Chapter 6 Mitosis

Question 1

Why do we need mitosis?

Answer 1

For growth, development and repair.

Question 2

In mitosis, what occurs in prophase?

Answer 2

Chromatin fibres coils and condenses to form chromosomes.
Nuclear membrane breaks down.
Protein microtubules form spindle fibres connected to the poles of the cells.
The spindle fibres attach to the centromere on the chromosomes, to start moving the chromosomes to the centre of the cell.

Question 3

In mitosis, what occurs in the metaphase stage?

Answer 3

Chromosomes are moved by spindle fibres to the centre of the cell, called the metaphase plate.

Question 4

In mitosis, what occurs in the anaphase stage?

Answer 4

The shortening of spindle fibres causes chromatids in each chromosome to be separated and reach opposite ends of the cell.

Question 5

In mitosis, what occurs in the telophase stage?

Answer 5

The chromatids at either ends of the cell uncoils and a nuclear membrane forms around each set of DNA.

Question 6

In mitosis, what occurs in the cytokinesis stage?

Answer 6

The cell separates into two cells. These cells are referred to as ‘daughter cells’ and are genetically identical to the parent cell. In animal cells, cytokinesis occurs when a cleavage furrow forms around the middle of the cell (when the cell surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle).

Question 7

Relative to all the other stages in mitosis, how much of mitosis is taken up by the interphase stage?

Answer 7

Interphase takes up most of mitosis- it takes up around 95% of the cell cycle.

Question 8

State the stages chronologically that occur in interphase (and checkpoints).

Answer 8

G1
G1 checkpoint
S phase
G2
G2 checkpoint

Question 9

What occurs in the G1 phase?

Answer 9

In the G1 phase:
Organelles replicate.
Increase in cell size.
Protein synthesis.
Respiration occurs.

Question 10

What does the G1 checkpoint check for?

Answer 10

G1 checks if:
Enough replication of organelles has occurred.
Checks cell size.
Checks for DNA damage before any DNA replication occurs- it could lead to a faulty cell.

Question 11

What occurs in S (synthesis) stage?

Answer 11

DNA replication occurs.

Question 12

What is G0?

Answer 12

G0 is also referred to as cell arrest, and a cell may enter G0 after entering the G1 checkpoint. A cell may enter G0 temporarily or permanently because:
The cell is already differentiated.
There is DNA damage.
The cell has already reached the maximum number of divisions.

Question 13

What occurs in G2 phase?

Answer 13

Organelles replicate.
Increase in cell size.
Protein synthesis.
Respiration.

Question 14

What is checked in the G2 checkpoint?

Answer 14

The G2 checkpoint occurs after S phase, so it checks if there were any errors in the DNA replication- as this may lead to mutations.

Question 15

After interphase, mitosis and cytokinesis occurs. Within the mitosis stage, there is a spindle fibre checkpoint. What does this check for?

Answer 15

This checkpoint makes sure the spindle fibres are correctly attached to the centromeres of the chromosomes. If not, this can lead to uneven splitting of chromosomes, and hence the daughter cells would not be genetically identical.

Question 16

A cancer cell is a cell that initially has DNA damage. Theoretically, where in the cell cycle should it stop?

Answer 16

As a cancer cell is a cell with DNA damage, it should theoretically stop at the G1 checkpoint and enter G0 (cell arrest). However, cancer is known as the ‘uncontrolled division of cells’. This means the cancer cell has somehow surpassed the G1 checkpoint and gone through the whole cell cycle.

Question 17

In meiosis, what occurs in prophase 1?

Answer 17

Chromosomes coils and condenses.
Nuclear membrane breaks down.
Protein microtubules start to form spindle fibres.
The maternal and paternal chromosomes undergo crossing over. The crossing over means that alleles from the mum may be swapped with the dad and vice versa. This leads to genetic variation. (The point at which the alleles cross over is called the chiasmata).
Alleles are genes coding for the same general characteristic; a gene coding for a blue eye colour, and a gene coding for a green eye colour are alleles of each other.

Question 18

What occurs in metaphase 1 of meiosis?

Answer 18

Homologous pairs of chromosomes lines up along the metaphase plate (with guidance of spindle fibres). There is the independent assortment of each chromosome pair- this just means the orientation of each chromosome pair is random, and hence which end of the cell they end up at, is random. This leads to genetic variation.

Question 19

What are the products of the first division of meiosis?

Answer 19

2 haploid cells are produced.

Question 20

What occurs in prophase 2 of meiosis?

Answer 20

Prophase 2 initiates with each haploid produced from division 1 of meiosis.
The nuclear membrane around the DNA breaks down.
The DNA coils and condenses.
Protein microtubules form spindle fibres.

Question 21

What occurs in metaphase 2 of meiosis?

Answer 21

The spindle fibres (attached to centromeres of chromosomes) guides the chromosomes to the metaphase plate.
In this stage, the independent assortment of chromosomes occur. This just means the orientation of each chromosomes is random- so at which pole of the cell that the chromatids end up is random. This leads to genetic variation.

