Cells

Cells Cells make up every living thing around us, whether it be an as complex as a human or as simple as a amoeba. Cells are the building blocks of life, without them, there is no us or they. The Cell Theory states that all living things are made of cells, which are the basic units of life, and that cells come from other cells. Cells are divided into two groups; Eukaryotic and Prokaryotic. Eukaryotic (multicellular) cells have a nucleus and organelles that are surrounded by membranes. Each organelle does a specific cell function. All species in the Eukaryota domain (protists, fungi, plants, and animals) have eukaryotic cells. Prokaryotic (unicellular) cells are simple in structure, with no recognizable organelles. They have an outer cell wall that gives them shape. Just under the rigid cell wall is the more fluid cell membrane. Each cell has a job to do, it needs to survive on its own, and maintain itself to carry out its specialized function. It needs to take in nutrients and convert that to energy, carry out special functions, and reproduce when necessary. All cells share these abilities: o Reproduction by cell division o Use of enzymes and proteins coded by the DNA and carry out instructions from RNA o Metabolism, including taking in raw materials, building cell components, converting energy, and releasing byproducts (waste) o Response to internal and external stimuli e.g. temperature o And that cell contents are contained within a cell membrane Osmosis is the movement of water molecules across from a semi-permeable membrane from a region of low concentration to an area of high solute potential. Many plant cells perform osmosis. This is because the osmotic entry of water is opposed and eventually equaled by the pressure exerted by the cell wall, creating a steady state. In fact, osmotic pressure is the main cause of support in plant leaves. In other words, it takes in the water so it can stay alive. If we put an animal or a plant cell in a solution of sugar or salt in water o If the medium surrounding the cell has a higher water concentration than the cell, the cell will gain water through osmosis. Such a solution is called a hypotonic solution. o If the medium has exactly the same water concentration, there will be no net movement of water across the cell membrane. Such a solution is called a isotonic solution. o If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution the cell will lose water by osmosis. Such a solution is called a hypertonic solution. Cell division is the biological base of life. For simple unicellular organisms such as the Amoeba, one cell division makes an entire organism. On a larger scale,

cell division can create children from multicellular organisms, such as plants that grow from cuttings. But most importantly, cell division enables sexually reproducing organisms to develop from the one-celled zygote, which was produced by cell division from gametes. And after growth, cell division allows for renewal and repair of the organism. The human body consists of cells, and each of its cells contains 23 pairs of chromosomes, i.e. a total of 46 chromosomes (gr. chroma: color und soma: body). They are thread-like parts of the cell’s nucleus, which can be dyed and thus made visible under a microscope. They carry the genetic information, and they play a important role in determining a person’s sex. There are different forms of gene called alleles. These different forms of gene are what cause variation amongst living things. There are two types of allele: dominant and recessive. One type of gene is for eye colour. The allele for brown is dominant and thus represented as 'B' (capital letter). The allele for blue eyes is recessive and represented as 'b'. Mrs Science has brown eyes and the genotype Bb Mr Science has blue eyes and the genotype bb. They are having a baby Because the baby gets an allele from each parent, you can work out the possible outcomes by representing it in a 'punnet square'.

Two of the outcomes are Bb. This means the baby has a 50% chance of having brown eyes. As, even though it has a b (blue eye) allele, it has one B allele, and as this is dominant the baby will have this characteristic. In order to have blue eyes, the baby must have two recessive blue alleles (bb). Chromosomes need to carry information accurately and it needs to be exact every time. The roles of chromosomes are quite simple; tell the cells what to do and what to produce. Without it, we would be nothing but humans. If one part of the chromosome is damaged, the whole body is affected. For example, Down syndrome is common and is a result of damaged chromosomes. Genes also tell the body what to chemicals to produce and determine our sexuality, if anything is altered the DNA tells things to the body that its not supposed to. Mitosis is a process of cell division which results in the production of two daughter cells from a single parent cell. The daughter cells are identical to one another and to the original parent cell. In a typical animal cell, mitosis can be divided into four principals stages: o Prophase: The chromatin, diffuse in interphase, condenses into chromosomes. Each chromosome has duplicated and now consists of two sister chromatids. At the end of prophase, the nuclear envelope breaks down into vesicles.

o Metaphase: The chromosomes align at the equitorial plate and are held in place by microtubules attached to the mitotic spindle and to part of the centromere. o Anaphase: The centromeres divide. Sister chromatids separate and move toward the corresponding poles. o Telophase: Daughter chromosomes arrive at the poles and the microtubules disappear. The condensed chromatin expands and the nuclear envelope reappears. The cytoplasm divides, the cell membrane pinches inward ultimately producing two daughter cells (phase: Cytokinesis). Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material. Meiosis comprises two successive nuclear divisions with only one round of DNA replication. Four stages can be described for each nuclear division. o Interphase: Before meiosis begins, genetic material is duplicated. o First division of meiosis o Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage. o Metaphase 1: Homologous chromosomes align at the equatorial plate. o Anaphase 1: Homologous pairs separate with sister chromatids remaining together. o Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair. o Second division of meiosis: Gamete formation o Prophase 2: DNA does not replicate. o Metaphase 2: Chromosomes align at the equatorial plate.

o Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. o Telophase 2: Cell division is complete. Four haploid daughter cells are obtained. One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different. Meiosis differs from mitosis because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes. "Normal" cells contain two full sets of chromosomes and are described as diploid. Diploid is often abbreviated as 2n to remind us that it is "double". Most of your cells are diploid - they contain two sets of chromosomes. Each set (n) of human chromosomes contains 23 chromosomes so most cells in your body have 46 chromosomes. However, there is one kind of cell that is not diploid and that you must completely understand. Mature gametes, the sex cells (sperm and egg), have only one full set of chromosomes. Cells with only one set of chromosomes are called haploid. Haploid is a single set of chromosomes, not a half set. Haploid is often abbreviated as 1n to remind us there's only one set of chromosomes in that cell. Meiosis has some similarities to mitosis and that will make it easier to learn meiosis. But meiosis has some very important differences too and they make meiosis very special. During meiosis and the cytokinesis that follow, one diploid mother cell (2n) makes four unique daughter cells and they are all haploid (1n)! Meiosis is often called "reduction division" because the genetic material is reduced - by half. Meiosis is extremely important not only for sexual reproduction, but also for creating the diversity upon which natural selection operates. Sexual reproduction combines the genetic information from two different cells that are usually from two different individuals. Each parent contributes a haploid (n) gamete (sex cell). These two gametes, a female egg and a male sperm, fuse to produce a genetically unique single cell called a zygote. The zygote is diploid (2n) having combined the genetic information provided by the gametes (n) of both parents. In a very real genetic sense, all offspring of sexual reproduction are "half identical" to each parent! Note that if diploid (2n) cells were fused the "zygote" would have FOUR sets of chromosomes. It would be 4n! Although this sometimes happens among plants, it is deadly for humans. [Why? Well, it has to do with the genetic regulation and management of all the extra materials. Just think of it as too much of a good thing.] If it were not for meiosis, sexual reproduction would double chromosome sets every generation! That's deadly and that's why meiosis is important.

Meiosis is also important because it creates unique gametes that go on to make unique zygotes - and the unique individuals that grow from unique zygotes.