Essential knowledge 2.E.1: Timing and coordination of specific events are necessary for the normal development of an organism, and these events are regulated by a variety of mechanisms http://www.youtube.com/watch?v=pa9uPnIeVK U&index=30&list=PLFCE4D99C4124A27A

Cell Differentiation is… the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of tissues and cell types.

Cell Differentiation

Differentiation is a common process in adults as well: adult stem cells divide and create fully differentiated daughter cells during tissue repair and during normal cell turnover. Blood cells are derived through adult stem cells

Observable cell differentiation results from the expression of genes for tissue-specific proteins. Every cell in our bodies contain our entire genome, yet on a few genes get expressed. Each cell type is defined by the particular genes that they express. Cell differentiation is thus a transition of a cell from one cell type to another and it involves a switch from one pattern of gene expression to another.

Induction of transcription factors(TFs) during development results in sequential gene expression. Transcription factors are one of the groups of proteins that read and interpret the genetic "blueprint" in the DNA. They bind to the DNA and help initiate a program of increased or decreased gene transcription. The regulation of gene target networks by specific TFs at different stages constitutes the transcriptional programs that direct cellular differentiation.

Homeotic genes are involved in developmental patterns and sequences. Homeotic genes are genes that determine which parts of the body form what body parts For example, homeotic genes are involved in determining where, when, and how body segments develop in flies. Alterations in these genes cause changes in patterns of body parts, sometimes causing dramatic effects such as legs growing in place of antennae or an extra set of wings or, in the case of plants, flowers with abnormal numbers of parts.

Embryonic induction in development results in the correct timing of events. Induction is the process by which the presence of one tissue influences the development of others. Certain tissues, especially in very young embryos, have the potential to direct the differentiation of adjacent cells. Absence of the inducing tissue results in lack of or improper development of the induced tissue.

An example of embryonic induction is… the development of the eye lens from epidermis under influence of the eye cup, which grows toward the skin from the brain. As the eye cup comes into contact with any neighboring epidermis, it transforms that particular region into a lens.

Temperature and the availability of water determine seed germination in most plants. Germination is the growth of an embryonic plant contained within a seed Water is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellular metabolism and growth can resume. Temperature affects cellular metabolic and growth rates. Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures.

Genetic mutations can result in abnormal development. To function correctly, each cell depends on thousands of proteins to do their jobs in the right places at the right times. Sometimes, gene mutations prevent one or more of these proteins from working properly. By changing a gene’s instructions for making a protein, a mutation can cause the protein to malfunction or to be missing entirely. When a mutation alters a protein that plays a critical role in the body, it can disrupt normal development or cause a medical condition. A condition caused by mutations in one or more genes is called a genetic disorder.

Genetic Transplantation Genetic transplantation experiments support the link between gene expression and normal development. John Gurdon transplanted nuclei from a tadpole cell into eggs from which the nuclei had been removed. This indicates that the nuclei of cells in later stages of development retain genetic information necessary to direct development of all other cells in an individual.

Genetic regulation by microRNAs plays an important role in the development of organisms and the control of cellular functions. MicroRNAs - post-transcriptional regulators; They are short RNAs, about 22 nucleotides long; bind to the mRNAs, usually silencing them. Wow that is short! Must be why they are called MicroRNAs!! What do they do again? That’s right, they bind to mRNAs before they have time to get translated—This usually silences them(stops them from translating into a protein).

Apoptosis Programmed cell death (apoptosis) plays a role in the normal development and differentiation.

normal development of the fingers and toes depends on death of the cells forming webs between them.

Apoptosis and the immune response Apoptosis of lymphocytes occurs in order to control an excessive immune response. When the immune system is activated, it produces lots of lymphocytes that work to fight the infection. If this immune response becomes too extreme, the excessive lymphocytes can begin to attack the body’s own normal cells. apoptosis also directly fights infection as part of the immune system’s first line of defense against foreign invaders such as bacteria, viruses, or fungi. As part of the first response to infection, apoptosis is activated in cells infected by the invader, leading to their destruction.

Apoptosis in development Nat Rev Mol Cell Biol. 2006 Feb;7(2):97-108. Developmental apoptosis in C. elegans: a complex CEDnario. Lettre G, Hengartner MO. Source Division of Genetics, Children's Hospital, 300 Longwood Avenue, Boston, Massachusetts 02115, USA. Abstract Apoptosis, an evolutionarily conserved programme of cellular self- destruction, is essential for the development and survival of most multicellular animals. It is required to ensure functional organ architecture and to maintain tissue homeostasis. During development of the simple nematode Caenorhabditis elegans, apoptosis claims over 10% of the somatic cells that are generated - these cells were healthy but unnecessary. Exciting insights into the regulation and execution of apoptosis in C. elegans have recently been made. These new findings will undoubtedly influence our perception of developmental apoptosis in more complex species, including humans.

Apoptosis & Flower Development The duration of the flower is species-specific and carefully tailored to its ecological requirements. This is important because firstly the flower can be a substantial sink on the plant’s resources, and as such is energetically expensive to maintain beyond its useful life. In addition, its architecture has been exploited by pathogens that use the stigma as a point of entry, and thus the flower poses an added risk of pathogen attack. Another important reason for floral death after pollination is to remove it from the population so that it does not compete for pollinators with the remaining blooms.

Essential knowledge 2.E.2: Timing and coordination of physiological events are regulated by multiple mechanisms. http://www.youtube.com/watch?v=dju6 tTb55Fw&list=PLFCE4D99C4124A27A

In plants, physiological events involve interactions between environmental stimuli and internal molecular signals. 2 examples: Phototropism, or the response to the presence of light Photoperiodism, or the response to change in length of the night, that results in flowering in long-day and short-day plants

Tropisms- plant response to environmental stimuli Phototropism Gravitropism Thigmotropism

Plant hormones, such as auxin, are responsible for plant movement ✘✘Memorization of the names, molecular structures and specific effects of all plant hormones are beyond the scope of the course and the AP Exam.

Photoperiodism

In fungi, protists and bacteria, internal and external signals regulate a variety of physiological responses that synchronize with environmental cycles and cues. Two illustrative examples of this are: Fruiting Body Formation in Fungus Quorum Sensing in Bacteria

Fruiting body formation in fungi, slime molds and certain types of bacteria Fruiting bodies are spore producing structures Fruiting body formation is influenced by external factors such as season (which affects temperature and air humidity)

Quorum sensing in bacteria Quorum sensing is a system of stimulus and response correlated to population density. Used to coordinate gene expression according to the density of their local population. Bacteria use it to coordinate certain behaviors based on the local density of the bacterial population. http://www.ted.com/talks/bonnie_bassler_on_ how_bacteria_communicate.html