CH 11 Sexual Reproduction and Meiosis
meiosis is characterized by four distinct features: 1. homologous pairing and _______ _____ joins maternal and paternal homologues during meiosis I 2. sister chromatids remain connected at the centromere and segregate ________ during anaphase I 3. kinetochores of sister chromatids are attached to the ______ pole(s) in meiosis I and to ________ pole(s) in mitosis 4. DNA replication is ________ between the two meiotic divisions
1. crossing over 2. together 3. same, opposite 4. suppressed
in prophase I, each chromosome is composed of __ sister chromatids
2
the zygote, like all of the cells ultimately derived from it, contains ___ copies of each chromosome
2
meiosis includes ___ round(s) of division
2 (meiosis 1 and 2)
gametes (eggs and sperm) each contain ___ chromosomes, but all of the no reproductive cells or somatic cells, of embryos and mature individuals each contain _____
2,4
The outstanding characteristic of sexual reproduction is that offspring inherit chromosomes from two parents ____ from mother (maternal homologue) ____from father (paternal homologue)
23,23
final result of meiosis is ____ cells containing _____ sets of chromosomes
4 haploid
bivalents align randomly along the metaphase plate possible number of different, random alignments=2^n where n = # of chromosomes per set humans: 23 chromosomes per set, 2^23 or over _________ possibilities
8 million
meiosis II is like a mitotic division without ______ __________
DNA replication
True or false: sister chromatids are identical after meiosis I
False
meiotic cells have an interphase period that is similar to mitosis with ___, ____ and ____ phases
G1, S, G2
following a brief interphase, with no S phase, meiosis ___ begins.
II
________ _: the pairs of homologous chromosomes align at the equator of the cell. Chiasmata keep homologous pairs together and microtubules from opposite poles attach to sister kinetochores of each homologue, producing tension. A kinetochore microtubule from one pole of the cell attaches to one homologue of a chromosome, while a kinetochore microtubule from the other cell pole attaches to the other homologue of a pair.
Metaphase I
________ ___: chromosomes consisting of sister chromatids joined at the centromere align along the metaphase plate in each cell. Now kinetochore microtubules from opposite poles attach to kinetochores of sister chromatid (as in mitosis)
Metaphase II
_______ _: the chromosomes begin to condense, and the spindle of microtubules begins to form. The DNA has been replicated and each chromosome consists of two sister chromatids attached at the centromere. The cell illustrated has four chromosomes, or two pairs of homologues. Homologous chromosomes pair along their entire length during synapsis. Crossing over occurs, forming chiasmata, which hold the homologous chromosomes together.
Prophase I
True or False: meiotic cells have an interphase period that is similar to mitosis with G1, S and G2 phases
True
in fruit fly (drosophila) males there is no recombination and yes meiosis proceeds accurately, a process called _____ _______ ("without chiasmata")
achiasmate segregation
_________ II: sister chromatids are separated from each other
anaphase
chiasmata are maintained until what phase
anaphase
________ _: microtubules of the spindle shorten homologues are separated from each other and move to opposite poles sister chromatids remain attached to each other at their centromeres
anaphase I
________ __: kinetochore microtubules shorten and homologous pairs are pulled apart. one duplicated homologue goes to one pole of the cell, and the other duplicated homologue goes to the other pole. sister chromatids do no separate. this is in contrast to mitosis, in which duplicated homologues line up individually on the metaphase plate and sister chromatids are pulled apart
anaphase I
during ________ __ homologues are pulled to opposite poles for each pair of chromosomes (**in contrast to mitosis, in which sister chromatids, not homologues, are pulled to opposite poles)
anaphase I
_______ ___: when microtubules shorten, sister chromatids are pulled to opposite poles of the cells (as in mitosis)
anaphase II
gametes with an improper number of chromosomes due to nondisjunction are called _______ gametes. in humans, this condition is the most common cause of spontaneous abortion
aneuploid
the association between the homologues persists throughout meiosis I and dictates the _______ of the chromosomes
behavior
________ - site of crossing over (contact maintained until anaphase I)
chiasmata
when crossing over is complete, the synaptonemal complex breaks down, and the homologous chromosomes become less tightly associated but remain attached by _______
chiasmata
the sites of crossing over are called _______, and these sites of contact are maintained until anaphase I
chiasmata (singular, chiasma)
crossing over allows the homologues to exchange ______ material
chromosomal
the process of crossing over (genetic recombination) literally allows the homologues to exchange ______ material
chromosomal
anaphase I comes about by the release of sister chromatid _________ along the chromosome arms, but not at the centromeres
cohesion
the four chromatids are held together in two ways: 1. the two sister chromatids of each homologue, the products of DNA replication, are held together by _____ proteins (sister chromatid cohesion) 2. exchange of material by crossing over between _______ locks all four chromatids together
cohesion homologues
