The Cell Cycle & Mitosis Chapter 12

Limits to Size Why can’t organisms just be one giant cell? – Diffusion cannot occur quickly and efficiently if the distances involved become too large. wastes would collect inside the cell and poison it nutrients could not reach organelles in time, so cells would die – Information overload would occur. DNA does not make copies as a cell grows – what it starts with is all that it has Must be enough DNA blueprint to allow for protein production

Ratio of Surface Area to Volume in Cells Volume is a three-dimensional unit – length X width X height Surface Area is a two-dimensional unit – length X width SO VOLUME INCREASES AT A FASTER RATE THAN DOES SURFACE AREA…Thus, SURFACE AREA IS THE LIMITING FACTOR IN THE SIZE OF A CELL

Functions of Cell Division 1. Reproduction of cells – all cells come from pre-existing cells – results in two identical cells except for size 2. Growth and Development of Organisms – tadpoles become frogs, ivy vines get longer 3. Tissue renewal – skin cells are being replaced, cuts and bruises heal

Cell Division: Distributing Identical Sets of Chromosomes to Daughter Cells Key to cell division is the copying and equal separation of chromosomes: – Chromosomes are carriers of the genetic material that is copied and passed from generation to generation: made up of DNA and protein cells of every organism have a specific number of chromosomes not visible in cells except during cell division

Cell Division Before it becomes too large, a growing somatic cell divides forming 2 “daughter” cells by a process known as cell division – Each daughter cell gets 1 complete set of genetic information during cell division and therefore will be IDENTICAL TO THE MOTHER CELL! A somatic cell is a non sex cell!

Chromosomes Spend most of their time as Chromatin – long strands of DNA that are wrapped around proteins and appear hazy and unorganized through the microscope – this loose arrangement is necessary for copying to occur. – When a cell gets ready to divide, the chromatin coils and condenses into what we call Chromosomes. When visible, chromosomes consist of two identical Sister Chromatids that are joined in the center at the Centromere

Chromosomes In eukaryotic cells, the genetic information that is passed on from one generation of cells to the next is carried by chromosomes – Chromosomes are made of DNA – The cells of every organism have a specific number of chromosomes (human cells have 46 chromosomes) Before cell division, each chromosome is replicated and consists of 2 identical “sister” chromatids – Each pair of chromatids is attached at an area called the centromeres centromeres sister chromatids

Figure 12.3 Chromosome duplication and distribution during mitosis A duplicating chromosome consists of 2 sister chromatids, which narrow at their centromeres. The DNA molecules of sister chromatids are identical. Chromosomes normally exist in the highly condensed state shown here only during the process of mitosis.

The Cell Cycle During the cell cycle, a cell grows, prepares for division, and divides to form 2 daughter cells, where each one of which begins a new cycle. The 5 phases of the cell cycle are: 1. Interphase – period of rest between cell division 2. Prophase 3. Metaphase 4. Anaphase 5. Telophase 6. Cytokinesis (division of the cytoplasm) phases of nuclear division (MITOSIS)

Events of the Cell Cycle nuclear division M Phase cell divides

Events of the Cell Cycle Interphase is divided into 3 phases: 1.G 1 – cell growth 2.S – DNA replication 3.G 2 – preparation for Mitosis During Interphase, chromosomes are in their “uncondensed” form and are called chromatin Mitosis (nuclear division) is the division of the nucleus and it occurs in 4 phases: 1.P = prophase – chromatin condenses into chromosomes, the centrioles separate & nuclear membrane breaks down 2.M = metaphase – chromosomes line up across center of cell and each chromosome is connected to a spindle fiber at its centromere 3.A = anaphase – sister chromatids separate into individual chromosomes and are pulled apart 4.T = telophase – chromosomes gather at opposite ends of the cell and 2 new nuclear membranes form around them Cytokinesis – division of cytoplasm

includes is divided into Concept Map of all events of Cell Cycle Cell Cycle M phase (Mitosis) Interphase G 1 phaseS phaseProphaseG 2 phaseMetaphaseTelophaseAnaphase Go to Section:

Interphase: G 1, S, and G 2 Interphase is very long (cells spend most of time here)… – G 1 phase – cell growth; cells increase in size and synthesize new proteins and organelles – S phase – chromosomes are replicated and the synthesis of DNA molecules takes place; key proteins associated with the chromosomes are synthesized during this time – G 2 phase – shortest of 3 phases; many of the organelles and molecules required for cell division are produced

INTERPHASE Nucleus well defined bounded by nuclear envelope. Easily identifiable nucleolus. Genetic material in uncondensed form of chromatin. – chromosomes cannot be seen.

