17 - cell cycle

Question 1

what does the cell cycle involve?

Answer 1

cells growing, performing routine activities needed to survive, and dividing to create new cells.

Question 2

cells arise only by the division of existing cells, why os this important?

Answer 2

• cell division is essential to all life
• it enables multi-cellular organisms to grow and repair damage
• is the basis of reproduction in every organism.

Question 3

what are chromosomes?

Answer 3

structures composed of DNA and proteins that carries the genetic information of an organism:
- provide continuity between one generation and the next.

Question 4

when do chromosomes become clearly visible?

Answer 4

they become clearly visible when the chromatin condenses during the cell cycle.

Question 5

if the chromosomes are separated out, they can be arranges into matching pairs, according to what?

Answer 5

size, position of the centromeres and banding pattern

Question 6

what are pairs of chromosomes called?

Answer 6

homologous pairs

the results are referred to as a karyotype.

Question 7

how many chromosomes do human cells usually have?

Answer 7

46 chromosomes:

• 22 homologous pairs of autosomal chromosomes (autosomes)
• 1 pair of sex chromosomes (allosomes)

Question 8

where do the chromosomes within the pair come from?

Answer 8

one chromosome in each pair comes from the individuals mother, the other from the father.

Question 9

what are cells that have the normal two sets of chromosomes called?

Answer 9

diploid. (2n)

haploid cells contain a single copy (n)

Question 10

…

Answer 10

on copies of the same chromosome, with the same banding pattern, the same gene will be at the same locus within the chromosome but possibly different alleles.

Question 11

what is a centromere?

Answer 11

chromosomes have a centromere which keeps the pairs aligned and acts as an attachment site during cell division.

• short arms - ‘p arm’
• long arms - ‘q arm’

it is the point where mitotic spindle attached during prometaphate.

Question 12

what are Idiograms?

Answer 12

When a chromosome is stained, is appears with a characteristic pattern of light and dark bands.

Idiograms are used to describe the location of genes on each chromosome.

Question 13

what is mitosis?

Answer 13

Before dividing, the cell must duplicate its components — most importantly,
its genome — so that it can split into two complete daughter cells.

There are two forms of nuclear division: mitosis and meiosis

In mitosis, there is one nuclear division stage resulting in two identical nuclei

Mitosis ensures that each successive cellular generation has the same genetic
information as the previous generation
(n.b. unless some form of mutation occurs)

Question 14

what are the new cells from mitosis used for?

Answer 14

growth, maintenance and repair of a cellular organsim

Question 15

what is a chromatid?

Answer 15

a longitudinal DNA subunit of a chromosome. The two
chromatids of a duplicated chromosome are held together at a region of
DNA called the centromere.

Question 16

what is meiosis?

Answer 16

Meiosis is the basis of sexual reproduction.
- gametes have only a single set of chromosomes e.g. they have half the number of chromosomes of somatic cells
- meiosis produces these haploid (n) gametes.

in meiosis the nucleus divides twice, producing 4 haploid nuclei.

the haploid gametes produced by meiosis fuse during fertilisation and create a diploid cell.

meiosis involves ‘recombination’ during which homologous chromosomes can exchange genetic material with one another.

Question 17

what is the cell cycle?

Answer 17

the cell cycle is an organised sequence of events that occurs from one cell division to the next in a proliferating cell.

Question 18

what are the three main stages of the cell cycle?

Answer 18

1. interphase
2. mitosis
3. cell division (or cytokinesis)

Question 19

what must the cell pass through that ensure conditions are favourable for division?

Answer 19

checkpoints

Question 20

what happens during interphase?

Answer 20

DNA replicates; RNA is synthesised constantly/proteins actively produced as the cell grows; and the centrioles duplicate.

Question 21

how long does interphase typically last in mammalian cells?

Answer 21

20-24 hours

Question 22

what three stages can interphase be divided into?

Answer 22

1. G1 (gap 1)
2. S (synthesis)
3. G2 (gap 2)

cells may also enter a resting phase, G0.

Question 23

what happens during the G0 phase?

Answer 23

a cell may leave the cycle and stop proliferating.

the cell will continue to perform all other functions associated with its cell type. this may be a temporary resting period or more permanent.

some mature cells (e.g. skeletal muscles, neurons) remain in G0 indefinitely.

