Three Cells Undergo Meiosis How Many Haploid Cells Are Produced

Three Cells Undergo Meiosis How Many Haploid Cells Are Produced

How is the same process responsible for genetic recombination and multifariousness too the cause of aneuploidy? Understanding the steps of meiosis is essential to learning how errors occur.

Organisms that reproduce sexually are thought to have an advantage over organisms that reproduce asexually, considering novel combinations of
genes
are possible in each generation. Furthermore, with few exceptions, each private in a
population
of sexually reproducing organisms has a distinct genetic composition. Nosotros have
meiosis
to thank for this variety.

Meiosis, from the Greek give-and-take
meioun, meaning “to brand small,” refers to the specialized process past which germ cells split up to produce
gametes. Because the
chromosome
number of a
species
remains the same from ane generation to the adjacent, the chromosome number of germ cells must exist reduced by one-half during meiosis. To reach this feat, meiosis, unlike
mitosis, involves a unmarried circular of
Deoxyribonucleic acid
replication
followed by two rounds of
prison cell
sectionalisation (Effigy one). Meiosis besides differs from mitosis in that it involves a procedure known every bit
recombination, during which chromosomes exchange segments with ane another. As a effect, the gametes produced during meiosis are genetically unique.

Researchers’ initial understanding of meiosis was based upon careful observations of chromosome behavior using light microscopes. Then, in the 1950s, electron microscopy provided scientists with a glimpse of the intricate structures formed when chromosomes recombine. More recently, researchers accept been able to place some of the molecular players in meiosis from biochemical analyses of meiotic chromosomes and from genetic studies of meiosis-specific mutants.

Meiosis Is a Highly Regulated Process

A schematic diagram shows key events in mitosis and meiosis during the development cycles of male and female sex cells in humans. During fetal development, cells undergo a period of mitotic proliferation. In females, the cells enter meiosis, followed by meiotic arrest. Cells exit meiotic arrest and are either lost before birth or undergo follicle formation after birth. After puberty, these cells are either ovulated each month, one at a time, or becomes atretic. During fetal development in males, proliferating cells enter mitotic arrest. After birth, they enter a second period of mitotic proliferation. After puberty, the cells undergo meiotic divisions to produce sperm cells.

Meiosis represents a survival mechanism for some simple eukaryotes such equally
yeast. When conditions are favorable, yeast reproduce asexually past mitosis. When nutrients get limited, withal, yeast enter meiosis. The commitment to meiosis enhances the
probability
that the next generation volition survive, considering genetic recombination during meiosis generates 4 reproductive spores per cell, each of which has a novel
genotype. The entry of yeast into meiosis is a highly regulated process that involves significant changes in cistron
transcription
(Lopez-Maury
et al., 2008). By analyzing yeast mutants that are unable to consummate the diverse events of meiosis, investigators take been able to identify some of the molecules involved in this complex process. These controls have been strongly conserved during
evolution, so such yeast experiments take provided valuable insight into meiosis in multicellular organisms besides.

In virtually multicellular organisms, meiosis is restricted to germ cells that are prepare aside in early on
development. The germ cells reside in specialized environments provided by the gonads, or
sex
organs. Within the gonads, the germ cells proliferate by mitosis until they receive the right signals to enter meiosis.

In mammals, the timing of meiosis differs profoundly betwixt males and females (Figure 2). Male germ cells, or spermatogonia, practice not enter meiosis until after puberty. Fifty-fifty then, only limited numbers of spermatogonia enter meiosis at any one time, such that developed males retain a puddle of actively dividing spermatogonia that acts as a stem cell population. On the other hand, meiosis occurs with quite different kinetics in mammalian females. Female germ cells, or oogonia, terminate dividing and enter meiosis within the fetal ovary. Those germ cells that enter meiosis become oocytes, the source of futurity eggs. Consequently, females are built-in with a finite number of oocytes arrested in the first meiotic
prophase. Within the ovary, these oocytes grow within follicle structures containing large numbers of support cells. The oocytes will reenter meiosis only when they are ovulated in response to hormones. Human females, for instance, are born with hundreds of thousands of oocytes that remain arrested in the first meiotic prophase for decades. Over time, the quality of the oocytes may deteriorate; indeed, researchers know that many oocytes die and disappear from ovaries in a procedure known as atresia.

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Meiosis Consists of a Reduction Division and an Equational Division

Ii divisions,
meiosis I
and
meiosis II, are required to produce gametes (Figure 3). Meiosis I is a unique
cell division
that occurs just in germ cells; meiosis II is similar to a mitotic partition. Before germ cells enter meiosis, they are generally
diploid, significant that they have two
homologous
copies of each chromosome. Then, but before a germ cell enters meiosis, it duplicates its Dna so that the cell contains four DNA copies distributed between two pairs of homologous chromosomes.

