Cell Division, Cell Diversity And Cellular Organisation

Question 1

State what the cell cycle is and outline it’s stages

Answer 1

Regulated cycle of division with intermediate growth periods
1. Interphase
2. Mitosis or meiosis
3. Cytokinesis

Question 2

Outline what happens during interphase

Answer 2

G1: cell synthesis proteins for respiration e.g. tubulin for spindle fibres 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 two genetically identical 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-shaped: two sister chromatids joined at a centromere)
2. Centrioles move to opposite poles of the cell (animal cells) and mitotic spindle fibres form
3. Nuclear envelope and nucleolus break down = chromosomes free in cytoplasm

Question 6

Outline what happens in 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 two distinct chromosomes and are pulled to opposite ends of the cell (looks like V shapes facing each other)
3. Spindle fibres break down

Question 8

What happens during telophase

Answer 8

1. Chromosomes decondense, becoming visible again
2. New nuclear envelope forms 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-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 checks for DNA damage (e.g. via action of p53 - a gene that codes for a protein that checks the fidelity of the DNA)
Between G2 and M the cell checks the chromosome replication
At the metaphase checkpoint the cell checks that the sister chromatids are connected to the spindle properly

Question 12

What is meiosis

Answer 12

Meiosis is a form of cell division that produces 4 genetically different haploid cells which only have half the number of chromosomes present
These are known as gamates

Question 13

What happens during meiosis

Answer 13

1. Homologous chromosomes pair to form bivalents
2. Crossing over (the exchange of sections of genetic material) occurs at chiasmata
3. Cell divides in two - homologous chromosomes seperate randomly to form two cells containing either the maternal or paternal copy of a gene

Question 14

What are homologous chromosomes

Answer 14

Pair of chromosomes with genes at the same locus. 1 maternal and 1 paternal chromosome present
Some alleles may be the same but some may be different which gives variation

Question 15

What happens during meiosis II

Answer 15

1. Independent segregation of sister chromatids
2. Each cell divides again which produces 4 haploid cells

Question 16

How does meiosis produce genetic variation

Answer 16

Both crossing over during meiosis 1 and the independent assortment of homologous chromosomes and sister chromatids mix up the genes in a new way resulting in a new combination of alleles

Question 17

How do cells become specialised

Answer 17

Some genes are expressed whilst 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

Move from the cytoplasm into nucleus
Bind to promoter region upstream of target gene
Makes it easier or more difficult for RNA polymerase to bind to 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 cell types

Question 21

Name and define the 4 types of stem cell

Answer 21

Totipotent - can develop into any type of cell including in the placenta and embryo
Pluripotent - can develop into any type of cell excluding in the placenta or embryo
Multipotent - can develop into only 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. Alzheimers or Parkinsons
Researching developmental biology e.g. formation of organs and embryos

Question 23

Describe the 2 groups of specialised cells in blood

Answer 23

Erythrocytes (RBCs): biconcave, no nucleus, lots of haemoglobin to carry oxygen
Leucocytes (WBCs): Lymphocytes, eosinophils, neutrophils to engulf foreign material, monocytes

Question 24

How do the specialised cells form in the blood

Answer 24

Multipotent stem cells in the bone marrow differentiate into:
Erythrocytes, which have a short lifecycle 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 and flat with round nucleus) fixed in place by basement membrane Ciliated epithelium: made of ciliated epithelial cells (column-shaped with surface projections 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 membrane 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: From 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 plants 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 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 cytosplasm links, allowing substances to flow

Question 32

Describe the primary cell types in xylem tissue

Answer 32

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 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 cartliage

Answer 34

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 35

Name the three muscles in the body and where they are located

Answer 35

Cardiac : exclusively in the 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