Lab Exam: Questions

Why choose an embryonic mass of cells to study the stages of mitosis

• Unlike most cells in an adult body, an embryonic mass of cells is always dividing. Most cells in the adult body is quiescent and will not divide unless signals have been given to them to divide, and many cells such as muscle and nerve cells have even lost the ability to divide.

What stage of mitosis of often associated with the beginning of cytokinesis?telophase

Does the cell cycle have a beginning and an end?

• cell cycle, the ordered sequence of events that occur in a cell in preparation for cell division. The cell cycle is a four-stage process in which the cell increases in size (gap 1, or G1, stage), copies its DNA (synthesis, or S, stage), prepares to divide (gap 2, or G2, stage), and divides (mitosis, or M, stage). The stages G1, S, and G2 make up interphase, which accounts for the span between cell divisions. On the basis of the stimulatory and inhibitory messages a cell receives, it “decides” whether or not it should enter the cell cycle and divide

spermatogenesis

• The seminiferous tubules, in which the sperm are produced, constitute about 90 percent of the testicular mass. In the young male the tubules are simple and composed of undeveloped sperm-producing cells (spermatogonia) and the Sertoli cells. In the older male the tubules become branched, and spermatogonia are changed into the fertile sperm cells after a series of transformations called spermatogenesis. The Sertoli cells found in both young and adult males mechanically support and protect the spermatogonia.

• Each seminiferous tubule of the adult testis has a central lumen, or cavity, which is connected to the epididymis and spermatic duct (ductus deferens). Sperm cells originate as spermatogonia along the walls of the seminiferous tubules. The spermatogonia mature into spermatocytes, which mature into spermatids that mature into spermatozoa as they move into the central lumen of the seminiferous tubule. The spermatozoa migrate, by short contractions of the tubule, to the mediastinum testis; they are then transported through a complex network of canals (rete testis and efferent ductules) to the epididymis for temporary storage. The spermatozoa move through the epididymis and the spermatic duct to be stored in the seminal vesicles for eventual ejaculation with the seminal fluid. Normal men produce about one million spermatozoa daily.

• In animals that breed seasonally, such as sheep and goats, the testes regress completely during the nonbreeding season and the spermatogonia return to the state found in the young, sexually immature males. Frequently in these animals the testes are drawn back into the body cavity except in the breeding season, when they again descend and mature; this process is known as recrudescence.

Spermatogenesis

• Spermatogenesis: the process by which stem cells develop into mature spermatozoa. There are three phases: (1) Spermatocytogenesis (Mitosis), (2) Meiosis, and (3) Spermiogenesis

Spermatogenesis in the Sexually Mature Male

• A. The gametogenic function of the testes is to produce the male gametes or spermatozoa. This process is termed, spermatogenesis. The sites of spermatozoa production are the seminiferous tubules. The spermatozoa originate from precursor cells that are called spermatogonia, and these cells line the basement membrane of the seminiferous tubule. Spermatogenesis can be divided into three portions:

• spermatocytogenesis -- proliferative phase

• meiosis -- production of the haploid gamete

• spermiogenesis -- "metamorphosis" of spermatids into spermatozoa

oogenesis

• in the human female reproductive system, growth process in which the primary egg cell (or ovum) becomes a mature ovum.

oogenesis

• The egg cell remains as a primary ovum until the time for its release from the ovary arrives. The egg then undergoes a cell division. The nucleus splits so that half of its chromosomes go to one cell and half to another. One of these two new cells is usually larger than the other and is known as the secondary ovum; the smaller cell is known as a polar body. The secondary ovum grows in the ovary until it reaches maturation; it then breaks loose and is carried into the fallopian tubes. Once in the fallopian tubes, the secondary egg cell is suitable for fertilization by the male sperm cells

Oogenesis Dr. Charlotte Ownby

Stages of mitosis

• Prior to the onset of mitosis, the chromosomes have replicated and the proteins that will form the mitotic spindle have been synthesized. Mitosis begins at prophase with the thickening and coiling of the chromosomes. The nucleolus, a rounded structure, shrinks and disappears. The end of prophase is marked by the beginning of the organization of a group of fibres to form a spindle and the disintegration of the nuclear membrane.

