Ploidy and Human Cell Types Cell Cycle and Mitosis Chapter 12 Pg. 228 245 Cell Types Somatic cells (body cells) have 46 chromosomes, which is the diploid chromosome number. A diploid cell is a cell with two copies of every chromosome, designated 2n. Gametes (sex cells, sperm and egg) have 23 chromosomes, which is the haploid chromosome number. A haploid cell is one with one copy of each chromosome, designated n. A cell that begins mitosis in the diploid state will end mitosis with daughter cells still in the diploid state, each with two identical copies of every chromosome. However, each chromosomes will consist of only one chromatid (one DNA molecule). During the S phase, the second DNA molecule is replicated from the first. DNA and Chromosomes In a non-dividing state, cells contain DNA in chromatin form. Chromatin: DNA associated with proteins, called histones. Before the cell actually divides, chromatin must condense into chromosomes. Chromosome: a long strand of DNA, associated with histones, tightly coiled in on itself Chromosomes are rod-shaped and consist of many genes hundreds to thousands. After replication (before cell division), chromosomes contain two identical halves called sister chromatids, joined at the centromere. At the centromere, there are kinteochores that help spindle fibers attach and organize DNA during cell division. In diploid cells, there are TWO copies of EVERY chromosome, forming a pair called homologous chromosomes. One of the homologous pair comes from the mother, the other from the father. Humans have 46 chromosomes, or 23 homologous pairs. Before the cell divides, there are a total of 92 chromatids in the cell. Where It All Began You started as a cell smaller than a period at the end of a sentence. and now look at you! How did you ever make it this far?! Getting from here to there: The original fertilized egg has to divide and divide and divide. 1
Why Do Cells Divide? Cell Cycle Reproduction Asexual reproduction of one-celled organisms For growth From fertilized egg to multi-celled organism For repair and renewal Replace cells that die from normal wear and tear or from injury Describes the sequence of events that occurs during the life of most eukaryotic cells. Interphase The period before and after mitosis when the cell is not dividing. Chromatin is not condensed, and enclosed in a clearly-defined nuclear envelope. One or more nucleoli are visible in the nucleus. Outside the nucleus, 2 microtubule-organizing centers (MTOCs) lie adjacent to one another. (These are centrosomes/centrioles in animal cells.) Major growth period, divided into three phases: G 1 : intense growth and biochemical activity S: growth, DNA synthesis Starting with S phase, each chromosome that appears will be two sister chromatids. G 2 : growth; materials for the next mitotic division are prepared; completes preparations for cell division More than 90% of a cells life is spent in interphase. Interphase: Mitochondrial Division Organelles do it too! S phase: DNA Synthesis Dividing cell replicates DNA Must separate DNA copies correctly to 2 daughter cells Human cell duplicates about 3 meters of DNA Each daughter cell gets complete identical copy Error rate is about 1 per 100 million bases 3 billion base pairs in mammalian genome About 30 errors per cell cycle, leading to mutations of somatic cells After this stage is complete, the chromatin begins to condense. Mitotic Chromosomes Duplicated chromosome 2 sister chromatids Narrow at centromeres Contain identical copies of original DNA homologous chromosomes homologous chromosomes sister chromatids 2
Two phases: Cell Division Nuclear division: divides the genetic material in the nucleus Mitosis: divides the nucleus so that both daughter cells are genetically identical Meiosis: reduction division that produces genetically variable daughter cells that contain half the genetic information of the parent cell Cytokinesis: divides the cytoplasm Daughter Cells Cell division results in genetically identical daughter cells Cells duplicate genetic material before division, so that each daughter cell receives an exact copy of DNA A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells Mitosis and Cytokinesis Mitosis: division of the nucleus to form 2 nuclei Prophase Metaphase Anaphase Telophase Please note: the AP exam does not require you to use the names, and you will get the same credit for just explaining the events of mitosis correctly, in order. Cytokinesis: division of the cytoplasm to form two new cells, each with one nucleus Both occur during the M phase of the cell cycle. Prophase (Condensation) Nucleoli disappear Chromatin condenses into chromosomes Nuclear envelope breaks down Mitotic spindle is assembled from actin and myosin MTOCs move to poles of cell, developing microtubules Microtubules of each MTOC connect to the kinetochore, which move the chromosomes 3
Prometaphase (transition state) Some textbooks list this phase as an intermediate between prophase and metaphase. In this phase The nuclear envelope fragments and microtubules attach to chromosomes. Chromatids of each chromosome are held together by protein kinetochores in the centromere. The microtubules will attach to the kinetochores. Note: if not mentioned, these events are considered to occur in prophase. Metaphase (Alignment) Chromosomes are aligned across the metaphase plate (the equator ) Ends when the microtubules, attached to kinetochores, pull each chromosome apart into two chromatids. Each chromatid is complete with a centromere and a kinetochore. Once separated, each chromatid is called a chromosome. To count the number of chromosomes at any one time, count the number of centromeres. Anaphase (Separation) Begins after the chromatids are separated Microtubules shorten, effectively pulling the chromosomes to opposite poles. Motor proteins pull chromosomes by walking along shortening microtubules this step requires a lot of ATP Microtubules not attached to chromosomes interact to push the poles farther apart to lengthen the cell At the end, each pole has a complete set of chromosomes like the original cell. Telophase (Restoration) Concludes nuclear division Chromosomes arrive at opposite poles Nuclear envelope is restored around each pole, forming two nuclei Chromosomes within each nuclei disperse into chromatin, and the nucleoli reappear Spindle fibers disperse 4
Cytokinesis Cytokinesis divides the cytoplasm to form two cells. Differs in plants and animals by the formation of two kinds of structures: Cell plate Vesicles from the Golgi move to the space between the two new nuclei, fuse to form a cell plate, and a cell wall develops between the membranes. Cleavage furrow Constriction belt of actin tightens around the equator of the cell and splits it in two. Mitosis in Whitefish Blastula Cell Cycle in Animal Cells 5
Cell Cycle in Plants Nucleus Chromatin condensing Nucleolus Chromosomes Cell plate 10 µm Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is starting to form. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore microtubules shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell. Prokaryotes v. Eukaryotes While eukaryotic cells typically multiply using mitosis, prokaryotic cells (bacteria and Archaea) use a process called binary fission. They use the following, much simpler process to double: Replicate DNA (one chromosome) Attach the original and replicate chromosomes to separate parts of the cell membrane Divide in half Specific Cells The timing and rate of cell division are CRUCIAL to homeostasis. In humans, frequency of cell division varies with the cell type. Bone marrow cells are always dividing to make RBCs and WBCs. Liver cells are arrested in G 0 but can be induced if damaged. Human intestine cells normally divide about twice per day to renew tissue destroyed during digestion. Specialized cells, like nerve cells, do not divide at all. In every case, however, the entire process is tightly regulated by a complex mechanism involving kinases and allosteric interactions. If something goes awry, the result can be uncontrolled cell division characteristic of cancer. Limiting Cell Size and Volume Two important factors limit cell size and promote cell division: Ratio of the volume of a cell to the surface area: as a cell grows, its volume increases faster than the cell membrane size (surface area). This makes nutrient and waste exchange more difficult, determining at what size the cell actually divides. Capacity of the nucleus to control the cell: the nucleus provides information to produce substances to meet the cell s needs. The larger the cell, the more difficult it will be to supply it with what it needs. Cells that have evolved as a strategy to exist as large, active cells do exist in several kingdoms. 6