and devoid of those factors through the selection process.
what is occuring in the following pathway?
Activated G1 Cdk and G1/S complex phosphorylates RB protein and inactivates it and allows for cell to continue to S phase and transcribe
what is p53?
a major tumor suppressor protein that helps cells sense the DNA damage and temporarily halt the cell cycle at G1 until the DNA repair system is engaged
what happens if the cell is unable to repair DNA?
it triggers p53-dependent apoptosis
when is p53 stable?
p53 is stable once it is phosphorylated
what happens to p53 in the absence of DNA damage?
it is degraded in proteasomes
what happens when DNA is damaged?
protein kinases phosphorylate p53
p53 is stabilized and activated
active p53 binds to regulatory region of p21 gene
p21 gene is transcribed and translated
p21 Cdk inhibitor protein binds and inactivates G1/S-Cdk and S-Cdk complex
what controls S phase?
S-Cdk complex
what does the S-Cdk complex do during S phase?
it initiates replication and blocks the repetition of replication
what is necessary for the prereplication complex to initiate replication at ORI?
it must be activated by S-Cdk
what does S-Cdk do to helicase?
S-Cdk phosphorylates helicase and triggers the assembly of functional replisome
what happens if replication is incomplete at the end of S phase?
the cell can be halted at G2 phase
what does the presence of active replication during S phase do?
it prevents Cdc25 from activation, which is necessary to dephosphorylate and thus inactivating M-Cdk
what is the prereplication complex (preRC)?
the complex made up of DNA helicase and the ORC that initiates replication at the ORI
it holds together until it is phosphorylated by the S-Cdk complex
what happens to Cdks at the beginning of G1?
they are all inhibited
how are all Cdks inhibited at the beginning of G1?
While anaphase-promoting complex (APC) marks M-cyclin for degradation, it is also activated by M-Cdk at the same time.
The lack of activator and the gene expression depletes the cell from eventually all Cdks completely at the end of mitosis.
what causes the rise of Cdks?
rise of Cdks starts during G1 with accumulation of G1 cyclins that triggers the transition to S phase
how does the S-Cdk complex control S phase?
- it phosphorylates the prereplication complex, initiating replication
- it phosphorylates helicase, triggering the assembly of functional replisome
it blocks the repetition of replication
what does M-Cdk do in regards to mitosis?
it drives entry into M phase and Mitosis
how does M-Cdk work?
activated M-Cdk self-regulate (amplifies) its own activity by activating Cdc25 (positive feedback) and shutting down Wee1
what are the stages of M phase (mitosis)?
- Prophase
- Prometaphase
- metaphase
- mitosis
- anaphase
- telophase
- cytokinesis
what are the stages of interphase?
- G1
- S
- G2
what happens during interphase?
- centrosomes are duplicated
- tubulins and other proteins are synthesized
- DNA is replicated but is still in decondensed state
- Before replication, each chromosome is diploid (has a homologous pair)
- after S phase, each cell becomes tetraploid
what is the result of mitosis in most eukaryotes?
2n ⇢ 4n ⇢ 2n+2n
what is the result of mitosis in some eukaryotes, like haploid yeast)?
n ⇢ 2n ⇢ n+n
what is the result of mitosis in some eukaryotes like plants?
2Xn ⇢4Xn ⇢ 2Xn + 2Xn
what is the result of binary division (prokaryotes)?
n ⇢ 2n ⇢ n + n
what happens during prophase?
duplicated chromosomes (each with 2 sister chromatids) condense
mitotic spindle assembles outside nucleus between two centrosomes
what helps configure duplicated chromosomes for segregation?
cohesins and condensins
what are cohesins?
they tie together two adjacent sister chromatids, preventing them from breaking apart
what are condensins?
they help to coil and thus condense each sister chromatid
where else besides mitosis is cohesion and condensation important?
meiosis - generation of gametes
what happens when there are defects in the cohesion of sister chromatids?
defects in sister chromatids cohesion leads to their missegregation and results in abnormal number of chromosomes (aneuploidy)
what activates cohesins and condensins?
