Cell Division: Mitosis and Cytokinesis

• Cell division is a process in which one cell divides into two new cells

Objectives:

• Diagram and label cell division
• Identify cells in each stage

Key Roles of Cell Division

• The ability of organisms to reproduce distinguishes living things from nonliving matter.
• The continuity of life is based on the reproduction of cells.
• In unicellular organisms, the division of one cell reproduces an entire organism.
• Cell division for multicellular organisms is essential for development from a fertilized cell, growth, and repair.
• Cell division is an integral part of the cell cycle, from formation to its own division.

Genetically Identical Daughter Cells

• Most cell divisions result in daughter cells with identical genetic information, DNA.
• Eukaryotic cell division consists of mitosis and cytokinesis.
• Mitosis is the division of the nucleus.
• Cytokinesis is the division of the cytoplasm.
• Somatic cells are non-reproductive cells, and have two sets of chromosomes.
• A special type of division produces nonidentical daughter cells (gametes, or sperm and egg cells).
• Meiosis yields daughter cells that have only one set of chromosomes, half as many as the parent cell
• Gametes, which are reproductive cells (sperm and eggs), have half as many chromosomes as somatic cells.

Genetic Material Organization

• All DNA in a cell constitutes the cell's genome.
• Genomes can consist of a single DNA molecule in prokaryotic cells, or a number of DNA molecules in eukaryotic cells.
• DNA molecules in a cell are packaged into chromosomes.
• Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division.
• Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.
• Humans have 46 chromosomes.

Eukaryotic Cell Division

• In preparation for cell division, DNA is replicated and chromosomes condense.
• Each duplicated chromosome has two sister chromatids which separate during cell division.
• The centromere is the narrow "waist" of the duplicated chromosome, where the two chromatids are most closely attached.

Cell Cycle Phases

• The cell cycle consists of: mitotic (M) phase and interphase.
• Mitotic (M) phase includes mitosis and cytokinesis.
• Interphase includes cell growth and copying of chromosomes in preparation for cell division.
• Interphase can be divided into subphases: G1 phase ("first gap"), S phase ("synthesis"), and G2 phase ("second gap").
• The cell grows during all three phases for interphase.
• Chromosomes are duplicated only during the S phase.
• Mitosis is conventionally divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase.
• Cytokinesis is well underway by late telophase.

Walther Flemming

• In 1882, German anatomist Walther Flemming developed dyes to observe chromosomes during mitosis and cytokinesis.
• Flemming coined the term mitosis: mitos = "wrap thread" and osis = "act or process".
• This term was based on the warped thread appearance of the chromatin in the cell's nucleus in the first stages of mitosis

Animal Cell Mitotic Division

• The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis.
• During prophase, assembly of spindle microtubules begins in the centrosome.
• The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell with spindle microtubules growing out from them.
• An aster, a radial array of short microtubules, extends from each centrosome.
• The spindle includes the centrosomes, the spindle microtubules, and the asters.
• During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes.
• At metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle's two poles.
• In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell.
• The microtubules shorten by depolymerizing at their kinetochore ends.
• Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell
• During telophase, genetically identical duaghter nuclei form at opposite ends of the cell

Cytokinesis

• In animal cells, cytokinesis occurs by cleavage, forming a cleavage furrow.
• In plant cells, a cell plate forms during cytokinesis.

Mitosis in a Plant Cell

• During prophase the chromatin is condensing and nucleolus begins to disappear.
• During late prophase We can now see chromosomes and the nuclear envelope will fragment.
• During metaphase The spindle is complete, and the chromosomes attached to the microtubules are at metaphase plate.
• During anaphase the chromatids of each chromosome have separated to the end of the cell as the kinetochore shortens
• During telephase Nuclei are beginning to form and cytokinesis has started: The cell plate will divide the cytoplasm in two.

Bacterial Cell by Binary Fission

• Prokaryotes reproduce by binary fission.
• The chromosome replicates (at origin of replication), and the two daugher chromosomes go apart.

Other Notes on Cell Division

• asexual reproduction is the creation of offspring from a single parent.
• Binary fission produces two daughter cells genetically identical to the single parent cell.
• Binary fission occurs in prokaryotes.
• A hypothetical sequence for the evolution of mitosis exists
• Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission.
• Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis
• Chromosomes contain genetic information.
• Mitosis cell division begins in interphase, when the chromosomes duplicate.
• Mitosis cells must divide for asexual reproduction.
• Mitosis daughter cells are genetically identical to the parent cell of the same kind and number of chromosomes.
• Mitosis occurs in somatic or body cells such as liver, heart, skin, and stomach cells.
• Every organism has its own number of chromosomes
• Human have 46 chromosomes, called a diploid, writting as 2N
• Adult organisms have 2 sets of homologous chromosomes that match, one from a female and one from a male parent.
• In meiosis, the number of chromosomes in the original cells are reduced by HALF by separating homologous chromosomes
• Male meiosis occurs in the testes(XY) while female meiosis occurs in ovaries(XX)
• Males meiosis begins at puberty and females meiosis finish before birth.
• Chromosome numbers are unique to each kind of organism.

Unique Events in Meiosis:

• Homologous chromosomes pair up before the 1st cell division
• Homologous chromosomes: are look alikes that code for same traits, receive one from each parent
• During 1st division, homologous chromosomes exchange genes during a process called crossing over", during meiosis the gametes will be different.
• Crossing over increases genetic variation and is the reason why siblings look different
• No crossing over = daugher identical to parent, crossing over = daughters not identical to parent.

Comparing Mitosis and Meiosis

Explore cell division, focusing on mitosis and cytokinesis. Learn how cells divide into genetically identical daughter cells. Understand the key roles of cell division in organismal development, growth, and repair.