Specialised cells and cell division

Question 1

What are the stages of the cell cycle?

Answer 1

1. Interphase
2. Mitosis
3. Cytokinesis

Question 2

What happens in the interphase?

Answer 2

There are 3 stages in the interphase. The first stage is Growth 1 (G1) and consists of the cell growing, developing and carrying out all of its metabolic functions including producing enzymes and excreting them. The second stage is Synthesis (S) and consists of the DNA in the nucleus replicating into 2 copies. Each chromosome now consists of 2 sister chromatids joined at centromere. The third stage is Growth 2 (G2) and is similar to G1, however, the mitochondria (or chloroplast) in this stage are also duplicating; preparing for cell division.

Question 3

How does the body check DNA for ‘mutations’?

Answer 3

Special enzymes check all newly replicated strands of DNA molecules for any mistakes or ‘mutations’. These may lead to disastrous consequences for the body if not found and eliminated.

Question 4

What are the 4 stages of Mitosis?

Answer 4

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Question 5

What happens in prophase?

Answer 5

1. Chromosomes coil up tightly and condense to become visible under the CLM. This makes them easier to move about during mitosis, but also deactivates DNA.
2. Nuclear envelope breaks down.
3. Centriole divides into 2 and move to opposite poles and produce microtubules called spindles.
4. Nucleolus disappears.

Question 6

What happens in metaphase?

Answer 6

1. Chromosomes line up along the equator of the cell.

2. Each sister chromatid in chromosomes attach to opposite poles by spindle structure.

Question 7

What happens in anaphase?

Answer 7

1. Cenromeres split and the two sister chromatids making up each chromosome separate.
2. Sister chromatids are pulled towards to opposite poles by the spindles shortening.
3. Chromatids become v-shaped due to being led towards the poles by the centromere.
4. There are now 2 sets of 46 chromosomes in the nucleus.

Question 8

What happens in telophase?

Answer 8

1. Chromatids (chromosomes) reach the opposite poles.
2. Chromosomes uncoil and decondense to become almost invisible again.
3. Spindle structures break down.
4. Nucleolus reappears and divides into 2.
5. 2 separate nuclear envelopes are formed around the 2 sets of 46 chromosomes and there are now 2 identical nuclei within the cell.

Question 9

What does mitosis do?

Answer 9

1. One parent cell creates two genetically identical daughter cells.
2. Daughter cells contain the same number of chromosomes as the parent cell.
3. Daughter cells are genetically identical to parent cell so are able to perform the same tasks.

Question 10

What is mitosis used for in nature?

Answer 10

1. Growth. Cells divide to create more identical cells which slowly increases the volume of the organism.
2. Repair. When cells are damaged in the body, mitosis creates new cells which are identical to the damaged ones to replace them.
3. Replace. Some cells, like red blood cells, become worn out after a while and need to be replaced. Mitosis constantly produces more red blood cells to replace the worn out ones.
4. Asexual reproduction. Some organisms reproduce by asexual reproduction in which they produce genetically identical offspring by mitosis (clones). This doesn’t produce variation but does mean only one parent is required.

Question 11

What is meiosis?

Answer 11

Meiosis is a process that happens in the gonads of an organism. During meiosis, a diploid parent cells divides into 4 haploid daughter cells. During meiosis, 2 other important processes happen:
1. Reassortment/ independent assortment.
2. Crossing-over/ recombination.
All this ensures that the genetic information in the daughter cells are randomly different from each other. During sexual reproduction, two haploid gametes will fuse to produce a diploid zygote which is genetically different from both parents. This ensures variation.

Question 12

What is cytokinesis?

Answer 12

Cytokinesis is the process of the cell itself dividing to produce 2 daughter cells.
In animal cells, cytokinesis begins pinching into a cell along what is known as cleavage furrows and eventually splits the cytoplasm, nuclei and organelles of a parent cell into 2 identical daughter cells.
In plant cells, cleavage furrows cannot form due to the cell walls so instead, vesicles line up along the equator of the plant cell and break open to release cell wall materials which come together to form a cell plate that grows into a new cell wall, splitting the plant cell into 2 plant cells.
In some microorganisms like yeast, cytokinesis occurs via a process called budding. The parent cell produces what is known as a bud, a small vesicle like projection out of the cell in which a nucleus enters as the result of mitosis. The bud eventually splits from the parent cell and grows into a full sized adult yeast cell.