Question 22

What are the products at the end of the end of the second division of meiosis?

Answer 22

4 haploids cells are produced (they are genetically different).

Question 23

Why does meiosis occur?

Answer 23

Meiosis is used to produce gametes.

Question 24

State 3 ways in which genetic variation occurs in meiosis and in which stage they occur.

Answer 24

Crossing over- Prophase 1.
Independent assortment of homologous pairs of chromosomes- Metaphase 1.
Independent assortment of chromosomes- Metaphase 2.

Question 25

What are stem cells? When do they stop being ‘stem cells’?

Answer 25

Stem cells are undifferentiated cells. Once they differentiate, they become specialised and permanently enter G0 phase.

Question 26

What does differentiation do to a cell?

Answer 26

Differentiation makes a cell specialised- this means the shape and size of the cell (as well as the organelles) change to become adapted to carry out its function.

Question 27

What is potency with respect to cell differentiation?

Answer 27

Potency is a cell’s ability to differentiate into different cells. The greater the number of cell types, the greater the potency.

Question 28

What does it mean if a stem cell is totipotent?

Answer 28

If a stem cell is totipotent, it can differentiate into any cell type. A fertilised cell contains totipotent cells, which eventually produce a whole organism.

Question 29

What does it mean if a stem cell is pluripotent?

Answer 29

If a stem cell is pluripotent it can different into all tissue types but cannot form a whole organism. These cells are present in early embryos.

Question 30

What does it mean if a stem cell is multipotent?

Answer 30

A stem cell is multipotent if it can only differentiate into a range of cells within a certain type of tissue. Haematopoietic stem cells in the bone marrow in multipotent as it can only differentiate into varying blood cells.

Question 31

Why do red blood cells need to be replaced constantly?

Answer 31

Erythrocytes have fewer organelles to allow there to be space for haemoglobin (for it to carry oxygen). The lack of a nucleus means an erythrocyte has a shorter life span (of 120 days) than normal cells.
Stem cell colonies in the bone marrow produce around 3 billions of erythrocytes per kilogram of body mass per day.

Question 32

Where can we source totipotent animal stem cells from?

Answer 32

Totipotent stem cells can be found in fertilised egg (embryonic stem cells). After 7 days, a mass of cells called a blastocyst is formed, which is now in a pluripotent state.

Question 33

Where can adult stem cells be sourced from?

Answer 33

Adult stem cells multipotent stem cells and are present from birth. They can be found in places like the bone marrow.

Question 34

State how stem cells can be used.

Answer 34

Testing drugs on cultured cells in laboratory (in vitro).
Studying biology development and disease development (in vitro).
A treatment to burns.
Bone marrow transplant to replace stem cells damaged/destroyed during cancer treatment.

Question 35

What are the ethics behind using embryonic stem cells for therapy and treatment?

Answer 35

The removal of stem cells from embryos normally results in the destruction of embryos (so perhaps considered ‘taking a life’)- this leads to religious and moral objections.

Question 36

What are scientists doing to find a new source of embryonic stem cells?

Answer 36

They are using techniques to reprogram differentiated adult cells back into pluripotent cells, as the use of stem cells can lead to treatment of many incurable diseases.

Question 37

How are erythrocytes adapted to their function?

Answer 37

A flattened biconcave shape gives a high surface area to volume ratio.
No nuclei gives more space for haemoglobin, which binds to oxygen.
The erythrocyte is flexible which allows it to squeeze through narrow capillaries.

Question 38

How are neutrophils adapted to their function?

Answer 38

A multi-lobed nucleus means the neutrophil can squeeze through gaps to get to the site of infection.
Lysosomes in cytoplasm- Contains enzymes used to attack pathogens.

Question 39

How are sperm cells adapted to their function?

Answer 39

A flagellum makes it capable of movement/swimming.
Lots of mitochondria supplies the cell with a lot of ATP to swim.
Ac acrosome is found on the head of the sperm cell, which contains digestive enzymes to break through the protective layers of the egg cell.

Question 40

How are palisade cells adapted to their function?

Answer 40

Palisade cells are found in the mesophyll of the cell.
The have:
Chloroplasts to absorb a lot of sunlight for photosynthesis.
Thin cell walls to increase rate of diffusion of carbon dioxide.
A large vacuole maintains turgor pressure.

Question 41

How are root hair cells adapted to their function?

Answer 41

Root hair cells are found in the growing tips of the roots.
These cells have long extensions called root hairs, used to increase surface area- this means there would be the maximum uptake of water and mineral ions from the soil.

Question 42

How are guard cells adapted to their function?

Answer 42

Guard cells form a small opening called a stomata, which helps with carbon dioxide entering the plant. When the plant is losing a lot of water, the guard cells become less swollen due to osmotic pressure. This changes the shape of the stomata and it closes to prevent further loss of water.