Cell cycle (meiosis) Flashcards
contain two slightly different copies,
or homologs, of each chromosome, one from
each parent
diploids
carrying only a single copy of each
chromosome → gametes
haploid
sperm and egg fuse →
zygote
reduces the chromosome number by
half
Meiosis
duplication of chromosomes
S phase
_ successive rounds of chromosome
segregation
two
segregate the homologs
meiosis I
duplicated paternal and maternal
homologs pair up alongside each other
pair of homologs
no further DNA replication;
the sister chromatids pulled apart and
segregated
meiosis II
produces four haploid daughter cell
(either maternal or paternal copy)
meiosis II
homologs ____each other and
associate ____in order for the
maternal and paternal homologs to be
bi-oriented on the first meiotic spindle
recognize
physically
prolonged period; homologs begin
pairing (interactions between pairing
sites)
meiotic prophase (prophase I)
four-chromatid structure
bivalent
homolog pairs are then locked together
by
homologous recombination
the DNA of a chromatid
crosses over to become continuous with
the DNA of a homologous chromatid
crossovers
paired homologs are brought into close
______, with their structural axes (axial cores)
juxtaposition
assembles on
a double
-strand break in a chromatid,
binds the matching DNA sequence in
the nearby homolog and helps reel in
this partner
recombination complex
followed by synapsis
presynaptic alignment
axial core of a homolog becomes tightly linked to the axial core of its partner by a closely packed array of transverse filaments
synapsis
bridges the gap
synaptonemal complex
morphological changes that occur during homolog pairing are the basis
for dividing meiotic prophase into five sequential stages
leptotene, zygotene,
pachytene, diplotene, and diakinesis
homologs condense and pair and genetic recombination begins
leptotene
synaptonemal complex begins to assemble at sites where the homologs are
closely associated and recombination events are occurring
zygotene
assembly process is complete, and the homologs are synapsed along their
entire lengths
pachytene
disassembly of the synaptonemal complexes and the concomitant
condensation and shortening of the chromosomes
diplotene
segregation of homologs
diakinesis
individual crossover events between nonsister chromatids can be seen
as inter-homolog connections called
chiasmata
three features of meiosis I that
distinguish it from mitosis
1st: both sister kinetochores in a homolog
must attach stably to the same spindle
pole
2nd: crossovers generate a strong
physical linkage between homologs,
allowing their bi-orientation at the
equator of the spindle
3rd: cohesion is removed in anaphase I
only from chromosome arms and not
from the regions near the centromeres,
where the kinetochores are located
Cohesins near the centromeres are protected from separase in meiosis I by
a kinetochore-associated protein called
shugoshin
acts by recruiting a protein phosphatase that
removes phosphates from centromeric cohesins.
Shugoshin
functions of crossing-over in meiosis
- helps hold homologs together so that they
are properly segregated to the two
daughter nuclei produced by meiosis I - contributes to the genetic diversification
of the gametes that are eventually
produced
highly regulated
crossing-over
each pair of human homologs is linked
by about how many crossovers
two or three
DNA is accessible
hot spots
heterochromatin regions
(centromeres and telomeres)
cold spots
How mamy crossover forms between the
members of each homolog pair
at least one crossover f
the presence of
one crossover event inhibits another from
forming close by
crossover interference
mistakes can occur in allocating the chromosomes during
meiosis
Mistakes are especially common in human female meiosis, which arrests for years after ____: meiosis I is completed only at_____
diplotene
ovulation
chromosome
segregation errors during egg development are the most common cause of both
spontaneous abortion (miscarriage) and mental retardation in humans
when homologs fail to separate; some of the
resulting haploid gametes lack a particular chromosome, while
others have more than one copy of it
nondisjunction
caused by an extra copy of chromosome 21
Down syndrome
