2.1.6: Cell division, cell diversity and cellular organisation

Question 1

State what the cell cycle is and outline its stages.

Answer 1

Regulated cycle of division with intermediate growth periods.

1. interphase
2. mitosis or meiosis (nuclear division)
3. cytokinesis (cytoplasmic division)

Question 2

Outline what happens during interphase.

Answer 2

G1: cell synthesises proteins for replication e.g. tubulin for spindle fibres & cell size doubles.
S: DNA replicates = chromosomes consist of 2 sister chromatids joined at a centromere.
G2: Organelles divide.

Question 3

What is the purpose of mitosis?

Answer 3

Produces 2 genetically identical daughter cells for:
● growth
● cell replacement/ tissue repair
● asexual reproduction

Question 4

Name the stages of mitosis.

Answer 4

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

Question 5

Outline what happens during prophase.

Answer 5

1. Chromosomes condense, becoming visible.
   (X-shaped: 2 sister chromatids joined at centromere).
2. Centrioles move to opposite poles of cell (animal cells) & mitotic spindle fibres form.
3. Nuclear envelope & nucleolus break down = chromosomes free in cytoplasm

Question 6

Outline what happens during metaphase.

Answer 6

Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres

Question 7

Outline what happens during anaphase.

Answer 7

Requires energy from ATP hydrolysis

1. Spindle fibres contract = centromeres divide.
2. Sister chromatids separate into 2 distinct chromosomes & are pulled to opposite poles of cell. (looks like ‘V’ shapes facing each other).
3. Spindle fibres break down.

Question 8

Outline what happens during telophase.

Answer 8

1. Chromosomes decondense, becoming invisible again.

2. New nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome.

Question 9

What happens during cytokinesis?

Answer 9

1. Cell membrane cleavage furrow forms.

2. Contractile division of cytoplasm.

Question 10

How is the cell cycle regulated?

Answer 10

Checkpoints regulated by cell-signalling proteins ensure damaged cells do not progress to next stage of cycle.
Cyclin-dependent kinase enzymes phosphorylate proteins that initiate next phase of reactions.

Question 11

Describe what happens at each key checkpoint in

the cell cycle.

Answer 11

Between G1 & S, cell checks for DNA damage (e.g. via action of p53). After restriction point, cell enters cycle.
Between G2 & M, cell checks chromosome replication.
At metaphase checkpoint, cell checks that sister chromatids have attached to spindle correctly.

Question 12

What is meiosis?

Answer 12

A form of cell division that produces four genetically different haploid cells (cells with half the number of chromosomes found in the parent cell) known as
gametes.

Question 13

What happens during meiosis I?

Answer 13

1. Homologous chromosomes pair to form bivalents.
2. Crossing over (exchange of sections of genetic material) occurs at chiasmata.
3. Cell divides into two. Homologous chromosomes separate randomly. Each cell contains either maternal or paternal copy

Question 14

What are homologous chromosomes?

Answer 14

Pair of chromosomes with genes at the same locus. 1 maternal & 1 paternal.
Some alleles may be the same while others are different

Question 15

What happens during meiosis II?

Answer 15

1. Independent segregation of sister chromatids.

2. Each cell divides again, producing 4 haploid cells.

Question 16

How does meiosis produce genetic variation?

Answer 16

● Crossing over during meiosis I.
● Independent assortment (random segregation) of homologous chromosomes & sister chromatids.
Result in new combinations of alleles.

Question 17

How do cells become specialised?

Answer 17

Some genes are expressed while others are silenced due to cell differentiation mediated by transcription factors. Cells produce proteins that determine their
structure & function.

Question 18

What is a transcription factor?

Answer 18

A protein that controls the transcription of genes so that only certain parts of the DNA are expressed, e.g. in order to allow a cell to specialise.

Question 19

How do transcription factors work?

Answer 19

1. Move from the cytoplasm into nucleus.
2. Bind to promoter region upstream of target gene.
3. Makes it easier or more difficult for RNA polymerase to bind to gene. This increases or decreases rate of transcription.

Question 20

What is a stem cell?

Answer 20

Undifferentiated cells that can divide indefinitely and turn into other specific cell types.

