the cell cycle

Question 1

in unicellular organisms, each cell cycle gives rise to ?

Answer 1

2 new organisms

Question 2

outline the phases of the celll cycle

Answer 2

G1- the cell at the beginning of the cell cycle. During G1, this is the preparation phase of the cell cycle and during this time, there is growth and preparation for S phase.

S phase- DNA synthesis, the DNA on the chromosome is duplicated by the process of DNA replication. At the end of this, all of the chromosomes that are contained within the nucleus, have been copied.

G2- at the centromere, the sister chromatids remain attached to one another all the way through G2. The cells with duplicated chromosomes are held together at the centromere. During G2, which is another preparation phase, the cell organelles are duplicated and increase the amount of cytoplasm. Growth and preparation for M phase.

M phase – Mitotic phase. The division phase of the cell cycle. In this process, there is division of the nucleus so that there are two nuclei, and the division of the cell itself and division of the cytoplasm. Mitosis + cytokinesis.
The two new daughter cells can then go and perform another round of the cell cycle to produce more cells.

Question 3

what is the G0 phase

Answer 3

a resting phase where no cell division is taking place.

Question 4

describe the 3 states cells in our bodies can be in .

Answer 4

• some cells, Cell cycle re-entry not possible, they are always in g0 cell cycle. this means If you have an injury to these particular cells, they are unable to regenerate and the function of those cells will be lost. e.g. nerve cells
• Maintained in Go ( not going through the cell cycle) unless stimulated to divide , eg if the liver Is damaged and needs to replenish e.g. hepatocytes
• Constantly in the cell cycle e.g. epithelial cells of the gut, skin cells.

Question 5

what occurs in the mitotic phase

Answer 5

Prophase: Chromosomes that have been in interphase and decondensed into chromosome territories begin to condense and separate and become more tightly packaged, centrosomes move to opposite poles, mitotic spindle forms

Prometaphase: breakdown of nuclear envelope, chromosomes attach to mitotic spindle. Under a microscope, the chromosomes have a very long appearance because they aren’t fully condensed.

metaphase: centrosome are at opposite poles, chromosome are at their most condensed and line up at the equator of the mitotic spindle. This is also called a metaphase plate, a section through the midpoint of the cell where all of the chromosomes lined up.
anaphase: sister chromatids separate synchronously, each new daughter chromosome moving to the opposite spindle pole
telophase: chromosome arrives at the spindle poles, chromosomes decondense, nuclear envelope reforms around the two new nuclei.
cytokinesis: cytoplasm divides

Question 6

what is cytokenesis

Answer 6

Cytokinesis: cytoplasmic division
•at the position of the metaphase plate
• a contractile ring of actin and myosin II constrict the cell into two new cells

• Accumulates between the poles of the mitotic spindle beneath the plasma membrane
• Ring contracts and forms an indentation or cleavage furrow, dividing the cell into two new cells

Question 7

describe the mitotic spindle

Answer 7

→ bipolar array of microtubules.
•Starts to assemble during prophase from the centrosomes
•Function: separate the duplicated chromosomes
•Constructed from microtubules
•3 types of spindle microtubules

–astral microtubules: projecting out from the poles and they have a role in orientation and the positioning of the spindle within the cell because it’s a symmetrical structure. The site of kinesis (where the cells are dividing ) is influenced by the positioning of the astral microtubules.

–kinetochore microtubules: make contact with the kinetochore. It attaches to the microtubules

–interpolar microtubules: they extend and project from the poles in the zone of interdigitation. they are interacting in the middle.

Question 8

when can the cell cycle and mitosis proceed ?

Answer 8

It isn’t until the kinetochore microtubules are making contact to all of the kinetochore, with all of the chromosomes in the cell that the cell cycle and mitosis can proceed.

Question 9

what is the fibrous corona

Answer 9

• The fibrous corona – an assembly of dozens of proteins that participate in microtubule capture and spindle assembly. It needs to be disassembled before anaphase in order to avoid mistakes in chromosome segregation.

Question 10

what do the centrosomes consist of ?

Answer 10

microtubule-organizing centre in animal cells

•centrosome consists of a pair of centrioles surrounded by pericentriolar material

Question 11

outline the steps in the centrosome cycle

Answer 11

During interphase stages (s / G2 phase ) - we have a pair of centrioles surrounded by pericentriolar material - a Centrosome consists of a pair of centrioles
They are duplicated during interphase and early prophase
The centrosome are moving towards the poles then Migrating into place in preparation for metaphase - here we have the full mitotic spindle formed, in metaphase this is when all of the chromosomes are attached to a kinetochore microtubule at this point the cell is ready to speatte and complete the division of the nuclear DNA ( the chromosomes ) -
Process by which kinetochore microtubules are shortening towards the poles dragging the chromosomes along with them at the same time overlapping microtubules are also lengthening increasing the distance between the poles - the sister chromatids are pulled apart.

