BIOL #10: Cell Cycle & Mitosis Flashcards

Question

Binary Fission

Answer 1

Prokaryotes reproduce by doubling their size and dividing in half – a process known as binary fission Does not include mitosis Bacteria do not undergo cytokinesis, but instead divide via fission, a process similar to animal cytokinesis.

Answer 2

Cell cycle length can vary greatly among cell types; variation in the length of G1 phase is responsible for these differences. - G1 phase is essentially eliminated in rapidly dividing cells. - Nondividing cells get permanently stuck in G1 phase; this arrested stage is called the G0 state (e.g. nerve or muscle cells) The rate of cell division can also respond to changes in environmental conditions. Variations in cell cycle length suggest that the cell cycle is regulated and that regulation varies among cells and organisms. Cell Cycle control system - Experimental evidence suggest that the cell cycle is driven by specific signaling molecules present in the cytoplasm - This control system involves a cyclically operating set of molecules that both trigger and coordinate key events in the cell cycle - Includes checkpoints regulated by both internal and external signals

Answer 3

Mitosis-promoting factor (MPF) is present in the cytoplasm of M phase cells and induces mitosis in all eukaryotes. MPF is composed of two distinct subunits: a protein kinase and a cyclin. - The protein kinase is an enzyme that catalyzes the transfer of a phosphate group from ATP to a target protein (phosphorylation). - The cyclin subunit functions as a regulatory protein.

Answer 4

The MPF protein kinase is a cyclin-dependent kinase (Cdk) that is active only when bound to the cyclin subunit. Thus, when cyclin concentrations are high, more MPF is active and more target proteins are phosphorylated, initiating mitosis. The concentration of MPF cyclin increases during interphase, then peaks in M phase before decreasing again.

Answer 5

After it binds to cyclin, MPF’s Cdk subunit becomes phosphorylated at two sites, rendering it inactive. Late in G2 phase enzymes cause one of the phosphate groups on the Cdk subunit to drop off. - This dephosphorylation reaction changes MPF’s shape, activating it. Once MPF is activated, it triggers a chain of events, culminating in the condensation of chromosomes and formation of the mitotic spindle apparatus.

Answer 6

During Anaphase, an enzyme complex begins degrading MPF’s cyclin subunit. In this way, MPF triggers a chain of events that leads to its own destruction. The Cdk persists in the cell, inactive, until new cyclin molecules are made during the S and G2 phases of the next cell cycle

Answer 7

A checkpoint in the cell cycle is a control point where ‘stop’ and ‘go ahead’ signals can be regulated - Such signals involve signal transduction pathways There are three distinct cell-cycle checkpoints during the four phases of the cell cycle (G1, G2 and Metaphase checkpoints) Interactions among regulatory molecules at each checkpoint allow a cell to “decide” whether to proceed with division. If these regulatory molecules are defective, the checkpoint may fail and cells may start dividing in an uncontrolled fashion. The three cell-cycle checkpoints prevent the division of cells that are damaged or that have other problems, and they prevent the growth of mature cells that should stay in the G0 state.

Answer 8

The first and most important checkpoint occurs late in G1. This checkpoint determines whether the cell will continue through the cycle and divide, or exit the cycle and enter G0. Four factors affect whether cells pass the G1 checkpoint: 1) Cell size 2) Nutrient availability 3) Social signals from other cells 4) Health of DNA

Answer 9

Cells must be large enough to split into two functional daughter cells. Food must be sufficient for cell growth. Cells in multicellular organisms pass (or do not pass) through the G1 checkpoint in response to signaling molecules from other cells. - Certain cells release proteins called growth factors that stimulate other cells to divide - High density can inhibit division via growth-inhibiting signals between adjacent cells

Answer 10

The p53 protein either pauses the cell cycle or initiates apoptosis – programmed cell death – if the DNA is physically damaged. - p53 is an example of a tumor suppressor (checkpoint regulatory molecule) -- damage to the p53 gene can lead to uncontrolled cell division.

Answer 11

The second checkpoint is after chromosome replication, between the G2 and M phases. Cells stop growing at this check point if chromosome replication has not proceeded properly or if DNA is damaged. Activated MPF must be present for mitosis to begin (i.e. high level of cyclin)

Answer 12

The third and final checkpoint is during the M phase. - Cell division ceases during metaphase if the chromosomes are not properly attached to the mitotic spindle. - This mechanism prevents incorrect chromosome separation that could give daughter cells the wrong number of chromosomes.

Answer 13

Cancer is a common, sometimes lethal disease that affects many humans. Cancer is a complex family of diseases caused by cells that grow in an uncontrolled fashion, that invade nearby tissues, and that spread to other sites in the body. Cancers vary widely in time of onset, growth rate, seriousness, and cause. Despite their differences, all cancers arise from cells in which cell-cycle checkpoints have failed (e.g. p53). - Cancers are thought to arise from cells with defects in the G1 checkpoint.

Answer 14

Cancerous cells have two types of defects: 1) Defects that make the proteins required for cell growth active when they should not be 2) Defects that prevent tumor suppressor genes (e.g. p53) from shutting down the cell cycle Most cancers result from multiple defects in cell-cycle regulation. - Most cancers develop only after several genes have been damaged. - This combined damage is enough to break cell-cycle control and induce uncontrolled growth and metastasis. Each type of cancer is caused by a unique combination of errors.

Answer 15

A tumor forms when one or more cells in a multicellular organism begin to divide in an uncontrolled fashion.

Answer 16

When cancer cells detach from the original tumor and invade other tissues, this is called metastasis.

Answer 17

Benign tumors are noninvasive and noncancerous. | - Do not transform adjacent tissues and do not spread

Answer 18

Malignant tumors are invasive. They can spread throughout the body via the blood or lymph, and initiate secondary tumors.

Answer 19

Cells respond to signals from other cells, so that cells divide only when their growth benefits the whole organism. This is known as social control. Social control is based on growth factors–polypeptides or small proteins released by cells that stimulate division in other cells (paracrine signaling). - Generally, cell cultures will not grow unless growth factors are present. Cancer cells, however, divide without growth factors. They are no longer subject to social control at the G1 checkpoint.