Chapter 12 Cell Cycle

What is Cyclin?

A protein that gets its name from its cyclically fluctuating concentration in the cell. These proteins activate cyclin dependent kinases. Cyclin level rises during the S and G2 phases and then falls during the M phase

What is an aster?

A radial array of short microtubules that extend from the centrosomes which are at the opposite ends of the cell. Function of them is to anchor the centrosomes in place to the poles of the cells

What is a checkpoint in the cell cycle?

A control point in the cell cycle where stop and go-ahead signals can regulate the cycle. Three important checkpoints are found in the G1, G2 and M phases.

What is the cell cycle control system?

A cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle. These molecules include protein kinases and cyclins.

What is the G0 phase?

A non-dividing state in the cell. Most cells in the human body are in the G0 phase. Mature nerve cells and muscle cells never divide.

Describe the cell cycle

A cell grows during interphase, subphase G1. Continues to grow as it duplicates it chromosomes S phase. Continues to grow as it prepares for cell division G2. And finally divides M phase.

What is the centromere?

A region of the chromosomal DNA where the chromatid is attached most closely to its sister chromatid. This attachment is mediated by proteins bound to the centromeric DNA. Other bound proteins condense the DNA and gives the duplicated chromsome a narrow waist.

What is the mitotic spindle?

A structure which begins to form in the cytoplasm during prophase. The structure consists of fibers which are made of microtubules and associated proteins. The structure is formed while the microtubules of the cytoskeleton are disassembled, these same microtubules provide the structure for the mitotic spindle. The spindle microtubules lengthen by incorporating more subunits of the protein tubulin and shorten by losing these same subunits.

What is genome?

All the DNA in a cell. In a prokaryotic cell this can be one DNA molecule and in a eukaryotic cell it can be several DNA molecules.

Describe the G1 checkpoint

Also known as the restriction point. Is the most important checkpoint in the cell. If the cell passes this checkpoint, it will go onto the other phases. If it does not pass this checkpoint, it remains in the G0 Phase.

How do Cyclin-CDK complexes regulate the activity of cell behavior at the G1 checkpoint?

Animal cells have at least 3 CDK proteins and several types of cyclins at this checkpoint. Different cyclin-CDK complex fluctuations are of major important controlling all the stages of the cell cycle.

How does the cell die via apoptosis?

Apoptosis if it does not pass the checkpoints Chromatin condenses Cells shrink Preservation of organelles and cell membranes Rapid engulfment of by neighboring cells preventing inflammation DNA fragmentation

What happens in the M-phase checkpoint?

By the end of anaphase, the former sister chromatids AKA individual chromosomes are on the opposite ends of the cell attached to kinetochore microtubules and being pulled toward the centrosome. However, this process of separating the sister chromatids from each other does not begin until each individual chromosome is attached to the spindle at the metaphase plate, that it each kinetochore on a sister chromatid, is attached to a kinetochore microtubule. Until this process occurs, the appropriate regulatory protein complex does not become activated. Once it does become activated, the molecule sets off a chain of molecular events that activates the enzyme separase. This checkpoint prevents the daughter cells from missing chromosomes or having extra chromsomes

Distinguish the purpose of cell division in a unicellular organism to a multi-cellular organism?

Cell division in a unicellular organism results in reproduction of the entire cell (since it gives rise to a new organism). Cell division in a multi-cellular organism is used for many purposes such as reproduction, growth and development, tissue renewal.

What happens in metaphase of the mitotic phase?

Centrosomes are at the opposite ends of the cell. Chromosomes as a result of tug and war by opposite centrosomes have arrived at the metaphase plate, which is equidistant between the spindle's two poles. Centromeres of the chromosomes lie on this plane. Each chromsome's kinetochore is attached to a microtubule from the opposite end of the cell.

What happens in prophase of the mitotic phase?

Chromatin fibers become more coiled and form discrete chromosome units that are observable. Nucleoli has disappeared. There are now sister chromatids joined by their centromere regions. Mitotic spindle is beginning to from each centrosome. Centrosomes are starting to move away from each other via the microtubules growing out of them.

What happens to chromosomes during cell division in the most general terms?

Chromosomes, prior to cell division, are in the form of a long, thin chromatin fiber. Making them hard to see. After DNA replication, however, the chromosomes condense as a part of cell division. Each of these chromatin fibers that compose the chromosomes become densely coiled and folded, which make the chromosomes much shorter and thick enough to see with a light microscope.

What happens during anaphase?

