Meiosis

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Meiosis is a type of cell division. It can only occur in [[Diploid|diploid]] cells, resulting in four unidentical [[Haploid|haploid]] [[Daughter cells|daughter cells]].This contrasts to mitosis which can occur in both [[Haploid|haploid]] and [[Diploid|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. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase.&nbsp;<br>  
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Meiosis is a type of [[Cell|cell]] division which the process that is characteristic of [[Sexual reproduction|sexual reproduction ]] occur only in [[Eukaryotes|eukaryotes]]<ref>Encyclopedia.com,1 December 2015 (cited by 2003) Schmidt, Silke, available from http://www.encyclopedia.com/topic/meiosis.aspx</ref>. It also can only occur in [[Diploid|diploid]] cells, resulting in four unidentical [[Haploid|haploid]] [[Daughter cells|daughter cells]]. This contrasts with [[Mitosis|mitosis]] which can occur in both [[Haploid|haploid]] and [[Diploid|diploid]] cells, producing only two identical daughter cells. Therefore, meiosis results in a variation whereas 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 occurring once. During prophase I, meiosis I involve recombination or [[Crossing over|crossing over ]] of chromosomes<ref>Encyclopedia.com,1 December 2015 (cited by 2003) Schmidt, Silke, available from http://www.encyclopedia.com/topic/meiosis.aspx</ref>. Like mitosis, [[Chromosomes|chromosomes]] in meiosis have duplicated in [[Interphase|Interphase]], during S phase and also ends with [[Cytokinesis|cytokinesis]]. Meiosis has 2 important purposes are its keep the number of [[Chromosomes|chromosomes]] from doubling each generation and introduce genetic diversity in [[Gametes|gametes]]<ref>Daniel L.Hartl and Maryellen Ruvolo,(2012), Genetic, Analysis of genetic and genome, 8th edition, United States of America: Jones and Bartlett Learning.</ref>. In meiosis, if the chromosomes fail to segregate equally and correctly this can lead to genetic disorders. This is called [[Non disjunction|non-disjunction]]. As a result of this, [[Gametes|gametes]] contain the incorrect number of [[Chromosomes|chromosomes]] and they are said to be [[Aneuploidy|aneuploid]] gametes<ref>http://education.seattlepi.com/can-happen-meiosis-goes-wrong-5596.html</ref>.
  
= Meiosis I <ref>Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers</ref>  =
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= Meiosis I<ref>Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers</ref>. =
  
Meiosis 1 is separated into 4 stages. These are Prophase I, Metaphase I, Anaphase I and Telophase I.  
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Meiosis I 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  ==
 
== Prophase I  ==
  
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.<br>
+
In most higher organisms, prophase 1 can last several days. During prophase, I chromosomes pair, condense and crossing over occurs between non-sister chromatids. It is separated into 5 different stages. Similar to mitosis, [[Centrioles|centrioles]] move to opposite poles and [[Spindle fibres|spindle fibres]] start to form.  
  
 
=== Leptotene  ===
 
=== 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.
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Leptotene, which is also known as leptonema is the first stage of Prophase I. During the Leptotene stage chromosomes coil and start to condense into long strands in the nucleus<ref>https://www.biology-online.org/dictionary/Leptotene</ref>.&nbsp;This is when the chromosomes first become visible. The threadlike chromosomes are called chromatonemata<ref>http://www.macroevolution.net/prophase-details.html</ref>.&nbsp;Then the two [[Chromatids|chromatids]] are joined together at the [[Centromere|centromere]]. At this stage, the telomeres of the chromosomes are turned toward, and mostly attached to the same region of nuclear envelope<ref>http://www.macroevolution.net/prophase-details.html</ref>.
  
