Prophase - definition and stages of mitosis and meiosis | biology (2023)

Prophase-Definition

Prophase is the initial stage of cell division in eukaryotes. Prophase is recognized in both mitosis and meiosis by the compaction of chromosomes and the separation of centrioles at the centrosome. This organelle controls the microtubules in the cell and each centriole is half of the organelle. During prophase, they separate to provide microtubule centers in each new cell. Organelles, chromosomes, and many other substances and structures move through the cell via microtubules and associated proteins. These structures are collectively known asmolecular motorsand drive many cellular processes. Prophase initiates cell division by separating the centers of these molecular motors and condensing the chromosomes.

prophase in mitosis

Mitosis occurs after interphase in eukaryotes. During interphase, DNA is replicated along with the organelles needed for division. This includes the centrosome, where the centriole replicates. With a replicated genome and organelles, the cell can initiate mitosis. The first stage of mitosis is prophase and it sets the stage for the later stages of mitosis. During the interphase that precedes mitosis, the chromosomes are loose. At the beginning of prophase, the DNA-associated proteins are activated and the DNA coils around them and clumps into dense shapes. This stage can be seen under the microscope as a darkening of various parts of the cell nucleus.

Another part of the cell nucleus.nucleolus, disappears during prophase. The nucleolus is a dark spot in the nucleus of the cell that contains RNA and proteins responsible for the formation of ribosomes. Ribosomes are the large complexes of proteins and RNA that give rise to other proteins. As the cell attempts to transcribe new RNA during cell division, this complex can be broken down and distributed to each new cell. It is reassembled after cell division.

In plants, prophase is preceded by a stage in which the nucleus moves toward the center of the cell and the large water-filled vacuole moves out of the way. Some plants also lack centrosomes and organize their microtubules in different ways. During pre-prophase, these mechanisms are established. As in animals, the chromosomes condense and become visible, and the nucleolus disappears during prophase.

After the prophase of mitosis, the microtubule-bearing chromosomes arranged in the middle of the cell and the sister chromatids of each chromosome separate into new cells. In this way, the cells are identical to the cell that existed before the DNA was copied. Each cell will still havecounterpart toof chromosomes that carry different alleles for each gene. During the prophase of mitosis, molecular motors and the necessary DNA condensation allow the sister chromatids to separate.

prophase in meiosis

There are two stages of prophase during meiosis. Meiosis consists of two cell divisions separated by what is called a phase.questioned.

Prophase I

The first division of meiosis,Means Ibegins after a cell passes through interphase. As in the case of interphase before mitosis, each chromosome replicates during interphase synthesis. Thus, when meiosis begins, there are two copies of each chromosome linked together as sister chromatids on the same chromosome. During prophase I, chromosomes condense as in mitosis. However, homologous chromosomes also pair up in prophase I. When homologous chromosomes pair up, parts of the DNA on different chromosomes can be swapped. This recombines the alleles present on each chromosome as seen in the image below.

homologous recombinationis responsible for the law of independent assortment. In addition to genes that are closely linked on a chromosome, almost any combination of alleles can result from homologous recombination. For example, if your father has brown hair and blue eyes and your mother has blonde hair and brown eyes, you could get any combination of these traits. These features are different parts of the chromosomes and get mixed up during recombination. The law of independent sorting created in prophase I ensures that homologous chromosomes are conserved from the parents.

After prophase I, meiosis I continues to divide the cell contents. Unlike mitosis, in meiosis I the sister chromosomes stay together, but the homologous chromosomes are separated. Each homologous chromosome carries different alleles for each gene. During meiosis I, these homologous chromosomes separate, leading to the law of separation. This ensures that sexual organisms can transmit any allele for any gene, regardless of the phenotypic relationship between the alleles. Thus, there may be recessive alleles in the organism, which act as "carriers" of the allele and do not show the phenotypic effects of the allele, but transmit them to the offspring. This ensures that each generation can contain variability that can help populations survive changes in the environment.

Prophase II

After meiosis I, the two daughter cells take a short break, calledquestioned. During this interruption, the DNA is not replicated because it is already made up of duplicate strands of DNA joined together in place.Centromere. Although DNA exists as double sister chromatids, only half of each homologous pair is present in each cell. As in prophase of mitosis, the centrosome, which contains microtubules, separates on both sides of each cell during prophase II. The nucleus begins to disintegrate and the chromosomes condense again in preparation for being separated.

During the remainder of meiosis II, the sister chromatids of each chromosome separate into two new cells. At the end of meiosis, there will be a total of 4 cells from a single starting cell. Although prophase II is very similar to the prophase of mitosis, the intricate processes involved in prophase I allow sexually reproducing organisms to reduce their ploidy in preparation for a fertilization event between cells from two different organisms.

• preprofase– The stage immediately before prophase in plants.
• Centromere– A region on chromosomes that joins sister chromatids and provides a site for microtubule attachment.
• centrossoma– The organelle responsible for organizing microtubules.
• molecular motors– Proteins and microtubules that can work together to transport structures through the cytosol of a cell.

Trial

1. Bacterial cells also divide. However, bacteria do not need to condense their little rings of DNA before separation takes place. Do bacteria have prophase?
A.Y
B.NO
C.Only in higher bacteria.

2. A cell undergoes meiosis. The first cell division took place, the cell was inactive for a while, and the chromosomes began to condense again. What stage is the cell in?
A.Prophase
B.Prophase I
C.Prophase II

3. In fungi, chromosomes condense and centrosomes separate. However, the nuclear envelope never collapses. Mitosis or meiosis continues, but inside the nucleus, until the cell divides. When does prophase occur in fungi?
A.Never
B.At the beginning of cell division.
C.After the nucleus splits