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11.1C: Meiosis II - Biology

11.1C: Meiosis II - Biology


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During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes.

LEARNING OBJECTIVES

Describe the stages and results of Meiosis II

Key Points

  • During prophase II, chromsomes condense again, centrosomes that were duplicated during interphase I move away from each other toward opposite poles, and new spindles are formed.
  • During prometaphase II, the nuclear envelopes are completely broken down, and each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.
  • During metaphase II, sister chromatids are condensed and aligned at the equator of the cell.
  • During anaphase II sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles.
  • During telophase II and cytokinesis, chromosomes arrive at opposite poles and begin to decondense; the two cells divide into four unique haploid cells.

Key Terms

  • meiosis II: the second part of the meiotic process; the end result is production of four haploid cells from the two haploid cells produced in meiosis I

Meiosis II

Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis II resembles a normal mitosis. In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II together. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.

Prophase II

If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interphase I move away from each other toward opposite poles and new spindles are formed.

Prometaphase II

The nuclear envelopes are completely broken down and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.

Metaphase II

The sister chromatids are maximally condensed and aligned at the equator of the cell.

Anaphase II

The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell.

Telophase II and Cytokinesis

The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly-formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover.


11.1C: Meiosis II - Biology

In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis.


Interphase

Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase is the first phase of interphase and is focused on cell growth. In the S phase, the DNA of the chromosomes is replicated. Finally, in the G2 phase, the cell undergoes the final preparations for meiosis.

During DNA duplication of the S phase, each chromosome becomes composed of two identical copies (called sister chromatids) that are held together at the centromere until they are pulled apart during meiosis II. In an animal cell, the centrosomes that organize the microtubules of the meiotic spindle also replicate. This prepares the cell for the first meiotic phase.


The Phases of Meiosis I

After Interphase I meiosis I occurs after Interphase I, where proteins are grown in G phase and chromosomes are replicated in S phase. Following this, four phases occur. Meiosis I is known as reductive division, as the cells are reduced from being diploid cells to being haploid cells.

1. Prophase I

Prophase I is the longest phase of meiosis, with three main events occurring. The first is the condensation of chromatin into chromosomes that can be seen through the microscope the second is the synapsis or physical contact between homologous chromosomes and the crossing over of genetic material between these synapsed chromosomes. These events occur in five sub-phases:

  • Leptonema – The first prophase event occurs: chromatin condenses to form visible chromosomes. Condensation and coiling of chromosomes occur.
  • Zygonema – Chromosomes line up to form homologous pairs, in a process known as the homology search. These pairs are also known as bivalents. Synapsis happens when the homologous pairs join. The synaptonemal complex forms.
  • Pachynema – The third main event of prophase I occurs: crossing over. Nonsister chromatids of homologous chromosome pairs exchange parts or segments. Chiasmata form where these exchanges have occurred. Each chromosome is now different to its parent chromosome but contains the same amount of genetic material.
  • Diplonema – The synaptonemal complex dissolves and chromosome pairs begin to separate. The chromosomes uncoil slightly to allow DNA transcription.
  • Diakinesis – Chromosome condensation is furthered. Homologous chromosomes separate further but are still joined by a chiasmata, which moves towards the ends of the chromatids in a process referred to as terminalization. The nuclear envelope and nucleoli disintegrate, and the meiotic spindle begins to form. Microtubules attach to the chromosomes at the kinetochore of each sister chromatid.

2. Metaphase I

Homologous pairs of chromosomes align on the equatorial plane at the center of the cell. Independent assortment determines the orientation of each bivalent but ensures that half of each chromosome pair is oriented to each pole. This is to ensure that homologous chromosomes do not end up in the same cell. The arms of the sister chromatids are convergent.

3. Anaphase I

Microtubules begin to shorten, pulling one chromosome of each homologous pair to opposite poles in a process known as disjunction. The sister chromatids of each chromosome stay connected. The cell begins to elongate in preparation for cytokinesis.

4. Telophase I

Meiosis I ends when the chromosomes of each homologous pair arrive at opposing poles of the cell. The microtubules disintegrate, and a new nuclear membrane forms around each haploid set of chromosomes. The chromosomes uncoil, forming chromatin again, and cytokinesis occurs, forming two non-identical daughter cells. A resting phase known as interkinesis or interphase II happens in some organisms.


11.1C: Meiosis II - Biology

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We said that meiosis is the two step division process by which sperm and eggs are produced. But there are several concepts that need to be addressed before we look at that process.

We also said that humans have 46 chromosomes in each somatic cell. However, since humans have two of every type of chromosome (not counting gender differences), that means that we really have two sets of 23 chromosomes. One set came from your mother and one set came from your father. Notice here that each parent is contributing only 23 chromosomes, which is one half of the total in somatic cells. Therefore, meiosis has to produce specialized reproductive cells called gametes in general, sperm and eggs specifically, which have one of every type of chromosome. After fertilization, then the number of chromosomes in the developing zygote is returned to 46.

