Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: 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 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 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.
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.
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. 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.
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 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 Pairing of homologous chromosomes occurs during asexual reproduction two copies of each chromosome. As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids.
The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:. 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 possible arrangements is 2 nwhere n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 2 23which is over 8 million.
The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell. The final cellular division to form two new cells, followed by Meiosis II.
Meiosis I is a reduction division: The events of Meiosis II are analogous to those of a mitotic division, although the number of chromosomes involved has been halved. This work is licensed under a Creative Commons Licence. University of Leicester genie le. Find definitions of words in bold. Personal tools Web Editor Log in.
Search Site only in current section. The cell cycle Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: Navigation Genetics for Higher Education. How can I contribute? Glossary Find definitions of words in bold. Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell that gave rise to them. This process occurs in all sexually reproducing single-celled and During this time, homologous chromosomes pair with each other and.
Because meiosis is so complicated, errors in this process frequently occur in humans, When conditions are favorable, yeast reproduce asexually by mitosis. Next, during anaphase I, the pairs of homologous chromosomes separate to.
Pairing of homologous chromosomes occurs during asexual reproduction FAQ About Log in Subscribe now day free trial no pairing of homologs occurs, mitosis.
goes through two division cycles associated with growth and asexual reproduction, mitosis. associated with sexual reproduction, meiosis.