Cohesins holding the sister chromatids together AKA the sister chromatid cohesion is cleaved by an enzyme called separase. Now the separated sister chromatids are known as individual chromosomes and move towards opposite ends of the cell. By the end of this phase, the chromosomes have arrived at the opposite ends of the elongated cell.

What happens in cytokinesis of the mitotic phase?

Cytokinesis begins during anaphase or telophase. Occurs via a process known as cleavage. First sign of cleavage is the cleavage furrow, a shallow groove in the cell surface nearly the old metaphase plate. Actin microfilament contractile ring forms in the cytoplasm where the metaphase plate was. Myosin interacts with the actin filaments causing the ring of microfilaments to contact until the cleavage furrow deepens enough to produce two completely separated cells.

How does MPF regulate itself?

During anaphase, MPF switches itself off by initiating a process that leads to the destruction of its own cyclin.

What occurs during the checkpoints?

G1 checkpoint checks for cell size, and the environment (are there enough nutrients in the environment for the cell ) They regulate cell division through 4 different checkpoints, G1, S, G2, and M phase checkpoints G1 Checkpoints check for cell/outside environment (nutrients) S- Was the DNA duplicated correctly ? G2- Cell Size and cell environment M phase- Specifically in metaphase specifically checking in metaphase whether or not. Are the kinetochores attached and are they coming from the opposite ends M phase checks for the correct binding of kinetochore microtubules

What are gametes and how many chromosomes do they contain?

Human reproductive cells such as sperm and eggs which have one set or half as many chromosomes as somatic cells. Human gametes have one set of 23 chromosomes.

What happens when a cell cannot pass a checkpoint?

If a cell cannot pass a checkpoint and cannot fix whatever is wrong, then the cell should die Two different ways Apoptosis Cell die by DNA fragmentation Necrosis: Cells burst

When does cell division not result in the equal distribution of identical genetic material to two daughter cells?

In Meiosis because this is a special type of eukaryotic cell division that produces sperm and eggs

What are the time lengths of the phases in the cell cycle?

M phase typically lasts in an hour in a human cell. S Phase is usually 10-12 hours. G1 and G2 phase are usually 4-6 hours each. G1 is the most variable in length, depending on different organisms.

What happens in anaphase of the mitotic phase?

Is the shortest stage of mitosis, and only happens to last a few minutes. Begins when the cohesins holding the sister chromatids together are cleaved by an enzyme called separase. Sister chromatids that have separated are now individual chromosomes. Daughter chromosomes begin to move to opposite ends of the cell as the kinetochore microtubules begin to shorten. Centromeres are moving first, being dragged almost. Cell begins to elongate as the nonkinetochore microtubules lengthen. By the end of anaphase, opposite ends of the cell have complete collection of chromosomes.

Where does the assembly of the mitotic spindle begin?

It begins at the centrosome, a region containing material that functions throughout the cell cycle to organize the cell's microtubules. Also a type of MTOC. Has a pair of centrioles in the middle but this is not essential for the division of the cell.

How does MPF work?

It phosphorylates a variety of proteins, initiating mitosis. MPF acts both directly as a kinase and indirectly by activating other kinases. Ex: MPF phosphorylates various proteins in the nuclear lamina which result in the fragmentation of the nuclear envelope during prometaphase of mitosis. MPF also contributes to molecular events required for chromosome condensation and spindle formation during prophase.

What are sister chromatids?

Joined copies of the original chromosome which are a result of duplication. They each contain an identical DNA molecule.

What is MPF?

Maturation promoting factor, or M-phase promoting factor. It is a complex of cyclin-CDK. Peaks of MPF activity depend on the peaks of cyclin concentration since it varies while CDK concentration remains constant, however, inactive. Activity of CDK rises and falls with changes in the concentration of its cyclin partner. Cyclin level rises during the S and G2 phases and then falls during the M phase

How does the cell die via necrosis?

Necrosis Nuclear swelling Cell Swelling DIsruption or organelles Rupture of cell and release of cellular contents Inflammatory response

What is the mitotic phase?

One part of the cell cycle, which includes mitosis and cytokinesis and which is usually the shortest part of the cell cycle.

What are chromosomes?

Packaged structures of DNA that are composed of chromatin which is a complex of DNA and proteins. These proteins maintain the structure of the chromosomes and help control the activity of the gene. The DNA molecules can carry several hundreds to a thousand genes.

What are kinetochores?

