1.6- Cell division

Question 1

How many chromosomes do humans have?

Answer 1

46 (23 pairs of homologous chromosomes)

Question 2

When do chromosomes become visible?

Answer 2

when chromatin condenses prior to cell division, after each DNA molecule has replicated and made an exact copy of itself

Question 3

What are chromatids?

Answer 3

when a single thread of DNA becomes 2 identical threads

Question 4

What is the centromere?

Answer 4

the region where the sister chromatids join

Question 5

How many daughter cells does mitosis produce?

Answer 5

2 (identical)

Question 6

What is a cell cycle?

Answer 6

Dividing cells undergo a regular pattern of events

Question 7

What are the 4 stages of mitosis?

Answer 7

Prophase
Metaphase
Anaphase
Telophase

Question 8

What phase is crucial to the cell cycle?

Answer 8

interphase

Question 9

What happens during interphase?

Answer 9

Replication of DNA
Replication of organelles which have their own DNA – (mitochondria and chloroplasts)
Making new organelles
Synthesis of ribosomal material
Synthesis of ATP.
Synthesis of proteins.
Increase in cell size

Question 10

Where does mitosis take place in plants?

Answer 10

meristem

Question 11

What is seen at interphase?

Answer 11

nothing
chromosomes have not condensed and the chromatids are not visible

Question 12

What happens in prophase in mitosis?

Answer 12

Prophase is the first stage of mitosis
During prophase the DNA condenses (becomes shorter and thicker) forming chromosomes
Chromatids become visible. In animal cells the centrioles move to opposite poles of the cell
Protein microtubules form from each centriole and the spindle develops, extending from pole to pole. Towards the end of prophase the nuclear membrane disintegrates and the nucleolus disappears
Pairs of chromatids can clearly be seen lying free in the cytoplasm.

Question 13

What happens in metaphase in mitosis?

Answer 13

During metaphase the chromosomes arrange themselves at the equator of the spindle
The chromosomes become attached to the spindle fibres at the centromere
Contraction of the spindle fibres draws the individual chromatids apart
the chromosomes are now maximally condensed

Question 14

What happens in anaphase in mitosis?

Answer 14

Anaphase is a very rapid stage
The centromere splits and the spindle fibres contract
The chromatids separate and are pulled to opposite poles of the cell; the centromeres lead the way

Question 15

What happens in telophase in mitosis?

Answer 15

Telophase is the final stage of mitosis
The chromatids have now reached the poles of the cells and are referred to as chromosomes again
The chromosomes uncoil and lengthen
The spindle breaks down
The nucleolus reappears and the nuclear membrane reforms

Question 16

What is cytokinesis?

Answer 16

In animal cells cytokinesis occurs by constriction of the centre of the parent cell from the outside inwards (cytokinesis means cytoplasm splitting).
In plant cells, a cell plate forms across the equator of the parent cell from the centre outwards and a new cell wall is laid down

Question 17

What is mitosis used for?

Answer 17

growth, the repair of tissues
replacement of dead or worn out cells
Asexual reproduction

Question 18

What is an advantage of asexual reproduction?

Answer 18

the ability to increase in numbers quickly to take advantage of an ideal environment

Question 19

What is an disadvantage of asexual reproduction?

Answer 19

the lack of genetic variation, leading to an inability to adapt if the environment changes

Question 20

How do you calculate the mitotic index?

Answer 20

(No of cells in prophase + metaphase + anaphase + telophase/Total number of cells) x 100

Question 21

What is uncontrolled mitosis?

Answer 21

cancer

Question 22

What can damage/mutate cells?

Answer 22

radiation
viruses
some chemicals

Question 23

How do tumours form?

Answer 23

Cancerous cells divide repeatedly, out of control

Question 24

Where does meiosis take place?

Answer 24

reproductive organs

Question 25

What does meiosis result in?

Answer 25

the formation of gametes with half the normal chromosome number; this is the haploid number

Question 26

What happens during prophase 1?

Answer 26

first stage of meiosis
DNA condenses (becomes shorter and thicker) forming chromosomes
Chromatids become visible
In animal cells the centrioles move to opposite poles of the cell
Protein microtubules form from each centriole and the spindle develops, extending from pole to pole
Paternal and maternal chromosomes associate as homologous pairs (this process is called synapsis); each pair is called a bivalent
Towards the end of prophase the nuclear membrane disintegrates and the nucleolus disappears

Question 27

What do bivalents consist of?

Answer 27

4 strands, consisting of 2 chromosomes, each with 2 chromatids
These chromatids wrap around each other and then partially repel each other, but remain joined at certain parts called chiasmata

Question 28

What is crossing over?

Answer 28

The swapping of pieces of chromosomes
At the chiasmata the chromatids may break and recombine with a different but equivalent chromatid

Question 29

Is crossing over a source of genetic variation?

Answer 29

yes

Question 30

What happens during metaphase 1?

Answer 30

the homologous chromosomes arrange themselves randomly on the equator of the spindle
Chance determines how the homologous chromosomes are arranged on the equator
This random distribution and consequent independent assortment of chromosomes produces new genetic combinations
The homologs remain in their bivalent pairs at this stage

Question 31

What happens during anaphase 1?

Answer 31

rapid stage
The spindle fibres contract
The chromosomes in each bivalent separate and are pulled to opposite poles of the cell
because of the random arrangement of homologous pairs at metaphase I this means that each pole has a random mixture of paternal and maternal chromosomes

Question 32

What happens during telophase 1?

Answer 32

marks the end of the first meiotic division
The chromosomes have reached opposite poles
The nuclear envelope reforms around each group of haploid chromosomes
The chromosomes remain in their condensed form. In animal cells cytokinesis occurs after telophase I
Meiosis II follows on immediately

Question 33

What happens during prophase 2?

Answer 33

begins with the 2 daughter cells produced by the first meiotic division
if there was a nuclear envelope formed, then it will begin to break down again during this stage
the centrioles have replicated and are moving toward the poles
again a spindle apparatus is growing outward from the centrioles, at right angles to the old spindle

Question 34

What happens during metaphase 2?

Answer 34

the chromosomes line ups the equator/metaphse plate, with each chromosome attached to a spindle fibre by its centromere
independent assortment happens because the chromatids of the chromosomes can face wither pole

Question 35

What happens during anaphase 2?

Answer 35

the centromeres divide and the spindle fibres shorten, pulling the sister chromatids to opposite poles

Question 36

What happens during telophase 2?

Answer 36

the chromosomes reach opposite poles
cytokinesis occurs
the two cells produced by meiosis 1 divide to form 4 haploid daughter cells, and nuclear envelope forms
the spindle fibres disintegrate
meiosis is now complete

Question 37

How many division in mitosis?

Answer 37

1

Question 38

How many division in meiosis?

Answer 38

2

Question 39

How many chromosomes are in the daughter cells of mitosis ?

Answer 39

same as parent cell

Question 40

How many chromosomes are in the daughter cells of meiosis ?

Answer 40

half of parent cell