Meiosis I

A multi-panel diagram (labeled a through i) shows illustrations of a cell in five phases of Meiosis I and four phases of Meiosis II. Meiosis I begins with interphase, when a cell duplicates its DNA. Meiosis I then proceeds through prophase I, metaphase I, anaphase I, and telophase I. Meiosis I is followed by meiosis II. The stages of meiosis II include prophase II, metaphase II, anaphase II, and, finally, telophase II. At the end of Meiosis II, the single cell has divided to form four genetically unique daughter cells.

Compared to mitosis, which can take place in a thing of minutes, meiosis is a slow process, largely because of the time that the jail cell spends in
prophase I. During prophase I, the pairs of homologous chromosomes come up together to grade a
tetrad
or
bivalent, which contains four chromatids. Recombination can occur between whatsoever two chromatids within this tetrad structure. (The recombination process is discussed in greater detail afterwards in this commodity.) Crossovers between homologous chromatids can exist visualized in structures known every bit chiasmata, which appear belatedly in prophase I (Effigy 4). Chiasmata are essential for accurate meioses. In fact, cells that fail to class chiasmata may not be able to segregate their chromosomes properly during
anaphase, thereby producing aneuploid gametes with abnormal numbers of chromosomes (Hassold & Hunt, 2001).

At the end of prometaphase I, meiotic cells enter
metaphase I. Here, in sharp contrast to mitosis, pairs of homologous chromosomes line up opposite each other on the
metaphase plate, with the kinetochores on
sister chromatids
facing the aforementioned pole. Pairs of
sex chromosomes
also align on the metaphase plate. In human males, the
Y chromosome
pairs and crosses over with the X chromosome. These crossovers are possible considering the 10 and Y chromosomes take small regions of similarity near their tips. Crossover betwixt these homologous regions ensures that the sex chromosomes volition segregate properly when the cell divides.

Next, during
anaphase I, the pairs of homologous chromosomes separate to unlike daughter cells. Before the pairs can divide, even so, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sis chromatids. Failure to separate the pairs of chromosomes to dissimilar girl cells is referred to as
nondisjunction, and it is a major source of
aneuploidy. Overall, aneuploidy appears to be a relatively frequent event in humans. In fact, the
frequency
of aneuploidy in humans has been estimated to be as loftier as 10% to 30%, and this frequency increases sharply with
maternal
age (Hassold & Hunt, 2001).

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Meiosis II

An illustration of two homologous chromosomes shows crossing over during meiosis. One chromosome is green, and the other is orange. Each chromosome consists of two sister chromatids, which look like strands of pasta, connected at a junction called the centromere. The chromatids are shown crossing over each other at two places, which are labeled chiasmata. At these locations, the chromatids change color either from orange to green, or vice versa, to show the exchange of DNA between chromosomes during recombination.

Following meiosis I, the daughter cells enter meiosis Two without passing through
interphase
or replicating their Deoxyribonucleic acid. Meiosis 2 resembles a mitotic sectionalisation, except that the chromosome number has been reduced by half. Thus, the products of meiosis 2 are 4
haploid
cells that comprise a single copy of each chromosome.

In mammals, the number of viable gametes obtained from meiosis differs between males and females. In males, iv haploid spermatids of similar size are produced from each
spermatogonium. In females, withal, the cytoplasmic divisions that occur during meiosis are very asymmetric. Fully grown oocytes within the ovary are already much larger than sperm, and the time to come
egg
retains about of this book equally it passes through meiosis. As a effect, only i functional oocyte is obtained from each female person meiosis (Effigy ii). The other iii haploid cells are pinched off from the oocyte every bit polar bodies that contain very little cytoplasm.

Recombination Occurs During the Prolonged Prophase of Meiosis I

A schematic diagram shows the process by which double-stranded DNA breaks are fixed. A leftward pointing, horizontal arrow at the bottom of the diagram represents an increasing degree of interaction between the homologous chromosomes. During the leptotene portion, two homologous DNA strands are aligned. After a double-stranded break, one broken strand aligns with the complementary strand on the homologous DNA. Entering the zygotene phase, a bridge forms between the broken DNA and the complementary DNA strand. The broken strand then invades the complete strand, forming a synaptonemal complex. Then, in the pachytene phase, the broken strand is extended by DNA synthesis based on the complementary homologous strand. The synaptonemal complex is then stabilized by formation of a double Holliday junction.