Question 12

what does meiosis start with and what does it end up with, and what does it consist of?

Answer 12

• Meiosis is a specialised cell division that starts with one diploid cells and ends with 4 haploid cells
• One round of DNA replication during S phase and two rounds of cell division

→purpose is to produce gametes:
sperm and egg (sex cells)

Question 13

outline the steps in meiosis

Answer 13

o in meiosis I homologous chromosomes line up on the spindle and separate to opposite spindle poles. The homologous chromosomes line up at the same point and results In one homologous chromosome going to one pole and the other homologous chromosome going to the other pole. The resulting cells each have 2 sister chromatids from chromosome 1.

o in meiosis II sister chromatids line up on the spindle and separate to opposite spindle pole, resulting in one copy of chromosome 1 in each cell.

Question 14

compare mitosis and meiosis

Answer 14

Mitosis:

• two cells
• one round of cell division
• diploid
• no recombination

Meiosis:

• four cells
• Two rounds of cell division
• haploid
• recombination

Question 15

what is nondisjunction

Answer 15

• nondisjunction- failure of homologues to separate from one another, either at meiotic division I or meiotic division II

Question 16

what are the three requirements for the cell cycle?

Answer 16

• Entry into the cell cycle must be strictly controlled
• Each phase must occur only once per cell cycle
• Phases must be in the correct order
  * G1-S-G2-M
• Phases must be non-overlapping

Question 17

list the checkpoints in the cell cycle and their functions

Answer 17

G1 checkpoint : check DNA for damage, check extracellular environment

G2: Check DNA for damage, check DNA replicated

metaphase checkpoint : Are all chromosomes aligned on the mitotic spindle

Question 18

which checkpoint will stop the cells dividing further until all the chromosomes are aligned ?

Answer 18

metaphase checkpoint

Question 19

what are the cell-cycle regulators and what do they do ?

Answer 19

•Cyclin dependent kinases (Cdks):
- When unbound – cdks are inactive.
o active when bound to a cyclin
o phosphorylates target proteins

•Cyclins:
o different cyclins produced at each phase of the cell cycle. there are different cylins produced at each phase of the cell cycle, controlling whether cdks are turned on or off.

Question 20

describe the cell cycle control in yeast

Answer 20

• • in yeast the single CDK controls the cell cycle by binding successively to different cyclins as it progresses through the cell cycle.
• In G1, the Cdk is bound by s-cylin, which triggers the s phase allowing it to duplicate the DNA.
• S-cyclin is degraded leaving Cdk free to progress through the cell cycle to be bound by M-cyclin which triggers M phase- results mitotic cell division resulting in 2 new cells. The M-cylcin is degraded leaving the cdk free to go another round of the cell cycle.

Question 21

describe the cell cycle control in humans

Answer 21

• Different classes of Cdk and cyclins at each stage of the cell cycle.

Question 22

what are the basic principles of cell cycle control ?

Answer 22

• Cdk levels fairly stable throughout the cell cycle
• Cyclin levels vary as part of the cell cycle, will move a cell eg from g1 to s phase.
• Cdk bound to cyclin is active and can phosphorylate target protein, and only when its active can it perform its function.
• Cdk activation triggers the next step in the cell cycle such as entry into S phase, M phase
• Cyclin degradation terminates Cdk activity

Question 23

what is neoplasia

Answer 23

uncoordinated cell growth
• Cells escape normal cell cycle regulation - no barriers to prevent uncontrolled progression through the cell cycle
• Many genes mutated in human cancers are regulators of the cell cycle such as p53 and pRB, known as tumour suppressor genes

Question 24

when do centrioles duplicate ?

Answer 24

prophase

Question 25

What are the three trisomies?

Answer 25

→Trisomy 21 (downs)
→trisomy 18 (edwards)

→trisomy 13 (patau)

Question 26

what are cdks

Answer 26

→enzymes that phosphorylates target proteins

Question 27

what do kinases do

Answer 27

phosphorylate proteins and activate them

Question 28

what is an example of an important kinase?

Answer 28

→tyrosine kinase

Question 29

how does the CDK work in prokaryotes?

Answer 29

there is only one CDK

Question 30

What is the kinetochore and what does it do?

Answer 30

→protein structure formed on a chromatid

→where the spindle fibres attach to pull the chromatids apart during cell division.

Question 31

what are some non viable autosomal monosomies?

Answer 31

sex chromosomes:
→XO (Turner’s syndrome)

→ XXX (Triple X syndrome)
→ XXY (Klinefelter’s syndrome)

Question 32

what is homologous recombination

Answer 32

Homologous Recombination occurs between homologous chromosomes. The resulting haploid cells are not identical