M-Cdk
what form the mitotic spindle?
microtubules
why is the mitotic spindle important?
it is crucial for the separation of sister chromatids and cytokinesis
what cytoskeletal structures mediate M phase in animal cells?
mitotic spindle
contractile ring
how do microtubules separate sister chromatids?
microtubules get attached to centromeres of sister chromatids through the protein complex called kinetochore and separate them at the beginning of anaphase
what structure is responsible for the physical separation of cells?
contractile ring
what is the structure of mictotubules?
microtubules undergo constant polymerization and depolymerization by the addition and removal of tubulin alpha/beta tubulin heterodimers at the plus end
with minus end attached to centrosome
why do microtubules only grow and shrink from the plus end?
the minus end normally remains intact and attached to the microtubule-organizing center (centrosome)
what is the plus end of a microtubule?
beta tubulin end
what is the minus end of a microtubule?
alpha tubulin end
how do microtubules grow?
they grow at their plus ends from γ-tubulin ring complexes of the centrosome
what is the structure of centrosomes?
a pair of centrioles in the center
centrosome matrix surrounding the centrioles
nucleating sites ( γ-tubulin ring complexes) on the surface of the centrosome matrix where microtubules attach
what is dynamic instability?
the ability of microtubules to grow and shrink
responsible for chromosome segregation and cell polarization onto two daughter cells during the cell division
driving force behind the cell division
when are microtubules stable?
stabilization of plus end by plasma membrane-capping protein stabilizes the cell shape during cell division
in which phase does the mitotic spindle start to assemble?
prophase
how does the mitotic spindle start to assemble in prophase?
centrosomes duplicate during the interphase, and during prophase they start outgrowing microtubules, forming mitotic spindle
interpolar microtubules are perpendicular to the equator and their associated (+) ends push centrosomes further apart
other microtubules become astral microtubules
what does shrinkage and growth of microtubules allow?
it allows them to find other microtubules and connect with them
what is prometaphase?
prometaphase starts abruptly with the breakdown of the nuclear envelope
chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement
what does phosphorylation of the nuclear envelope during prometaphase do?
promotes the break down of nuclear envelope onto separate lamellas and vesicles
when do chromosomes attach to the mitotic spindle?
at prometaphase
what is a kinetochore?
a protein complex that recognizes the special DNA sequence present at the centromere
attached to kinetochore microtubules
how many microtubules does each human kinetochore bind?
20-40
how many microtubules does each yeast chromatid bind?
one
what makes up a fully formed mitotic spindle?
kinetochore microtubules
astral microtubules
interpolar microtubules
what are astral microtubules?
microtubules that connect between centrosome and cytosolic part of mitotic spindle
what are interpolar microtubules?
microtubules that interact with each other in the mitotic spindle
what are kinetochore microtubules?
microtubules that attach to the kinetochore of sister chromatids
centrosome duplicates during interphase forming two centrosomes and thus two poles of mitotic spindle
as mitosis begins, two centrosomes separate, each nucleating a radial array of microtubules called aster. this separation is driven by interpolar microtubules
mitotic spindle starts to assemble in prophase
what drives the separation of centrosomes during mitosis?
interpolar microtubules
what is the centrosome cycle?
the process of centrosome duplication and separation
what happens during metaphase?
chromosomes are aligned at the equator of the spindle, midway between the spindle poles
the kinetochore microtubules on each sister chromatid attach to opposite poles of the spindle
how is the metaphase plate formed?
duplicated sister chromatids align half way between the two spindle poles, forming metaphase plate
why is the alignment of sister chromatids important?
the arrangement is highly important for synchronous separation and equal distribution of sister chromatids between daughter cell, but its mechanism is still poorly understood
what happens during anaphase?
the sister chromatids synchronously separate and are pulled slowly toward the spindle pole to which they are attached
the kinetochore microtubules get shorter, and the spindle poles also move apart, both contributing to chromosomes segregation
what is the fastest stage in mitosis?