 
=== Zygotene  ===
 
=== Zygotene  ===
  
During Zygotene the homologous chromosomes pair up and undergo synapsis where the [[Synaptonemal Complex|synaptonemal complex]] between the homologous chromosomes starts to form.  
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During Zygotene the [[Homologous chromosomes|homologous chromosomes]] pair up and undergo [[Synapsis|synapsis]] where the [[Synaptonemal Complex|synaptonemal complex]] between the homologous chromosomes start to form. Where a [[Synapse|synapse]] has formed between homologous chromosomes, they are referred to as a [[Bivalent|bivalent.]]&nbsp;At the end of synapsis, the fused homologs will be double chromosomes but it looks like single chromosome under light microscope<ref>http://www.macroevolution.net/prophase-details.html</ref>.
  
 
=== Pachytene  ===
 
=== Pachytene  ===
  
Synapsis is now complete and the a [[Bivalent|bivalent]]&nbsp;(pair of homologous chromosomes) is formed. Chiasmata (singular: chiasma)&nbsp;form between non-sister chromotids of homolohous chromosomes. This is the point where crossing over occurs and DNA exchange occurs.<br>
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During the third substage of meiosis, the chromosomes continue to condense. [[Crossing over|Crossing over]] takes place at this stage and at each point of crossing over a [[Chiasma|chiasma]] is formed (singular: chiasma) between non-sister chromatids of homologous chromosomes. Each bivalent will have one chiasma, however, chromosomes that are longer will have at least three chiasmas.  
  
=== Diplotene<br> ===
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=== Diplotene  ===
  
The snaptonemal complex breaks down, allowing the chromosomes to separate. The chiasmata are now visible and are point at which the chromosomes are still held together.<br>
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The [[Synaptonemal Complex|synaptonemal complex]] breaks down, allowing the synapsed chromosomes to separate. The chiasmata are now visible and are the point at which the chromosomes are still held together.  
  
=== Diakensis<br> ===
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=== Diakinesis ===
  
During diakenesis the nuclear envelope starts to breakdown. The bivalents are now ready for metaphase.<br>
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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 move towards the ends of the tetrad. Toward the end of diakinesis, the [[Nucleolus|nucleolus]] and [[Nuclear envelope|nuclear envelope]] start to break down and spindle fibres begin to form in preparation for metaphase I.  
  
== Metaphase I<br> ==
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== Metaphase I  ==
  
Bivalents or tetrads (four chromatids)&nbsp;align on on the metaphase plate (the equator of the cell)&nbsp;and spindle fibres attach to the kinetichores; protein structures located at the centromeres.&nbsp;In this stage, the nuclear envelope has been fully disintegrated.<br>
+
Bivalents or tetrads (four chromatids) align on on the metaphase plate (the equator of the cell) and spindle fibres attach to the [[Kinetochores|kinetichores]]; protein structures located at the centromeres. In this stage, the nuclear envelope has been fully disintegrated.  
  
== Anaphase I<br> ==
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== Anaphase I  ==
  
Disjunction occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles. <br>
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[[Disjunction|Disjunction]] occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles.  
  
== Telophase I<br> ==
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== Telophase I  ==
  
Nuclear envelope may form around the dyads and cytokinesis (cell division)&nbsp;occurs.<br>
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The nuclear envelope may form around the dyads and [[Cytokinesis|cytokinesis]] (cell division) occurs.  
  
== Interphase<br> ==
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== Interphase  ==
  
Interphase does not have to occur between Telophase I and Prophase II, but it can occur. Unlike traditional interphase, there is no DNA&nbsp;replication but growth and biosynthetic activities can still occur. <br>
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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<br>&nbsp; =
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= Meiosis II  =
  
= Prophase II<br>  =
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As meiosis forms 4 gametes from a single cell a second round of cell division must take place, Meiosis II resembles mitosis more than meiosis I as it is a simple division with no crossing over. The process Meiosis I generates haploid cells (chromosome number already halved) therefore no DNA replication occurs between Meiosis I and II and the number of chromosomes remains unchanged throughout meiosis 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. Centroiles move to opposing poles and spindle fibres start to form.<br>
+
== 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  ==
 
== Metaphase II  ==
  
The dyads align on the metaphase plate and spindle fibres attach to the kinetechores.  
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The dyads align on the metaphase plate and spindle fibres attach to the [[Kinetochores|kinetechores.]]
  