Protein structures found on each of the sister chromatids on specific regions of DNA that face opposite directions. These kinetochores will attach to microtubules that extend from the centrosome aka the mitotic spindle during prometaphase. These are known as kinetochore microtubules. Each kinetochore will attach to a microtubule from both centrosomes resulting in the sister chromatids being pulled in one direction, and then being pulled again in the opposite direction. Essentially a tug of war, which eventually results in a draw. The sister chromatids end up in the middle in a region known as the metaphase plate. This placement in the metaphase plate occurs during metaphase.

What are somatic cells and how many chromosomes do they contain?

Somatic cells are all the cells in the body except the reproductive cells. Each somatic cell contains 46 chromosomes made up of two sets of 23, one set inherited from each parent.

What is cytokinesis?

The division of the cytoplasm.

What is mitosis?

The division of the genetic material in the nucleus.

What is the cell cycle?

The life of a cell from the time it is formed during division of a parent cell until its own division into two daughter cells.

What is interphase?

The longest part of the cell cycle which accounts for 90% of the cell cycle which includes phases like G1, S, G2 phases. Intense growth and metabolic activity occur throughout interphase. Cell grows by producing proteins and cytoplasmic organelles like the mitochondria and the endoplasmic reticulum.

Describe the activity of the molecules that trigger and coordinate key events in the cell cycle.

The molecules are protein kinases(enzymes that activate other proteins by phosphorylating them) and cyclins. Many of these kinases are actually present at constant concentration, however, they are in the inactive form and depend on another protein. Ex: CDK and Cyclin

What happens in prometaphase of the mitotic phase?

The nuclear envelope has complete fragmented. Microtubules from each centrosome have invaded the nuclear area. Sister chromatids are now even more condensed. Each sister chromatid now has a kinetochore at the centromere region, facing opposite directions. Microtubules attach to the kinetochore regions and jerk the chromsomes back and forth. Non-kinetochore microtubules are interacting with opposite non-kinetochore microtubules.

What is the arm of a chromosome?

The portion of a chromatid to either side of the centromere. Unduplicated chromsomes have one centromere.

What does the centrosome do during interphase in animal cells?

The single centrosome duplicates, forming two centrosomes which remain near the nucleus. These two centrosomes begin to move apart during prophase and prometaphases as spindle microtubules are growing out of them. By the end of prometaphase, the centrosomes are at opposite ends of the cell.

Explain the bonds of sister chromatids

The sister chromatids are attached all along their lengths by protein complexes called cohesins. The entire attachment is known as a sister chromatid cohesion.

What are the stages of Mitosis?

There are five stages in mitosis. Prophase, prometaphase, metaphase, anaphase and telophase. Cytokinesis occurs in the latter stages of mitosis and it completes the mitotic phase.

What happens in G2 phase of interphase?

There is a nuclear envelope which encloses the nucleus. Contains more than one nucleoli. Two centrosomes have formed by duplication. Chromosomes which were duplicated during S phase cannot be seen individually because they are not yet condensed.

How does cytokinesis differ in animal cells and in plant cells?

There is no cleavage furrow. During telophase in plant cells, vesicles from the golgi apparatus move along the microtubules to the center of the cell where they produce a cell plate. Cell plate grows and contains cell wall materials. Enlarges to the point of fusing with the plasma membrane. Two daughter cells are formed as a result.

What happens to sister chromatids after they are formed?

They are separated and move into two new nuclei, which are being formed at the opposite ends of the cell. Once the sister chromatids have separated, they are no longer known as sister chromatids but are now known as individual chromsomes. This results in a doubling of the number of chromsomes in the cell.

What do non-kinetochore microtubules do?

They lengthen and elongate and by metaphase they overlap each other and interact with other non-kinetochore microtubules. By metaphase the microtubules of the asters have reached the plasma membrane and the mitotic spindle is complete. They elongate the entire cell and use ATP to move away from one another to elongate the cell The Non kinetochore microtubules attach to each other and pull on each other to make the cell bigger , elongating the cell, allowing for cytokinesis to occur

What happens in telophase of the mitotic phase?

Two daughter nuclei form in the cell. Nuclear envelope begins to arise from the fragments of the parent cell's nuclear envelope. Nucleoli begin to reappear. Chromosomes are now beginning to become less condensed. Spindle microtubules begin to depolymerize. Mitosis is now complete.

How are the chromosomes moving towards opposite ends of the cell?

Via motor proteins and two mechanisms. A pacman like mechanism in which the microtubules depolymerize as the kinetochore walks the chromosome along the microtubules and another mechanism in which the poles of the spindle are reeling the chromosomes in.