Meiosis

From The School of Biomedical Sciences Wiki
Revision as of 06:52, 2 December 2015 by Nnjm2 (Talk | contribs)
Jump to: navigation, search

Meiosis is a type of cell division which the process that is characteristic of sexual reproduction  occur only in eukaryotes[1]. It also can only occur in diploid cells, resulting in four unidentical haploid daughter cells.This contrasts to mitosis which can occur in both haploid and diploid cells, producing only two identical daughter cells. Therefore meiosis results in variation where as mitosis produces exact copies of the parent cell.  There are two stages of Meiosis, Meiosis I and II.This leads to the nucleus dividing twice but chromosome replication only occuring once. During prophase I, meiosis I involve recombination or crossing over  of chromosomes[2]. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase and also ends with cytokinesis.  Meiosis have 2 important purposes are its keep the number of chromosomes from doubling each generation and introduce genetic diversity in gametes[3]             .       

Contents

Meiosis I [4]          

Meiosis 1 is often referred to as the reductional phase, this is because the chromosome number is halved in meiosis 1. Meiosis 1 is separated into 4 stages. These are Prophase I, Metaphase I, Anaphase I and Telophase I.

Prophase I

In most higher organisms, prophase 1 can last several days. During prophase I chromosomes pair, condense and crossing over occurs between non-sister chromotids. It is separated into 5 different stages. Similar to mitosis, centrioles move to opposite poles and spindle fibres start to form.

Leptotene

Leptotene is the first stage of Prophase I. During the Leptotene stage chromosomes coil and condense. This is when the chromosomes first become visible. The two chromatids are joined together at the centromere.

Zygotene

During Zygotene the homologous chromosomes pair up and undergo synapsis where the synaptonemal complex between the homologous chromosomes starts to form. Where a synapse has formed between homologous chromosomes, they are referred to as a bivalent.

Pachytene

During the third substage of meiosis, the chromosomes continue to condense. Crossing over takes place in this stage and at each point of crossing over a chiasma is formed (singular: chiasma) between non-sister chromotids of homologous chromosomes. Each bivalent will have one chiasma, however chromosomes that are longer will have at least three chiasma.

Diplotene

The synaptonemal complex breaks down, allowing the synapsed chromosomes to separate. The chiasmata are now visible and are point at which the chromosomes are still held together.

Diakinesis

Diakinesis is the final substage of prophase 1. In this stage, the homologous chromosomes move further apart, however non-sister chromatids are connected via the chiasmata. Terminalization occurs as the chiasmata moves towards the ends of the tetrad. Toward the end of diakinesis, the nucleolus and nucleus envelope starts to breakdown and spindle fibres begin to be formed in preparation for metaphase I.

Metaphase I

Bivalents or tetrads (four chromatids) align on on the metaphase plate (the equator of the cell) and spindle fibres attach to the kinetichores; protein structures located at the centromeres. In this stage, the nuclear envelope has been fully disintegrated. 

Anaphase I

Disjunction occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles.

Telophase I

Nuclear envelope may form around the dyads and cytokinesis (cell division) occurs.

Interphase

Interphase does not have to occur between Telophase I and Prophase II, but it can occur. Unlike traditional interphase, there is no DNA replication but growth and biosynthetic activities can still occur.

Meiosis II

Prophase II

Unlike Prophase I no chiasmata form and no crossing over occurs. If a nuclear envelope has formed during Telophase I, it is broken down. Centrioles move to opposing poles and spindle fibres start to form.

Metaphase II

The dyads align on the metaphase plate and spindle fibres attach to the kinetechores.

Anaphase II

The dyads are pulled apart by spindle fibres and the myads arrive at opposite poles.

Telophase II

The nuclear envelope reforms around the myads. Cytokenesis occurs and the cells divide. This leaves us with 4 non identical daughter cells, also known as gametes.[5] [6]

References

  1. Encyclopedia.com,1 December 2015 (cited by 2003) Schmidt, Silke, available from http://www.encyclopedia.com/topic/meiosis.aspx
  2. Encyclopedia.com,1 December 2015 (cited by 2003) Schmidt, Silke, available from http://www.encyclopedia.com/topic/meiosis.aspx
  3. Daniel L.Hartl and Maryellen Ruvolo,(2012), Genetic, Analysis of genetic and genome, 8th edition, United States of America: Jones and Bartlett learning.
  4. Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers
  5. Hartl DL and Ruvolo M (2012), Genetics: Analysis of genes and genomes, eighth edition, Jones and Bartlett learning
  6. Hartl DL and Ruvolo M (2012), Genetics: Analysis of genees and genomes, eighth edition, Jones and Bartlett learning
Personal tools
Namespaces
Variants
Actions
Navigation
Toolbox