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.

Comparison of Mitosis and Meiosis

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.

Mitosis

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.

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.

Chromosomes

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.

Prophase

Metaphase

• Chromosomes align at the equatorial plane of the cell.

Metaphase

Anaphase

• Sister chromatids move toward opposite ends of the cell.

Anaphase

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

Meiosis

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.

Meiosis

Prophase II

• Chromosomes condense.• Spindle and spindle fibers form.• Nuclear membranes disassemble.

Metaphase II

• Chromosomes align in an unpaired manner.

Anaphase II

• Chromatids separate as chromosomes begin to move to the cell’s poles.

Telophase II

• Nuclear membrane re-forms.• Chromosomes uncoil.• Nucleoli reappear.• Spindle fibers disassemble.