During leptonema, the diffuse chromatin starts condensing into chromosomes. Each of these chromosomes is double stranded, consisting of two identical sister chromatids which are held together by a centromere; this arrangement will later give each chromosome a variation on an X-like shape, depending on the positioning of the centromere.
In the next substage, zygonema, there is further condensation of the chromosomes. As they come into closer contact, a protein compound called the synaptonemal complex forms between each pair of double-stranded chromosomes. As Prophase I continues into its next substage, pachynema, the homologous chromosomes move even closer to each other as the synaptonemal complex becomes more intricate and developed.
This process is called synapsis, and the synapsed chromosomes are called a tetrad. The tetrad is composed of four chromatids which make up the two homologous chromosomes. During pachynema and the next substage, diplonema, certain regions of synapsed chromosomes often become closely associated and swap corresponding segments of the DNA in a process known as chiasma.
At this point, while still associated at the chiasmata, the sister chromatids start to part from each other although they are still firmly bound at the centromere; this creates the X-shape commonly associated with condensed chromosomes.
The nuclear membrane starts to dissolve by the end of diplonema and the chromosomes complete their condensation in preparation for the last substage of prophase I, diakinesis. During this part, the chiasmata terminalize move toward the ends of their respective chromatids and drift further apart, with each chromatid now bearing some newly-acquired genetic material as the result of crossing over.
Simultaneously, the centrioles, pairs of cylindrical microtubular organelles, move to opposite poles and the region containing them becomes the source for spindle fibers. These spindle fibers anchor onto the kinetochore, a macromolecule that regulates the interaction between them and the chromosome during the next stages of meiosis.
The kinetochores are attached to the centromere of each chromosome and help move the chromosomes to position along a three-dimensional plane at the middle of the cell, called the metaphase plate. The cell now prepares for metaphase I, the next step after prophase I. During metaphase I, the tetrads finish aligning along the metaphase plate, although the orientation of the chromosomes making them up is random.
The chromosomes have fully condensed by the point and are firmly associated with the spindle fibers in preparation for the next step, anaphase I. During this third stage of meiosis I, the tetrads are pulled apart by the spindle fibers, each half becoming a dyad in effect, a chromosome or two sister chromatids attached at the centromere.
Assuming that nondisjunction failure of chromosomes to separate does not occur, half of the chromosomes in the cell will be maneuvered to one pole while the rest will be pulled to the opposite pole. This migration of the chromosomes is followed by the final and brief step of meiosis I, telophase I, which, coupled with cytokinesis physical separation of the entire mother cell , produces two daughter cells. Each of these daughter cells contains 23 dyads, which sum up to 46 monads or single-stranded chromosomes.
Meiosis II follows with no further replication of the genetic material. The chromosomes briefly unravel at the end of meiosis I, and at the beginning of meiosis II they must reform into chromosomes in their newly-created cells. This brief prophase II stage [isEmbeddedIn] is followed by metaphase II, during which the chromosomes migrate toward the metaphase plate.
During anaphase II, the spindle fibers again pull the chromosomes apart to opposite poles of the cell; however, this time it is the sister chromatids that are being split apart, instead of the pairs of homologous chromosomes as in the first meiotic step. A second round of telophase this time called telophase II and cytokinesis splits each daughter cell further into two new cells. Each of these cells has 23 single-stranded chromosomes, making each cell haploid possessing 1N chromosomes.
As mentioned, sperm and egg cells follow roughly the same pattern during meiosis , albeit a number of important differences. More complex organisms, including humans, produce specialised sex cells gametes that carry half of the genetic information, then combine these to form new organisms.
The process that produces gametes is called meiosis. During meiosis in humans, 1 diploid cell with 46 chromosomes or 23 pairs undergoes 2 cycles of cell division but only 1 round of DNA replication. The result is 4 haploid daughter cells known as gametes or egg and sperm cells each with 23 chromosomes — 1 from each pair in the diploid cell. At conception, an egg cell and a sperm cell combine to form a zygote 46 chromosomes or 23 pairs.
This is the 1st cell of a new individual. The halving of the number of chromosomes in gametes ensures that zygotes have the same number of chromosomes from one generation to the next. This is critical for stable sexual reproduction through successive generations. Replication of DNA in preparation for meiosis.
After replication, each chromosome becomes a structure comprising 2 identical chromatids. The chromosomes condense into visible X shaped structures that can be easily seen under a microscope, and homologous chromosomes pair up. Recombination occurs as homologous chromosomes exchange DNA. At the end of this phase, the nuclear membrane dissolves. The pairs of chromosomes separate and move to opposing poles.
Either one of each pair can go to either pole. Nuclear membranes reform. In kind means that the offspring of any organism closely resemble their parent or parents. Sexual reproduction requires fertilization: the union of two cells from two individual organisms. Haploid cells contain one set of chromosomes.
Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again or there will be a continual doubling in the number of chromosome sets in every generation.
Therefore, sexual reproduction includes a nuclear division that reduces the number of chromosome sets. Offspring Closely Resemble Their Parents : In kind means that the offspring of any organism closely resemble their parent or parents. The hippopotamus gives birth to hippopotamus calves a. Joshua trees produce seeds from which Joshua tree seedlings emerge b. Adult flamingos lay eggs that hatch into flamingo chicks c. Sexual reproduction is the production of haploid cells gametes and the fusion fertilization of two gametes to form a single, unique diploid cell called a zygote.
All animals and most plants produce these gametes, or eggs and sperm. In most plants and animals, through tens of rounds of mitotic cell division, this diploid cell will develop into an adult organism. Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis.
Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid, so the resulting cells have half the chromosomes as the original. To achieve this reduction in chromosomes, meiosis consists of one round of chromosome duplication and two rounds of nuclear division.
Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned.
In meiosis I, the first round of meiosis, homologous chromosomes exchange DNA and the diploid cell is divided into two haploid cells.
Meiosis is preceded by an interphase consisting of three stages. The G 1 phase also called the first gap phase initiates this stage and is focused on cell growth. The S phase is next, during which the DNA of the chromosomes is replicated. This replication produces two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins.
The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. Finally, during the G 2 phase also called the second gap phase , the cell undergoes the final preparations for meiosis. During prophase I, chromosomes condense and become visible inside the nucleus.
As the nuclear envelope begins to break down, homologous chromosomes move closer together. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations, spreading to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis.
In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. The synaptonemal complex also supports the exchange of chromosomal segments between non-sister homologous chromatids in a process called crossing over.
The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to an exchange of DNA between chromosomes. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.
Crossover between homologous chromosomes : Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.
Synapsis holds pairs of homologous chromosomes together : Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.
The key event in prometaphase I is the formation of the spindle fiber apparatus where spindle fiber microtubules attach to the kinetochore proteins at the centromeres. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes at the kinetochores. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole.
In addition, the nuclear membrane has broken down entirely. During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles.
The homologous pairs orient themselves randomly at the equator. This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set.
There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per set.
Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition.
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