Cell Cycle

Key Roles of Cell Division Functions in asexual reproduction, growth, embryonic development, tissue repair. Distributes identical sets of chromosomes to daughter cells. Genetic information, packaged as DNA, is called a genome. Prokaryotes: the genome is a single circular DNA molecule. Eukaryotes: genome consists of several DNA molecules. Bacterial cell

DNA packaged as chromosomes Eukaryotic species have a characteristic number of chromosomes in the nucleus. Human somatic cells (body cells) have 46 chromosomes. Human gametes (sperm or eggs) have 23 chromosomes. Each chromosome has hundreds or thousands of genes, the units that specify an organism's inherited traits. Chromatin: Eukaryotic DNA + the associated proteins that maintain its structure and help to control gene activity.

Chromosomes made of DNA Eukaryotic DNA is wrapped around proteins to form chromatin. Chromatin Chromatin condenses (super-coils) to become a chromosome visible with a light microscope.

Overview of mitosis Chromosomes are duplicated in interphase. Duplicated chromo- somes consist of 2 “sister” chromatids which contain identical copies of DNA. Strands condense; the region where they connect shrinks and be- comes the centromere.

Overview of mitosis Then chromosomes are separated into “daughter” cells. Sister chromatids are later pulled apart, re- packaged into 2 new nuclei at opposite ends of the parent cell. This process is called mitosis. It is fol- lowed by division of the cytoplasm, called cytokinesis.

Stages in the cell cycle The mitotic (M) phase of the cell cycle alternates with the much longer interphase. The M phase includes mitosis and cytokinesis. Interphase: 90% of the cell cycle; a very active period. During inter- phase the cell grows by pro- ducing proteins & cytoplasmic organelles, cop- ies its chromo- somes, and pre- pares for cell division.

Stages in the cell cycle Mitosis is broken into 4 sub-phases: Interphase Prophase Metaphase Anaphase Telophase

Stages in the cell cycle Prophase Late interphase: Chromosomes duplicated but loosely packed. Centrosomes duplicated and organize microtubules into an aster ("star"). Prophase: Chromosomes tightly coil; sister chromatids join. Nucleoli disappear; mitotic spindle forms and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell. Late Interphase Prophase

Stages in the cell cycle Late Prophase Late Prophase: Nuclear envel- ope fragments, and microtub- ules from the spindles coming from each pole attach to 1 of 2 kinetochores, special regions of the centromere. Centrosomes are pushed to opposite ends of the cell due to the lengthening microtubules. Prophase Late Prophase

Stages in the cell cycle Metaphase Metaphase: Spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equi- distant between the poles.

Stages in the cell cycle Anaphase Anaphase: Centromeres divide, sep- arating sister chromatids into chromosomes again. As spindle fibers shorten, each chromosome is pulled toward the pole to which it is attached. The 2 poles will then have equivalent collec- tions of chromosomes.

Stages in the cell cycle Telophase Telophase: The cell continues to elongate as free spindle fibers from each centrosome push off each other. Two nuclei begin to form, sur- rounded by the fragments of the parent's nuclear envelope. Nucleoli begin to reappear as spindle apparatus disappears. Chromatin becomes less tightly coiled. Cytokinesis, division of the cytoplasm, begins.

Cytokinesis divides the cytoplasm Cytokinesis, division of the cytoplasm, follows mitosis. In animals a cleavage furrow forms in the cell surface near the old metaphase plate. Contraction of the ring pinches the cell in two. Cytokinesis in plants involves building a cell wall. During telophase, Golgi vesicles form a cell plate. The plate membranes fuse with the plasma membrane; contents of vesicles form new wall material.

Genetically identical nuclei By this method, each daughter cell ends up with one copy of every chromosome. An abnormal number of chromosomes in a cell is usually fatal.

Uncontrolled cell growth Cancer cells have escaped from cell cycle controls. They result from uncontrolled cell division. Cells divide excessively, invade other tissues. They can occur in any organ or tissue They may divide indefinitely if they have a constant food supply. In contrast, most mam- malian cells divide 20 to 50 times under culture conditions before they stop, age, or die. Cancer cells may be "immortal".

Uncontrolled cell growth Cancer results from cell mutations.

Uncontrolled cell growth The immune system recognizes & destroys transformed cells. Cells that evade destruction proliferate to form a tumor. If the abnormal cells remain at the originating site, the lump is called a benign tumor: remove by surgery. If cells lose attachment to nearby cells, they are carried by blood & lymph system to other tissues (metastasis). Malignant tumor: cells leave original site and form new tumors that impair the functions of one or more organs.

Uncontrolled cell growth Breast cancer treatment DCIS = ductal carcinoma in situ

Uncontrolled cell growth Skin cancer from excessive UV light

Uncontrolled cell growth Lung cancer from smoking

Uncontrolled cell growth Brain cancer