DNA and MITOSIS

Question 1

What is a germ cell and how is it different from a gamete?

Answer 1

> germ cell = a reproductive cell capable when mature of being fertilised and reproducing an entire organism
gamete = a mature male or female reproductive cell - the mature form of a germ cell

Question 2

Describe the phases of the cell cycle

Answer 2

G1 phase:

• cell growth and differentiation
• checkpoint: determines whether cell is ready for DNA synthesis - checking for mutations etc

G0 phase:

• cell leaves cycle and does not divide (neurons)
• called Quiescent

S phase:

• DNA synthesis
• checkpoint again, repair enzymes if DNA coded incorrectly

G2 phase:

• cell continues to grow
• checkpoint: ensures DNA Synthesis is complete and cells are ready for mitosis

M Phase:

• mitosis occurs and cell divides
• goes back into cycle at G1 or out of cycle for G0

Question 3

describe the stage of mitosis and the main proteins involved

Answer 3

Prophase:

• microtubule asters form and extend from each centrosome (made of two centrioles)
• chromosomes condense and become visible
• centrioles begin to move to opposite poles

Prometaphase:

• late prophase
• nucleur envelope breaks don
• chromosomes start to move as mircotubules bind to kinetochores present on the centromere region of the chromotids
• microtubule asters form the mitotic spindle

Metaphase:

• chromosomes line up along the equatorial plane of the cell
• form the metaphase plate

Anaphase:

• sister chromatids separate at the kinetochore to form single chromosomes
• these chromosomes migrate to opposite poles
• poles move apart, microtubules slide over one another, shortening

Telophase:

• supercoiling begins to decondense (becomes chromatin again
• the nuclear envelop begins to reform
• microtubules are no longer visible
• chromosomes are no longer visible

cytokinesis
- a contractile ring forms to pull plasma membrane, pinching in so one cell can become two cells

Question 4

Which part of the cell cycle do cancer drugs target?

Answer 4

> taxol interferes with cell division by preventing the break down of microtubules
therefore they cannot be shortened, so cell are arrested in metaphase

Question 5

Describe the steps in DNA replication

Answer 5

1) DNA helicase locally denatures ds DNA into ss DNA by breaking H bonds
2) DNA toperisomerase (gyrase in bact.) removes suprcoil that results from opening of strand
3) DNA polymerase III adds new free nucleoides to the 3’ end of the newly forming strand (leading strand) - is highly accurate but mistakes are made and it proof reads itself
4) on the lagging strand, primase adds primers of RNA and DNA to create an initation point on lag strand for DNApoly III to add necleotides to, creating fragments (okazaki)
5) DNA poly I replaces RNA sections of primers with DNA
5) Okazaki fragments on the lagging are joined together by DNA ligase

> DNA replication enzymes and proteins are targets for antiviral, bacterial and cancer drugs.