anaphase
how does anaphase begin?
abruptly with the breakage of cohesin linkage that holds sister chromatids together
what destroys the cohesion linkage to begin anaphase?
a protease called separase
what prevents separase from destroying the cohesion linkage and beginning anaphase?
the protease is held in inactive state by an inhibitory protein called securin
how does separase destroy cohesin?
in order to destroy cohesion and start anaphase, securin has to be removed from separase
what does the APC complex do?
marks M cyclin for degradation
triggers separation of chromatids by promoting the destruction of cohesions through the cascade of reactions, destroying securin and beginning anaphase
how does the APC complex degrade and destroy securin to begin anaphase?
inhibitory securin binds to inactive separase
active APC/C removes securin from separase
this causes ubiquitylation and degradation of securin and activation of separase
active separase cleaves and dissociates cohesins all at once
anaphase begins
how are daughter chromatids pulled apart during anaphase?
daughter chromosomes are pulled toward opposite poles as kinetochore microtubules depolymerase
how fast do sister chromatids move?
with a speed of about 1 mm per minute
what creates the driving force to separate sister chromatids?
disassembly of microtubules and motor proteins, kinesins and dyneins
kinesins and dyneins causing sliding
what are anaphase A and anaphase B?
they are two anaphase processes that occur simultaneously and involve kinetochore on one hand and interpolar and aster microtubules on the other
what happens during Anaphase A?
chromosomes** are pulled **poleward
kinetochore microtubules shorten, dragging chromosomes toward their spindle pole
what happens during anaphase B?
poles are pushed and pulled apart
a sliding force between interpolar microtubules from opposite poles (1) pushes the poles apart; a pulling force at the cell cortex (2) drags the two poles apart
how do kinesins work?
they move cargo from ‘minus’ end to the ‘plus’ end, therefore, from the cell center toward the periphery
how do dyneins work?
they move cargo from the ‘plus’ end to the ‘minus’ end and thus in the opposite direction as kinesins
what kind of transport do kinesins provide?
anterograde
(movement of molecules outward, away from cell body towards the axon terminal)
pictured in red
what kind of transport do dyneins provide?
retrograde
(movement of molecules backwards, toward the cell body away from the axon terminal)
pictured in blue
what happens if the tail domain of motor proteins is attached to the immobile substrate (surface)?
soluble microtubules can be moved back and forth along the substrate
what does dynein cause in isolated doublet microtubules?
dynein produces microtubule sliding
what do kinesins do during anaphase B?
kinesins “walk” from the ‘minus’ to the ‘plus’ end, it provides sliding of overlapping interpolar microtubules
(process #1 - pushes poles apart from pulling force at cell cortex)
what do dyneins do during anaphase b?
dyneins are immobilized by their tails at the cell cortex that underlies the plasma membrane
they “walk” from the ‘plus’ to the ‘minus’ end on astral microtubules, bringing centrosomes closer to the plasma membrane and pulling the poles apart
(process #2 - drags poles apart)
what helps push the poles apart?
microtubule growth at plus ends of interpolar microtubules
what are crucial for the functioning of mitotic spindle?
motor proteins
what happens during telophase?
the two sets of chromosomes arrive at the poles of the spindle
a new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis
the division of the cytoplasm begins with the assembly of the contractile ring
when does cytokinesis start?
at the end of telophase
during telophase…
- the spindle disassembles
- the new nuclear envelope is formed from the nuclear membrane vesicles
once the envelope is formed, chromosomes decondense
nuclear envelope reappears and mitotic spindle disassembles
what happens during cytokinesis?
the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches the cell into two daughters, each with one nucleus
what begins cytokinesis?
a cleavage furrow initiated by overlapping interpolar microtubules during anaphase
cytokinesis and mitosis overlap in time
when does cytokinesis end?
when telophase is over and the formation of the contractile ring is completed
when do cytokinesis and mitosis overlap?
during anaphase to the end of telophase
what is the cytokinesis cleavage furrow made of?
