Meiosis when do cells become haploid

Diploid cell: XX Haploid cell: X During meiosis I, the cell is diploid because the homologous chromosomes are still located within the same cell membrane. Possible Answers: The cytoplasm splits and forms two diploid daughter nuclei.

The cell's chromatin condenses and forms chromosomes. DNA is replicated, resulting in two identical sister chromatids attached at the centromere. The cell grows in size, prepares mRNA and proteins, and prepares to divide. The cell enters a state in which it neither divides, nor is preparing to divide. Correct answer: DNA is replicated, resulting in two identical sister chromatids attached at the centromere. Explanation : The S phase occurs between the G1 and G2 phases and is the stage during which DNA is replicated, and then checked for defects.

During which phase of meiosis does DNA begin to condense? Possible Answers: Prophase I. Correct answer: Prophase I. Explanation : Mitosis is also known as "karyokinesis. Possible Answers: diploid. Correct answer: haploid. Explanation : Meiosis is the process by which a haploid cell is formed from a diploid cell. Which three events most accurately describes what occurs in meiosis I? Possible Answers: Homologous chromosomes are duplicated, pair, then separate. Homologous chromosomes pair, cross over, then separate.

Sister chromatids pair, cross over, then separate. Sister chromatids are duplicated, pair, then cross over. Correct answer: Homologous chromosomes pair, cross over, then separate. Explanation : In meiosis I, the homologous chromosomes have already been duplicated in S phase of interphase. Possible Answers: three. Correct answer: four.

Explanation : Meiosis involves two divisions and results in four unique daughter cells called gametes. Homologous chromosomes pair up in which stage of meiosis? Possible Answers: Metaphase II. Explanation : In prophase I chromosomes become compact and homologous chromosomes pair up.

Homologous chromosomes line up along the cell's equator in which stage? Possible Answers: Anaphase II. Correct answer: Metaphase I. Explanation : In metaphase I, homologous chromosomes line up along the center of the cell in order to be pulled apart.

Sister chromatids separate in which stage of meiosis? Correct answer: Anaphase II. Explanation : Chromatid disjunction occurs in anaphase II after the chromosomes line up along the equator during metaphase II. Copyright Notice. View Tutors. Simone Certified Tutor. Samantha Certified Tutor. Allegheny College, Bachelor of Science, Neuroscience. The recombination process is discussed in greater detail later in this article.

Crossovers between homologous chromatids can be visualized in structures known as chiasmata, which appear late in prophase I Figure 4. Chiasmata are essential for accurate meioses. 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 same 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 because the X and Y chromosomes have small regions of similarity near their tips. Crossover between these homologous regions ensures that the sex chromosomes will segregate properly when the cell divides. Next, during anaphase I , the pairs of homologous chromosomes separate to different daughter cells.

Before the pairs can separate, however, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to separate the pairs of chromosomes to different daughter 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.

Meiosis II resembles a mitotic division, except that the chromosome number has been reduced by half. Thus, the products of meiosis II are four haploid cells that contain a single copy of each chromosome.

In mammals, the number of viable gametes obtained from meiosis differs between males and females. In males, four haploid spermatids of similar size are produced from each spermatogonium.

In females, however, the cytoplasmic divisions that occur during meiosis are very asymmetric. Fully grown oocytes within the ovary are already much larger than sperm, and the future egg retains most of this volume as it passes through meiosis.

As a consequence, only one functional oocyte is obtained from each female meiosis Figure 2. The other three haploid cells are pinched off from the oocyte as polar bodies that contain very little cytoplasm. Prophase I is the longest and arguably most important segment of meiosis, because recombination occurs during this interval.

For many years, cytologists have divided prophase I into multiple segments, based upon the appearance of the meiotic chromosomes. Thus, these scientists have described a leptotene from the Greek for "thin 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 "two threads" phases.

In recent years, cytology and genetics have come together so 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 difficult problem 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 act as pairing centers for alignment. In some cases, alignment appears to begin as early as interphase, when homologous chromosomes occupy the same territory within the interphase nucleus Figure 5.

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, because spo11 mutants fail to sporulate. As the invading strand is extended, a remarkable structure called synaptonemal complex SC develops around the paired homologues and holds them in close register, or synapsis.

The stability of the SC increases as the invading strand first extends into the homologue and then is recaptured by the broken chromatid, forming double Holliday junctions. Investigators have been able to observe the process of SC formation with electron microscopy in meiocytes from the Allium plant Figure 6. Bridges approximately nanometers long begin to form between the paired homologues following the DSB.

Only a fraction of these bridges will mature into SC; moreover, not all Holliday junctions will mature into crossover sites. Gerton, J. Homologous chromosome interactions in meiosis: Diversity amidst conservation. Nature Reviews Genetics 6 , — doi Hassold, T.

To err meiotically is human: The genesis of human aneuploidy. Nature Reviews Genetics 2 , — doi Lopez-Maury, L. Tuning gene expression to changing environments: From rapid responses to evolutionary adaptation. Nature Reviews Genetics 9 , — doi Marston, A. Meiosis: Cell-cycle controls shuffle and deal. Nature Reviews Molecular Cell Biology 5 , — doi Page, S. Chromosome choreography: The meiotic ballet. Science , — Petes, T. Meiotic recombination hot spots and cold spots.

Zickler, D. Meiotic chromosomes: Integrating structure and function. Annual Review of Genetics 33 , — Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer.

Karyotyping for Chromosomal Abnormalities. Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes. 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. In this case, there are two possible arrangements at the equatorial plane in metaphase I.

The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes.

In anaphase I, the microtubules pull the attached chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart.