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Flashcards in 12 The Cell Cycle Deck (67):

What regulates the activity of a cell?

The cell cycle.


What is the entirety of a cells genetic information called?

Its genome


How does prokaryote DNA often structured?

As a single loop of one DNA strand


What is DNA packaged into in eukaryotes?



What are chromosomes composed of and what does this contain?

Chromatin which is made up of DNA and its associated proteins.


Before division how does a cells DNA appear?

As long strands of chromatin that are not coiled into chromosomes


How are sister chromatids attached?

By cohesins along the length of the chromosomes, and by the centromere.


What is the centromere composed of?

centromeric DNA


How is the centromere's attachment controlled?

By proteins bound to the centromeric DNA


What is the part of the chromatid either side of the centromere called?

The arm


What is the division of the cytoplasm called?



What is the cell cycle divided into?

The mitotic (M) phase and Interphase


What does the mitotic phase include?

Mitosis and Cytokinesis


What does "M phase" refer to?

The mitotic phase of the cell cycle


What happens first: mitosis or cytokinesis?



In order, what does Interphase include?

G1, S and G2


What stage of the cell cycle (interphase or mitosis) takes longer and to what degree?

Interphase which constitutes 90% of the time of the cell cycle.


Broadly speaking what happens during interphase?

The DNA and organelles are duplicated in preparation of mitosis.


What is G1 phase known as?

"First gap"


What is S phase known as?

"synthesis" phase


What is G2 phase known as?

"Second gap"


About how frequently does a human cell undergo mitotic division?

every 24 hours-ish


What happens during the various sub phases of interphase?

During all three phases (G1, S and G2) the cell grows by producing proteins and organelles like mitochondria and ER.

Only during S phase is the DNA duplicated in preparation for mitosis.


Do all cells continuously divide?

No. Those that aren't dividing stay in G1 phase or a related G) phase and do their normal functions i.e. pancreas cells release enzymes.


What structure guides mitosis?

Mitotic spindles


When do mitotic spindle form?



What are mitotic spindles composed of?

Fibres made of microtubules and associate proteins.


Where do the microtubules for the mitotic spindle come from?

The cytoskeleton which partially disassemble to provide the microtubules.


What are the stages of mitosis in order?

Prophase, prometaphase, metaphase, anaphase and telophase. (5 + cytokinesis)


In animal cells where are the mitotic spindles assembled?

In the centrosomes


Do plants form mitotic spindle?

Yes, but without centrosomes.


How many microtubules attach to a single kinetochore?

It varies between species from one in yeast to 40 in some mammals.


How does the cell appear after G2?

A nuclear envelope encloses the nucleus which contains one or more nucleoli.

The chromosomes were duplicated during S phase but are not yet condensed.

If animal: Two centrosomes have been formed from the duplication of a single one, each of which contain a pair of centrioles


What happens during prophase of mitosis?

The chromatin fibres begin condensing forming distinct chromosomes consisting of sister chromatids.

The nucleoli disappear.

The mitotic spindle begins to forms but does not connect to the chromosomes. The radial arrays of microtubules that extend from the centrosomes are called "asters"

The centrosomes move farther apart due to the lengthening microtubules between them.


What happens during pro metaphase of mitosis?

The nuclear envelope fragments.

Each chromatid is attached to the mitotic spindle. They are joined by kinetochores, which are protein structures that anchor them together.

The mitotic spindle fibres attached to the kinetochores jerk the chromosomes.

Some mitotic spindle fibres do not attach to kinetochores but continue to span between the centromeres.


What happens during metaphase of mitosis?

The centrosomes are now at opposite poles of the cell. The chromosomes convent at the "metaphase plate" that is at the midpoint of the cell.

For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.


What happens during anaphase of mitosis?

The cohesion proteins are cleaved so that the sister chromatids are separated.

They are then pulled apart as the mitotic spindle fibres attached to kinetochores shorten.

The mitotic spindle fibres not attached to kinetochores become longer, causing the cell to elongate.


What happens during telophase of mitosis?

Two daughter nuclei form in the cell, each with the same set of chromosomes.

The nuclei reappear and the chromosomes become less condensed. The spindle microtubules are broken down

Meanwhile cytokinesis occurs and splits the cytoplasm. In animals a cleave furrow forms.


How do the mitotic spindle fibres attached to kinetochores shorten?

