Genetic polymorphism (03/16/2008)
... Such ‘heterosis’ is seen in human sickle-cell anaemia where malaria is prevalent. The malaria parasite attacks red blood cells. The sickle-cell mutation gives rise to red cells that are physiologically imperfect and misshapen. However, sickle-cell heterozygotes are fittest because they suffer only slightly from anemia and are little affected by malaria; but they continually generate homozygotes that are either dangerously anemic (two sickle-cell genes) or susceptible to malaria (no sickle-cell genes). None the less, the superior fitness of the heterozygote maintains both types of gene in the population (that is, a polymorphism)....

Climatic changes (03/16/2008)
... The same picture is repeated in European profiles. As the number of pollen records has increased, it has become possible not only to plot the changes in vegetation at a point in space, but to begin to map the movements of the various species as they have spread across the continents. In the invasions that followed the retreat of the ice in eastern North America, spruce was followed by jack pine or red pine, which spread northwards at a rate of 350–500 m year-1 for several thousands of years. White pine started its migration about 1000 years later, at the same time as oak. Hemlock was also one of the rapid invaders (200–300 m year-1), and arrived at most sites about 1000 years after white pine....

Definition and scope of ecology (03/15/2008)
... Krebs’ definition has the merit of pinpointing the ultimate subject matter of ecology: the distribution and abundance of organisms – where organisms occur, how many occur there, and why. This being so, it might be better still to define ecology as: the scientific study of the distribution and abundance of organisms and the interactions that determine distribution and abundance. As far as the subject matter of ecology is concerned, ‘the distribution and abundance of organisms’ is pleasantly succinct. But we need to expand it. The living world can be viewed as a biological hierarchy that starts with subcellular particles, and continues up through cells, tissues and organs....

Mitosis Phases (03/10/2008)
... Microtubules tug on the kinetochore, moving the chromosomes back and forth, toward one pole, then the other. In addition to these microtubules, the completed spindle also includes other microtubules from each MTOC that overlap at the center of the spindle and do not attach to the chromosomes. 2. Metaphase begins when the chromosomes are distributed across the metaphase plate, a plane lying between the two poles of the spindle. Metaphase ends when the microtubules, still attached to the kinetochores, pull each chromosome apart into two chromatids....

Meiosis Phases (03/10/2008)
... Unlike mitosis, however, once the chromosomes are condensed, homologous chromosomes pair, a process called synapsis. These pairs of homologous chromosomes are variously referred to as tetrads (a group of four chromatids) or bivalents. During synapsis, corresponding regions along nonsister chromatids form close associations called chiasmata (singular, chiasma). Chiasmata are sites where genetic material is exchanged between nonsister homologous chromatids, a process called crossing over. A tetrad together with chiasmata and crossover events is referred to as a synaptonemal complex....

Pleiotropy (03/10/2008)
... Many disease-causing genes exhibit pleiotropy. Sickle-cell anemia, a human blood disease, is caused by an allele that incorrectly codes for hemoglobin. As a result, the abnormal hemoglobin molecule causes the red blood cell, usually circular, to become sickle shaped. In response, red blood cells do not flow through capillaries freely and oxygen is not adequately delivered throughout the body. As a result, there is a general breakdown throughout the entire body, including damage to the heart, lungs, kidneys, brain, and other organs, which in turn promotes various disorders including anemia, pneumonia, heart and kidney failure, bone abnormalities, and impaired mental functioning....

Noncyclic Photophosphorylation (03/09/2008)
... Electron transport chain. Electrons pass through an electron transport chain. This chain consists of proteins that pass electrons from one carrier protein to the next. Some carrier proteins, like ferredoxin and cytochrome, include nonprotein parts containing iron. 4....

Mitosis versus Meiosis (03/09/2008)
... Note that one copy of each chromosome in the zygote originates from one parent, and the second copy from the other parent. Thus, a pair of homologous chromosomes in the diploid zygote represents both maternal and paternal heritage. The life cycle of a human illustrates the production of gametes by meiosis and subsequent growth by mitosis.Note that the number of chromosomes in diploid and haploid cells is indicated by 2n and n, respectively. Human cells (except gametes) contain 46 chromosomes (23 homologous pairs)....