While homologues are paired during prophase I, another process unique to meiosis occurs: genetic recombination or _______ ________
crossing over
genetic recombination between non-sister chromatids
crossing over
in most animals, _____ state dominates
diploid
somatic cells of adults have 2 sets of chromosomes
diploid
both of the somatic cells and the gamete-producing germ-line cells are _____, but whereas somatic cells undergo mitosis to form genetically identical, ______ daughter cells, gamete-producing germ-line cells undergo meiosis to produce _______ gametes
diploid diploid haploid
two sets of chromosomes are present in the somatic cells of adult individuals, making them ______ cells, but only one set is present in gametes, which are thus ______
diploid haploid
the key to understanding meiosis: the second meiotic division is like mitosis with no chromosome _______
duplication
the fusion of gametes to form a new cell is called ________, or syngamy
fertilization
After crossing over occurs and the synaptonemal complex breaks down, there are ____ chromatids for each type of chromosome (two homologous chromosomes, each of which consists of two sister chromatids)
four
sister chromatid cohesion also occurs in mitosis, but in meiosis the cohesion complex contains a meiosis-specific cohesion. This brings all ____ chromatids for each set of paired homologues into close association
four
After interphase, ______-______ cells enter meiosis I. In prophase I, the DNA coils tighter and individual chromosomes first become visible as threads. Because the DNA has already replicated before the onset of meiosis, each of these threads actually consists of two sister chromatids joined at their centromeres. In prophase I, homologous chromosomes become closely associated in ______, exchange segments by crossing over, and then separate.
germ-line synapsis
the cells that will eventually undergo meiosis to produce gametes are set aside from somatic cells early in the course of development
germ-line cells
gametes have only 1 set of chromosomes
haploid
eggs and sperm are both ______ and are fertilized to become a _____ zygote
haploid, diploid
________ pairing is specific to meiosis
homologous
meiosis I is characterized by the segregation of ______ not ______ during anaphase
homologues, sister chromatids
________ ________ of maternal and paternal chromosomes occurs during anaphase I
independent assortment
because of the random orientation of homologous chromosomes on the metaphase plate, a pole may receive either the maternal or the paternal homologue from each chromosome pair. As a result, the genes on different chromosomes assort independently, that is meiosis I results in the ___________ _________ of maternal and paternal chromosomes into gametes
independent assortment
the physical connection of homologues due to crossing over and the continued connection of the sister chromatids _____ homologues together
lock
sister chromatid cohesion is _______ through meiosis I but ________ in meiosis II
maintained, released
______ occurs during gamete formation, producing cells with half the normal number of chromosomes
meiosis
anaphase in ______: microtubules pull the homologous chromosomes apart, but sister chromatids are held together at the centromere
meiosis (anaphase I)
metaphase in _______: crossovers and sister chromatid cohesion lock homologues together. microtubules connect to the kinetochores of sister chromatids so that homologues are pulled toward opposite poles
meiosis (metaphase I)
after interphase, germ-line cells enter _________ _
meiosis 1
the key distinction between meiosis and mitosis being the maintenance of sister chromatid cohesion at the centromere during all of ________ __, but the loss of cohesion from the chromosome arms during _______ __
meiosis I anaphase I
meiosis in a diploid organism consists of two rounds of division called ______ and ______, with each round containing prophase, metaphase, anaphase and telophase stages
meiosis I and meiosis II
_______ __ resembles a mitotic division
meiosis II
________ II: chromosomes align on plate
metaphase
________ ___: microtubules from opposite poles attach to each homologue (NOT each sister chromatid as in mitosis)
metaphase I
during _______ __ , the paired homologues move to the plate and become oriented with homologues of each pair attached to opposite poles of the spindle (** by contrast, in mitosis homologues behave independently of each other)
metaphase I
during synapsis (in prophase I) crossing over occurs between non sister chromatids which remain attached at the chiasmata and move to the end of the chromosome arm before _______ __
metaphase I
anaphase in ______: microtubules pull sister chromatids apart
mitosis
metaphase in _______: homologues do NOT pair; kinetochores or sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere
mitosis
zygote first undergoes _____ to produce diploid cells
mitosis
failure of chromosomes to move to opposite poles during either meiotic division is called ________ and it produces one gamete that lacks a chromosomes and one that has two copies
nondisjunction
crossing over involves a complex series of event in which DNA segments are exchanged between _________ chromatids
nonsister
Because of the events of prophase I, each of the paired homologues are locked together as a bivalent. As these capture spindle fibers, they move to the center of the cell, where they are aligned as _______ ______ and not _______ ________
pair homologues not individual chromosomes
meiosis differs from mitosis in the metaphase stage because in metaphase I microtubules attach to each ________ instead of ________
