Starfish Description

Starfish are classified under the phylum Echinodermata. They exhibit 5 rayed radial symmetry, or sometimes bilateral symmetry, characteristic that developed through evolution. The body wall consists of three germ cell layers. The outer layer, the epidermis,  is a single layer of cells that covers the entire animal including its various spines. The middle layer, the dermis, is much thicker. This layer is composed of connective tissue and contains the organism's exoskeleton. For Starfish, the exoskeleton is a set of free moving small plates called ossicles. The third layer is also a single layer of ciliated cells that encloses the animal's coelom, separating the animal's internal organs from the skin. This is related to the phylum's characteristic of having a body cavity of a true coelom. The third layer creates what is known as the "coelomic lining".
Starfish possess a through gut with an anus with a headless body that can vary in shape. They have a poorly defined open circulatory system but a fairly good nervous system with a circum-oesophageal ring.  Starfish feed on fine particles of the water, detritus, or even other animals. Their diet includes oysters and clams and proves to make Starfish a pest of commercial clam and oyster beds. Interestingly, species of the phylum Echinodermata possess no excretory organs. Species of Starfish can reproduce by means of sexual reproduction and gonochoristic reproduction.
There are five living classes:
Crinoidea: Includes Sea lilies, Feather stars, and comatulids.

Image of Crinoidea structure from http://tolweb.org/tree/ToLimages/crinoid.gif

Ophiocistioidea: A class of extinct echinoderms from the Palaeozoic. They had a body structure similar to that of a sea urchin.

Image of Ophiocistioidea fossil from http://www.geomuseum.uni-goettingen.de/people/mreich/pdf/images/neuweb_seegurken11.gif

Astroidea: Harmless algal grazers and detrivores. Includes Starfish.

Image of Astroidea Starfish from http://www.reefs.org/hhfaq/starfish/star.jpg/view

Echinoiudea: Includes Sea urchins, which are spiny, globular animals. Sand dollars are closely related.

Image of Sea urchin from http://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Riccio_Melone_a_Capo_Caccia_adventurediving.it.jpg/250px-Riccio_Melone_a_Capo_Caccia_adventurediving.it.jpg

Holothuoidea: Includes Sea cucumbers, which have leathery skin and an elongated body containing a single, branched gonad.

Image of Sea cucumber from http://en.wikipedia.org/wiki/File:Espardenya_(animal).jpg

Genome Chapter 2- Species

This chapter of Genome begins by explaining how people had mistaken the human genome to have 24 chromosomes. Later studies have shown that humans have only 23 chromosomes due to the fact that Chromosome 2  is actually the fusion of two medium sized chromosomes. Chromosome has been subject to much speculation, with some believing that at some part of the chromosome near the centromere lies the code that creates the human soul. The chapter further explores the slight difference in genes that exists between humans and chimps, with only about a 2% difference in the genes. At some point in time a population of chimpanzees was isolated and a genetic mutation occurred. This mutation created a new species, hence the chapter title, and prevented the new species from breeding with the chimps. Ridley concludes the chapter pondering at the possibility that genes have some sort of influence over behavior. He explains that in the end he isn't sure how genes can play a role in behavior but he is open to the possibility that genes are not constrained to only determining anatomy.

Genome Chapter 1- Life

Chapter 1 of Genome looks into the "word" that builds up the world. Ridley points out that many would mistaken DNA as this word when in reality, the word is RNA. He describes RNA as a kind of bridge between DNA and proteins worlds. Its presence is extremely necessary in the most primitive and basic functions of a cell, acting as catalyst or replication device. Another important topic brought up in the chapter was the discussion on LUCA, the Last Universal Common Ancestor. LUCA is first described as an organism that looked like a bacterium that lived in a warm pond, possibly near a hot spring. However, it is later revealed that LUCA was most likely a protozoan.
The reason I believe this chapter is titled "Life" is that it still amazes us how our lives are basically determined by genetic code. Our lives literally hang by a thread of four letters put into specific sequences that map out our appearance and ability to carry out vital processes.

Double Fertilization

Double fertilization is an essential characteristic in the sexual reproduction of angiosperms. In sexual reproduction, a haploid sperm cell fuses with a haploid egg cell, forming a diploid zygote that soon develops into an embryo. Following this fertilization process is a second event that involves another sperm cell and a second cell found in the female's reproductive tissue creating a triploid cell as a product. This product develops into what would be the embryo's food supply. Without double fertilization, angiosperms would be incapable of successfully reproducing due to an inability to fertilize and a lack of resources for the embryo.

Fertilization starts between sperm cells, which are transferred through pollen grains produced in the anthers, and two cells within an ovule, the reproductive organ of the female. The reproductive cell found in the ovule is a diploid (2n) megaspore mother cell, which eventually undergoes meiosis to produce four haploid (n) megaspores. Only one of the four megaspores remain in most species while the other three degenerate. The surviving megaspore goes through three rounds of mitosis to form eight haploid nuclei that share the same cytoplasm, forming the embryo sac. Cell walls form between the nuclei to form three antipodal cells opposite the micropyle and near the micropyle; the ones near the micropyle are distinguished as two synergids and an egg. The two remaining nuclei, called polar nuclei, remain together in a single large central cell.

Before the two polar nuclei can take place in double fertilization, the sperm must travel within the female's reproductive organs to the cells. A pollen grain lands on the stigma and begins to germinate, sending a long pollen tube through the style and ovary. The generative cell, a haploid, travels down the pollen tube behind the tube nucleus and divides by mitosis to form two haploid sperm cells.

After the pollen tube reaches the micropyle and makes its way into one of the synergids, the sperms cells are released, which degenerates the synergid and sends one of the two sperm cells to fertilize the egg cell. The second sperm cell fuses with both of the polar nuclei, forming a triploid cell. This cell develops into the endosperm and serves as the embryo's food supply as the zygote develops into an embryo.

Genome Chapter 4- Fate

This chapter of Genome focused on chromosome 4. Chromosome 4 is linked to degenerative diseases such as Wolf-Hirschhorn syndrome and the notorious Huntington's disease. Both of these diseases are caused by a gene that contains the repetition of the "word" CAG (glutamine). The chapter focuses mainly around Huntington's disease, which is caused by a mutation of the previously mentioned gene. Usually the longer the repetition, the more prone you are to the disease. The Huntington gene was actually located quite recently by a woman named Nancy Wexler. Finding the gene was compared to "looking for a needle in a haystack the size of America" but Wexler pushed herself to locate that needle.

Going back to the disease itself, Huntington's disease is known to strike at earlier ages in people with longer repetitions of glutamine. The disease is extremely destructive, causing a loss of muscle control and, inevitably, control of your mind. Symptoms as usually not apparent until it is too late. The inevitable result: Death. No case of Huntington's has been cured at the moment.
The chapter is titled "Fate" for a very obvious reason. It is constantly repeated that no one can escape their fate even if what will happen is known. The Greek allusion to Tiresias, the blind seer, caught my attention. The allusion spoke the fact that knowing the future (or fate) is truly not a gift since nothing can be done to change it. In this case, a person who has Huntington's disease knows that he/she has the inescapable fate of a slow but premature death, whether it be by their own hand or by the disease itself.