Meiosis - High School Biology

When chromatids "cross over," homologous chromosomes trade pieces of genetic material, resulting in novel combinations of alleles, though the same genes are still present. Crossing over occurs during prophase I of meiosis before tetrads are aligned along the equator in metaphase I.

By meiosis II, only sister chromatids remain and homologous chromosomes have been moved to separate cells. Recall that the point of crossing over is to increase genetic diversity. If crossing over did not occur until sometime during meiosis II, sister chromatids, which are identical, would be exchanging alleles. Since these chromatids are identical, this swap of material would not actually change the alleles of the chromatids.

Crossing over is a process that happens between homologous chromosomes in order to increase genetic diversity. During crossing over, part of one chromosome is exchanged with another. The result is a hybrid chromosome with a unique pattern of genetic material. Gametes gain the ability to be genetically different from their neighboring gametes after crossing over occurs. This allows for genetic diversity, which will help cells participate in survival of the fittest and evolution.

During crossing over, homologous chromosomes come together in order to form a tetrad. This close contact allows the nonsister chromatids from homolgous chromosomes to attach to one another and exchange nucleotide sequences. The word "nonsister" implies that the chromatids have the same genes, but are not exact copies of one another, as they come from separate chromosomes.

The crossing over of homologous chromosomes occurs in prophase I of meiosis. Prophase I of meiosis is characterized by the lining up of homologous chromosomes close together to form a structure known as a tetrad. A tetrad is composed of four chromatids.

Anaphase I is marked by the separation of homologous chromosomes, whereas in anaphase II there is the separation of sister chromatids. In anaphase I sister chromatids are still intact and connected at the centromere. Prophase II is similar to prophase in mitosis in that there is the break down of the nuclear membrane and the formation of spindle fibers in preparation for the separation of sister chromatids.

The tetrad, which divides into non-sister chromatids, exchanges genetic information in order to make the genetic pool more variant, and result in combinations of phenotypic traits that can occur outside of linked genotypic coding.

During prophase I, homologous chromosomes pair with each other and exchange genetic material in a process called chromosomal crossover. The exchange occurs in segments over a small region of homology (similarity in sequence, ie., the same alleles). The new combinations of DNA created during crossover provide a significant source of genetic variation.

Crossing over occurs when chromosomal homologs exchange information during metaphase of Meiosis I. During this stage, homologous chromosomes line up on the metaphase plate and exchange genetic information.

Crossing over occurs during prophase I when parts of the homologous chromosomes overlap and switch their genes.

When a primary oogonium undergoes meiosis, it will only result in one viable germ cell, or egg. The other smaller cells are called polar bodies and typically disappear following division. Spermatogenesis will result in four separate sperm cells, each capable of producing offspring.

Both eggs and sperm are haploid, and both processes can involve crossing over during meiosis.

Down's syndrome is caused by an extra copy of chromosome 21. This trisomy is a result of nondisjunction, which can occur during either meiosis I or meiosis II. Nondisjunction most often occurs during anaphase I of meiosis. Note that most other trisomies and monosomies are lethal.

Oogonia are the primordial oocytes formed inside females either during or shortly after birth. At this time, the formation of primary oocytes occurs during dictyate, which lasts until the surge of luteinizing hormone (LH) just before the onset of puberty. After menarche, a few of these cells will further develop each period into secondary oocytes, which are halted in metaphase II until fertilization. At the end of meiosis II, both polar bodies created to discard extra haploid sets of chromosomes disintegrate, leaving behind the oocyte which can then mature into an ovum. Thus, the correct order is the following order: oogonium, primary oocyte, secondary oocyte, and ovum.

Follicular atresia is a hormone-controlled, apoptotic (cell-suicide) process by which immature follicles degenerate and are resorbed into the main body of the ovary, leaving one out of typically 20 primary follicles standing as a secondary follicle. This process, moderated by follicle stimulating hormone and tumor necrosis factor alpha (TNF), assists the body in forming the corpus luteum out of the remaining follicle following ovulation, as the body would otherwise not be able to generate enough progesterone to continue the process.

The female gamete is called the ovum (ova, plural). A zygote is the cell that results from fertilization occurs between two gametes. An ovary is the female reproductive organ that produces ova. Sperm is the male gamete. The uterus is the organ where the fetus develops.

After completing meiosis II, ova are haploid cells containing one chromosome. Haploid cells have half the number of chromosomes (n) as a diploid cell. Haploid cells like ova and sperm will merge during fertilization and form a diploid cell with two complete sets of chromosomes (2n).

An oocyte develops in the ovaries during female gametogenesis. If the oocyte eventually becomes an ovum and is fertilized in the fallopian tubes, the resulting gamete will be implanted in the uterus. The placenta is an organ that connects the developing fetus to the uterine wall, and the cervix is the lower portion of the uterus that separates the vagina from the uterus.

The release of the secondary oocyte from the ovaries is ovulation. This occurs after the follicles surrounding the oocyte mature and rupture, releasing the cell to be available for fertilization. Menstruation occurs after ovulation - if the ovum fails to be fertilized, the uterus will shed its lining. Oogenesis, the creation of an ovum, is the female form of gametogenesis (creation of a gamete). Meiosis is the process of cell division in which the number of chromosomes is halved.

Primary oocytes enter meiosis I and replicate their genomes, but they do not make their first meiotic division. They remain in prophase I until a female begins her first menstrual cycle. Then, each month, one primary oocyte resumes the process of meiotic division.

A primary oocyte becomes a secondary oocyte when the follicle grows and matures and the primary oocyte completes its first meiotic division. Shortly after, the follicle will rupture and release the secondary oocyte to be fertilized. Later, after the oocyte develops into an ovum and is fertilized, it becomes a diploid zygote, which develops into an embryo through the process of mitosis. The ovary is the reproductive organ where the ova are produced, and a gamete is the haploid cell (ovum or sperm) that is fertilized and forms a zygote.

During the menstrual cycle, typically only one follicle matures and is fertilized. The follicles that do not mature are called polar bodies and degenerate. However, sometimes multiple follicles remain and are available to be fertilized, resulting in genetically distinct embryos, known as fraternal twins.