Cell Division Chapter 9
Cell Division • Cell division is the process in which a cell becomes two new cells. • Cell division allows organisms to grow and to reproduce.
Cell Division • In single celled organisms cell division is the method for increasing their numbers, • In multicellular organisms, the process of cell division leads to growth, the replacement of lost cells, the healing of injuries, and the formation of reproductive cells.
3 Types of Cell Division • Binary Fission • Mitosis • Meiosis
Binary Fission • Prokaryotic cells utilize binary fission. • A single loop of DNA replicates, a membrane forms in between the two molecules, and the cell divides. • The daughter cells are genetically identical to the parent cell.
Mitosis • Eukaryotic cells utilize mitosis. • Eukaryotic cells have several chromosomes that are replicated and divided. • The daughter cells are genetically identical to the parent cell.
Meiosis • Eukaryotic cells also utilize meiosis to reproduce daughter cells with half of the genetic information of the parent cells.
Asexual Reproduction • Binary fission and mitosis are methods utilized by single-celled organisms for asexual reproduction. • Only one parent cell is necessary.
Asexual Reproduction • The parent cell divides and results in two organisms that are genetically identical to the parent. • Most prokaryotes use binary fission. • Most eukaryotic organisms are multicellular. In multicellular organisms, mitosis produces new cells.
Sexual Reproduction • Sexual reproduction is the combining of genetic information from two parents. This results in a genetically unique individual. • Meiosis is the process that produces the cells for sexual reproduction.
Cell Cycle • The cell cycle consists of all the stages of growth and division for a eukaryotic cell. • The cell cycle includes the stages in which the cell spends its time engaged in metabolism.
Interphase • Interphase is the longest stage of the cell cycle. • During this stage, the cell engages in metabolic activities to prepare for the next cell division. • Interphase is broken down into 3 distinct stages: • G1, S, and G2
G1 Stage of Interphase • During the G1 stage of interphase, the cell gathers nutrients. • This allows the cell to carry out its normal functions and to grow in volume. • If a cell stays in the G1 phase for an extended period, it is often renamed G0 because the cell is not moving forward in the cell cycle.
S Stage of Interphase • During the S Stage of interphase, DNA synthesis (replication) occurs. • The DNA in chromosomes is wrapped around histone proteins to form nucleosomes.
S Stage of Interphase • The nucleosomes are then coiled to form chromatin. • As the chromatin becomes coiled, it becomes visible as a chromosome. • A chromatid is one of two parallel parts of a chromosome.
DNA Synthesis • Before DNA synthesis, each chromatid contains one DNA molecule.
DNA Synthesis • DNA synthesis occurs resulting in 2 DNA molecules, one in each chromatid. Sister chromatids are the 2 chromattids of the chromosome that were produced by replication. • The sister chromatids are attached at the centromere.
G2 Stage of Interphase • The final stage of interphase is G2. • The cell makes all of the cellular components that it will need for division. • The chromatin has replicated, but has not coiled, so it is not visible.
Mitosis • There are two distinct events in Mitotic cell division: • 1. The replicated genetic information of the cell is equally distributed in mitosis. • 2. After mitosis, the cytoplasm of the cell divides into two new cells. This division is called cytokinesis (cell splitting).
Stages of Mitosis • Prophase • Metaphase • Anaphase • Telophase
Prophase • Chromosomes condense. • Spindle and spindle fibers form. • Nuclear membrane disassembles.
Metaphase • Chromosomes align at the equatorial plane of the cell.
Anaphase • Sister chromatids move toward opposite ends of the cell.
Telophase • Spindle fibers disassemble. • Nuclear membrane re-forms. • Chromosomes uncoil. • Nucleolus re-forms.
Cytokinesis • After telophase, the cell has two nuclei. • Cytokinesis creates two daughter cells. • Cytokinesis is the process whereby the contents of the cell are split between the two daughter cells.
Cytokinesis • Animal cells form a cleavage furrow, which is an indentation of the plasma membrane that pinches in towards the center of the cell. • In plant cells, a cell plate begins to form at the center of the cell. • Cytokinesis marks the end of a round of cell division. The cell then returns to interphase at G1.
Determination and Differentiation • Determination is the cellular process of determining the genes a cell will express when mature. • A cell commits to becoming a certain cell type. • When a cell reaches the end of this path, it is said to be differentiated. It has become a particular cell type.
Cell Division and Sexual Reproduction • Meiosis is a form of cell division that aids sexual reproduction. • Mitosis is responsible for growth and repair of tissues. • Meiosis is responsible for the production of eggs and sperm. • The cells of sexually reproducing organisms have two sets of chromosomes and thereby two sets of genetic information.
Cell Division and Sexual Reproduction • One set is received from the mother’s egg and the other set from the father’s sperm. • Therefore, gametes containing only one set of chromosomes must be formed. • Haploid cells carry only one complete copy of their genetic information. • Diploid cells carry two complete copies of their genetic information.
Cell Division and Sexual Reproduction • In many sexually reproducing organisms, meiosis takes place in the gonads. • The gonads in females are known as ovaries. The gonads in males are know as testes.
Cell Division and Sexual Reproduction • A gamete is a reproductive cell like eggs and sperm. They are also referred to as germ cells. • Fertilization is the joining of genetic material from two haploid cells.
Meiosis I • Meiosis I is a reduction division, in which the chromosome number in the two cells produced is reduced from diploid to haploid. • The sequence is divided into four phases: • Prophase I, metaphase I, anaphase I, and telophase I.
Prophase I • Chromosomes condense. • Spindle and spindle fibers form. • Nuclear membrane disassembles. • Synapsis and crossing-over occur.
Metaphase I • Chromosomes align on equatorial plane as synapsed pairs.
Anaphase I • Homologous chromosomes separate from each other. • Chromosomes move towards cell’s poles. • Reduction occurs.
Telophase I • Spindle fibers disassemble. • Chromosomes uncoil. • Nuclear membrane re-forms. • Nucleoli reappear.
Meiosis II • The two daughter cells formed from Meiosis I both continue through Meiosis II so that four cells result. • Meiosis II includes four phases:
Meiosis II • Prophase II, metaphase II, anaphase II, and telophase II. • No DNA replication occurs between telophase I and meiosis II. • The events in the division sequence of meiosis II are the same as those that occur in meiosis.
Prophase II • Chromosomes condense. • Spindle and spindle fibers form. • Nuclear membranes disassemble.