7. Cell Division

Question 1

What is mitosis?

Answer 1

A type of nuclear division that produces daughter cells genetically identical to each other and to the parent cell.

Question 2

What is cytokinesis?

Answer 2

Cytoplasmic division following nuclear division, resulting in two new daughter cells

Question 3

What is interphase?

Answer 3

Phase of the cell cycle when the cell is not dividing.

It is subdivided into cell growth and DNA synthesis phases.

Question 4

What is the purpose of checkpoints?

Answer 4

There are two main checkpoints the G1/S checkpoint and the G2/M checkpoint
They prevent uncontrolled division that would lead to tumours (cancer), they ensure the cell cycle cannot be reversed, they allow the detection and repair of damage to DNA and ensure that DNA is only duplicated once each cell cycle.

Question 5

What are the 5 phases of the cell cycle (in order)?

Answer 5

M phase, G0 phase, G1 phase (also called the growth phase), S (synthesis) phase of interphase, G2 phase of interphase

Question 6

What happens in the M phase?

Answer 6

A checkpoint chemical triggers condensation of chromatin, cell growth stops, nuclear division (mitosis) happens and cytokinesis happens

Question 7

What happens in the G0 phase?

Answer 7

Some cells (e.g. epithelial cells) do not have this phase.
Cells may undergo differentiation and some cells remain in this phase for a long time or indefinitely.

Question 8

What happens in the G1 phase?

Answer 8

Ensures that the cell is ready to enter the S phase and begin DNA synthesis.
All cell contents except the chromosomes are duplicated.
P 53 (tumour suppressor) gene controls this phase

Question 9

What happens in the S phase of interphase?

Answer 9

DNA replicates, each chromosome now consists of a pair of identical sister chromatids, this phase is rapid as DNA base pares are more susceptible to mutations.

Question 10

What happens in the G2 phase of interphase?

Answer 10

The cell grows and stimulates proteins involved in making chromatin condense.

Question 11

Why do all living organisms need to divide by mitosis?

Answer 11

For asexual reproduction, growth and tissue repair.

Question 12

What is asexual reproduction?

Answer 12

when organisms divide by mitosis to produce new individuals

Question 13

What are the four main stages of mitosis?

Answer 13

Prophase, metaphase, anaphase and telophase

Question 14

What happens during prophase?

Answer 14

The chromosomes shorten and thicken as DNA super coils (they become visible).
Nuclear envelope breaks down
The centriole found in the centrosome divides and the two daughter centrioles move to opposite poles of the cell.
Tubulin threads form a spindle between the centrioles.

Question 15

What happens during metaphase?

Answer 15

Pairs of chromatids attach to the spindle treads at the equator region by their centromeres.

Question 16

What happens during anaphase?

Answer 16

The centromere of each chromosome splits
Motor proteins pull each sister chromatid in opposite directions towards the opposite poles
The chromatids are now called chromosomes.

Question 17

What happens during telophase?

Answer 17

The separated chromosomes reach the poles
A new nuclear envelope forms around each set of chromosomes
The cell now contains two nuclei each genetically identical to each other and to their parent cell

Question 18

What happens during cytokinesis?

Answer 18

The cells’ cytoplasm splits into two.

Each new cell contains a nucleus.

Question 19

Why is genetic variation important?

Answer 19

Genetic variation within a population increases its chances of survival when the environment changes, as some individuals will have characteristics that enable them to be better adapted to the change.

Question 20

How many chromosomes do haploid and diploid cells in humans have?

Answer 20

Haploid= 23
Diploid= 46

Question 21

What are homologous chromosomes?

Answer 21

Matching chromosomes, containing the same genes in the same places.
One is paternal and one is maternal.

Question 22

What happens before meiosis?

Answer 22

Each chromosome is duplicated and its DNA replicated.
Each chromosome consists of two sister chromatids.
The chromosomes pair up in their homologous pairs.

Question 23

What are the stages of meiosis?

Answer 23

In the first division, there are 4 stages…
Prophase 1, metaphase 1, anaphase 1, and telophase 1
The cell may enter a short interphase.
The second meiotic division also has four stages…
Prophase 2, metaphase 2, anaphase 2, and telophase 2
This takes place at a right angle to that of the first division.
Then cytokinesis may occur.

Question 24

What happens during prophase 1?

Answer 24

Chromosomes condense as DNA super coils.
Crossing over occurs.
The nuclear envelope breaks down.
Spindle threads of tubulin protein form between the centrioles.

Question 25

What happens during metaphase 1?

Answer 25

Pairs of homologous chromosomes, still crossed over, attach along the equator of the spindle by their centromeres Homologous pairs are arranged randomly, this arrangement determines how they will segregate once pulled apart

Question 26

What happens during Anaphase 1?

Answer 26

The homologous chromosome pairs are pulled apart by their centromere. The crossed over areas separate from each other. Each chromosome consists of two chromatids.

Question 27

What happens during telophase 1?

Answer 27

Nuclear envelopes may form around the new chromosomes sets The cell divides by cytokinesis and there’s a short interphase when chromosomes uncoil. Plant cells go straight to prophase 2

Question 28

What happens during prophase 2?

Answer 28

If nuclear envelopes have re-formed, they now break down The chromosomes coil and condense each one consisting of two chromatids The chromatids of each chromosome and no longer identical, due to crossing over in prophase 1 Spindles form

Question 29

What happens during metaphase 2?

Answer 29

The chromosomes attach by their centromere to the equator of the spindle Again, the chromatids of each chromosome are randomly arranged.

Question 30

What happens during anaphase 2?

Answer 30

The centromeres divide The chromatids of each chromosome and are pulled apart by motor proteins towards opposite poles The chromatids are randomly segregated

Question 31

What happens in telophase 2?

Answer 31

Nuclear envelopes form around each of the four haploid nuclei In animals the two cells now divide to give four haploid cells. In plants a tetrad of four haploid cells is formed

Question 32

Give 4 ways that meiosis produces genetic variation?

Answer 32

Crossing over during prophase 1 shuffles alleles. Independent assortment of chromosomes in metaphase 1 and 2 leads to a random distribution of chromosomes. The haploid gametes produced can undergo fertilisation with gametes derived from another organism.

Question 33

What is differentiation?

Answer 33

The process by which stem cells become specialised into different types of cells.

Question 34

Which types of organisms require specialised cells?

Answer 34

Single-cell organisms are small and have a large SA:V ratio. Oxygen can diffuse across the plasma membrane and throughout the cell. However, multicellular organisms have smaller SA:V ratios, most of their cells are not in contact with the external environment. They need specialised cells to carry out specific functions.

Question 35

How are specialised cells made in animals?

Answer 35

A zygote is produced after fertilisation. After several mitotic divisions, many embryonic stem cells are formed. These have the ability to differentiate to produce any type of cell (pluripotent). Each cell is specialised for a particular function.

Question 36

How large is the average cell?

Answer 36

10–20 μm

Question 37

What is crossing over?

Answer 37

Where two non-sister chromatids wrap around each other, forming a bivalent, and swap sections so that alleles are shuffled. Where the chromosomes touch is called the chiasma.

Question 38

How do you calculate the number of possible combinations of genes in the gametes after meiosis?

Answer 38

Calculate 2 to the power of n | Where n is the haploid number of the cell.

Question 39

How do you calculate the number of mitotic divisions needed to produce a certain number of cells?

Answer 39

Use the formula 2 to the power of n= number of cells made Where n is the number of mitotic divisions.