actin and myosin filaments
cytokinesis is a ___ process regardless of the presence of the spindle
irreversible
what happens to cells during cytokinesis?
cells normally round up and loose their contacts with the surrounding cells
what happens to cells once cytokinesis is completed?
the cell changes shape and expresses attachment proteins integrins that allow them to reestablish their contacts with the substrate (neighboring cells and extracellular matrix), so that the new cells can be accommodated within the tissue
what happens if clumping occurs during cytokinesis?
tumors forms
what guides cytokinesis in plant cells?
a specialized microtubule-based structure called phragmoplast
why is cytokinesis in higher plants different from cytokinesis in animal cells?
because the plant cells are surrounded by the rigid cell wall and cannot form cleavage furrows
how does cytokinesis occur in plants because the cleavage furrow cannot form?
the remnants of interpolar microtubules arrange membrane, polysaccharides- and glycoproteins-filled vesicles from Golgi in the middle of dividing cells at the beginning of telophase, forming phragmoplast and allowing accumulated vesicles to fuse later with the formation of two membranes
what is sexual reproduction?
a development of a new organism from a fusion of two cells that come from two organisms, paternal and maternal
sexual reproduction = meiosis + fertilization
what is fertilization?
the fusion of parental cells
a restoration of diploid set of chromosomes through the fusion between an egg and a sperm, resulting in the formation of a fertilized egg (zygote) capable of diving and developing into embryo and further into adult organism.
what is asexual reproduction?
reproduction that does not involve a sexual partner
what can asexual reproduction cause?
in many instances it creates substantial genetic variability by shuffling the organism’s own genes through the process of homologous recombination that occurs during meiosis
what is meiosis?
a specialized cell division that reduces the number of chromosomes
only happens in sexual organs of multicellular organisms
what is homologous recombination?
a type of genetic recombination that occurs during meiosis where paired chromosomes from the parents align so that similar DNA sequences from the paired chromosomes cross over each other
what are somatic cells?
cells that are normally diploid (2n), each chromosome has a homologous pair that came form either father or mother
what are gametes?
(eggs and sperm)
haploid cells (n) that are produced in specialized sex organs (gonads), ovaries, and testes
they have only one out of two homologous chromosomes present
what are zygotes?
fertilized eggs
what happens to the zygote after the first division?
it becomes an embryo that further develops into adult organism
how do somatic cells and gametes/organs and their systems develop?
from a single cell, a fertilized egg with restored diploid number of chromosomes
what are the gametes produced by males and females?
sperms and eggs
what are eggs in sexual reproduction?
normally carry lots of nutrients necessary for the development of an embryo upon fertilization
what is a sperm cell (spermatozoon) in sexual reproduction?
small and highly mobile cells
what provides mobility to animal sperm cells?
flagellum
what kind of mobility does nematode spermatozoa have?
amoeba-like crawling cells
what kind of sexual reproduction do hermaphroditic nematode C. elegance utilize?
self-fertilization
is self - fertilization still considered a form of sexual reproduction?
yes because it involves haploid gametes and fertilization
what are sequential hermaphrodites?
some fish species change sex and the types of gametes produced depending on their age
younger age (female) - produce eggs
older age (male) - produce sperm
who discovered meiosis?
Theodore Boveri
what is the difference between meiosis and mitosis?
meiosis involves one round of DNA replication similar to mitosis but, unlike mitosis, two rounds of cell division
where does meiosis begin in humans and other multicellular eukaryotes?
meiosis begins in specialized diploid germ-line cells that resides in female and male gonads, ovaries (produce eggs) and testes (produce sperm)
what happens in yeast?
the whole organism undergoes meiosis triggered by starvation
what happens to chromosomes in meiosis that doesn’t happen in mitosis?
after duplication, duplicated paternal chromosomes align along duplicated maternal homologs (process called pairing)
what does pairing during meiotic mitosis form?
bivalent
what is a bivalent?
formed by pairing
a structure formed by the sticking together of sister chromatids at the beginning of the first meiotic division (meiosis I)
what happens as a result of pairing followed by the first cell division (meiosis I)?
each daughter cell receives either paternal or maternal duplicated chromosomes, and for each chromosome the choice
how many rounds of division occur in meiosis?
two
what does the independent and random distribution of paternal chromosomes among gametes result in?
