2.1.6 - cell division, cell diversity, 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/meiosis (nuclear division)
3. cytokinesis (cytoplasmic division)

Question 2

outline what happens during interphase

Answer 2

G1: cell synthesises proteins for replication and cell size doubles
S: DNA replicates - chromosomes consist of 2 sister chromatids joined at 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) and mitotic spindle fibres form
3. nuclear envelope and 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 mitotic spindle by their centromeres

Question 7

outline what happens during anaphase

Answer 7

1. spindle fibres contract = centromeres divide
2. sister chromatids separate into 2 distinct chromosomes and are pulled to opposite poles of the cell
3. spindle fibres break down
   process requires energy from ATP hydrolysis

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 phosphororylate proteins that initiate next phase of reactions

Question 11

describe what happens at each key checkpoint of the cell cycle

Answer 11

• between G1 and S, cell checks for DNA damage (e.g. via action of p53), 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 4 genetically different haploid cells (cells with half number of chromosomes found in 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 same whilst 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 and 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 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 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/more difficult for RNA polymerase to bind to the gene - this increases/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 cells

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 one 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 and 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 the blood form?

Answer 24

multipotent stem cells in the bone marrow differentiate into:
- erythrocytes, which have a short lifespan and 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 epithelia 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 - flagellum bound by plasma membrane propels cell - spiral shaped mitochondrion - acrosome secretes enzymes to penetrate ovum coat

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 by spirals 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 and phloem sieve tubes - classified as apical (at root and shoot tips), intercalary (stem) or lateral (in vascular areas)

Question 31

describe the structure of a vascular bundle

Answer 31

- cambium (meristematic tissue) - phloem tissue - xylem tissue

Question 32

describe the structure of phloem tissue

Answer 32

- 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 33

describe the structure of xylem tissue

Answer 33

- 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 34

describe the additional cell types in xylem tissue

Answer 34

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

Question 35

describe the structure of cartilage

Answer 35

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

Question 36

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

Answer 36

- cardiac: exclusively found in the heart - smooth: walls of blood vessels and intestines - skeletal: attached to incompressible skeleton by tendons

Question 37

describe the gross structure of skeletal muscle

Answer 37

- 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