Prophase I is the longest and arguably most important segment of meiosis, because recombination occurs during this interval. For many years, cytologists accept divided prophase I into multiple segments, based upon the advent of the meiotic chromosomes. Thus, these scientists have described a
leptotene
(from the Greek for “sparse threads”) phase, which is followed sequentially by the
zygotene
(from the Greek for “paired threads”),
pachytene
(from the Greek for “thick threads”), and
diplotene
(from the Greek for “ii threads”) phases. In contempo years, cytology and genetics have come together and then that researchers now understand some of the molecular events responsible for the stunning rearrangements of
chromatin
observed during these phases.

Recall that prophase I begins with the alignment of homologous chromosome pairs. Historically, alignment has been a hard trouble to approach experimentally, but new techniques for visualizing individual chromosomes with fluorescent probes are providing insights into the process. Recent experiments suggest that chromosomes from some species have specific sequences that human action as pairing centers for alignment. In some cases, alignment appears to begin as early as interphase, when homologous chromosomes occupy the same territory inside the interphase
nucleus
(Effigy 5). However, in other species, including yeast and humans, chromosomes exercise not pair with each other until double-stranded breaks (DSBs) appear in the Deoxyribonucleic acid (Gerton & Hawley, 2005). The formation of DSBs is catalyzed by highly conserved proteins with
topoisomerase
activity that resemble the Spo11 protein from yeast. Genetic studies have shown that Spo11 activity is essential for meiosis in yeast, considering
spo11
mutants fail to sporulate.

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Following the DSBs, one Deoxyribonucleic acid strand is trimmed dorsum, leaving a 3′-overhang that “invades” a homologous sequence on another
chromatid. Every bit the invading strand is extended, a remarkable structure called
synaptonemal complex
(SC) develops around the paired homologues and holds them in shut register, or
synapsis. The
stability
of the SC increases as the invading strand get-go extends into the homologue and so is recaptured by the broken chromatid, forming double Holliday junctions. Investigators accept been able to observe the procedure of SC formation with electron microscopy in meiocytes from the
Allium
establish (Figure half-dozen). Bridges approximately 400 nanometers long begin to form betwixt the paired homologues post-obit the DSB. Only a fraction of these bridges will mature into SC; moreover, non all Holliday junctions will mature into crossover sites. Recombination volition thus occur at only a few sites forth each chromosome, and the products of the crossover will become visible as chiasmata in diplotene later the SC has disappeared (Zickler & Kleckner, 1999).

A series of electron photomicrographs shows the gradual formation of synaptonemal complex patches following double-stranded breaks in DNA. The photomicrographs are shown in a row from left to right. The three photomicrographs at left are enclosed in a pink box labeled \"nascent DSB; partner complex.\" The two photomicrographs at center are enclosed in a green box labeled \"onset of stable strand exchange.\" A final photomicrograph at right is enclosed in an orange box labeled \"CO nodule plus SC patch.\" Nascent DNA (pink box) appears as two horizontal black lines arranged in parallel with a bridge beginning to form between the two lines. When stable strand exchange occurs (green box), the upper DNA strand overlaps across the lower DNA strand, forming an X-shape. CO nodules and SC patches (orange box) hold the two recombined DNA strands closely together. The DNA looks like two horizontal, parallel lines with vertical lines connecting and spanning the space between them.

Effigy 6: Visualization of chromosomal bridges in Allium fistulosum and Allium cepa (plant) meiocytes.

The sites of double-stranded intermission (DSB) dependent homologue interaction tin exist seen as approximately 400 nm bridges betwixt chromosome axes. These bridges, which probably contain a DSB that is already engaged in a nascent interaction with its partner DNA, occur in large numbers. Their formation depends on the RecA (recombination protein) homologues that are expressed in this species. In the side by side phase of homologue interaction, these nascent interactions are converted to stable strand-invasion events. This nucleates the formation of the synaptonemal complex (SC).

© 2005 Nature Publishing Group Gerton, J. L. & Hawley, R. South. Homologous chromosome interactions in meiosis: variety amidst conservation.
Nature Reviews Genetics
half-dozen,
481 (2005). All rights reserved.

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References and Recommended Reading


Gerton, J. L., & Hawley, R. S. Homologous chromosome interactions in meiosis: Diversity amidst conservation.
Nature Reviews Genetics
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Hassold, T., & Hunt, P. To err (meiotically) is human: The genesis of human aneuploidy.
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Lopez-Maury, L., Marguerat, S., & Bahler, J. Tuning gene expression to changing environments: From rapid responses to evolutionary adaptation.
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Three Cells Undergo Meiosis How Many Haploid Cells Are Produced

Source: http://www.nature.com/scitable/topicpage/meiosis-genetic-recombination-and-sexual-reproduction-210