2n different gametes, where n is a number of chromosomes (for humans, 223 = 8,388,608)
what is the difference between meiosis and mitosis in terms of duration?
mitosis and meiosis II are usually accomplished within hours
germ-line cells may stay in meiosis I for days, months or even years because of the long time they spend in prophase I
overview of mitosis vs. meiosis
meiosis
- has 1 round of DNA replication
- produces four haploid nuclei
- begins in diploid germ-line cells from gonads, ovaries, and testes
- pairing of duplicated paternal chromosomes align along duplicated maternal homologs
- forms bivalent
- daughter cells receive either paternal or maternal duplicated chromosomes
- independent and random distribution of paternal chromosomes (2n)
- meiosis I can last days, months, or years
- meiosis II is done in hours
mitosis
- has 2 rounds DNA replication
- produces two diploid nuclei
- accomplished in hours
what is crossing over? when does it occur?
non-sister chromatids in each bivalent swap segments during meiosis I
how are chiasmata formed?
they are formed when paternal and maternal chromatids undergo homologous recombination and form multiple chiasmata
what does the synaptonemal complex do?
it is a complex that helps stabilize and move chiasmata along DNA chains
how are genes shuffled?
through controlled double stranded breaks
what are chiasmata?
a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.
why is the crossover between non-sister chromatids in each bivalent important?
it introduces almost infinite genetic variability
major source of genetic variation in genetic reproducing species
what happens to the synaptonemal complex by the time prophase I ends?
the synaptonemal complex has disassembled allowing the homologs to separate along the most of either length
what do chiasmata do?
help to proper segregate duplicated homologs during the first mitotic division
before anaphase I, few remaining chiasmata resist the pulling, helping to position and stabilize all bivalents together at the metaphase I plate
at the centromeres, they still hold chromatids together
what stabilizes bivalents?
they are stabilized by cohesion proteins along their entire length that degrade all of a sudden by proteolysis to start anaphase I
what is the purpose of the second meiotic division?
the second meiotic division produces haploid daughter cells
it occurs without any further DNA replication and simply separates each pair of duplicated chromatids into separated gametes
are the haploid cells produced in meiosis identical?
NO
what are the two kinds of genetic assortment?
- independent and random distribution of paternal and maternal chromosomes between gametes
- exchange of segments between maternal and paternal chromosomes through homologous recombination during crossing over
what is oogenesis?
about ~20,000 primary oocytes are arrested at meiosis I at birth
meiosis in human oocytes is completed only if fertilization occurs
asymmetrical and intermittent
secondary oocyte is arrested in meiosis II
what is spermatogenesis?
starts in puberty
each cycle is completed from start to finish in 74 days
what are the two arrest phases in oogenesis?
primary oocyte is arrested in early meiosis 1 present at birth
secondary oocyte is arrested in meiosis 2
when is meiosis complete?
only if fertilization occurs
why are errors common in meiosis?
the frequency of chromosome missegregation during gametogenesis is quite high
in females, nondisjunction during meiosis occurs in 10% of human oocytes
what is nondisjunction in females oocytes responsible for?
in females, nondisjunction during meiosis occurs in 10% of human oocytes and is largely responsible for the high rate of miscarriages during early pregnancy
are errors caused by nondisjunction during meiosis always fatal?
no, in some cases, like chromosome 21, the abnormal embryos survive, and this results in Down Syndrome
what is nondisjunction?
the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.
what is zona pellucida?
a protective layer surrounding the egg
has to be digested by sperm proteases
what happens once the sperm digests through the zona pellucida?
happens fast
triggers a massive influx of Ca2+ into the cell, resulting in a massive exocytosis which lifts zona pellucida off the egg plasma membrane and hardens it
this prevents the entry of additional sperm