 
== Anaphase II  ==
 
== Anaphase II  ==
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== Telophase II  ==
 
== Telophase II  ==
  
The nuclear envelope reforms around the myads. Cytokenesis occurs and the cells divide. This leaves us with 4 unidentical daughter cells, also known as gametes.<br>  
+
The nuclear envelope reforms around the myads. [[Cytokinesis|Cytokenesis]] occurs and the cells divide. This leaves us with 4 non-identical daughter cells, also known as [[Gametes|gametes]]<ref>Hartl DL and Ruvolo M (2012), Genetics: Analysis of genes and genomes, eighth edition, Jones and Bartlett learning</ref><ref>Hartl DL and Ruvolo M (2012), Genetics: Analysis of genes and genomes, eighth edition, Jones and Bartlett learning</ref>.
  
 
= References  =
 
= References  =
  
<references /><br>
+
<references />

Latest revision as of 09:26, 10 December 2018

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 with mitosis which can occur in both haploid and diploid cells, producing only two identical daughter cells. Therefore, meiosis results in a variation whereas 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 occurring 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 has 2 important purposes are its keep the number of chromosomes from doubling each generation and introduce genetic diversity in gametes[3]. In meiosis, if the chromosomes fail to segregate equally and correctly this can lead to genetic disorders. This is called non-disjunction. As a result of this, gametes contain the incorrect number of chromosomes and they are said to be aneuploid gametes[4].

Contents

Meiosis I[5].

Meiosis I 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 chromatids. It is separated into 5 different stages. Similar to mitosis, centrioles move to opposite poles and spindle fibres start to form.

Leptotene

Leptotene, which is also known as leptonema is the first stage of Prophase I. During the Leptotene stage chromosomes coil and start to condense into long strands in the nucleus[6]. This is when the chromosomes first become visible. The threadlike chromosomes are called chromatonemata[7]. Then the two chromatids are joined together at the centromere. At this stage, the telomeres of the chromosomes are turned toward, and mostly attached to the same region of nuclear envelope[8].

Zygotene

During Zygotene the homologous chromosomes pair up and undergo synapsis where the synaptonemal complex between the homologous chromosomes start to form. Where a synapse has formed between homologous chromosomes, they are referred to as a bivalent. At the end of synapsis, the fused homologs will be double chromosomes but it looks like single chromosome under light microscope[9].

Pachytene

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

Diplotene

The synaptonemal complex breaks down, allowing the synapsed chromosomes to separate. The chiasmata are now visible and are the 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 move towards the ends of the tetrad. Toward the end of diakinesis, the nucleolus and nuclear envelope start to break down and spindle fibres begin to form 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

The 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

As meiosis forms 4 gametes from a single cell a second round of cell division must take place, Meiosis II resembles mitosis more than meiosis I as it is a simple division with no crossing over. The process Meiosis I generates haploid cells (chromosome number already halved) therefore no DNA replication occurs between Meiosis I and II and the number of chromosomes remains unchanged throughout 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[10][11].

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. http://education.seattlepi.com/can-happen-meiosis-goes-wrong-5596.html
  5. Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers
  6. https://www.biology-online.org/dictionary/Leptotene
  7. http://www.macroevolution.net/prophase-details.html
  8. http://www.macroevolution.net/prophase-details.html
  9. http://www.macroevolution.net/prophase-details.html
  10. Hartl DL and Ruvolo M (2012), Genetics: Analysis of genes and genomes, eighth edition, Jones and Bartlett learning
  11. Hartl DL and Ruvolo M (2012), Genetics: Analysis of genes and genomes, eighth edition, Jones and Bartlett learning
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