Figure 13.8 Exploring Meiosis in an Animal...
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Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Sister chromatids Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Sister chromatids remain attached Homologous chromosomes separate Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example. Chromosomes line up by homologous pairs. Each pair of homologous chromosomes separates. Two haploid cells form; each chromosome still consists of two sister chromatids. Cleavage furrow Centrosome (with centriole pair) Figure 13.8 Exploring Meiosis in an Animal Cell Prophase I During early prophase I, before the stage shown above: • Chromosomes begin to con- dense, and homologs loosely pair along their lengths, aligned gene by gene. • Paired homologs become physically connected to each other along their lengths by a zipper-like protein structure, the synaptonemal complex; this state is called synapsis. • Crossing over, a genetic rearrangement between non- sister chromatids involving the exchange of corresponding segments of DNA molecules, begins during pairing and synaptonemal complex forma- tion, and is completed while homologs are in synapsis. At the stage shown above: • Synapsis has ended with the disassembly of the synaptonemal complex in mid-prophase, and the chromosomes in each pair have moved apart slightly. • Each homologous pair has one or more X-shaped regions called chiasmata (singular, chiasma). A chiasma exists at the point where a crossover has occurred. It appears as a cross because sister chromatid cohesion still holds the two original sister chromatids together, even in regions beyond the crossover point, where one chromatid is now part of the other homolog. • Centrosome movement, spin- dle formation, and nuclear envelope breakdown occur as in mitosis. Later in prophase I, after the stage shown above: • Microtubules from one pole or the other attach to the two kinetochores, protein structures at the centromeres of the two homologs. The homologous pairs then move toward the metaphase plate. Metaphase I • Pairs of homologous chromo- somes are now arranged at the metaphase plate, with one chromosome in each pair facing each pole. • Both chromatids of one homolog are attached to kinetochore microtubules from one pole; those of the other homolog are attached to microtubules from the opposite pole. Anaphase I • Breakdown of proteins re- sponsible for sister chromatid cohesion along chromatid arms allows homologs to separate. • The homologs move toward opposite poles, guided by the spindle apparatus. • Sister chromatid cohesion per- sists at the centromere, caus- ing chromatids to move as a unit toward the same pole. Metaphase I Anaphase I Telophase I and Cytokinesis Prophase I MEIOSIS I: Separates homologous chromosomes Telophase I and Cytokinesis • At the beginning of telo- phase I, each half of the cell has a complete haploid set of duplicated chromo- somes. Each chromosome is composed of two sister chro- matids; one or both chro- matids include regions of nonsister chromatid DNA. • Cytokinesis (division of the cytoplasm) usually occurs simultaneously with telophase I, forming two haploid daughter cells. • In animal cells like these, a cleavage furrow forms. (In plant cells, a cell plate forms.) • In some species, chromo- somes decondense and nuclear envelopes form. • No chromosome duplication occurs between meiosis I and meiosis II. 254 UNIT THREE Genetics
Transcript of Figure 13.8 Exploring Meiosis in an Animal...
Chiasmata
Spindle
Homologouschromosomes
Fragmentsof nuclearenvelope
Sisterchromatids
Centromere(with kinetochore)
Metaphaseplate
Microtubuleattached tokinetochore
Sister chromatidsremain attached
Homologouschromosomesseparate
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example.
Chromosomes line upby homologous pairs.
Each pair of homologouschromosomes separates.
Two haploid cellsform; each chromosomestill consists of twosister chromatids.
Cleavagefurrow
Centrosome(with centriole pair)
! Figure 13.8
Exploring Meiosis in an Animal Cell
Prophase IDuring early prophase I, beforethe stage shown above:• Chromosomes begin to con-
dense, and homologs looselypair along their lengths,aligned gene by gene.
• Paired homologs becomephysically connected to eachother along their lengths by azipper-like protein structure,the synaptonemal complex;this state is called synapsis.
• Crossing over, a geneticrearrangement between non-sister chromatids involving theexchange of correspondingsegments of DNA molecules,begins during pairing andsynaptonemal complex forma-tion, and is completed whilehomologs are in synapsis.
At the stage shown above:• Synapsis has ended with
the disassembly of thesynaptonemal complex inmid-prophase, and the
chromosomes in each pairhave moved apart slightly.
• Each homologous pair hasone or more X-shaped regionscalled chiasmata (singular,chiasma). A chiasma exists atthe point where a crossoverhas occurred. It appears as across because sister chromatidcohesion still holds the twooriginal sister chromatidstogether, even in regionsbeyond the crossover point,where one chromatid is nowpart of the other homolog.
• Centrosome movement, spin-dle formation, and nuclearenvelope breakdown occuras in mitosis.
Later in prophase I, after thestage shown above:• Microtubules from one pole
or the other attach to thetwo kinetochores, proteinstructures at the centromeresof the two homologs. Thehomologous pairs then movetoward the metaphase plate.
Metaphase I• Pairs of homologous chromo-
somes are now arranged atthe metaphase plate, withone chromosome in each pairfacing each pole.
