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 specialised structure of a spermatozoon.

Answer 27

Specialised to fertilise an ovum during sexual reproduction in mammals. Haploid nucleus so fertilisation restores diploid chromosome number. Acrosome secretes enzymes to penetrate ovum coat. Spiral-shaped mitochondrion. Flagellum bound by plasma propels cell.

Question 28

Describe the structure and function of palisade cells and guard cells in plants.

Answer 28

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 byspirals of cellulose.

Question 29

Describe the structure and function of root hair cells.

Answer 29

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 30

What are meristems?

Answer 30

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 31

Describe the structure of phloem tissue.

Answer 31

● 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 32

Describe the structure of xylem tissue.

Answer 32

● 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 33

Describe the additional cell types in xylem tissue.

Answer 33

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

Question 34

Describe the structure of cartilage.

Answer 34

● 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 35

Name the 3 types of muscle in the body and where they are located.

Answer 35

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

Question 36

Describe the gross structure of skeletal muscle.

Answer 36

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.