• The chromosomes, each of which is a double structure consisting of duplicate chromatids, line up along the midline of the cell at metaphase. In anaphase each chromatid pair separates into two identical chromosomes that are pulled to opposite ends of the cell by the spindle fibres. During telophase, the chromosomes begin to decondense, the spindle breaks down, and the nuclear membranes and nucleoli re-form. The cytoplasm of the mother cell divides to form two daughter cells, each containing the same number and kind of chromosomes as the mother cell. The stage, or phase, after the completion of mitosis is called interphase

meiosis

• Prior to meiosis, each of the chromosomes in the diploid germ cell has replicated and thus consists of a joined pair of duplicate chromatids. Meiosis begins with the contraction of the chromosomes in the nucleus of the diploid cell. Homologous paternal and maternal chromosomes pair up along the midline of the cell. Each pair of chromosomes—called a tetrad, or a bivalent—consists of four chromatids. At this point, the homologous chromosomes exchange genetic material by the process of crossing over (see linkage group). The homologous pairs then separate, each pair being pulled to opposite ends of the cell, which then pinches in half to form two daughter cells. Each daughter cell of this first meiotic division contains a haploid set of chromosomes. The chromosomes at this point still consist of duplicate chromatids.

• In the second meiotic division, each haploid daughter cell divides. There is no further reduction in chromosome number during this division, as it involves the separation of each chromatid pair into two chromosomes, which are pulled to the opposite ends of the daughter cells. Each daughter cell then divides in half, thereby producing a total of four different haploid gametes. When two gametes unite during fertilization, each contributes its haploid set of chromosomes to the new individual, restoring the diploid number

Rhizobium and legumes

• Rhizobium organisms in the soil recognize and invade the root hairs of their specific plant host, enter the plant tissues, and form a root nodule. This process causes the bacteria to lose many of their free-living characteristics. They become dependent upon the carbon supplied by the plant, and, in exchange for carbon, they convert nitrogen gas to ammonia, which is used by the plant for its protein synthesis and growth. In addition, many bacteria can convert nitrate to amines for purposes of synthesizing cellular materials or to ammonia when nitrate is used as electron acceptor

rhizobidium

• Nitrogen Fixation by Legumes• Legume nitrogen fixation starts with the

formation of a nodule. A common soil bacterium, Rhizobium, invades the root and multiplies within the cortex cells. The plant supplies all the necessary nutrients and energy for the bacteria. Within a week after infection, small nodules are visible with the naked eye. In the field, small nodules can be seen 2-3 weeks after planting, depending on legume species and germination conditions. When nodules are young and not yet fixing nitrogen, they are usually white or grey inside. As nodules grow in size they gradually turn pink or reddish in color, indicating nitrogen fixation has started. The pink or red color is caused by leghemoglobin (similar to hemoglobin in blood) that controls oxygen flow to the bacteria.

Steps of Gram stain

• Lab 6: Gram Stain• 1. The bacteria are first stained with the basic dye

crystal violet. Both Gram-positive and Gram-negative bacteria become directly stained and appear purple after this step.

• 2. The bacteria are then treated with Gram's iodine solution. This allows the stain to be retained better by forming an insoluble crystal violet-iodine complex. Both Gram-positive and Gram-negative bacteria remain purple after this step.

• 3. Gram's decolorizer, a mixture of ethyl alcohol and acetone, is then added. This is the differential step. Gram-positive bacteria retain the crystal violet-iodine complex while Gram-negative are decolorized.

• 4. Finally, the counterstain safranin (also a basic dye) is applied. Since the Gram-positive bacteria are already stained purple, they are not affected by the counterstain. Gram-negative bacteria, which are now colorless, become directly stained by th e safranin. Thus, Gram-positive appear purple, and Gram-negative appear pink.