There are two mechanisms depending on the cell:

1) The kinetochores contain motor proteins which walk the kinetochore up the spindle fibre (kind of like climbing a ladder) - The spindle left behind is broken down so this is called the Pac man mechanism

2) Motor proteins at the spindle points i.e. centromeres "reel" the chromosomes in by shortening the spindles.


How do the mitotic spindle fibres not-attached to kinetochores lengthen?

During metaphase they overlap. As they need to lengthen motor proteins reduce this overlap and thus cause the structure to elongate.


How does cytokinesis in animal cells occur?

With a cleavage furrow.


How does cytokinesis differ in plant cells?

Their cell walls makes a cleavage furrow impossible so instead they use 'Cell plate formation'


How does cell plate formation occur?

The two nuclei move to opposite ends of the cell. In the middle of the cell vesicles align to form a 'cell plate'. This cell plate further develops into to new cell wall and thus splits the cell in half, thus causing cytokinesis.


What is cell division known as in bacteria?

Binary fission.


How is genetic material carried in bacteria?

In a single 'bacterial chromosome' that is a single circular loop of DNA and proteins.

Note that the loop is still folded and coiled.


Describe binary fission in bacteria.

Replication begins in a specific region known as the "origin of replication" here DNA is duplicated, causing it to 'bubble out'. Two origins are formed as there is two copies of the origin. This bubble continue to go round the loop.

When the bubble finishes the two origin regions are identified and one is pulled to onside and the other origin region is pulled to the other side. Thus when cytokinesis occurs each daughter cell has one DNA loop.


What regulates the frequency of cells undergoing the cell cycle and dividing?

The cell-cycle control system.


How does the cell-cycle control system regulate division?

It operates a set of checkpoints that ensures the cell only continues if it need to divide and is ready to do so.


What checkpoints are there for the cel cycle?

G1 (before S phase), G2 (before M phase) and M (before G1)


At which checkpoint do cells commit to division and what happens if they decide not to?

At the G1 checkpoint they can either continue onto S phase or exit the cell cycle and goes into the G0 phase.


What cells are in the G0 phase?

Most somatic cells that do not need to divide


Can cells exit the G0 phase?

Some can't like muscle or nerve cells which never divide.

Others can like liver cells.


What conditions might trigger a liver etc. cell to exit G0?

External chemical signals such as 'growth factors' released after injury.


Which cells never divide?

Nerve cells, muscle cells and others.


How do checkpoints operate chemically?

Protein kinases trigger the checkpoint to open. However they are typically inactive. To activate them cyclin is released which binds to the kinases, known as cyclin-dependant kinases. This forms a cyclin-Cdk complex, known as MPF (maturation promoting factor) which triggers the checkpoint.

The cyclins are degraded so that the cell will not reenter the cell cycle unless more cyclins are released.


What are Cdks?

Cyclin-dependant kinases


How do the checkpoints operate?

They are not physical barriers but more conceptual.

The MPF's actually trigger a range of effects directly, such as the kinases phosphorylating the nuclear lamina and thus causing it to fragment.


What conditions cause inhibition and activation of the cell cycle?

Low nutrient availability and a high density suppress cyclin release and thus delay the cell cycle.

The presence of growth factors stimulates the cell cycle.


Why is regulation of the cell cycle important?

1) It prevents the cell from wasting nutrients by dividing if it doesn't need it.
2) It ensure the mitosis is ready to continue: for example if any kinetochores are not attached to mitotic spindle fibres mitosis will not continue.


What factors are necessary for animal cells to divide?

They exhibit two types of dependance: anchorage dependance and density-dependant inhibition.

Thus they must be attached to something i.e extracellular matrix and not in overly cramped conditions to undergo division.


How is cancer related to the cell cycle?

It is caused when the cell cycle erroneously repeats causing rapid division.


What is the process of a normal cell becoming cancerous called?



How do benign and malignant tumours differ?

Benign tumours have too few genetic changes to survive at different sites of the body so they remain in one place whereas malignant tumours can spread.


How do platelets trigger the repair of damaged tissued?

They secrete "platelet-derived growth factor" (PDGF) which triggers nearby fibroplasts, a form of connective tissue, to divide and thus plug the hole.


What does a typical cell growth medium include?

Glucose, amino acids, salts and antibiotics (to prevent bacterial growth if not wanted.)


How do cells grow in a single sheet?

They use anchorage dependancy so that only those in content with an extracellular matrix can how and thus a single sheet forms.


How can cancer cells form tumours?

They do not exhibit density or anchorage dependancy so can grow in a large clump i.e. a tumour.