Cellular Division - AP Biology

Once the soon-to-be daughter DNA has reached opposite poles of the cell, new nuclear envelopes form around them during telophase. Also, in telophase, the spindle apparatus breaks down and the DNA begins to condense. Remember that telophase is essentially the opposite of prophase.

Prophase I is the correct answer. During oogenesis in mammals, meiosis I occurs during the prenatal age. When the germ cells reach prophase I, the cell cycle is arrested, and the cells are frozen in prophase until puberty.

During puberty, the female will begin to ovulate. This means that one egg cell will progress from prophase I to metaphase I and complete meiosis on a cyclical basis, known as the menstrual cycle.

Mitosis is used by somatic cells throughout the body. The goal of mitosis is to replace older cells with newer, healthier cells. In order for this replacement to be effective, the daughter cells must be identical to the parent cell. Somatic cells, or "body cells," are diploid, meaning that they carry two copies of each allele. Each round of mitosis produces two daughter cells after one division.

Meiosis only takes place in the gonads and is used to produce gametes. Gametes fuse to form a diploid zygote, but each individual gamete carries only half of the genetic information to form this zygote; as such, all gametes are haploid and carry only one copy of each allele. Gametes are not identical to the parent cell for this reason (the parent cell is diploid). Genetic variation (crossing over) can also occur during meiosis to enhance genetic diversity. Each round of meiosis produces four daughter cells after two divisions.

There are two events that occur in prophase I that do not occur in any other stage: chiasmata (crossing over) and synapsis (pairing of the chromosomes). Note that chiasmata does not occur during prophase of mitosis, but synapsis does occur.

In prophase I, a process called synapsis involves the pairing of chromosomes. Chromosome pairs are referred to as a tetrad, homologous pair, or as bivalents.

Meiosis involves two cell divisions. During the first division, pairs of homologous chromosomes align at the center of the cell and are separated into two daughter cells. During the second division, single chromosomes align at the cell center (as they would during mitosis) and sister chromatids are separated into the daughter cells.

When homologous chromosomes align during the first division there are a total of four chromatids in each set, forming a tetrad. The alignment of chromosomes at the equatorial plate takes place during metaphase. Since we are looking at the alignment of chromosomes during the first meiotic division, the correct answer will be metaphase I.

The separation of sister chromatids is the only item of the answer choices that occurs in both mitosis and meiosis. Prophase II and metaphase II only occur in meiosis, as does recombination between homologous chromosomes. Recombination between sister chromatids does not occur (they are identical).

Separation of the sister chromatids by the microtubules pulling them to opposite poles occurs in anaphase. Also, during late anaphase, cytokinesis begins. This is the process of the cell dividing, since the nucleus has already divided (karyokinesis).

Meiosis results in cells with 23 chromosomes (22 autosomes and 1 sex chromosome). Note that these cells are haploid since n=23, and nonidentical, due to crossing over during prophase I.

Gametes are made via meiosis which produces cells with n=23 instead of diploid cells. If gametes were produced instead by mitosis each gamete would be diploid not haploid. During fertilization of diploid gametes, the zygote would become 4n=92. With each new generation the number of chromosomes would double.

The processes of mitosis and meiosis have many characteristics in common (i.e. cytokinesis, chromosome condensation, and nuclear membrane reformation); however, there are key differences that distinguish the processes from one another. One of these differences is the cell type that result from each respective process. Mitosis results in two daughter cells with the same number of chromosomes as the parent cell. Meiosis, on the other hand, yields four daughter cells with half of the number of chromosomes as the parent cell—“haploid” cells.

The processes of mitosis and meiosis have many differences between them. These differences include the genetic recombination event called crossing over unique to meiosis, the fact that only mitotic daughter cells are genetically identical to parent cells, and the paring of homologous chromosome pairs during metaphase I of meiosis. One characteristic that is common to both processes is that they occur in all animals.

During prophase I of meiosis, a crossing over event allows for genetic recombination. Crossing over ensures the formation of gametes with different genetic combinations. The process involves the swapping of genetic material from one homologous chromosome pair to another, and is facilitated by double stranded breaks in the DNA helix and recombinase enzymes. The structure formed during crossing over is called the “Holliday junction”.

During meiosis, the “chiasma” is the point of chromatid contact that precedes crossing over. The crossing over event then occurs at this point. Synapsis refers to the pairing of homologs during prophase I of meiosis.

During the anaphase I stage of meiosis, homologous chromosome pairs separate to opposite poles of the cell, and the cell elongates. The sister chromatids remain attached at the centromeres. This maintenance of sister chromatids is the key difference between meiosis and mitosis. In mitosis, anaphase features the separation of sister chromatids, which is what will happen during anaphase II of meiosis. Note however, that the sister chromatids in meiosis are not identical, due to crossing over in prophase I.

Meiosis is the process that forms haploid gametes, or sex cells, that will go on to form zygotes after fertilization. Meiosis is performed by germ line cells, which, in humans, are located in the gonads (ovaries of females and in the testicles of males).

Meiosis produces four daughter cells that are haploid, meaning that they contain half the number of chromosomes as the parent cell. Human parent somatic cells have 46 chromosomes, or 23 chromosome pairs, meaning that gametes produced during meiosis have 23 chromosomes.

Synapsis is the pairing of homologous chromosomes during prophase I of meiosis. Due to synapsis, pairs of homologous chromosomes are arranged in a way that allows for crossing over to occur, which leads to genetic recombination, and genetic diversity.

Meiosis I is the first cell division of the entire meiosis process. Following segregation of homologous chromosome pairs, or tetrads, the two resulting daughter cells have half the number of chromosomes as the parent cell. These chromosomes have two sister chromatids per chromosome, which are nonidentical due to crossing over.

Prophase II marks the beginning of the second cell division of meiosis, called meiosis II. Unlike prophase I, the chromosomes are not duplicated prior to the start of prophase II. However, during prophase II, the centrioles replicate, resulting in two pairs of centrioles per cell. The two centriole pairs separate to opposite cell poles. Other features of prophase II are chromosome condensation, the degradation of the nuclear membrane, and meiotic spindle formation.