2.6 Cell division and organisation

Question 1

State what the cell cycle is and outline its stages

Answer 1

1. Interphase
2. mitosis or meiosis
3. cytokinesis

Question 2

Outline what happens during interphase

Answer 2

G1: cell synthesis proteins for replication and 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

Prophase
Metaphase
Anaphase
Telophase

Question 5

Outline what happens during prophase

Answer 5

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

Question 6

Outline what happens during metaphase

Answer 6

Sister chromatids line up at the 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 and centromeres divide.
2. Sister chromatids seperate into 2 distinct chromosomes and are pulled to opposite poles of the cell.
Spindle fibres break down.

Question 8

Outline what happens during telophase.

Answer 8

1. chromosomes decondense, becoming invisble again.
2. New nuclear envelopes form around each set of chromosomes and form 2 new nuclei, each with one 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 are regulated by cell-signalling proteins to ensure damaged cells o not progress to the next stage of the cycle.
Cyclin-dependant kinase enzymes phosphorylate proteins that initiate the next phase of reactions.

Question 11

Describe what happens at each key checkpoint in the cell cycle.

Answer 11

Between G1 and S, cell check for DNA damage. After restriction point, cell enters cycle.
Between G2 and 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 and 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.
  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 and 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 cells to specialise.

Question 19

How do transcription factors work?

Answer 19

1. Move from the cytoplasm into the nucleus.
2. Bind to promoter region upstream of the target gene.
3. Makes it easier or more difficult for RNA polymerase to bind to the gene. This increases or decreases the 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 the 4 types of stem cell.

Answer 21

Totipotent
Pluripotent
Multipotent
Unipotent

Question 22

Totipotent

Answer 22

Can develop into any cell type including the placenta and embryo.

Question 23

Pluripotent

Answer 23

Can develop into any cell type excluding the placenta and embryo.

Question 24

Multipotent

Answer 24

Can only develop into a few different types of cell.

Question 25

Unipotent

Answer 25

Can only develop into one type of cell.

Question 26

Uses of stem cells

Answer 26

• Repair of damaged tissue
• Drug testing on artificially grown tissues.
• Treating neurological diseases.
• Researching developmental biology.

Question 27

Describe the 2 groups of specialised cells in blood.

Answer 27

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 28

How do the specialised cells in blood form?

Answer 28

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

Question 29

Describe the structure of squamous and ciliated epithelia.

Answer 29

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

Question 30

Describe the specialised structure of a sperm cell.

Answer 30

Haploid nucleus so fertilisation restores diploid chromosome number.
Acrosome secreted enzymes to penetrate the ovum coat. Spiral-shaped mitochondrion.
Flagellum bound by plasma membrane propels cell.

Question 31

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

Answer 31

Palisade cells: specialised to absorb light energy for photosynthesis, so contain many chloroplasts. Packed closely together.
Guard cells: form stoma. When turgid, stoma opens; when flaccid, stoma closes. Walls are thickened by spirals of cellulose.

Question 32

Describe the structure and function of root hair cells.

Answer 32

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 33

What are meristems?

Answer 33

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

Question 34

Describe the structure of phloem tissue.

Answer 34

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

Question 35

Describe the structure of xylem tissue.

Answer 35

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

Question 36

Describe the additional cell types in xylem tissue.

Answer 36

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