Question 24

what happens in the G1 phase?

Answer 24

cell increase in size in G1 and actively synthesise proteins required for growth and subsequent division.

Question 25

what is the S phase?

Answer 25

To produce two daughter cells, the complete set of genetic information in the cell must be duplicated. DNA replication occurs during this synthesis phase.

Question 26

what happens during the G2 phase?

Answer 26

during the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins.

Question 27

when does G1 begin?

Answer 27

when the cell senses growth factors or other mitogens.

Question 28

what checkpoint is there during G1?

Answer 28

During G1, a critical checkpoint ensures environmental conditions are favourable for replication. - If conditions are not favourable, the cell may enter G0

Question 29

what does the cell cross in G1 that is the point of no return?

Answer 29

The cell crosses a restriction point ca. 8-10 hours into G1 - this is the point of no return, the cell commits to divide or die

Question 30

what do checkpoints in G2 do?

Answer 30

At end of G2, checkpoint to ensure DNA is structurally intact & properly replicated - The cell may pause at this point to allow time for DNA repair, if necessary

Question 31

what do cyclin-dependant Kinases (CDKs) do?

Answer 31

The main proteins involved in cell cycle regulation are a family of proteins known as cyclins and their associated cyclin-dependent kinases (CDKs): various combinations of these two components regulate orderly progression through the cell cycle They signal by phosphorylating other proteins e.g. controlling the activity of various transcription factors. Their concentrations vary throughout cell cycle. Hartwell, Hunt and Nurse received the 2001 Nobel Prize for their description of the cyclin and cyclin-dependent kinase regulatory mechanisms

Question 32

when does nuclear division take place?

Answer 32

during mitosis (M phase)

Question 33

what five stages is mitosis divided into?

Answer 33

prophase (when the chromosome condenses) prometaphase (when the nuclear envelope breaks down) metaphase (when the chromosomes are lined up along the midline of the cell) anaphase (when the chromosomes separate) telophase (when the chromosomes are encapsulated into two new nuclei)

Question 34

what are the two types of chromatin?

Answer 34

heterochromatin - tightly packed, not very active in transcription. euchromatin - less tightly packed, gene rich and usually active in transcription.

Question 35

explain prophase

Answer 35

begins when the chromatin starts condensing into chromosomes, which become visible. each chromosome structure comprises two sister chromatids, attached at the centromere, with the same genetic information.

Question 36

how do microtubules of the cytoskeleton disassemble in prophase?

Answer 36

- the building blocks of these microtubules are used to grow new microtubules called spindle fibres that extend between the pair of centrosomes (interpolar microtubules) - other microtubules attach to the plasma membrane (aster microtubules) or radiate outwards into the cytoplasm.

Question 37

explain prometaphase

Answer 37

in this stage the nuclear envelope breaks down so there is no longer a recognisable nucleus. some mitotic spindle fibres (kinetochore microtubules) elongating from the centrosomes attach to kinetochores: - kinetochores are protein structures that associate with the centromere on each sister chromatid other elongating spindle fibres, instead of attaching to chromosomes, overlap each other at the centre of the cell.

Question 38

what happens during metaphase?

Answer 38

begins as the chromosomes move to a narrow central zone at the equator of the cell (the metaphase plate) - orchestrated by mechanical forces applied via the spindle fibres.

Question 39

explain anaphase

Answer 39

the centromere splits and the kinetochore microtubules shorten pulling the individual chromatids apart. - the astral microtubules and the interpolar microtubules pull the poles further apart and exert additional pull on the chromosomes.

Question 40

explain telophase

Answer 40

Daughter chromosomes arrive at the poles and nuclear membranes reform. The chromosomes uncoil and the spindle fibres that have pulled them apart disassemble.

Question 41

explain cytokinesis

Answer 41

Spindle fibers that were not attached to chromosomes begin breaking down until only the portion of overlap is left. A contractile ring is formed and cleaves the cell into two daughter cells. - microtubules then reorganise into a new cytoskeleton for the return to interphase.

Question 42

what are telomeres?

Answer 42

repetitive sequences of DNA are found at the end of chromosomes. telomeres shorten with each cell division. - once its telomeres reach a critical length, a cell can no longer divide. - in cancer and stem cells, telomerase repairs telomerase and cells do not stop dividing.