Mutations (03/09/2008)
... If a DNA error is not repaired, it becomes a mutation. A mutation is any sequence of nucleotides in a DNA molecule that does not exactly match the original DNA molecule from which it was copied. Mutations include an incorrect nucleotide (substitution), a missing nucleotide (deletion), or an additional nucleotide not present in the original DNA molecule (insertion). When an insertion mutation occurs, it causes all the subsequent nucleotides to be displaced one position, producing a frameshift mutation. Radiation or chemicals that cause mutations are called mutagens....

Prokaryotes and Eukaryotes Cells (03/08/2008)
... Prokaryotic ribosomes are smaller (70S, with 50S and 30S subunits) than those of eukaryotes (80S, with 60S and 40S subunits). 4. The cell walls of bacteria, when present, are constructed from peptidoglycans, a polysaccharide-protein molecule. 5. Flagella, when present in prokaryotes, are not constructed of microtubules....

Why Cells Divide (03/08/2008)
... This situation is alleviated by cell division. A second reason for dividing is the limited capability of the nucleus. The genetic material (chromosomes) in the nucleus, collectively called its genome, “controls” the cell by producing substances which make enzymes and other biosynthetic substances. These substances, in turn, regulate cellular activities. The capacity of the genome to do this is limited by its finite amount of genetic material....

Epistasis (03/08/2008)
... The phenotypic expressions of CCBB, CCBb, CcBB, and CcBb are all black, and the expressions of CCbb and Ccbb are both brown. However, whenever cc is inherited, no pigment is produced and the fur is white regardless of the color encoded by the B allele. ......

Sex Linked Inheritance (03/07/2008)
... In contrast, however, a male (XY) will inherit only one copy of the gene because only the X chromosome delivers the gene. There is no similar gene delivered by the Y chromosome. As a result, whichever allele is on the X chromosome of a male, regardless of whether it is dominant or recessive, is the allele whose trait expressed. Hemophilia is caused by a sex-linked, recessive gene (h) in humans. Hemophiliacs cannot properly form blood clots and in the worst cases can die from minor injuries by bleeding to death....

Movement of Substances (03/02/2008)
... 4. Solute concentrations between two areas may be compared. A solute may be hypertonic (a higher concentration of solutes), hypotonic (a lower concentration of solutes), or isotonic (an equal concentration of solutes) relative to another region. 5. The movement of substances may be passive or active....

Life at high temperatures (02/22/2008)
... The perennial plant, desert honeysweet (Tidestromia oblongifolia), grows vigorously in such an environment despite the fact that its leaves are killed if they reach the same temperature as the air. Very rapid transpiration keeps the temperature of the leaves at 40–45°C, and in this range they are capable of extremely rapid photosynthesis. Most of the plant species that live in very hot environments suffer severe shortage of water and are therefore unable to use the latent heat of evaporation of water to keep leaf temperatures down. This is especially the case in desert succulents in which water loss is minimized by a low surface to volume ratio and a low frequency of stomata. In such plants the risk of overheating may be reduced by spines (which shade the surface of a cactus) or hairs or waxes (which reflect a high proportion of the incident radiation)....

Ectotherms and endotherms (02/13/2008)
...g. Philodendron) use metabolic heat to maintain a relatively constant temperature in their flowers; and, of course, birds and mammals use metabolic heat almost all of the time to maintain an almost perfectly constant body temperature. An important distinction, therefore, is between endotherms that regulate their temperature by the production of heat within their own bodies, and ectotherms that rely on external sources of heat. But this distinction is not entirely clear cut. As we have noted, apart from birds and mammals, there are also other taxa that use heat generated in their own bodies to regulate body temperature, but only for limited periods; and there are some birds and mammals that relax or suspend their endothermic abilities at the most extreme temperatures....