Organisms or cells with two of every type of chromosomes are called diploids. Organisms or cells with one of every type of chromosomes are called haploids. Human somatic cells are diploid while the gametes are haploid. Meiosis then is a process by which haploid cells are produced.

There is an additional concept which uses the letter N to represent the haploid number of chromosomes. Therefore, gametes are 1N while somatic cells are 2N.

Also, the letter C is used to represent a haploid amount of DNA in a cell. Remember that unreplicated chromosomes have only one piece of DNA each while replicated chromosomes have two pieces of DNA each (refer to previous key word list). Therefore, a 1N cell can be 1C or 2C and a 2N cell can be 2C or 4C depending on the replication state of their chromosomes.

A cell ready to undergo meiosis (2N, 2C), enters in to one last Interphase where its chromosomes are replicated (2N, 4C). If this were mitosis, it would remain in this state until Anaphase when the chromatids separate forming two daughter cells (2N, 2C each). However, in the first meiotic division (called meiosis I), during Prophase while the chromosomes are condensing, each type of chromosome pairs up lengthwise with the other chromosome of that type ultimately forming 23 pairs of chromosomes. This pairing process is called synapsis. Chromosomes of the same type are called homologous chromosomes and each member of a pair is referred to as a homolog or homologue. At Anaphase of meiosis I, rather than chromatids separating, the homologs separate with one of each pair going to a different daughter cell (1N, 2C). In this way, the chromosome number is reduced in half during meiosis I and two haploid cells are produced. Because of this, meiosis I is called the Reductional Division.

After meiosis I, there is no additional Interphase. Sometimes the cells produced in meiosis I will go in to a prolonged resting state which is referred to as interkinesis. In meiosis II, each of the two daughter cells produced in meiosis I will divide essentially as occurs in mitosis. It is in meiosis II that the chromatids separate forming a total of four daughter cells (1N, 1C) for every cell entering in to meiosis. Meiosis II is called the Equational Division.

In males, four sperm are produced for each cell undergoing meiosis. But in females, certain special events occur through which only one mature egg will be produced per starting cell. Meiosis I takes place as above. However, of the two potential cells to be produced, one of them keeps the bulk of the cytoplasm to itself leaving the other nucleus without cytoplasm. This nucleus which is called a primary polar body will degenerate and the chromosomes it contains will be lost. The same thing happens in meiosis II forming what is called a secondary polar body which is also lost. So, for each cell starting meiosis in females, only one mature egg is produced carrying most of the cytoplasm of the original cell. This is necessary since it is the egg which provides the cytoplasm from which the fetus will develop. There is not enough in the starting cell to produce four eggs.


What is Meiosis?

Meiosis [1] is a type of cell division that involves the reduction in the number of the parental chromosome by half and consequently the production of four haploid daughter cells. This process is very essential in the formation of the sperm and egg cells necessary for sexual reproduction. When the haploid sperm and egg fuse, the resulting offspring acquires the restored number of chromosomes.

Meiosis is highly ubiquitous among eukaryotes as it can occur in single-celled organisms like yeast as well as multi-cellular ones like humans.

The process of meiosis is very essential in ensuring genetic diversity [2] through sexual reproduction.

In humans, two distinct types of daughter cells are produced by males and females (sperm and egg cells respectively).


Prophase II

In prophase II of meiosis, the following events occur:

  • The nuclear membrane and nuclei break up while the spindle network appears.​
  • Chromosomes do not replicate any further in this phase of meiosis.​
  • The chromosomes begin migrating to the metaphase II plate (at the cell's equator).

At the end of prophase II of meiosis, the cell enters into metaphase II.


Mei·o·sis

The process of nuclear division whereby cells divide without replicating chromosomes, producing mature eggs and sperm with a haploid number of chromosomes. Meisosis is a type of cell division required for sexual reproduction, which consists of two nuclear divisions:
(1) In the first division, the chromosomes undergo recombination, forming different genetics in each daughter gamete&mdasheach of which has a full (diploid) complement of chromosomes&mdashwhich is essentially what occurs in mitosis
(2) In the second division, the diploid complement is reduced to a haploid number.

The resulting cells contain one part of each pair of homologous chromosomes, which allows the haploid daughter cell from the mother (ovum) to combine with a haploid daughter cell from the father (sperm).


6.6 Meiosis II

After the first meiotic cell division, the resulting cells have been reduced from diploid to haploid because they now only have either a maternal or a paternal copy of each gene. However, each of these chromosomes still has two complete copies of DNA (the sister chromatids). The second meiotic division divides the sister chromatids, ensuring that only one copy is passed down.