Question 13

What are homologous pairs of chromosomes?

Answer 13

In a human cell, the nucleus contains 46 chromosomes. However, apart form the sex chromosomes, all other chromosomes are paired with the same chromosomes into 22 pairs. These chromosomes contain the same genes as each other but different versions called alleles which produce variation. These pairs of chromosomes are called homologous pairs and there are 23 of them (including the sex chromosomes).

Question 14

What is the structure of chromosomes?

Answer 14

Each chromosome consists of one DNA molecule wrapped around proteins called histones. Each DNA molecule contains sections which code for a specific polypeptide called genes. Genes have specific locations on a chromosome called their loci.

Question 15

What limits the size of a cell?

Answer 15

As size increases, surface area to volume ratio decreases. This means that the exchange surface to volume ratio also decreases. If a cell gets too big, it may not be able to absorb nutrients and oxygen quickly enough to sustain itself. This is why cells have a maximum size.

Question 16

Why do multicellular organisms need a transport system?

Answer 16

In single cellular organisms, all of the organism is exposed to the external medium where it can acquire nutrients and oxygen. However, in multicellular organisms, surface to area volume ratio is much smaller and only a small proportion of cells are exposed. This means that a transport system is needed to carry nutrients and oxygen to all cells, ensuring they all live and are able to carry out their functions.

Question 17

What is differentiation?

Answer 17

When cells in a multicellular organism undergo structural changes and become different types of cells in order to perform specific functions.

Question 18

How can cells differentiate?

Answer 18

1. Different numbers of particular organelles.
2. Different shapes and sizes.
3. Different contents altogether.

Question 19

What are stem cells?

Answer 19

Stem cells are unspecialised cells found in most multicellular organisms. They are characterised by two very important characteristics:

1. Self-renewal: The cell has to be able to divide by mitosis but still maintain its unspecialised state.
2. Potency: The cell has to be able to differentiate into different cells.

Question 20

What are the levels of potency?

Answer 20

Totipotent: When a stem cell is able to become any cell it’s capable of becoming. In humans, these cells are found in the early embryo.
Pleuripotent: When a stem cell is able to become most cells but not all cells. This refers to the cells in the inner layer of a blastocyst as they are able to become all human tissue cells, but not placenta cells.
Oligopotent: When a stem cell is able to produce some different cells, but has lost its ability to produce many.
Unipotent: These are the most common stem cells in adults and can only produce one cell typle. However, they still fit the definition of a stem cell.

Question 21

How is differentiation shown in plants and animals?

Answer 21

In plants, the cells that divide are called meristem cells. Special meristems called cambium allow plants to grow side ways. These cells are capable of producing xylem and phloem cells.
In animals, stem cells in the bone marrow are able to produce red blood cells as well as neutrophils (white blood cells) which are clearly different in structure and purpose.

Question 22

How are erythrocytes (red blood cells) specialised?

Answer 22

1. They have a biconcave shape to maximise surface area and oxygen carrying potential.
2. They are lacking most of their organelles; including the nucleus, mitochondria, rough ER and smooth ER so more space is available for haemoglobin.
3. Their cytoplasm is rich in haemoglobin so they are capable of carrying oxygen.
4. Enlarged surface area allows for oxygen to diffuse in and out of cells.
5. Lack of mitochondria also means that no oxygen is used up.
6. Flexible so can fit through capillaries.
7. 6-8 micrometers wide.

Question 23

How are neutrophils specialised?

Answer 23

1. Presence of many granules in the cytoplasm (of which most are lysosomes) so that they are capable of excreting and digesting pathogens.
2. Lobed nucleus and irregular shape allows the neutrophil to fit through tight spaces like pores in blood vessels as well as perform phagocytosis to destroy pathogens.
3. 10-14 micrometers wide.

Question 24

How are sperm cells specialised?

