When do cells become haploid

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. Full chromosomes are pulled to each pole during anaphase I, resulting in two haploid cells at the end of meiosis I.

During prophase II, sister chromatids align at the center of the cell in singular chromosome structures. These sister chromatids are separated during anaphase II, resulting in a total of four haploid cells. A diploid cell will have two copies of each chromosome, known as a homologous pair.

A haploid cell will only have one copy of each chromosome, though the chromosome may consist of two sister chromatids. During meiosis I, the cell is diploid because the homologous chromosomes are still located within the same cell membrane. 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. The quick answer to your question is that meiosis, with its two divisions, requires more energy than mitosis. As we mentioned, mitosis is an equational form of cell division, and meiosis is a reductional form of cell division. Meiosis I is the reductional division step during which the number of chromosomes is reduced by half, and meiosis II is an equational division step that resembles mitosis.

During prophase I of meiosis I, homologous chromosomes i. Meiosis produces haploid cells with new allele combinations different from those of either parent thanks in large part to the events that occur during meiosis I. We encourage you to focus in particular on the events that occur during prophase I. You might also be interested to read that mistakes occurring during mitosis and meiosis are linked to cancer and chromosomal abnormalities e.

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?