M phase: Mitosis - PMAT Prophase Metaphase Anaphase Telophase nimations/content/mitosis.html nimations/content/mitosis.html

Prophase: 1 st and Longest Phase Chromatin coils and condenses – becomes visible as chromosomes; centrioles separate and take up positions on opposite sides of the nucleus; chromosomes become attached to spindle fibers; nucleolus disappears, nuclear membrane breaks down

PROPHASE

Prometaphase

Metaphase: Shortest Phase Chromosomes line up in center of cell along metaphase plate For each chromosome, the kinetochores of the sister chromatids are attached to microtubles coming from opposite poles of the cell

METAPHASE

Figure 12.6 The mitotic spindle at metaphase

Anaphase Anaphase begins suddenly when the paired centromeres that join the sister chromatids separate from each other. NOW EACH CHROMATID IS A SEPARATE CHROMOSOME….they begin moving toward opposite poles of the cell. Chromosomes continue to move until they have separated into two groups near the poles of the spindle. Anaphase is over when the chromosomes stop moving!

ANAPHASE

Telophase: Final Phase of Mitosis Two daughter nuclei form at the two poles of the cell. Chromosomes begin to relax back down into chromatin. Nuclear envelope re-forms around each cluster of chromatin. Spindle begins to break apart and nucleolus reappears in each daughter cell MITOSIS IS NOW COMPLETE, BUT NOT CELL DIVISION!

TELOPHASE

Figure 12.5x Mitosis

Cytokinesis Division of the cytoplasm itself. – Can take place in a number of ways: in animal cells – “draw-string” effect forms cleavage furrow (which pinches the cell into two parts) in plant cells – cell plate forms from inside out, and cell wall begins to appear

CYTOKINESIS

Figure 12.8 Cytokinesis in animal and plant cells

Figure 12.5 The stages of mitotic cell division in an animal cell: G 2 phase; Prophase; Prometaphase

Figure 12.5 The stages of mitotic cell division in an animal cell: Metaphase; Anaphase; Telophase and Cytokinesis.

How do cells know when to divide? Closeness of neighboring cells. Presence of proteins called CYCLINS: – 2 types: internal regulators external regulators

Close Contact Between Cells WHEN CELLS COME INTO CONTACT WITH OTHER CELLS, THEY STOP GROWING!

Cyclins Regulate the timing of the cell cycle in eukaryotic cells – TYPES: Internal regulators: respond to events inside the cell; – Ex. Don’t begin mitosis until all chromosomes are copied External regulators: respond to events outside the cell; direct cells to speed up or slow down the cell cycle. – Ex. Growth factors that stimulate the growth and division of cells are external regulators (REFER TO YOUR BOOK AND KNOW SOME SPECIFIC EXAMPLES)

Uncontrolled Cell Growth If growth is not controlled, then crowding and even tissue damage may result: – Ex. Cancer: disorder where body’s own cells lose the ability to control growth; can crowd and even damage tissue in surrounding area – forms tumor. – Benign vs. malignant tumors – benign are localized and not spreading; malignancies are capable of breaking off and starting up in another location – metastasis.

Figure The growth and metastasis of a malignant breast tumor

Figure 12-17x1 Breast cancer cell

Figure 12-17x2 Mammogram: normal (left) and cancerous (right)

Treatments for Cancer: surgery radiation therapy – expose to x-rays – releases free ions/free radicals, these attach to DNA, prevents cell division chemotherapy – causes nausea, lowering of immune system due to healthy cells that are affected – venchristine – attaches to mitotic spindle – methotrexate – anti-metabolite attaches to DNA, prevents DNA from replicating used to treat leukemia, brain tumors, testicular tumors