2. the cell cycle

Question 1

how do prokaryotes reproduce?

Answer 1

through binary fission.

Question 2

what are the stages of binary fission?

Answer 2

1. dna is replicated
2. cell elongates (widens) and chromosomes move to opposite ends
3. cell undergoes cytokinesis by pinching inwards
4. cell wall + membrane formed down middle of cell

Question 3

what are the stages of the eukaryotic cell cycle?

Answer 3

interphase, mitosis, cytokinesis.

Question 4

what is interphase?

Answer 4

first stage of the eukaryotic cell cycle. it consists of the G1 phase, the G0 phase, the S phase and the G2 phase.

Question 5

what happens in the G1 phase?

Answer 5

the cell grows by
- increasing cytosol volume
- synthesising proteins for DNA replication
- replicating its organelles

Question 6

what happens in the G0 phase?

Answer 6

cells that are not required to replicate rest in this phase.

Question 7

what happens in the S phase?

Answer 7

the cell replicates its DNA turning one chromosome into two identical sister chromatids.

Question 8

what happens in the G2 phase?

Answer 8

continues to grow and prepare for mitosis
- increasing cytosol volume
- synthesising proteins for mitosis

Question 9

what is mitosis?

Answer 9

involves the separation of chromosomes into two new nuclei.
consists of four stages:
- prophase
- metaphase
- anaphase
- telophase

Question 10

what happens in prophase?

Answer 10

chromatin is condensed around histones into chromosomes.
centrioles move to opposite ends of the cell.
nuclear membrane breaks down and nucleolus disappears.

Question 11

what happens in metaphase?

Answer 11

spindle fibres fully form and attach to centromere of each chromosome.
this allows spindle fibres to the equator where they line up.

Question 12

what happens in anaphase?

Answer 12

spindle fibres contract, splitting centromere and pulling sister chromatids to opposite ends of the cell.

Question 13

what happens in telophase?

Answer 13

chromosomes pack densely together at each end.
nuclear membranes form.
spindle fibres disintegrate.

Question 14

what happens in cytokinesis?

Answer 14

the cytoplasm divides and organelles evenly distribute themselves before separating into two daughter cells.

Question 15

how is the cell cycle regulated?

Answer 15

the cycle has three checkpoints in which the cell inspects itself for errors before proceeding.
these checkpoints are located:
- at G1
- at G2
- during metaphase

Question 16

what happens at the G2 checkpoint?

Answer 16

• ensures replication of chromosomes is complete
• all chromosomes have started to condense
• occurs to make sure no DNA is missing

Question 17

what happens at the mitosis checkpoint?

Answer 17

• ensures spindle fibres have attached to centromeres
• ensures chromosomes will separate easily

Question 18

what happens at the G1 checkpoint?

Answer 18

• ensures cells are large enough to proceed to S phase
• checks for DNA damage before replication

Question 19

what happens at a checkpoint if a cell has an error?

Answer 19

either:
- the problem is resolved
- the cell is killed (apoptosis)

Question 20

what is a nucleotide?

Answer 20

a monomer that makes up DNA.

Question 21

what are nucleotides made up of?

Answer 21

• phosphate
• sugar
• nitrogen-containing base

Question 22

what are the nitrogen-containing bases?

Answer 22

adenine, thymine, guanine, cytosine

Question 23

how is a phosphodiester bond formed?

Answer 23

when the OH group (3 prime) and the H group (5 prime) combine, releasing H2O and forming a phosphodiester bond.

Question 24

what are the five steps in replicating DNA?

Answer 24

1. helicase (enzyme) unwinds helix.
2. DNA is unzipped by breaking hydrogen bonds between bases.
3. free nucleotides form hydrogen bonds with opposite nucleotide on DNA
4. DNA polymerase (enzyme) joins the sugar-phosphotes in the backbone by forming phosphodiester bond.
5. process repeats until whole length of DNA strand is copied

Question 25

what is binary fission? how does it occur?

Answer 25

process in which prokaryotes replicate.
1. chromosome is uncoiled and DNA is replicated
2. cell elongates (stretches out) and chromosomes migrate to opposite ends
3. cell undergoes cytokinesis. new cell wall and membrane are formed.

Question 26

what are stem cells?

Answer 26

cells that are yet to become specialised.

Question 27

what does potency mean?

Answer 27

potency refers to the types of cells that a stem cell can specialise into.

Question 28

what is totipotency? at what stage are cells totipotent?

Answer 28

totipotent means a stem cell can become any cell type of an organism. cells in the 2-cell stage are totipotent.

Question 29

what is pluripotency? at what stage are cells pluripotent?

Answer 29

pluripotent means a stem cell can become any cell type except placental cells. cells in the blastocyst (8-cell) stage are pluripotent.

Question 30

what is multipotency? at what stage are cells multipotent?

Answer 30

multipotent means a stem cell can become some cell types. cells after the blastocyst (8-cell) stage can be multipotent.

Question 31

what is unipotency? at what stage are cells unipotent?

Answer 31

unipotent means a stem cell can become one cell type. cells after the blastocyst (8-cell) stage can be unipotent.

Question 32

what are the first stages in apoptosis?

Answer 32

external signal:
1. death signal binds to death receptor
2. message sent to mitochondria
3. mitochondria releases caspases (enzyme) that break down DNA and cell cytoskeleton.

internal signal:
1. internal problem damages mitochondria
2. mitochondria releases caspases (enzyme)
3. caspases break down DNA and cytoskeleton

Question 33

how does apoptosis destroy the cell?

Answer 33

1. as cytoskeleton breaks down, cell will shrink
2. cell loses it’s shape, starts to bleb
3. blebs break off into apoptotic bodies
4. the apoptotic bodies are endocytosed by white blood cells.