2. New nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome.
what happens during cytokinesis
1. Cellmembrane cleavage furrow forms.
2. Contractile division of cytoplasm.
how is the cell cycle regulated
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.
describe what happens at each key checkpoint in the cell cycle
Between G1 and S cell checks 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
what is meiosis
a form of cel devision that produces genetically different haploid cells (cells with half the number of chromosomes found in the parent cell) known as gametes
what happens during meiosis 1
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.
What are homologous chromosomes?
pair of chromosomes with genes at the same locus. 1 maternal and 1 paternal. some alleles may be the same while others are different
what happens during meiosis 2
1. Independent segregation of sister chromatids.
2. Each cell divides again, producing 4 haploid cells.
how does meiosis produce genetic variation?
Crossing over during meiosis Ⅰ.
Independent assortment (random segregation) of homologous chromosomes & sister chromatids. Result in new combinations of alleles.
how do cells become specialised
Some genes are expressed while others are silenced due to cell differentiation mediated by transcription factors. Cells produce proteins that determine their structure & function.
what are transcription factors
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
how do transcription factors work
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.
what is a stem cell
Undifferentiated cells that can divide indefinitely and turn into other specific cell types.
name and define 4 types of stem cells
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
suggest some uses of stem cells
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.
describe the 2 groups of specialised cells in blood
erythrocytes: biconcave, no nucleus, lots of haemoglobin to carry oxygen
leucocytes: lymphocytes, eosinophils, neutrophils to engulf foreign material, monocytes
how do the specialised cells in blood form
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.
state the relationship between a system and specialised cells
specialised cells -> tissues that perform specific function-> organs made of several tissues -> organ systems
describe the structure of squamous and ciliated epithelia
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 called cilia that move in a synchronised pattern
describe the specialised structure of a sperm matozoon
specialised to fertilise an ovum during sexual reproduction
e.g flagellum bound by plasma membrane propels cell
describe the structure and function of palisade cells and guard cells in plants
palisade cells: specialised to absorb light energy for photosynthesis, so contain many chloroplasts. Pack closely together
guard cells: form stoma. when turgid, stomata opens; when flaccid, stomata closes. walls are thickened by spirals of cellulose
describe the structure and function of root hair cells
specialised to absorb water and low-concentration minerals from soil
hair-like projections increase surface are for osmosis/carrier proteins for active transport
many mitochondria produce ATP for active transport
what are meristems
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)
describe the structure of vascular bundle
cambium (meristematic tissue)
phloem tissue
xylem tissue
describe the structure of phloem tissue
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.
describe the primary cell types in xylem tissue
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.
describe the additional cell types in xylem tissue
xylem parenchyma: packing tissue with thin walls transmit turgidity
sclereids
sclerenchyma fibres: heavily lignified to withstand negative pressure
describe the structure of carilage
avascular smooth elastic tissue made of chondrocytes, which produce extensive extra cellular matrix (ECM)
ECM mainly contains collagen and proteoglycan
3 categories: hyaline, yellow elastic, white fibrous (depends on ratio of cells : ECM)
name the 3 types of muscles in the body and where they are located
Cardiac: exclusively found in heart
smooth: walls of blood vessels and intestines
skeletal: attached to incompressible skeleton by tendons
Describe the gross structure of a skeletal muscle.
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 endomycium: loose connective tissue with many capillaries.