D103 Cytokinesis and Meiosis (ALS 25. Vid 38)

phases of cytokinesis

early cytokinesis

mid-cytokinesis

late cytokinesis

overview of cytokinesis

in interphase cells, actin and myosin II filaments form a cortical network underneath the PM

• some cells can also form stress fibers

as cells enter mitosis, actin and myosin arrays disassemble

anter anaphase, a contractile ring of actin and myosin provides the force required to constrict the equator and divide the cytoplasm to form 2 daughter cells

early cytokinesis

new membrane inserted

acto-myosin contractile ring forms

midbody begins to form

mid-cytokinesis

reformation of interphase microtubule array

contractile ring forms cleavage furrow

• belt cinches and constricts to separate the 2

late cytokinesis

vesicle fusion drives separation (absicissin) of the 2 cells

• chromatin decondenses

• nuclear substructures reform

• belt disassembles and becoms the midbody

decrease in S and M cyclin activity

• complete daughter cells form

purpose of the contractile ring

actin and myosin II generate force required for cytokinesis

• little actin remains in periphery of the daughter cells

• myosin II colocalizes with the belt at the cleavage furrow

overview of contractile ring fuction

1. during anaphase - begins to asemble at the plane of the metaphase plate (in part due to activation of formins)

2. after anaphase - contractile ring begins to constrict via acto-myosin contraction similar to that in muscle

3. ring constricts → maintains the same thickness suggesting that its total volume and number of actin and myosin filaments gradually decreases

surface area of daughter cells

SA is 40% greater than the pre-division cell

• to provide the additional membrane required, vesicles are inserted adjacent to the leading edge of the cleavage furrow

what does the central spindle colocalize

molecules that stimulate Rho A activity to mediate formation of a contractile ring of actin and myosin II

where does the actin-myosin ii contractile ring form

overlapping interpolar MTs provide a signal for motor proteins to deliver components of contractile ring (signals also emanate from other locations)

centralspindlin

protein that recruits an activator (GEF) of Rho A to the cell cortex overlying the overlapping interpolar MTs

result of membrane localized Rho A

coordinates activation of formins that cause actin filament formation and Rho kinases that activate myosin II

• lack this protein = cannot complete cytookinesis

what completes cytokinesis

completed by abscission involving constriction and severing of the membranes on either side of the midbody

how does meiosis facilitate genetic diversity

contributes to diversity among members of a species

valuable trait for survival of a species

• genetic diversity primarily arises from the independent assortment of maternal and paternal homologs during meiosis

recombo for genetic diversity

permits new versions of chromosomes to be formed that have novel combos of different alleles of the gene s

• greatly increases the possible number of genetic combos of alleles

• helps hold homologs together, so they are correctly segregated to 2 daughter nuclei produced during meiosis I

mitosis vs meiosis

meiotic S phase

• both undergo dna replication

meiosis I (major difference)

• meiosis: homolog pairs line up on the spindle

• mitosis: duplicated chromosomes line up indvidiually on the spindle

meiosis II

• both segregate sister chromatids at anaphase

mitosis vs meiosis: cell divisions

mitosis

• one cell division, resulting in 2 daughter cells

meiosis

• 2 cell divisions, resulting in four products of meiosis

mitosis vs meiosis: chromosome number

mitosis

• chromosome number per nucleus maintained (diploid, 2n)

meiosis

• chromosome number halved in the products (2n → n)

mitosis vs meiosis: s phase

mitosis: one premitotic S phase per cell division

meiosis: one premeiotic S phase for both cell divisions

mitosis vs meiosis: pairing of chromosomes

mitosis: normalliy, no pairing of homologous chromosomes in prophase (no tetrads)

meiosis: full synapsis of homologous chromosomes in prophase (tetrads)

mitosis vs meiosis: recombo

mitosis: norecombo in prophase

meiosis: at least 1 recombo between nonsister chromatids

mitosis vs meiosis: orientation

mitosis: bi-oriented sister kinetochores; loss of cohesion between sister chromatid arms during metaphase

meiosis: co-orientation of sister kinetochores; maintainence of cohesion between sister chromatid arms during metaphase of meioses I

mitosis vs meiosis: centromeres

mitosis: centromeres divide at anaphase

• conservative process: daughter cells’ genotypes identical with parental genotype