• Both chromatids of onehomolog are attached tokinetochore microtubulesfrom one pole; those of theother homolog are attachedto microtubules from theopposite pole.
Anaphase I• Breakdown of proteins re-
sponsible for sister chromatidcohesion along chromatidarms allows homologs toseparate.
• The homologs move towardopposite poles, guided by thespindle apparatus.
• Sister chromatid cohesion per-sists at the centromere, caus-ing chromatids to move as aunit toward the same pole.
Metaphase I Anaphase I Telophase I andCytokinesisProphase I
MEIOSIS I: Separates homologous chromosomes
Telophase I andCytokinesis
• At the beginning of telo-phase I, each half of thecell has a complete haploidset of duplicated chromo-somes. Each chromosome iscomposed of two sister chro-matids; one or both chro-matids include regions ofnonsister chromatid DNA.
• Cytokinesis (division of thecytoplasm) usually occurssimultaneously withtelophase I, forming twohaploid daughter cells.
• In animal cells like these, acleavage furrow forms. (Inplant cells, a cell plate forms.)
• In some species, chromo-somes decondense andnuclear envelopes form.
• No chromosome duplicationoccurs between meiosis I andmeiosis II.
254 U N I T T H R E E Genetics
Sister chromatidsseparate
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.
Haploid daughter cellsforming
Prophase II• A spindle apparatus forms.• In late prophase II (not
shown here), chromosomes,each still composed of twochromatids associated at thecentromere, move toward themetaphase II plate.
Metaphase II• The chromosomes are posi-
tioned at the metaphase plateas in mitosis.
• Because of crossing over inmeiosis I, the two sister chro-matids of each chromosomeare not genetically identical.
• The kinetochores of sisterchromatids are attached tomicrotubules extending fromopposite poles.
Anaphase II• Breakdown of proteins hold-
ing the sister chromatidstogether at the centromereallows the chromatids to sep-arate. The chromatids movetoward opposite poles asindividual chromosomes.
Telophase II andCytokinesis
• Nuclei form, the chromo-somes begin decondensing,and cytokinesis occurs.
• The meiotic division of oneparent cell produces fourdaughter cells, each with ahaploid set of (unduplicated)chromosomes.
• The four daughter cells aregenetically distinct fromone another and from theparent cell.
Visit the Study Areaat www.masteringbiology.comfor the BioFlix® 3-D Animation onMeiosis.
ANIMATION
Prophase II Metaphase II Anaphase II Telophase II andCytokinesis
MEIOSIS II: Separates sister chromatids
Look at Figure 12.7 and imagine the twodaughter cells undergoing another round of mitosis, yielding four cells.Compare the number of chromosomes in each of those four cells, aftermitosis, with the number in each cell in Figure 13.8, after meiosis. Whatis it about the process of meiosis that accounts for this difference, eventhough meiosis also includes two cell divisions?
MAKE CONNECTIONS
C H A P T E R 1 3 Meiosis and Sexual Life Cycles 255
256 U N I T T H R E E Genetics
Daughter cells of meiosis II
Chromosomeduplication
Chromosomeduplication
Sisterchroma-tidsseparateduringanaphase II.
n
MITOSIS MEIOSIS
Parent cell(before chromosome duplication)
Mitosis Meiosis
Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins
Enables multicellular adult to arise fromzygote; produces cells for growth, repair, and, in some species, asexual reproduction
Produces gametes; reduces number of chromosome sets by halfand introduces genetic variability among the gametes
Two, each diploid (2n) and geneticallyidentical to the parent cell
Four, each haploid (n), containing half as many chromosomesas the parent cell; genetically different from the parentcell and from each other
One, including prophase, prometaphase, metaphase, anaphase, and telophase
Two, each including prophase, metaphase, anaphase, andtelophase
Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion
SUMMARY
Property
DNAreplication
Role in theanimal body
Number ofdaughter cellsand geneticcomposition
Number ofdivisions
Does not occurSynapsis ofhomologouschromosomes
n n n
Prophase
Metaphase
AnaphaseTelophase
Anaphase ITelophase I
Metaphase I
MEIOSIS II
MEIOSIS I
Prophase I
Duplicated chromosome(two sister chromatids)
Chiasma (site ofcrossing over)
Homologous chromosomepair held together bychiasma and sisterchromatid cohesion
Individualchromosomesline up at themetaphase plate.
Pairs of homologous chromosomes line up at the metaphase plate.
Daughtercells of
meiosis I
Haploidn = 3
Daughter cellsof mitosis
Sister chromatidsseparate duringanaphase.
Homologsseparateduringanaphase I;sisterchromatidsremainattached atcentromere.
2n2n
2n = 6
! Figure 13.9 A comparison of mitosis and meiosis in diploid cells.Could any other combinations of chromosomes be generated during meiosis II from the specific
cells shown in telophase I? Explain. (Hint: Draw the cells as they would appear in metaphase II.)DRAW IT