paired homologue instead of sister chromatid
________ II: nuclear envelopes dissolve and new spindle apparatus forms
prophase
synapsis occurs during early______ _
prophase 1
_________ __ : chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms
prophase I
homologues are paired during _____ _
prophase I
________ __: a new spindle apparatus forms in each cell, and the nuclear envelope breaks down. in some species the nuclear envelope does not reform in telophase I, obviating the need for nuclear envelope break down
prophase II
in metaphase I homologues are aligned at the metaphase plate side-by-side and the orientation of each pair of homologues on the spindle is ______
random
orientation of chromosomes on the metaphase plate is ________. Each orientation produces gametes with different combinations of parental chromosomes
random
along with the synaptonemal complex that forms during prophase I, another kind of structure appears at the same time that recombination occurs. These are called __________ _________, and they are thought to contain the enzymatic material necessary to break and rejoin chromatids of homologous chromosomes
recombination nodules
first division of meiosis is termed the "__________" division - it results in daughter cells that contain one homologue from each chromosome pair. The second meiotic division does not further reduce the number of chromosomes; it will merely separate the sister chromatids for each homologue
reduction
the most obvious distinction between meiosis and mitosis is the simple observation that meiosis involves two successive divisions with no _________ ___ ______ _____ between them
replication of genetic material
meiosis and fertilization together constitute a cycle of ________
reproduction
during genetic recombination (crossing over) alleles of genes that were formerly on separate homologues can now be found on the ____ homologue
same
reproduction that involves this alternation of meiosis and fertilization is called _____ reproduction.
sexual
this associate joins homologues along their entire length. the sister chromatids of each homologue are also joined by the cohesin complex in a process called ____ ____ ______
sister chromatid cohesion
it is clear that both the maintenance of ______ ____ ______ at the centromere and ______ attachment are required for the segregation of homologues that distinguishes meiosis I from mitosis
sister chromatid cohesion monopolar
for meiosis to succeed in halving the number of chromosomes, DNA replication must be ________ between the two divisions
suppressed
during _______, homologous chromosomes become closely associated includes formation of synaptonemal complexes (also called tetrad or bivalents)
synapsis
during early prophase I of meiosis, homologous chromosomes find each other and become closely associated, a process called pairing, or _______
synapsis
during interphase in germ-line cells, the ends of the chromatids seem to be attached to the nuclear envelope at adjacent specific sites. During prophase I the members of each homologous pair of chromosomes are brought close together. Homologous pairs then align side by side, apparently guided by heterochromatin sequences, in the process of _________
synapsis
the process of homologous chromosomes pairing during meiosis includes the formation in many species of an elaborate structure called the _______ complex, consisting of homologues paired closely along a lattice of proteins between them
synaptonemal complex
________ II: nuclear envelope reforms around 4 sets of daughter chromosomes; cytokinesis follows
telophase
_______ __: nuclear envelope reforms sister chromatids are no longer identical because of crossing over (prophase I) cytokinesis may or may not occur after this phase
telophase I
_________ __: the separated homologues form a cluster at each pole of the cell, and the nuclear envelope reforms around each daughter cell nucleus. cytokinesis may occur. the resulting two cells have half the number of chromosomes of the original cell: In this example, each nucleus contains two chromosomes (versus four in the original cell). each chromosome consists of two sister chromatids, but sister chromatids are not identical because crossing over has occurred.
telophase I
________ __: the nuclear membranes reform around four different clusters of chromosomes. after cytokinesis, four haploid cells result. no two cells are alike due to the random alignment of the homologous pairs at metaphase I and crossing over during prophase I.
telophase II
the result of the synaptonemal complex is that all four chromatids of the two homologues are closely associated during this phase of meiosis. This structure is also called a _______ or bivalent
tetrad
the cosegregation of sister centromeres requires that the kinetochores of sister chromatids are attached to _________ during meiosis I
the same pole (monopolar)
diploid cells carry chromosomes from ____ parent(s)
two parents
the first meiotic division depends on each homologous pair behaving as a _____ and not as individual chromosomes, as they do in mitosis. This is accomplished by a complex set of processes that together join homologues until anaphase I, when they are separated
unit
an egg and a sperm (gametes), each containing half the complement of chromosomes found in other cells, fuse to produce a single cell called a ______
zygote
in animals, the single diploid _____ undergoes mitosis to give rise to all of the cells in the adult body
zygote
