The Cell Cycle, Mitosis and Meiosis

  • Cell cycleregular sequence of growth and division that cells undergo.cell cycle
  • Divided into three main stages:

Interphase – cell grows into its mature size, makes a copy of its DNA, and prepares for division.

Mitosis – one copy of the DNA is distributed into each of its daughter cells

Cytokinesis – the cytoplasm divides and organelles are distributed into the two new cells

  • Interphase is made up of 3 separate parts.

–G1-Cell doubles in size; Cell produces all of the structures it needs to carry out its functions

–S-Cell makes a copy of its DNA.This happens because the new cell needs all of the directions for its function and survival

–G2-Cell prepares to divide. Cell produces structures needed for cell division

Interphase is the stage that the cell is in for most of its life!

Mitosis

mitosis_2Mitosis 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.

PROPHASE: During the prophase the nucleoli in the nucleus disappear. The chromatin fibers become tightly coiled and condense into chromosomes. Chromosomes line up in pairs and are joined at the centromere. In the cytoplasm, the spindle fibers begin to form and are made of microtubules. The centrosomes move away from each other, propelled by the lengthening microtubules, or spindle fibers, between them.

METAPHASE: The centromes are at opposite ends of the cell poles. The chromosom es are lined up at the ‘metaphase plate’, an imaginary line equidistant between the two poles. The centromeres of the chromosomes are all aligned with one another. The chromatids are of each chromosomes are attached to a microtubule which form the spindle.

ANAPHASE: The paired chromosomes separate. They move along the microtubules toward opposite poles of the cell. The poles move farther apart. By the end of anaphase, the two poles of the cell each have a complete set of chromosomes.

TELOPHASE: The microtubules become even longer, and daughter nuclei begin to form at the two poles of the cell. Nuclear envelopes are formed, the nucleoli reappear, the chromatin of the chromosomes uncoils. Mitosis is now complete: one nucleus has divided into two genetically identical nuclei.

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.

Prophase I

The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:1

  • 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.

2

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

 

 

 

 

 

 

 

1.1

2.2

Resource:http://www2.le.ac.uk/  and biology book

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