2.6 - Cell Division, Cell Diversity & Cell Differentiation
What is interphase?
The phase of the cell cycle where the cell isn't dividing: it is divided into Growing phase and Synthesis phase
What is differentiation?
The process by which stem cells become specialised into different types of cell
What are the 4 sources of stem cells?
- Embryonic stem cells - In umbilical-cord blood - Adult stem cells found in blood, brain & bone - Induced pluripotent stem cells (iPS cells)
What does the cell look like before mitosis in the M-phase begins? (3)
- 4 chromosomes - Chromosomes consist of 2 sister chromatids - Chromatids joined at the centre by a centromere
What does connective tissue consist of?
- A non-living, extracellular matrix containing proteins (collagen & elastin) and polysaccharides (such as hyaluronic acid which traps water) - Matrix separates living cells within tissue & enables it to withstand forces such as weight
Describe what happens during the S-phase of the cell cycle (4)
- All DNA molecules are replicated (first housekeeping genes, then the rest) - Committed to completing cell cycle - Duplicated chromosomes consists of an identical pair of sister chromatids - Rapid
Why is meiosis important? (3)
- Allows sexual reproduction (haploid gametes > diploid cells) - Sexual reproduction produces genetic variation - Increases chances of survival when environment changes
Why is mitosis important? (3)
- Asexual reproduction, e.g fungi reproduce asexually - Growth, e.g. multicellular organisms grow - Tissue repair, e.g. wounds are repaired by proliferation of skin cells which divide by mitosis
What are the benefits of using stem cells in medicine in terms of developmental biology? (2)
- Can study how cells develop and see what goes wrong when they're diseased - Can research if its able to extend the capacity embryos have for growth and tissue repair into later life
What are the characteristics of epithelial tissue? (4)
- Cells close to each other & form continuous sheets - No blood vessels, cells receive nutrients by diffusion - Smooth surfaces, but some have cilia or microvilli - Specialised to carry out protection/absorption
Describe what happens during the G2-phase of the cell cycle (3)
- Cells grow - Special chemicals ensure cell is ready for mitosis - Chemicals stimulate proteins that condense chromosomes and in formation of the spindle
Explain what happens during Anaphase 2 of meiosis (2)
- Centromeres divide & chromatids of each chromosome are pulled apart my motor proteins - Chromatids dragged towards opposite poles and are randomly segregated
Explain what happens during mitosis in terms of Anaphase? (3)
- Centromeres divide, separating sister chromatids - Spindles contract & motor proteins walking along tubulin thread pulls sister chromatids of each pair to opposite poles - Chromatids are now V-shaped chromosomes
Explain what happens during Prophase 1 of meiosis (3)
- Chromatin condenses & chromosome supercoils - Nuclear envelope breaks down & tubulin spindle threads form from centriole - Chromosomes come together in homologous pair & non sister chromatids cross over so alleles are shuffled
Explain what happens during mitosis in terms of Prophase (3)
- Chromosomes condense as DNA supercoils - Centriole divides and daughter centrioles move to opposite poles of cell where tubulin threads form spindle between 2 centrioles - Nuclear envelope breaks down
Explain what happens during mitosis in terms of Metaphase? (2)
- Chromosomes line up in middle and attach to spindle equator region by their centromeres - Metaphase checkpoint checks all chromosomes are attached to spindle
How are sperm cells (spermatozoa) specialised to carry out their function? (4)
- Contain many mitochondria (respiration = ATP for undulipodium to move and propel cell towards ovum) - Small, long & thin so can move easily - Enzymes released from acrosome which digest outer protective covering of ovum - Head of sperm contains haploid gamete nucleus
Explain what meristematic tissue is and where it is found (3)
- Contains stem cells - All other plant tissues are derived by differentiation of these stem cells - Found at root/shoot tips & cambium in vascular bundles
How does meiosis cause genetic variation? (3)
- Crossing over of sister chromatids in Prophase 1 shuffles alleles - Independent assortment of chromosomes randomly distributes maternal & paternal chromosomes of each pair - Independent assortment of chromatids in Anaphase 2
Explain how cytokinesis occurs in animal cells (3)
- Cytoplasm divides as cleavage furrow forms to divide cell membrane - 2 new genetically identical daughter cells - Usually begins in anaphase and ends in telophase
What happens to the chromosome of a cell before meiosis? (2)
- Duplicated in S-phase - Each chromosome consists of 2 sister chromatids
Explain how cytokinesis occurs in plant cells
- End plate forms where equator of spindle was - New plasma membrane and cellulose cell wall material are laid down either side of the end plate
What are the 4 main types of tissue the human body has?
