Asexual and Sexual Reproduction Notes

Asexual and Sexual Reproduction Notes
from Starr and Taggart's Biology, Wadsworth Publishing, Co., 1992.

Mitosis

Meiosis

Meiosis Location

Development in Animals

Human Reproduction and Development

Plant Reproduction

Plant Growth and Development

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Cell Division and Mitosis

Each cell of a new generation must receive the required hereditary instructions and enough cytoplasm to operate on its own.
Cells of multicellular eukaryotes commonly have a diploid number of chromosomes. They have two of each type of chromosomes for that species. Each pair are said to be homologous which means that they are alike in size, shape, centromere location and which heritable traits they deal with.
Eukaryotic chromosomes are duplicated during interphase (between cell divisions). Whereas each was one DNA molecule (and associated proteins), now it consists of two, which temporarily stay attached as sister chromatids.
Eukaryotes employ different division mechanisms that serve different functions:

Mitosis is a type of nuclear division that maintains the parental number of chromosomes in each of two daughter nuclei. Thus if the parental cell's nucleus is diploid, the nucleus in each daughter cell also will be diploid.
Mitosis is the basis of bodily growth for multicellular eukaryotes. It also is the basis for asexual reproduction for some eukaryotes.
Meiosis is a type of nuclear division that reduces the parental chromosomes by half (to the haploid number) in each of four daughter nuclei.
Meiosis is the basis of sexual reproduction. It is a necessary precursor to the formation of the gametes.
For most organisms, actual cytoplasmic division or cytokinesis, occurs toward the end of nuclear division or at some point afterward.

A cell destined to divide by mitosis spends about 95% of the cell cycle in interphase, a period between mitotic divisions. During interphase, the cell increases in mass, doubles its cytoplasmic components and duplicates its chromosomes. G₁ is the period of cell growth before the DNA is duplicated, S is the period when the chromosomes are synthesized or replicated, and G₂ is the period after the DNA replication when the cell prepares for division.
Mitosis proceeds through five continuous stages

Prophase. Duplicated, threadlike chromosomes start to condense. New microtubules start to assemble in organized arrays near the nucleus; they will form the spindle fibers.

Metaphase. The nuclear envelope breaks up during the transition to metaphase. The chromosomes are free to capture microtubules of the forming spindle. Sister chromatids of each chromosome become oriented toward and attached to spindle fibers from opposite ends of the cell. All chromosomes are moved to and become aligned at the equator of the cell.

Anaphase. The two sister chromatids of each chromosome separate. Both are now independent chromosomes and they move to opposite ends of the cell.

Telophase. Chromosomes decondense to the threadlike form. A new nuclear envelope forms around the two parcels of chromosomes. Mitosis is completed.

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Meiosis

The life cycle of multicellular plants and animals generally includes meiosis and gamete formation, fertilization, and growth by way of cell divisions.

In animals, meiosis produces haploid gametes (sperm in males, eggs in females). Fusion of a sperm nucleus and an egg (ovum or oocyte) nucleus at fertilization produces a diploid cell (zygote), which develops into the multicellular individual by way of mitosis and cytokinesis.
In plants, meiosis gives rise to spores, which eventually undergo mitotic cell divisions that give rise to gamete production bodies.

Sexually reproducing organisms commonly have a diploid number of chromosomes (2N) or two of each type characteristic of the species. The two are "homologous" chromosomes wit the same length, centromere location and gene sequence. Homologous pair with each other during meiosis. The X and Y chromosomes vary in length, shape, and genes they carry, but they still pair as homologues.
Before meiosis, all chromosomes in a germ cell are duplicated, The duplicates remain attached (as sister chromatids) at the centromere.
Meiosis consists of two consecutive divisions that sort out the chromosomes in a germ cell. In meiosis I, each chromosome pairs with and then separates from its homologue with results in reductive division. In meiosis II, the sister chromatids of each pair separate from each other as in mitosis. In both cases, microtubules of a spindle fiber apparatus help move the chromosomes.
The following key events occur during meiosis I:

At prophase I, each chromosome comes into point-by-point alignment with its homologue (synapses) forming a tetrad. This allows crossing over to occur between non-sister chromatids. Crossing over breaks up old combinations of alleles and puts together new ones in the chromosomes. This genetic recombination leads to variation in traits among the offspring.
Late in metaphase I, all pairs of homologous chromosomes are aligned at the equator of the cell.
At anaphase I, each maternal chromosome is separated from its paternal homologue and moved to the opposite end of the cell.

