Cellular Reproduction

Transcription

1 Cellular Reproduction Ratio of Surface Area to Volume As the cell grows, its volume increases much more rapidly than the surface area. The cell might have difficulty supplying nutrients and expelling enough waste products. Transport of Substances Substances move by diffusion or by motor proteins. Diffusion over large distances is slow and inefficient. Small cells maintain more efficient transport systems. Cellular Communications The need for signaling proteins to move throughout the cell also limits cell size. Cell size affects the ability of the cell to communicate instructions for cellular functions. Why do cells divide One-celled organisms for reproduction asexual reproduction (clones) Multi-celled organisms for growth & development from fertilized egg to adult for repair & replacement replace cells that die from normal wear & tear or from injury Cell division: a quick overview A dividing cell duplicates its DNA creates 2 copies of all DNA separates the 2 copies to opposite ends of the cell splits into 2 identical daughter cells

2 Chromosome Number All cells in the human body (SOMATIC CELLS) have 46 or 23 pairs of chromosomes, called the DIPLOID or 2n number GAMETES (eggs & sperm) have only 23 chromosomes, called the MONOPLOID or 1n number The cell cycle has three main stages: they are described in the table below Stage Description Number of Cells Interphase The cell grows in size, performs normal functions, & copies its DNA. One Cell Mitosis The cell nucleus divides, & the chromosomes separate into the two One Cell nuclei Cytokinesis The cytoplasm of the cell divides, forming two daughter cells Two Cells INTERPHASE STAGES 1. G1 - Cells increase in size, synthesize new proteins & synthesize new cell organelles 2. S - Chromosomes are copied which gives the cell a duplicate set of DNA 3. G2 - Preparation for Mitosis Mitosis (M Phase) 1. It is the part of the cell cycle where the division of the nucleus and cytokinesis, (division of the cytoplasm), take place. 2. Mitosis is divided into 5 steps(phases) Prophase (Please) Metaphase (Make) Anaphase (Another) Telophase (Two) Cytokinesis (Cells) Prophase: 1 st and longest phase of mitosis Nuclear envelope disappears Chromosomes condense Now, you can see sister chromatids and centromere Spindle forms(microtubule structure that helps separate the chromosomes.) Metaphase: Chromosomes move to the equator of spindle

3 Each chromatid is attached to spindle with centromere Anaphase: Centromeres split Sister chromatids are pulled apart to opposite poles of the cell Each chromatid is now a separate chromosome Telophase Nuclear envelopes (2) reform Chromosomes begin to uncoil and return to a tangled looking mess. Cytokinesis Cytoplasm divides Two new daughter cells are now separate Cytokinesis usually occurs at the same time as telophase. Difference Between Plant and Animal Cell Mitosis Plant No centrioles Cell Plate forms to separate the two new cells in telophase. Cell Plate forms the cell wall. Animal Centrioles Pinching in of the cell membrane occurs in telophase to separate the two new cells.

4 When is mitosis a good thing? When you have to add or replace cells growth & development repair replacement When is mitosis a BAD thing When cells reproduce & they are not needed these cells take over organs, but don t do the right job they just keep making copies cancer :damages organs What is Cancer? Cancer is uncontrolled cell division (mitosis) that can t be stopped. A mass of cells called a tumor is a result of this uncontrolled cell division. There are 2 types of tumors: Benign = non-cancerous Malignant = cancerous and they invade and destroy other tissues. Benign tumor : abnormal cells remain at original site as a lump, most do not cause serious problems & can be removed by surgery Malignant tumor : cells leave original site carried by blood system to other tissues start more tumors damage functions of organs throughout body Apoptosis Programmed cell death Cells going through apoptosis actually shrink and shrivel in a controlled process. Stem Cells Unspecialized cells that can develop into specialized cells when under the right conditions Embryonic Stem Cells After fertilization, the resulting mass of cells divides repeatedly until there are about cells. These cells have not become specialized.

5 Chromosomes and Chromosome Number Human body cells have 46 chromosomes Each parent contributes 23 chromosomes Homologous chromosomes one of two paired chromosomes, one from each parent Chromosomes and Chromosome Number Same length Same centromere position Carry genes that control the same inherited traits Haploid and Diploid Cells An organism produces gametes to maintain the same number of chromosomes from generation to generation. Human gametes contain 23 chromosomes A cell with n chromosomes is called a haploid cell A cell that contains 2n chromosomes is called a diploid cell Meiosis The sexual life cycle in animals involves meiosis Meiosis produces gametes. When gametes combine in fertilization, the number of chromosomes is restored. Stages of Meiosis I Reduces the chromosome number by half through the separation of homologous chromosomes Involves two consecutive cell divisions called meiosis I and meiosis II Meiosis I Interphase Chromosomes replicate. Chromatin condenses. Prophase I Pairing of homologous chromosomes occurs. Each chromosome consists of two chromatids. The nuclear envelope breaks down. Spindles form. Crossing over produces exchange of genetic information. Crossing over chromosomal segments are exchanged between a pair of homologous chromosomes Metaphase I Chromosome centromeres attach to spindle fibers Homologous chromosomes line up at the equator. Anaphase I Homologous chromosomes separate and move to opposite poles of the cell.

6 Telophase I The spindles break down. Chromosomes uncoil and form two nuclei. The cell divides. Meiosis II Prophase II A second set of phases beginsas the spindle apparatus forms and the chromosomes condense. Metaphase II A haploid number of chromosomes line up at the equator. Anaphase II The sister chromatids are pulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell. Telophase II The chromosomes reach the poles, and the nuclear membrane and nuclei reform. Cytokinesis results in four haploid cells, each with n number of chromosomes. The Importance of Meiosis Meiosis consists of two sets of divisions Produces four haploid daughter cells that are not identical Results in genetic variation Meiosis Provides Variation Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine. Sexual Reproduction v. Asexual Reproduction Asexual reproduction The organism inherits all of its chromosomes from a single parent. The new individual is genetically identical to its parent. Sexual reproduction Beneficial genes multiply faster over time.