- Epithelial tissue (lining) - Connective tissue (blood, bone & cartilage) - Muscle tissue (contract for movement) - Nervous tissue (cells conduct electrical impulses)
What is epidermal tissue? (2)
- Forms protective covering over leaves, stems & roots - Some may be impregnated with a waxy substance (waxy cuticle) which can reduce water loss
Describe what happens during the G1-phase of the cell cycle (4)
- G1 checkpoint ensures cell is ready to enter S-phase - Cells grow & genes are transcribed to make RNA - Organelle duplication and biosynthesis occurs - p53 gene helps control this phase
Where are the 4 checkpoints located in the cell cycle?
- G1/S phase (restriction point) - G2/M phase - Halfway through mitosis - Early G1 phase
What are the 4 features of cells in meristematic tissue?
- Have thin walls containing little cellulose - Do not have any chloroplasts - Do not have a large vacuole - Can divide by mitosis and differentiate into other types of cells
Explain what happens during Metaphase 1 of meiosis (2)
- Homologous chromosome pairs in crossed over state attach along equator of spindle by centromere - Pairs arranged randomly & members of each pair face opposite poles of the cell (independent assortment)
What are the 3 types of cartilage and where are they found?
- Hyaline (forms embryonic skeleton) - Fibrous (discs between vertebrae in spine) - Elastic (forms pinna and epiglottis)
What are the useful features which root hair cells have? (3)
- Increase surface area for absorption - Contain carrier proteins in plasma membranes to actively transport mineral ions - Will produce ATP needed for active transport
Why is the p53 gene important to the cell cycle? (2)
- It triggers 2 main check points for regulation of cycle - Known as "tumor-suppressor" gene
How are erythrocytes adapted for their function? (3)
- Large SA:VR due to biconcave shape so O2 can diffuse across their membranes into all regions of cells - Well developed cytoskeleton means they're flexible and can fit through narrow capillaries - Most organelles lost at differentiation = allows more Hb
What are 4 plant organs and their accompanying functions?
- Leaf (photosynthesis) - Root (anchorage in soil/absorption of water & MI) - Stem (support, hold leaves upright so exposed to more sunlight, storage of photosynthesis products) - Flower (sexual reproduction)
How do guard cells open the stomata? (3)
- Light energy used to produce ATP - ATP actively transports Na+ ions from surrounding epidermal cells to guard cells, lowering water potential - Guard cells swell, tips of cellulose cell wall is flexible & rigid where it's thicker, so the tips bulge and the gap between them (stoma) enlarge
How are palisade cells within leaves adapted for photosynthesis? (3)
- Long & cylindrical so pack together closely with little space between them for air to circulate so CO2 in these spaces diffuses into cells - Large vacuole so chloroplasts positioned near periphery of cell, reducing diffusion distance of CO2 - Cytoskeleton threads and motor proteins to move chloroplasts
Explain what happens during Anaphase 1 of meiosis (3)
- Members of each pair of homologous chromosomes pulled apart by motor proteins - dragged along thread - Centromeres don't divide & each chromosome consists of 2 chromatids - Crossed over areas separate from each other = allele shuffling
Describe what happens in the M-phase of the cell cycle (3)
- Metaphase checkpoint ensures cell is ready for mitosis - Checkpoint chemical triggers condensation of chromatin - Cell growth stops & mitosis and cytokinesis occur
Explain what happens during Prophase 2 of meiosis (3)
- Nuclear envelopes break down & chromosomes coil and condense - Chromatids of each chromosome are no longer identical due to crossing over in Prophase 1 - Spindles form
Explain what happens during Telophase 2 of meiosis (2)
- Nuclear envelopes form around 4 haploid nuclei - 2 cells now divide to give 4 haploid cells
Explain what happens during Telophase 1 of meiosis (3)
- Nuclear envelopes form around each set of chromosomes & cell divides by cytokinesis - Short interphase when chromosomes uncoil - New nucleus = half original number of chromosomes, but each chromosome consists of 2 chromatids
What are the purposes of having checkpoints in the cell cycle? (4)
- Prevent uncontrolled division that could lead to tumors - Detect and repair damage to DNA - Ensure cell cycle cannot be reversed - Ensure DNA is only duplicated once in each cell cycle
What 3 things does the differentiation of embryonic stem cells lead to?
- Proportions of different organelles differs from those of other cells - Shape of cell changes - Some of the contents of the cell change
Describe what happens in the G0-phase of the cell cycle
- Resting phase during early G1 triggered by checkpoint chemical at restriction point - Cells can undergo apoptosis, differentiation or senescence
Explain what happens during mitosis in terms of Telophase? (3)
- Separated chromosomes reach poles - Nuclear envelope forms around each set of chromosomes - Cell now contains 2 new genetically identical nuclei, which are identical to the parent cell from which they arose
What are 2 useful features that epithelial cells have?