Figure 6.9 Meiosis II

In meiosis II the two cells from meiosis I are further divided into two cells each (for four cells total). Meiosis II looks a lot like mitosis: the sister chromatids separate and the chromosomes migrate into separate cells that are nearly identical to each other.

In a lifetime, a human female can produce hundreds of eggs and a human male hundreds of billions of sperm, essentially all of which will be genetically unique. However, most of these sex cells will never be used.


11.1C: Meiosis II - Biology

Chromosomal replication does not occur between meiosis I and meiosis II meiosis I proceeds directly to meiosis II without going through interphase. The second part of the meiosis, meiosis II, resembles mitosis more than meiosis I. Chromosomal numbers, which have already been reduced to haploid (n) by the end of meiosis I, remain unchanged after this division. In meiosis II, the phases are, again, analogous to mitosis: prophase II, metaphase II, anaphase II, and telophase II (see figure below). As shown in the figure below, meiosis II begins with two haploid (n = 2) cells and ends with four haploid (n = 2) cells. Notice that these four meiocytes are genetically different from one another. In humans (2n = 46), who have 23 pairs of chromosomes, the number of chromosomes remains unchanged from the beginning till the end of meiosis II (n = 23).

Prophase II

Spindle fibers reform and attach to centromeres in prophase II.

Metaphase II

The chromosomes align on the metaphase plate during metaphase II in preparation for centromeres to divide in the next phase.

Anaphase II

In anaphase II, chromosomes divide at the centromeres (like in mitosis) and the resulting chromosomes, each with one chromatid, move toward opposite poles of the cell.

Telophase II and Cytokinesis

Four haploid nuclei (containing chromosomes with single chromatids) are formed in telophase II. Division of the cytoplasm during cytokinesis results in four haploid cells. Note that these four cells are not identical, as random arrangements of bivalents and crossing over in meiosis I leads to different genetic composition of these cells.

In humans, meiosis produces genetically different haploid daughter cells, each with 23 chromosomes that consist of one chromatid. These haploid cells become unfertilized eggs in females and sperm in males. The genetic differences ensure siblings of the same parents are never entirely genetically identical.


Discover What Occurs During the Meiosis Stages

Meiosis 1 Phases

Interphase 1

One of the most important processes in this stage is chromosomal replication in which each chromosome produces an exact copy or replica of itself. The chromosomes are not visible as discrete structures but instead, they appear as a diffuse tangle of threads called chromatin. Another important process that takes place is the formation of new cellular organelles. There is also a build up of energy to be used up in the meiotic process.

This stage is manifested by the chromosomes becoming visible as distinct bodies as they get shorter and thicker and centrioles become arranged at opposite sides of the nucleus. As prophase progresses, homologous chromosomes lie side by side and become intertwined rather like a zipper forming pairs called bivalents in a process called synapsis. Chromosomes may become coiled around each other and their chromatids may remain in contact at points called chiasmata. During synapsis, homologous chromosomes exchange genetic material between one another. This exchange is called crossing over.

Metaphase 1

Nuclear membrane disappears completely making the chromosomes free in the cytoplasm. The spindles are already fully formed. Each pair of the homologous chromosomes moves to the equator of the spindle and attach to the spindles by their centromeres such that the two homologous chromosomes orientate towards opposite poles.

Homologous chromosomes separate and migrate to the opposite poles with their centromeres leading. This is because the spindle fibres shorten and thus the chromosomes are pulled. It is important to note that sister chromatids do not separate at this stage.

Telophase 1

Once the chromosomes reach the poles, they become densely packed together. The spindle apparatus breaks down and a nuclear membrane is formed around each set of chromosomes. The cell then divides into two across the middle. In some organisms, telophase 1 does not exist no nuclear membrane is formed and the cells proceed directly into meiosis 2.

Meiosis 2 Phases

Interphase 2

Once the first meiosis is complete, the daughter cells usually go into a short resting stage which is the interphase 2.

Chromosomes become shorter and thicker. New spindle fibres are formed.

Metaphase 2

Chromosomes migrate to the equator of the cell and attach to the spindle fibres at their centromeres. They then orient themselves towards the opposite poles.

The sister chromatids separate and migrate to the opposite poles.

Telophase 2

The spindle apparatus disappears. The nucleolus reappears and a nuclear membrane is formed around each set of chromatids. The chromatids become the chromosomes of the daughter cells they uncoil and regain their thread-like form. The cytoplasm divides across the middle.

Thus meiosis results into four daughter cells each having a haploid number of chromosomes.

Significance of meiosis

a) Helps to restore a constant diploid chromosomal constitution in a species at fertilization.

b) Provides opportunities for new combinations of genes to occur in the gamete cells leading to genetic variation in the offspring produced by the fusion of gametes.