Mitosis and Cell Reproduction

The reproduction of mankind is a great marvel and mystery. Had God consulted me in the matter, I should

have advised him to continue the generation of the species by fashioning them out of clay.

Martin Luther

Cell Nucleus• The nucleus contains the

hereditary material of the cell assembled into chromosomes.

• In addition, the nucleus usually contains one or more prominent nucleoli (dense bodies that are the site of ribosome synthesis)

• The nucleus is surrounded by a nuclear envelope that consists of a double membrane that is continuous with the ER.

• Transport of molecules btwn the nucleus and cytoplasm is accomplished through a series of nuclear pores lined with proteins that facilitate passage of molecules out of and into nucleus.

Chromatin and Eukaryotic Chromosomes

• The nuclear material consists of DNA

• DNA molecules are extremely long relative to the cell

• In chromosomes, the DNA is condensed and packaged into manageable bodies

• The mass of DNA material and its associated protein is called the chromatin.

Chromatin

• To form chromatin, the DNA molecule is wound around globules of protein called histone.

• The units formed in this way are called nucleosomes.

• Millions of nucleosomes are connected by short stretches of histone proteins much like beads on a string.

• The configuration of nucleosomes in a coil cause additional coiling of DNA and eventual formation of the chromosome.

Cell Cycle

• Cell cycle- many repetitions of cellular growth and reproduction. With few exceptions (for example Red Blood cells), all cells of living things undergo the cell cycle

Phases of the Cell Cycle

• Interphase- phase in which cells spends most time performing functions that makes it unique.

• Mitosis- phase of cell during which cell divides into two daughter cells.

Interphase Stages• G1- follows mitosis and is

period in which cell is synthesizing proteins and enzymes to perform its functions.

• During G1 phase, each chromosome consists of single molecule of DNA and associated histone protein. In human cells, there are 46 chromosomes per cell (diploid) except in sex cells (also referred to gametes) where there are 23 (haploid meaning half) and RBC which have no chromosomes

Interphase: S

• DNA replicates. During this phase, each chromosome is copied, so by the end of the S phase, there are two DNA molecules for each one formerly in the S phase.

Interphase Stage: G2

• Cell prepares for mitosis.

• Proteins organize themselves to form a series of fibers called a spindle, which is involved in chromosome movement during mitosis.

Mitosis

• Mitosis is derived from the term Latin stem mito meaning “threads.”

• During mitosis, the nuclear material becomes visible as threadlike chromosomes.

• The chromosomes organize in the center of the cell, then they separate, and 46 chromosomes move into each new cell that forms.

• Mitosis is a continuous process, but for convenience of denoting which portion of the process of taking place, scientists divide mitosis into a series of phases called Prophase, Metaphase, Anaphase, Telophase and cytokineses. (PMAT)

Functions of mitosis

• For simple organisms, it is a means of asexual reproduction. (For example cells of fungus)

• In multicellular organisms, it allows for entire cell to grow by forming new cells and replacing older cells.

• In certain species, mitosis is used to heal wounds or regenerate body parts.

• It is the universal process for cell division.

Stages

Prophase

• Begins with the condensation of chromosomes to form visible threads

• Two copies of chromosomes exist, each one called a chromatid. These are joined together at the region called the centromere.

• As this phase unfolds, chromatids become visible in pairs and spindles fibers form, the nucleoli disappear, and nuclear envelope dissolves.

Prophase continued

• In animal cells, microscopic bodies called centrioles begin to migrate to opposite sides of the cell. These are not present in most plant or fungal cells.

• As prophase continues, the chromatids attach to the spindle fibers that extend out from opposite poles of the cell.

• The spindles fibers attach at region of centromere called kinetochore.

• Eventually, all pairs of chromatids reach center of cell called the equatorial plate.

Metaphase

• Stage where pairs of chromatids line the equatorial plate.

• In the human cell, 92 chromosomes in 46 pairs align at the equatorial plate.

• Each pair is connected at the centromere where the spindle fiber is attached.

Anaphase

• Chromatids move apart from each other.

• Chromatids are called chromosomes after the separation.

• Chromosomes can be seen moving. They take on a rough V shape.

• Result of anaphase is an equal separation and distribution of chromosomes.

Telophase

• Chromosomes finally arrive at opposite poles of the cell.

• Distinct chromosomes begin to fade and chromatin are formed again.

• Spindle is dismantled, nucleoli reappear, and nuclear envelope is formed. (Opposite of prophase)

Cytokinesis

• Process in which cytoplasm divides into two separate cell forms.

• In animal cells, it begins with the formation of a furrow in the center of the cell.

• With formation of furrow, membrane begins to pinch into cytoplasm and formation of two cells begins. (Process referred to as cell cleavage).

Plant cell Final Stage

• Cleavage doesn’t take place.

• New cell wall is assembled at center of cell.