During which phase of meiosis are spindle fibers attached to both sides of the same centromere?

Spindle fibers are aggregates of microtubules that move chromosomes during cell division. Microtubules are protein filaments that resemble hollow rods. Spindle fibers are found in eukaryotic cells and are a component of the cytoskeleton as well as cilia and flagella.

Spindle fibers are part of a spindle apparatus that moves chromosomes during mitosis and meiosis to ensure even chromosome distribution between daughter cells. The spindle apparatus of a cell is comprised of spindle fibers, motor proteins, chromosomes, and, in some animal cells, microtubule arrays called asters. Spindle fibers are produced in the centrosome from cylindrical microtubules called centrioles.

Spindle fiber and cell movement occur when microtubules and motor proteins interact. Motor proteins, which are powered by ATP, are specialized proteins that actively move microtubules. Motor proteins such as dyneins and kinesins move along microtubules whose fibers either lengthen or shorten. The disassembly and reassembly of microtubules produces the movement needed for chromosome movement and cell division to occur.

Spindle fibers move chromosomes during cell division by attaching to chromosome arms and centromeres. A centromere is the specific region of a chromosome where duplicates are linked. Identical, joined copies of a single chromosome are known as sister chromatids. The centromere is also where protein complexes called kinetochores are found.

Kinetochores generate fibers that attach sister chromatids to spindle fibers. Kinetochore fibers and spindle polar fibers work together to separate chromosomes during mitosis and meiosis. Spindle fibers that don't contact chromosomes during cell division extend from one cell pole to the other. These fibers overlap and push cell poles away from one another in preparation for cytokinesis.

Spindle fibers are highly active during mitosis. They migrate throughout the cell and direct chromosomes to go where they need to go. Spindle fibers function similarly in meiosis, where four daughter cells are formed instead of two, by pulling homologous chromosomes apart after they have been duplicated to prepare for division.

Prophase: Spindle fibers form at opposite poles of the cell. In animal cells, a mitotic spindle appears as asters that surround each centriole pair. The cell becomes elongated as spindle fibers stretch from each pole. Sister chromatids attach to spindle fibers at their kinetochores.

Metaphase: Spindle fibers called polar fibers extend from cell poles toward the midpoint of the cell known as the metaphase plate. Chromosomes are held to the metaphase plate by the force of spindle fibers pushing on their centromeres.

Anaphase: Spindle fibers shorten and pull sister chromatids toward spindle poles. Separated sister chromatids move toward opposite cell poles. Spindle fibers not connected to chromatids lengthen and elongate the cell to make room for the cell to separate.

Telophase: Spindle fibers disperse as the chromosomes are separated and become housed within two new nuclei.

Cytokinesis: Two daughter cells are formed, each with the correct number of chromosomes because spindle fibers ensured this. The cytoplasm divides and the distinct daughter cells fully separate.

anaphase: the stage of mitosis during which sister chromatids are separated from each other

cell cycle: the ordered sequence of events that a cell passes through between one cell division and the next

cell cycle checkpoints: mechanisms that monitor the preparedness of a eukaryotic cell to advance through the various cell cycle stages

cell plate: a structure formed during plant-cell cytokinesis by Golgi vesicles fusing at the metaphase plate; will ultimately lead to formation of a cell wall to separate the two daughter cells

centriole: a paired rod-like structure constructed of microtubules at the center of each animal cell centrosome

cleavage furrow: a constriction formed by the actin ring during animal-cell cytokinesis that leads to cytoplasmic division

cytokinesis: the division of the cytoplasm following mitosis to form two daughter cells

G0 phase: a cell-cycle phase distinct from the G1 phase of interphase; a cell in G0 is not preparing to divide

G1 phase: (also, first gap) a cell-cycle phase; first phase of interphase centered on cell growth during mitosis

G2 phase: (also, second gap) a cell-cycle phase; third phase of interphase where the cell undergoes the final preparations for mitosis

interphase: the period of the cell cycle leading up to mitosis; includes G1, S, and G2 phases; the interim between two consecutive cell divisions

kinetochore: a protein structure in the centromere of each sister chromatid that attracts and binds spindle microtubules during prometaphase

metaphase plate: the equatorial plane midway between two poles of a cell where the chromosomes align during metaphase

metaphase: the stage of mitosis during which chromosomes are lined up at the metaphase plate

mitosis: the period of the cell cycle at which the duplicated chromosomes are separated into identical nuclei; includes prophase, prometaphase, metaphase, anaphase, and telophase

mitotic phase: the period of the cell cycle when duplicated chromosomes are distributed into two nuclei and the cytoplasmic contents are divided; includes mitosis and cytokinesis

mitotic spindle: the microtubule apparatus that orchestrates the movement of chromosomes during mitosis

prometaphase: the stage of mitosis during which mitotic spindle fibers attach to kinetochores

prophase: the stage of mitosis during which chromosomes condense and the mitotic spindle begins to form

quiescent: describes a cell that is performing normal cell functions and has not initiated preparations for cell division

S phase: the second, or synthesis phase, of interphase during which DNA replication occurs

telophase: the stage of mitosis during which chromosomes arrive at opposite poles, decondense, and are surrounded by new nuclear envelopes

Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides.

• G1 phase. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and moves into the S phase.
• S phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.
• G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis.
• M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis).
• The period between mitotic divisions - that is, G1, S and G2 - is known as interphase.

Mitosis

Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.

The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells.

In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation.

Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase.

The phases of mitosis

Prophase

Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organise the production of microtubules that form the spindle fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere.

Prometaphase

The chromosomes, led by their centromeres, migrate to the equatorial plane in the mid-line of the cell - at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibres bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense.

Metaphase

The chromosomes align themselves along the metaphase plate of the spindle apparatus.

Anaphase

The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions. The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.)

Telophase

The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibres disappear.

Cytokinesis

The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions.

Meiosis

Meiosis is the form of eukaryotic cell division that produces haploid sex cells or gametes (which contain a single copy of each chromosome) from diploid cells (which contain two copies of each chromosome). The process takes the form of one DNA replication followed by two successive nuclear and cellular divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids.

Meiosis I

Meiosis I separates the pairs of homologous chromosomes. 

In Meiosis I a special cell division reduces the cell from diploid to haploid.

Prophase I

The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:

• Leptotene: chromosomes start to condense.
• Zygotene: homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads).
• Pachytene: crossing over between pairs of homologous chromosomes to form chiasmata (sing. chiasma).
• Diplotene: homologous chromosomes start to separate but remain attached by chiasmata.
• Diakinesis: homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.

Prometaphase I

Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.

Metaphase I

Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.)

Anaphase I

The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell

Telophase I

The chromosomes become diffuse and the nuclear membrane reforms.

Cytokinesis

The final cellular division to form two new cells, followed by Meiosis II. Meiosis I is a reduction division: the original diploid cell had two copies of each chromosome; the newly formed haploid cells have one copy of each chromosome.

Meiosis II

Meiosis II separates each chromosome into two chromatids.

The events of Meiosis II are analogous to those of a mitotic division, although the number of chromosomes involved has been halved.

Meiosis generates genetic diversity through:

• the exchange of genetic material between homologous chromosomes during Meiosis I
• the random alignment of maternal and paternal chromosomes in Meiosis I
• the random alignment of the sister chromatids at Meiosis II