The following key events occur during meiosis II:

At metaphase II, all chromosomes are moved to the cell's equator.
At anaphase II, the sister chromatids of each chromosome are separated for movement to opposite poles. Once separated, they are chromosomes in their own right. And is in the unduplicated state.

Following cytokinesis (cytoplasmic division), there are four haploid cells, one or all of which may function as gametes (or give rise to gametes, in the case of spore producing plants).
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Meiosis Location in Higher Organisms

Meiosis occurs in germinal tissues: gonads in animals, and ovules and anthers in plants. Such tissues completely differentiate in the embryo of animals, but plants can arise from simple undifferentiated tissues whenever plants flower.
Germ line tissue gives rise to gametes. All other cells are called somatic cells.
In the human male embryo, germinal epithelium forms in the seminiferous tubules of the testes, but actual meiosis does not begin until puberty.
In the human female embryo, germinal epithelium develops in the ovaries and by the time of birth meiosis has begun in all germ line cells or oocytes. Meiosis ceases during prophase I, and remains suspended in each oocyte until it actually undergoes ovulation.
Though meiosis is suspended in oocytes, some DNA uncoils and transcribes RNA, actively producing protein.
While each meiosis in males produces four functional sperm, meiosis in females results in only one oocyte forming. The other cells are lost as polarbodies following unequal cytokinesis during telophase I and II.
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Reproduction and Development in Animals

Asexual reproduction in animals occurs most often by budding or fission. Sexual reproduction requires fusion of two gametes.
Animal development commonly proceeds through six stages:

Gametogenesis, during which the egg and sperm mature within the reproductive organs of the parents.
Fertilization, which begins when a sperm penetrates an egg and is completed when the sperm and egg nuclei fuse forming a zygote.
Cleavage, when the fertilized egg undergoes mitotic cell divisions that form the early multicellular embryo (the blastocyst or blastula). Most genes are inactive during cleavage, and the destiny of cell lineages is established in part by the sector of cytoplasm inherited at this time.
Gastrulation, when the organizational framework of the whole animal is laid out. Endoderm, ectoderm and usually mesoderm form; all the tissues of the adult body will develop from these germ layers.
Organogenesis, the onset of organ formation. The different organs start developing by a tightly orchestrated program of cell differentiation and morphogenesis that is controlled by the homeobox genes.
Growth and tissue specialization, when organs enlarge overall and acquire their specialized chemical and physical properties. The maturation of tissues and organs continues into post-embryonic stages.
Cell differentiation is a process whereby initial populations of genetically equivalent cells give rise to subpopulations of phenotypically different types of cells.
Starting at organogenesis, selective restrictions are placed on gene expression in different cell types. As a result, some cells remain in an undifferentiated state, and others proceed through differentiation and develop into specialized cell types.
Morphogenesis is a process whereby different cell types become organized into all the specialized tissues and organs of the body. It involves the growth, shaping, and arrangement of body parts according to prescribed patterns controlled by the homeobox genes, and it is brought about by cell divisions, cell migrations, changes in cell shapes, localized growth, and controlled cell death.
Embryonic cells sense their position relative to one another and respond to this information by producing ordered arrangements of differentiated tissues. This kind of developmental activity is called pattern formation.
Following embryonic development, some animals grow directly into the adult form. Other animals proceed through indirect development, in which, a larval stage is interposed between the embryo and the adult. Reactivate growth and development of the larva into the sexually mature adult form is called metamorphosis.
All complex animals gradually show changes in structure and a decline in efficiency (aging). Although aging is part of the life cycle of all animals having extensively specialized cell types, its cause is unknown.

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Human Reproduction and Development Female Reproductive System Text Male Reproductive System
from PennState Health Education 48 web page

Most animals reproduce sexually. Separation into male and female sexes involves specialized reproductive structures and forms of behavior that help assure successful fertilization and that lend nutritional support to offspring.
Humans have a pair of primary reproductive organs (sperm producing testes in males, egg producing ovaries in females), accessory ducts, and glands. Testes and ovaries also produce hormones that influence reproductive functions and secondary sexual traits.
The hormones testosterone, LH (luteinizing hormone), and FSH (follicle stimulating hormone) control sperm production. Testes and ovaries also produce hormones that influence reproductive functions and secondary sexual traits.
The hormones estrogen, progesterone, FSH and LH control egg maturation and release, as well as changes in the lining of the uterus (endometrium). They are part of the feedback loops involving the hypothalamus, anterior pituitary, and ovaries.
The following events occur during a menstrual cycle:

A follicle, which is an oocyte surrounded by a cell layer, matures in an ovary, and the endometrium starts to rebuild. (It breaks down at the end of each menstrual cycle when pregnancy does not occur.)
A mid cycle peak of LH triggers the release of a secondary oocyte from the ovary. THis event is called ovulation.
A corpus luteum forms from the remainder of the follicle. Its secretions prime the endometrium for implantation (the blastocyst attaching to the endometrium). When implantation occurs, the hormone HCG is secreted which causes the corpus luteum to be maintained, and thus its secretions help maintain the endometrium. Note: HCG is the hormone detected in home pregnancy tests.

