When does diploid become haploid

Haploid is the quality of a cell or organism having a single set of chromosomes. Organisms that reproduce asexually are haploid. Sexually reproducing organisms are diploid having two sets of chromosomes, one from each parent.

In humans, only their egg and sperm cells are haploid. Haploid refers to a cell or an organism that has only a single set of chromosomes. Most cells spend the majority of their time in interphase, which consists of the G1, S, and G2 phases.

During interphase, a cell grows, duplicates its chromosomal DNA, and prepares to divide. As its name suggests, DNA replication i. During mitosis, a cell divides and splits its entire contents including its chromosomes between two daughter cells. As you can imagine, the cell cycle is a tightly regulated process. There are checkpoints throughout the cell cycle to ensure that everything is in order at the end of each stage before the cell proceeds to the next stage.

As you might imagine, a failure to properly complete one stage of the cell cycle before jumping into the next could have drastic consequences for a cell! Although mitosis occupies only a small fraction of the entire cell cycle, it is an extremely important stage because it involves attaching the chromosomes to the mitotic spindle and precisely distributing one copy of each and every chromosome into the two resulting daughter cells.

Mitosis can be further subdivided into five phases: prophase, metaphase, anaphase, telophase, and cytokinesis. If any of the proteins involved in mitosis malfunction, or if the chromosomes do not segregate correctly, the result could be failed cell division or, at the other extreme, uncontrolled cell growth which can lead to cancer! As we mentioned, DNA replication occurs during the S phase of the cell cycle, before the cell enters the M phase and divides. You might be surprised to read that although the chromosomal DNA must be unwound during S phase to permit access by the DNA replication machinery, it becomes highly condensed and compacted as cells enter mitosis.

Indeed, mitotic chromosomes are often hundreds to thousands times more condensed than interphase chromatin. As a result, individual interphase chromosomes are often not visible whereas mitotic chromosomes are clearly observed under the microscope. Why are chromosomes condensed during mitosis?

Chromosome condensation plays a key role in the segregation of chromosomes between two daughter cells when a cell divides. The quick answer is that cells often produce a stockpile of metabolic enzymes capable of supporting metabolism even when the chromosomal DNA is being replicated.

Furthermore, the DNA is unwound at replication forks for only a brief moment in time before a complementary daughter strand of DNA is synthesized. As you can see, cell division is carefully regulated so that cells can maintain proper levels of metabolic activity while accurately segregating their DNA when they divide! Do transcription happen during DNA replication? Welcome back Sreeram, The timing of transcription depends largely on the gene. As you likely know, RNA transcription is initiated from promoter elements that target transcription factors to the DNA template.

RNA transcription is associated with a transcription bubble, which consists of an unwound template DNA that is accessible to RNA polymerases and transcription factors.

The short answer to your second question is no, transcription generally does not happen at the same time as DNA replication. In eukaryotic cells, DNA replication occurs during the S-phase of the cell cycle.

Or have cells evolved an efficient mechanism to deal with this molecular confrontation? Researchers have actively sought answers to these questions. Olavarietta et al. They found that a collision between DNA polymerase and RNA polymerase triggered deleterious effects on the structure of the DNA template, including the formation of knots behind the replication fork.

Ultimately, the cells exhibited incomplete replication of the plasmid and defective distribution of the plasmid to the daughter cells during mitosis. In other words, the cells could not recover from the collision between DNA polymerase and the transcription bubble. As it turns out, scientists have found very little evidence that this type of encounter occurs in nature, which is probably the result of the deleterious, and likely lethal, outcome. But there are some situations when DNA polymerase is known to pause at selected sites where RNA transcription may occur simultaneously.

Only after the first cytokinesis, when the daughter cells of meiosis I are fully separated, are the cells considered haploid. Following this first division, the cell begins meiosis II with prophase II, making this the first haploid meiotic stage.

The S phase occurs between the G1 and G2 phases and is the stage during which DNA is replicated, and then checked for defects. Depending on the level of nutrients and energy available, the cell will either enter the G0 phase or the M phase. During the G1 phase, the cell replicates organelles and grows in size. During the G2 phase, DNA is checked for damage and the cell prepares to divide.

The M phase refers to mitosis, while the G0 phase refers to quiescence—a period during which the cell is not preparing for division. Mitosis is also known as "karyokinesis. The "-kinesis" part of "karyokinesis" comes from the same roots as "kinetic" and refers to movement. Thus, mitosis is the movement of the nucleus.

Packing of the DNA occurs in prophase of mitosis so that it's easier to move rather than having to move the loose chromatin. Think of moving forty-six strands of hundreds of yards of yarn—we would want it to be tightly coiled to make it manageable. Meiosis is the process by which a haploid cell is formed from a diploid cell. The difference between haploid cells and diploid cells is that haploid cells contain one complete set of chromosomes, whereas diploid cells contain two complete sets of chromosomes.

Meiosis involves the division of a diploid 2n parent cell. The chromosomes are duplicated, but carry out two consecutive divisions. The result is four haploid n cells, each with half the number of chromosomes as the parent cell due to the separation of homologous pairs in meiosis I. In contrast, mitosis is the process by which a diploid parent cell produces two diploid daughter cells. In meiosis I, the homologous chromosomes have already been duplicated in S phase of interphase.

The sister chromatids are identical at this stage. Homologous chromosomes pair in prophase I, forming tetrads. The tetrads then cross over, exchanging genetic material. Then, the genetically-mixed tetrads line up on the metaphase plate and are separated in anaphase I. Note that after the first meiotic division, the two daughter cells are nonidentical and are haploid. Meiosis involves two divisions and results in four unique daughter cells called gametes.

Meiosis begins with one parent cell, after the first division there are two daughter cells, and then those each split, resulting in a total of four daughter cells. In prophase I chromosomes become compact and homologous chromosomes pair up. Also during prophase I, the nuclear membrane begins to break down and the spindle apparatus begins to form. In metaphase I, homologous chromosomes line up along the center of the cell in order to be pulled apart.

Recall that during meiosis I, homologous chromosomes pair, cross over, and separate. Meiosis II is when the sister chromatids are separated. Chromatid disjunction occurs in anaphase II after the chromosomes line up along the equator during metaphase II. The chromosomes are then pulled apart, with one chromatid moving north, and one moving south.

The next steps are telophase, and cytokinesis, which upon completion, will result in genetically distinct haploid gametes. If you've found an issue with this question, please let us know. With the help of the community we can continue to improve our educational resources.

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