Lecture 4 Wednesday, January 29, 2003 R.Jones Chapters 4, 8 and 16
1. Many protists, fungi and plants posses a cell wall laid down to the otside of the plasma membrane. In fungi the cell wall is composed of a polysaccharide called chitin, a polymer also found in the exoskeleton on insects and arthropods. Plants and many protisits have cellulose and hemicellulose polysaccharides in their cell walls. Plants also have a compound called lignin in the cell wall, and this is a polymer made up of phenolic molecules.
2. Multicellular organisms evolved from single-celled organisms. Materials (oxygen, carbon dioxide, sugars, amino acids etc.) move into and out of cells by the process of diffusion. This is a physical process occurring by random movement of molecules along concentration gradients. Diffusion across small distances (e.g. micromemters) can be relatively rapid (seconds to minutes) but movement of small moeclues over long distances (e.g meters) takes a very long time (years!). Cells therefore can not be too large otherwise they will be unable to exchange materials with their environment. To get bigger, an organism must divide its cells--that is, keep cell volume constant while increasing cell number.
3. Cellular reproduction. A feature that defines living organisms is their ability to reproduce. This means that two cells are produced from one and the daughter cells aquire identical genetic information.
4. Prokaryotic cells reproduce by the process of fission. First, the circular chromosome replicates then the cell body splits in two by fission allowing one circular DNA chromosome to acoompany one daughter cell and the other chromsome int othe other daughter cell.
5. In eukaryotes cell reproduction is complicated by the presence of a nucleus in which many chromosomes are found, as well as by the presence of organelles such as mitochondria and chloroplasts that contaqin their own DNAa and which must also divide/replicate so that daughter cells also have these organelles.
6. Chromosomes are highly compacted chromatin (chromatin is DNA + protein). Somatic cells of most eukaryotes have two identical sets of chromosomes inherited from two parents as a result of sexual reproduction. In humans we have 46 chromosomes made up of two sets of 23 homologous chromosomes. Somatic cells with 2 sets of chromosomes are referred to as diploid or 2N cells.
7. The eukaryotic cell cycle describes the process whereby the cell replicates and when new DNA is synthesized. The cell cycle is made up of four parts: Mitosis (M), the separation of chromosomes (karyokinesis) and the formation of two new cells (cytokinesis), and interphase. Interphase in turn is separated into Gap1 (G1) a resting period, S-the time when chromosomes replicate their DNA, follwoed by G2 a second resting period.
8. Cells that are about to replicate can do so only from G2, after the S phase of DNA synthesis. Chromosomes in cells at the end of G2 (or beginning mitosis) are made up of two daughter chromatids. For example, in a somatic human cell about to enter mitosis there will be 46 chromosomes each made up of two chromatids.
9. Mitosis is divided into 4 fairly distinct phases: prophase, metaphase, anaphase and telophase. In the interphase cell the nuclear membrane is intact and chromatin is dispersed throughout the nucleus and not organized into compact chromosomes. When the decision to enter mitosis is made a series of events occur.
10. Early prophase: the nuclear envelope begins to disintegrate, chromosomes are now compact and can be seen to be made up of two daughter chromatids. Centrioles, a pair of barrel-shaped structures that lie outside the nucleus divide and move to opposite sides of the cell with the help of microtubules. Microtubules also form in the area once occupied by the nuclues.
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