Human development proceeds through gamete formation, fertilization, cleavage, gastrulation, organ formation, and growth and tissue specialization.
All tissues and organs in the developing embryo arise from three germ layers: the endoderm (digestive system), the ectoderm (the skin and nervous system) and the mesoderm (muscle and skeletal systems) of the early embryo.
Embryonic development depends on the formation of four extraembryonic membranes:

Yolk sac: parts give rise to the embryo's digestive tube.
Allantois: its blood vessels function in oxygen transport.
Amnion: a fluid filled sac that surrounds and protects the embryo from mechanical shocks and keeps it from drying out.
Chorion: a protective membrane around the embryo and the other membranes; a primary component of the placenta.

The embryo and the mother exchange substances by way of the placenta (a spongy tissue of endometrium and extraembryonic membranes).
The placental barrier provides some protection for the fetus, but it may suffer harmful effects from the mother's nutritional deficiencies, infections, intake of prescription drugs, alcohol, and smoking.
At delivery, contractions initiated by the hormone oxytocin of the uterus dilate the cervical canal and expel the fetus and afterbirth. Estrogen and progesterone stimulate the growth of the mammary glands. After delivery, nursing causes the release of luteinizing hormone and oxytocin that stimulates milk production and release.
Control of human fertility raises important ethical questions. These questions extend to the physical, chemical, surgical or behavioral interventions used in the control of unwanted pregnancies.
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Plant Reproduction

Typically in flowering plant life cycles, a multicellular diploid stage (the sporophyte, or "spore producing plant") alternates with a multicellular haploid stage (the gametophyte, or "gamete producing plant"). The sporophyte is a vegetative body with roots, stems, leaves, and at some point, flowers.

http://www.kean.edu/~biology/botlab15.html

A flower consists of one or more carpels (female reproductive structures), stamens (male reproductive structures), petals, sepals, most or all of which are attached to the receptacle.

The anther of each stamen contains pollen sacs, the chambers in which pollen grains (immature male gametophytes) develop.
A carpel is a closed vessel in which eggs develop, fertilization takes place, and seeds mature. Its stigma is the landing platform for pollen.

In pollen sacs, haploid microspores form following meiosis. Each becomes a two-cell pollen grain. One cell produces the sperm, the other develops into a pollen tube.
An ovule is a stalked structure that forms inside a carpel, on the wall of the ovary. It develops into a female gametophyte with egg cell, a surrounding tissue (nucellus) and one or two protective layers (integuments).
Upon pollination (transfer of pollen grains to an appropriate stigma), the pollen grain germinates (resumes growth). A pollen tube develops from one of its cells and, later two sperm nuclei form when its sperm producing cell undergoes mitosis. A germinated pollen grain, with its two sperm nuclei, is a mature male gametophyte.
Many flowering plants coevolved with pollinators - insects, birds, and bats, mostly - which are agents that transfer pollen to the stigmas of flowers. Coeveolution is the joint evolution of two or more species interacting in close ecological fashion. When one evolves, the change affects selection pressures operating between them and the other species also evolves.
Fruits function to protect and disperse seeds. Fleshy fruits attract animals and other "dispersing" agents; light weight fruits can be dispersed by the winds. Some fruits have hooks and such that attach to animals.
Many flowering plants also reproduce asexually, as when new plants arise by mitotic divisions at nodes or buds along modified stems of the parent plant. New plants also arise by vegetative propagation.
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Plant Growth and Development

Following dispersal from the parent, the seeds of flowering plants germinate: the embryo inside the seed absorbs water, resumes growth, and its primary root breaks through the seed coat.

Following germination,the plant increases in volume and mass. Tissues and organs of the seedling develop, later flowers, fruits and new seeds form, then older leaves drop away from the plant. Plant hormones govern these developmental events.
Five hormones have been identified in most flowering plants:

Auxins promote elongation of coleoptile and stem cells.
Gibberellins promote stem elongation; they also may help sees and buds break dormancy.
Cytokinins stimulate cell division, promote leaf expansion, and retard leaf aging.
Abscisic acid promotes stmatal closure (retards water loss) and may trigger seed and bud dormancy.
Ethylene promotes fruit ripening, abscission, and other processes.

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