• cell undergoing mitosis can be diploid or haploid

meiosis: centromeres do not divide at anaphase I, but do at anaphase II

• promotes variation among the products of meiosis

• cell udnergoing meiosis is diploid

mitosis vs meiosis: prophase

prophase is much longer in meiosis than it is during mitosis

meiosis prophase I

requires the longest time

• increases local conc

• between homologous chromosomes

• haploid gamete formation

organization of synaptonemal complex during meiotic prophase I

1. pairing of homologous chromosomes

2. initiation of recombo

3. formation of synaptonemal complex

4. completion of recombo

steps fo synaptonemal complex during meiotic prophase I

1. homologs pair and recombo begns

2. synaptonemal complex begins to form at sites where recombination has been initiated (involves double strand breaks in chromatids)

3. assembly of synaptonamal complex complete; longest stage of meiosis

4. disassembly of synaptonemal complex; condensation and shortening of chromosomes; chiasmata become visible

impact of recombo events

generate diversity and tether homologous chromosomes together at chiasma where a crossover has occurred

• normally, every bivalent is linked by at least 1 chiasma (usually 2-3)

• linkage analyses of particular aneuploidies indicate reduced levels of recombo in the chromosomes involved in nondisjunction event

• in some studies, up to 2.5% of non-recombined bivalents for chromosome 16 have been identified

when are kinetochores of sister-chromatids are co-oriented

during meiosis I, homologous chromosomes rather than sister chromatids separate then segregate

• mammals: meikin (meiosis-kinetochore) is required for sister kinetochores to become associated with the same spindle pole (ex - co-oriented)

• meikin no longer functions during meiosis II when sister chromatids can be bioriented on the spindle as occurs during mitosis

rec8

meiosis specific conhesin subunits that facilitates stepwise removal of cohesins during meiosis

result of Rec8 replsces Scc1 in cohesion complex

the compelx does not dissociate in prophase when it becomes phosphorylated

rec8 in anaphase I

phosphorylated Rec8 on the chromosome arms is cleaved by separase to permit separation of sister chromatids distal to each crossover

• however, rec8 located at the centrosome is dephosphorylated and is protected from cleavage due to a phosphatase PP2A localized to the centromere by shugoskin (SgoI)

rec8 in anaphase II

phosphatase activity at the centromere is inhibited

• centromeric rec8 is no longer protected, and sister chromatids can separate following cleavage of phosphorylated Rec8

is meiosis error-prone

yes

• 20-40% of all human conceptuses are aneuploid

• majority of trisomies and monosomies result from non disjunction of homologs at maternal MI

• trisomy 16 is most common trisomy in human pregnancies

patau’s syndrome

trisomy 13

• variable phenotype (some individuals survive)

• mental and motor retardation

• polydactylyl (extra digits)

• microcephaly

• holoprosencephaly/ cyclopia

• heart defects

downs’s syndrome

• partial or complete trisomy for chromosome 21

• incidence increases dramatically with maternal age

evidence for age-related decrease of meiosis-specific cohesins in human and mouse oocytes

use fo immunostaining demonstrates decline in cohesins (Rec8 and SMC1B) as oocytes age in human and mice

• oocyte becomes less green over time = less fluorescence = less cohesin concentration

Which of the following processes in the cell cycle would be most likely to be affected by loss of the molecular motor myosin II ?

A. Capture of condensed chromosomes.

B. Cell division.

C. Separation of chromosomes at anaphase A

D. Separation of the spindle pole body at anaphase B

E. None of these processes would be affected by loss of myosin II

B. Cell division.

Identify the stage of cell division in the picture

A. Pro-metaphase of meiosis I

B. Pro-metaphase of meiosis II

C. Anaphase of meiosis I

D. Anaphase of meiosis II

C. Anaphase of meiosis I

A mammalian zygote is formed from fusion of a normal sperm with an egg that formed following non-disjunction of a chromosome during meiosis II. Which of the following terms best describes this zygote ?

A. Tetraploid.

B. Polyploid.

C. Haploid.

D. Diploid.

E. Aneuploid.

E. Aneuploid.