Cellular Reproduction and Genetics among Eukaryotes

Transcription

1 1 Cellular Reproduction and Genetics among Eukaryotes Overview The perpetuation of living things (reproduction) requires cell division - the splitting of one cell into two cells The events that take place inside the cell between one division and the next are collectively called the cell cycle And the net results of these events is cellular reproduction The Nucleus: Information for Directing Cell Growth The nucleus contains the genetic information needed to govern what a cell looks like and how it performs The definitive answer to the question (that the nucleus governed cell growth) came from some simple, yet elegant experiments conducted by the German biologist Joachim Hammerling using the algae Acetabularia Chromosomes Stained structures containing the hereditary information of the cell DNA exists as long threads called chromatin, that undergo compact folding and coiling at the time of cell division to form structures called chromosomes All along the chromosomes are discrete portions of the DNA, that represent heredity units that specify a organisms traits = genes The two members of each pair of chromosome are called homologous chromosomes Homologous chromosomes are maternal and paternal copies of the same chromosome As part of its preparation for cell division, a cell copies its entire genome by duplicating each of its chromosomes. After replication, each chromosome consists of 2 sister chromatids A specialized region of the chromosome, called the centromere, holds the 2 chromatids together

2 2 Mitosis versus Meiosis Among multicellular organisms, mitosis is the basis for growth through repeated divisions of the body's cells - called somatic cells Mitosis maintains the number of chromosomes in all the somatic cells of an individual In contrast, meiosis occurs only in the germs cells and is involved in the formation of the gametes Meiosis yields daughter cells that have half as many chromosomes as the parent cell; these cells are said to be haploid (N cell; where N is the number of chromosomes) The Cell Cycle of Mitosis Mitosis represents just a small part of the entire cell cycle. The cell spends about 95% of its time in that portion of the cell cycle called interphase It is during interphase that the cell grows, duplicates its DNA, and copies it cellular components in preparation for cell division Overview of the cell cycle: A. Interphase i) G 1 phase ii) S phase iii) G 2 phase B. M Phase Mitosis - the division of the nucleus (genetic material) of the cell - nuclear division i) Prophase ii) Metaphase iii) Anaphase iv) Telophase Mitosis is usually followed by the splitting of the cytoplasm of the cell to form 2 daughter cells - cytokinesis

3 3 Mitotic Spindle Apparatus Just prior to nuclear division, the original system of microtubules supporting the cell and organelles disassemble into subunits, and then reassemble into a new system of microtubules or the spindle apparatus Each spindle apparatus consists of a pair of barrel-shaped centrioles and 2 radial arrays of microtubules: 1. Asters, a small array of microtubules radiating toward the poles 2. Spindle fibers, long array of microtubules that attach to chromosomes toward the center and are responsible for chromosome movement during the different stages of mitosis Cytokinesis ("cell movement") In both plant and animal cells, new cell membranes form near the midline (= metaphasal plate) once occupied by the chromosomes during metaphase, thus separating the nuclei into 2 cells a. Animals Animal cells divide from the outside-in as a circle of micofilaments called a contractile ring, pinches each cell in two The constriction of the ring creates a dent or furrow in the cell surface This furrow deepens and eventually squeezes the cell in two b. Plants Plant cells retain their shape throughout the entire cell cycle, and divide from the inside-out During telophase, vesicles filled with cell wall and cell membrane precursors pinch off from the Golgi apparatus and collect in the cell's center The many vesicles gradually fuse, forming a central partition, or cell plate, made of cell membrane and cell wall material This central partition divides the cell into 2 daughter cells

4 4 Regulating the Cell Cycle The starting and stopping of cell division is often regulated by some kind of factor external to the cell It is apparent that most cells of plants and animals will not divide unless they are in contact with a solid surface - anchorage dependence May prevent cells from dividing inappropriately when separated from their usual surroundings Cells often stop dividing in response to contact with other cells - density-dependent (contact) inhibition Many cells begin to divide in response to chemical signals called growth factors Cancer Cancer cells don't stop growing after contact with other cells Cancer cells crawl over other cells, invade healthy tissues, and multiply into abnormal masses called tumors Causes - Believed to be related to changes in a cell's DNA that can alter how receptor proteins on the cell surface recognize other cells Other cancerous cells make their own growth factors in order to keep dividing Some seemingly do not require anchorage dependence Malignant tumors consist of cancerous cells; these tumors metastasize Benign tumors do not metastasize; they remain in the site of origin Cancers are named according to the tissue or organ of origin: - Carcinomas originate in the external or internal coverings of the body - Sarcomas arise in supportive tissues (e.g., bone and muscle) of the body - Leukemias are cancers of the blood vascular system, and lymphomas are cancers of the lymph nodes Two treatments for cancer include: radiation and chemotherapy

5 5 1. Asexual Reproduction New but genetically identical offspring grow directly from a few cells of a single parent The group of cells undergo mitotic division and often remain attached to the parent for a period of time There are many different types of asexual reproduction: a. Fission - the splitting off of new individuals from existing ones. b. Budding - in which a new individual grows out from the body of the original. c. Fragmentation - a breaking of the body into several pieces, each of which develops into a new organism. d. Regeneration - the replacements of parts of an organism that are lost due to injury. 2. Sexual Reproduction In this form of reproduction, parents generate specialized cells called gametes which eventually fuse together to zygotes Sperm cells and eggs cells come from a special group of cells called the germ cells The gametes of sexual reproduction are formed by a special kind of cell division called meiosis, a process that reduces the number of chromosomes that a cell possess in half If a cell has just one set of chromosomes it is said to be haploid Meiosis - The production of Haploid gametes Interphase I Meiosis I: prophase I, metaphase I, anaphase I, telophase I Cytokinesis Meiosis II: prophase II, metaphase II, anaphase II, telophase II Cytokinesis Meiosis resembles mitosis in that it has the same general phases But there is a significant difference in these 2 processes In meiosis, a cell undergoes 2 consecutive divisions: Meiosis I and Meiosis II, with each part is preceded by a period of interphase, producing 4 daughter cells result from the divisions rather than just 2 in mitosis But, note that there is only a single duplication of chromosomes Thus, the 4 daughter cells produced have half as many chromosomes as the starting cell

6 6 Genetic Variation Arises during Meiosis While mitosis leads to the production of identical daughter cells, the cells produced by meiosis can only be regarded as similar, not identical Because during meiosis, paternal and maternal chromosomes are reshuffled - genetic recombination - and new chromosome combinations occur Independent assortment The assortment of parental chromosomes is completely random during meiosis I And this random distribution process is called independent assortment Chromosomal crossing over This is a process in which parts of the homologous maternal and paternal chromosomes are exchanged The sites of crossing-over appear as X-shaped regions called chiasma During Meiosis I the homologous chromosomes are paired Summary: Mitosis, Meiosis, and Sexual Reproduction Asexual reproduction has several advantages: - only requires on individual to produce offspring - it requires very little energy expenditure - many offspring can be produced in a short period of time Theoretically this method of reproduction is most advantageous in stable, favorable environments because it perpetuates the exact same genetic combination over and over. Sexual reproduction can also be advantageous: - inheriting genes from the 2 parents and genetic recombination (independent assortment and crossing over) results in an increase in the genetic variability of offspring - genetic variability is particularly advantageous in unstable or unpredictable environments.