- Squamous epithelial cells (flattened) - Many epithelial cells have cilia
What are the benefits of using stem cells in medicine in terms of repair of damaged/lost tissues? (3)
- Stem cells have been used to treat diabetic mice by differentiating into pancreatic beta cells - Bone marrow stem cells have been developed into hepatocytes to treat liver disease - Stem cells can be directed to become nerve tissue to treat Parkinsons and Alzheimers disease
What are the benefits of using stem cells in medicine in terms of drug research? (2)
- Stem cells made into types of human tissue - New drugs can be tested on these tissues, rather than animals
What are the 4 features of stem cells?
- Undifferentiated - Pluripotent - Can express all their genes - Can divide by mitosis to provide more stem cells
What are the benefits of using stem cells in medicine in terms of bone marrow transplants? (2)
- Used in bone marrow transplants to treat blood diseases (e.g sickle cell anaemia and leukaemia) - Can restore patients blood system after treatment for specific types of cancer
What does vascular tissue in plants consist of? (2)
- Xylem vessels (water and mineral ions from roots to all parts of the plant) - Phloem sieve tubes (transfer of products of photosynthesis from leaves to parts of the plant that don't photosynthesise)
Explain how differentiation leads to an embryo (4)
- Zygote produced by fertilisation of ovum by spermatozoon - 2 haploid nuclei fuse to give diploid nucleus - Zygote is unspecialised and its genome is free to be expressed - After several mitotic divisions, an embryo is formed
What is a haploid cell?
A cell that has only one set of chromosomes, represented by the letter "n"
What is an organ?
A collection of tissues working together to perform a function/related functions
Define the term "tissue"
A tissue is a group of cells that work together to perform a specific function/set of functions
What is mitosis?
A type of nuclear division that produces daughter cells genetically identical to each other and to the parent cell
What is meiosis?
A type of nuclear division that results in the formation of cells containing half the number of chromosomes of the parent cell
What is the meristem?
An area of unspecialised cells within a plant that can divide & differentiate into other cell types
What is a stem cell?
An unspecialised cell that is able to express all of its genes and divide by mitosis
Explain the 3 processes cells can undergo during the G0-phase of the cell cycle
Apoptosis - programmed cell death Differentiation - cells differentiate Senescence - loss of cell's power of division and growth
Why are the chances of spontaneous mutations happening reduced in the S-phase of the cell cycle?
Because exposed DNA base pairs are more susceptible to mutagenic agents
What are induced pluripotent stem cells (iPS)?
Cells that are developed in a lab by reprogramming differentiated cells to switch on certain key genes to become undifferentiated
What are epithelial cells?
Cells that constitute lining tissue
What causes embryonic stem cells to differentiate?
Certain genes are switched off and other genes may be expressed
Explain what happens during Metaphase 2 of meiosis
Chromosomes attach by centromere to equator of spindle & chromatids are randomly arranged
What is cytokinesis?
Cytoplasmic division following nuclear division, resulting in 2 new daughter cells
What organs do gametes come from?
Gonads (ovaries and testes)
Explain the roles of cells within cartilage (2)
Immature cells = chondroblasts - divide by mitosis and secrete the extracellular matrix - After matrix has been synthesised, chondroblasts become mature, less active chondrocytes which maintain the matrix
What are homologous chromosomes?
Matching chromosomes containing the same genes at the same loci. They may contain different alleles for some of the genes
What is the difference between microvilli and cilia?
Microvilli = extensions of plasma membrane to increase surface area Cilia = hair-like organelles which beat and propel substances along epithelial surfaces
Why is specialisation and differentiation important?
Multicellular organisms are larger and have a small SA:VR, so need specialised cells to carry out particular functions as most cells aren't in direct contact with environment
What are myofilaments?
Organelles made from the proteins actin and myosin which allow muscle tissue to contract
What are chromatids?
Replicates of chromosomes
What are 3 functions of muscle?
Skeletal Muscles = joins bones to tendons, causes bones to move then they contract Cardiac Muscle = allows heart to beat and pump blood Smooth Muscle = propels substances along intestinal and urinary tracts
What is the collective term for sperm cells?
Spermatozoa
Where have erythrocytes and neutrophils derived from?
Stem cells in the bone marrow
What is regenerative medicine?
Using stem cells to populate a bioscaffold of an organ & then directing them to develop and grow into specific organs for transplanting