What happens when milk is pasteurized?

• heat-treatment process that destroys pathogenic microorganisms in certain foods and be

• The times and temperatures are those determined to be necessary to destroy the Mycobacterium tuberculosis and other more heat-resistant of the non-spore-forming, disease-causing microorganisms found in milk. The treatment also destroys most of the microorganisms that cause spoilage and so prolongs the storage time of food.verages.

• Pasteurization • Pasteurization is most important

in all dairy processing. It is the biological safeguard which ensures that all potential pathogens are destroyed. Extensive studies have determined that heating milk to 63° C (145° F) for 30 minutes or 72° C (161° F) for 15 seconds kills the most resistant harmful bacteria. In actual practice these temperatures and times are exceeded, thereby not only ensuring safety but also extending shelf life.

antibiotics

• chemical substance produced by a living organism, generally a microorganism, that is detrimental to other microorganisms.

• Antibiotics produce their effects through a variety of mechanisms of action. A large number work by inhibiting bacterial cell wall synthesis; these agents are referred to generally as β-lactam antibiotics. Production of the bacterial cell wall involves the partial assembly of wall components inside the cell, transport of these structures through the cell membrane to the growing wall, assembly into the wall, and finally cross-linking of the strands of wall material. Antibiotics that inhibit the synthesis of the cell wall have a specific effect on one or another phase. The result is an alteration in the cell wall and shape of the organism and eventually the death of the bacterium.

antibiotics

• Unlike bacteria, viruses mimic the metabolic functions of their host cells. Antibiotics are not effective against viruses.

Fungi

How can you tell whether Rhizopus is reproducing sexually or asexually?

• Rhizopus stolonifer (bread mold) is a zygomycete.

• Asexual reproduction in Rhizopus: Sporangia – note the root like hyphae (rhizoids) and horizontally-growing hyphae (stolons) at the base

• Sexual reproduction in Rhizopus: hyphae meeting (far left) and making a zygospore (far right).

Fungi - Biology

Rhizopus:Asexual reproduction in Rhizopus: Sporangia – note the root like hyphae (rhizoids) and horizontally-growing hyphae (stolons) at the base

Describe the relationship found in lichen.

• lichen is not a single organism the way most other living things are, but rather it is a combination of two organisms which live together intimately.

Lichens

Describe the relationship found in lichen.

• Most of the lichen is composed of fungal filaments, but living among the filaments are algal cells, usually from a green alga or a cyanobacterium

Lichens

Describe the relationship found in lichen.

• Lichens are formed from a combination of a fungal partner (mycobiont) and an algal partner (phycobiont).

• A lichen may absorb certain mineral nutrients from any of these substrates on which it grows, but is generally self-reliant in feeding itself through photosynthesis in the algal cells.

Life History and Ecology of Lichens

Describe the relationship found in lichen.

• Lichens growing in trees are simply using the tree as a home. Lichens growing on rocks, though, may release chemicals which speed the degradation of the rock into soil, and thus promote production of new soils.

Life History and Ecology of Lichens

Fungi: Benefits

• Lichens are symbioses involving fungi and unicellular algae

• Mycorrhizae are symbioses involving fungi and the roots of plants

Symbiotic Fungi

Multiclavula mucida, a lichenized basidiomycete (left) and Parmelia sp., a lichenized ascomycete (right)

The only distinction between a fungi spore and gamete is function

• Following a period of intensive growth, fungi enter a reproductive phase by forming and releasing vast quantities of spores. Spores are usually single cells produced by fragmentation of the mycelium or within specialized structures (sporangia, gametangia, sporophores, etc.). Spores may be produced either directly by asexual methods or indirectly by sexual reproduction. Sexual reproduction in fungi, as in other living organisms, involves the fusion of two nuclei that are brought together when two sex cells (gametes) unite. Asexual reproduction, which is simpler and more direct, may be accomplished by various methods.