Meiosis: Difference between revisions
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Meiosis is a type of [[Cell|cell]] division. It can only occur in [[Diploid|diploid]] cells, resulting in four unidentical [[Haploid|haploid]] [[Daughter cells|daughter cells]].This contrasts to [[Mitosis|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|chromosomes]] in meiosis have duplicated in Interphase, during S phase. <br> | Meiosis is a type of [[Cell|cell]] division. It can only occur in [[Diploid|diploid]] cells, resulting in four unidentical [[Haploid|haploid]] [[Daughter cells|daughter cells]].This contrasts to [[Mitosis|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|chromosomes]] in meiosis have duplicated in Interphase, during S phase. <br> | ||
= 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 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. | 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|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 chromotids. It is separated into 5 different stages. Similar to mitosis, [[Centrioles|centrioles]] move to opposite poles and spindle fibres start to form.<br> | ||
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=== Diakinesis === | === Diakinesis === | ||
Diakinesis is the final substage of prophase 1. In this stage, the homologous chromosomes move further apart, however | 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<br> == | == Metaphase I<br> == | ||
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.<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. <br> | ||
== Anaphase I<br> == | == Anaphase I<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. <br> | 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. <br> | ||
= Meiosis II<br> | = Meiosis II<br> = | ||
= Prophase II<br> = | == Prophase II<br> == | ||
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|Centrioles]] move to opposing poles and spindle fibres start to form.<br> | 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|Centrioles]] move to opposing poles and spindle fibres start to form.<br> | ||
| Line 61: | Line 61: | ||
== Telophase II == | == Telophase II == | ||
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.<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 genees and genomes, eighth edition, Jones and Bartlett learning</ref><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.<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 genees and genomes, eighth edition, Jones and Bartlett learning</ref><br> | ||
= References = | = References = | ||
<references /> | <references /> | ||
Revision as of 12:04, 10 January 2015
Meiosis is a type of cell division. It 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. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase.
Meiosis I [1]
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.[2] [3]
References
- ↑ Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers
- ↑ Hartl DL and Ruvolo M (2012), Genetics: Analysis of genes and genomes, eighth edition, Jones and Bartlett learning
- ↑ Hartl DL and Ruvolo M (2012), Genetics: Analysis of genees and genomes, eighth edition, Jones and Bartlett learning