Asexual and Sexual Reproduction

What are the differences between asexual and sexual reproduction?

Asexual reproductionglossary term (opens in a new window) occurs when only one parent contributes genes to the offspring; sexual reproductionglossary term (opens in a new window) is the creation of an offspring by combining the genetic material from two parents:

• The resulting offspring of asexual reproduction are genetically identical to the parent.
• Organisms that undergo asexual reproduction do not go through the same processes used in sexual reproduction including meiosisglossary term (opens in a new window), spermatogenesisglossary term (opens in a new window), oogenesisglossary term (opens in a new window), and fertilizationglossary term (opens in a new window).
• The genetic make-up of a sexually reproduced offspring is unique to both of its parents.

What are haploid and diploid cells?

Diploid cells  contain two complete sets of chromosomes inherited from the parent cell; haploid cells contain only a single set of chromosomes, or half the parental number:

• Somatic cells are diploid.
• Often, diploid cells are labeled as 2N.
• Gametes are haploid (N).
• When the haploid cells of the egg and sperm come together in sexual reproduction, the resulting cell is diploid.

What is the function of each stage of meiosis?

The haploid cells of gametes are created in a two-part cell division process called meiosisglossary term (opens in a new window):

• Only organisms that reproduce sexually undergo meiosis.
• During the two stages of meiosis, a parental diploid produces four haploid daughter cells.
• In meiosis I, homologous chromosome pair up, align at the center of the cell, separate to opposite ends, and divide into two daughter cells.
• The phases of meiosis are prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.
• During prophase I of meiosis I, chromosomes undergo crossing over in which entire chromosomes or parts of the chromatids cross over one another and switch genetic information.
• Crossing over is a major contributor to genetic diversity.
• In meiosis II, chromosomes condense, align at the center of the cell, separate at the centromere, move to opposite ends, and divide into two daughter cells.
• At the end of meiosis II, there are a total of four haploid cells.

What are the differences between mitosis and meiosis?

Meiosis II is similar to mitosisglossary term (opens in a new window), the process of cell division that occurs in body cells. However, there are major differences between the two processes:

• Chromosomes resulting from meiosis II are not identical because they have been recombined.
• Mitosisglossary term (opens in a new window) occurs in somatic cells while meiosis occurs in gametes.
• Mitosis results in diploid cells, meiosis in haploid cells.
• There is no crossing over of chromosomes in mitosis.

What are spermatogenesis and oogenesis?

Spermatogenesis is the process of sperm formation, and oogenesis is the process of egg formation:

• Cells in a male organism called spermatogonia produce sperm.
• During mitosis, one daughter cell becomes a spermatocyte, which will eventually become sperm.
• Spermatocytes undergo the stages of meiosis to produce two spermatids, or young spermatozoa.
• Spermatozoa develop into mature sperm cells.
• Oocytes undergo cell division to produce a haploid cellglossary term (opens in a new window), the ovum (egg).
• The ovarian cycle carries a female body through the steps necessary to mature and release an egg cell.
• Ovulation is the release of an egg cell.

What are fertilization and development?

Fertilization is the fusion of an ovum with a sperm in animals that reproduce sexually. Development is the process during which an organism forms via cell division and growth:

• During intercourse, sperm are released into a female. A single sperm is allowed to enter the egg.
• Following fertilization, the egg is activated and undergoes a series of metabolic changes.
• The one sperm that entered the egg penetrates the egg cytoplasm and the sperm's nuclear envelope disperses.
• Chromatinglossary term (opens in a new window) from both egg and sperm are surrounded by a nuclear membrane.
• The genomes of the two haploid (N) cells fuse to form a single diploid cellglossary term (opens in a new window) (2N) containing a set of chromosomes from each parent.
• The fertilized cell is called a zygote and will continue to develop into an embryo.
• Almost immediately, the one-celled zygote begins to undergo mitosis.
• The embryo undergoes several divisions progressing itself from a morula to a blastula.
• The blastula attaches to the wall of the uterus.
• As the embryo develops, the blastula divides into three layers in a process called gastrulation. 
• The three layers of cells consist of the ectoderm, mesoderm and endoderm. They will form all the tissues and organs of the developing organism.

What is Somatic Cell Nuclear Transfer (SCNT)?

Somatic cell nuclear transferglossary term (opens in a new window) (SCNT) allows scientists to implant the DNA held in the nucleus of one somatic cell into an egg in order to grow and develop the desired organism:

• A somatic cell nucleus is inserted into an enucleated egg cell, where it is reprogrammed as a diploid cell.
• The organism grows as a normal zygote.
• The DNA of the zygote is identical to the original organism that the somatic cell was taken from.
• SCNT is a controversial technology.

What are the effects of non-disjunction?

Non-disjunction is an error that can occur during meiosis:

• When chromosomes fail to separate properly, the number of chromosomes in the daughter cells is uneven.
• Aneuploidyglossary term (opens in a new window) is the discrepancy in chromosome number.
• The condition in which a daughter cell is missing a chromosome or part of a chromosome is called monosomyglossary term (opens in a new window).
• Trisomyglossary term (opens in a new window) is the condition in which a daughter cell receives an additional chromosome from one of the parent cells.
• Any discrepancy in chromosome number may cause birth defects and life-long disorders.

What are some examples of aneuploidy, monosomy, trisomy?

Aneuploidy may present in the form of missing chromosomes or extra chromosomes:

• Turner Syndrome is a condition caused by monosomy in which a female only has one X chromosome.
• Klinefelter syndrome results from trisomy when a male has an extra X chromosome.
• Down syndrome and Patau syndrome are other conditions caused by trisomy.