Answer 24

1. Their nucleus contains a haploid number of chromosomes so it can fuse with an egg to create a diploid zygote.
2. There is a special sack on the head of the sperm called an acrosome (basically a big lysosome) which allows the sperm to penetrate the outer layer of the egg.
3. The connecting piece of the sperm contains many mitochondria to provide the sperm with enough energy to move and get to the egg.
4. Its tail pulsates to allow the sperm to ‘swim’ and travel to the egg.
5. Sperm is small and thin which makes it very streamlined and move easily

Question 25

How are palisade cells specialised?

Answer 25

1. They are packed with chloroplasts to maximise photosynthesis.
2. They are located near the surface of leaves to be exposed to the maximum amount of sunlight.
3. They are regularly shaped like blocks so can be packed tightly together without wasting space.
4. Chloroplasts are capable of moving up or down the cell depending on light levels.

Question 26

How are root hair cells specialised?

Answer 26

1. Single, ‘hair-like’ projection significantly increases its surface area and exchange surface, allowing for the max amount of water and nutrients to be absorbed.
2. Thin cell walls to make diffusion and active transport easy.
3. Doesn’t contain any chloroplast as no photosynthesis takes place in it.
4. Water potential always kept low to allow or osmosis.
5. May excrete acids to brea down minerals to ions in order to be absorbed.

Question 27

How are guard cells specialised?

Answer 27

1. Always comes in pairs to form the stomata on the endodermis of the leaf.
2. Contains chloroplasts, unlike most cells in the endodermis.
3. Cell walls thicker on the inside so that when the cells are turgid, only outside wall stretches, closing the guard cells on each other and closing the stomata.
4. Allows carbon dioxide to diffuse into the leaf for photosynthesis.

Question 28

How are xylem vessels specialised?

Answer 28

1. Meristem cells produce cells called parenchyma that elongate (these will become xylem elements). As they grow older, their cell walls get reinforced with lignin, which makes them impermeable to water, so they die.
2. The end cell walls eventually disappear to form a continuous vessel.
3. Due to the impermeability of lignin, xylem vessels are perfect for carrying water up the plant.
4. Cells have no organelles or cytoplasm to offer minimum obstruction to flowing water.
5. Pits in the walls allow for water to exit and reach other parts of the plant.
6. Spiral and annular lignification happens in growing parts of the plant whereas scalariform, reticulated and pitted lignification happens in parts of the plant that don’t grow.

Question 29

How are phloem sieve cells specialised?

Answer 29

1. Alive so it can transport sugars and amino acids up and down a plant.
2. Cytoplasm and organelles pressed tightly against sides to minimise obstruction to flow. Nucleus, vacuole and ribosomes are lost to do this.
3. Plasmodesma enlarge on the end walls to produce sieve plates which make flow of contents easier.
4. Sieve plates support structure of phloem and stops it from collapsing.

Question 30

How are phloem companion cells specialised?

Answer 30

1. No chloroplasts or vacuole in cytoplasm to make space for more mitochondria.
2. Very metabolically active as it needs to keep sieve cells alive as well as itself.
3. Next to sieve cells and connected by plasmodesma for ease of transport of metabolic products.

Question 31

How are squamous epithelial cells specialised?

Answer 31

1. Large and thin as they usually make up exchange surfaces like capillaries and alveoli to reduce distance of diffusion for molecules.
2. Smooth so fluid can pass over easily.
3. Lie on a basement membrane made out of collagen and glycoproteins by cells themselves.

Question 32

How are ciliated epithelial cells specialised?

Answer 32

1. Hundreds of hair-like petrusions from the top surface of cell which pulsate in a regular rhythm to move an entity.
2. Nucleus located at bottom, leaving large mitochondria concentration in top of cell to provide cilia with energy.
3. Regular, rectangular structure allows lots to be packed together in small space.

Question 33

Why is important in multicellular organisms?

Answer 33

In plants, leaves are well adapted organs for photosynthesis. The epidermis is transparent to allow for max amount of light to reach palisade cells that are packed with chloroplast. Spongy mesophyll layer beneath is full of air spaces to allow for ease of gaseous diffusion. Guard cells on the endodermis regulates gas exchange and transpiration. Vascular bundle of xylem and phloem constantly bring more water and carrying away products of photosynthesis.

Question 34

How are organisms organised?

Answer 34

Tissues: Groups of similar cells with a common function. They may be connected but not always.
Organs: A group of tissues performing one particular function.
Organ systems: Groups of organs working together to perform an overall life function.