Question 21

Name and define the 4 types of stem cell.

Answer 21

● Totipotent: can develop into any cell type including the placenta and embryo.
● Pluripotent: can develop into any cell type excluding the placenta and embryo.
● Multipotent: can only develop into a few different types of cell.
● Unipotent: can only develop into once type of cell.

Question 22

Suggest some uses of stem cells.

Answer 22

`● Repair of damaged tissue e.g. cardiomyocytes after myocardial infarction.
● Drug testing on artificially grown tissues.
● Treating neurological diseases e.g. Alzheimer’s & Parkinson’s.
● Researching developmental biology e.g. formation of organs, embryos.

Question 23

Describe the 2 groups of specialised cells in blood.

Answer 23

Erythrocytes (red blood cells): biconcave, no nucleus, lots of haemoglobin to carry oxygen.
Leucocytes (white blood cells): lymphocytes, eosinophils, neutrophils to engulf foreign material, monocytes.

Question 24

How do the specialised cells in blood form?

Answer 24

Multipotent stem cells in the bone marrow differentiate into:
● Erythrocytes, which have a short lifespan & cannot undergo mitosis since they have no nucleus.
● Leucocytes, including neutrophils.

Question 25

State the relationship between a system and

specialised cells.

Answer 25

specialised cells → tissues that perform
specific function → organs made of
several tissue types → organ systems

Question 26

Describe the structure of squamous and ciliated

epithelia.

Answer 26

Simple squamous epithelium: single smooth layer of
squamous cells (thin & flat with round nucleus) fixed in
place by basement membrane.
Ciliated epithelium: made of ciliated epithelial cells
(column-shaped with surface projections called cilia
that move in a synchronised pattern).

Question 27

Describe the structure and function of palisade cells

and guard cells in plants.

Answer 27

palisade cells: Specialised to absorb light energy for
photosynthesis, so contain many chloroplasts. Pack
closely together.
guard cells: Form stoma. When turgid, stoma opens;
when flaccid, stoma closes. Walls are thickened by
spirals of cellulose.

Question 28

Describe the structure and function of root hair cells.

Answer 28

Specialised to absorb water and low-concentration minerals from soil.
Hair-like projections increase surface area for osmosis / carrier proteins for active transport.
Many mitochondria produce ATP for active transport.

Question 29

What are meristems?

Answer 29

Totipotent undifferentiated plant cells that can develop into various types of plant cell, including xylem vessels & phloem sieve tubes.
Classified as apical (at root & shoot tips), intercalary (stem) or lateral (in vascular areas).

Question 30

Describe the structure of phloem tissue.

Answer 30

● Sieve tube elements: form a tube to transport
sucrose in the dissolved form of sap.
● Companion cells: involved in ATP production
for active loading of sucrose into sieve tubes.
● Plasmodesmata: gaps between cell walls
where the cytoplasm links, allowing substances
to flow.

Question 31

Describe the structure of xylem tissue.

Answer 31

● Vessel elements: lignified secondary walls for mechanical strength & waterproofing; perforated end walls for rapid water flow.
● Tracheids: tapered ends for close packing; pits for lateral water movement; no cytoplasm or nucleus.

Question 32

Describe the additional cell types in xylem tissue.

Answer 32

● xylem parenchyma: packing tissue with thin walls transmit turgidity
● sclereids
● sclerenchyma fibres: heavily lignified to withstand negative pressure

Question 33

Describe the structure of cartilage.

Answer 33

● Avascular smooth elastic tissue made of chondrocytes, which produce extensive extracellular matrix (ECM).
● ECM mainly contains collagen & proteoglycan.
● 3 categories: hyaline, yellow elastic, white fibrous
(depends on ratio of cells: ECM).

Question 34

Name the 3 types of muscle in the body and where

they are located.

Answer 34

Cardiac: exclusively found in heart.
Smooth: walls of blood vessels and intestines.
Skeletal: attached to incompressible skeleton by tendons

Question 35

Describe the gross structure of skeletal muscle.

Answer 35

Muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils).
Arrangement ensures there is no point of weakness
between cells.
Each bundle is surrounded by endomysium: loose
connective tissue with many capillaries.