Module 2.6 Cell Division, Cell Diversity and Cell Differentiation Flashcards
Purpose of checkpoints in the cell cycle
To prevent uncontrolled division
To detect and repair damaged DNA
M phase
A checkpoint chemical triggers condensation of chromatin
1/2way through the cycle, the metaphase checkpoint ensures that the cell is ready to complete mitosis
Events within cell during M phase
Cell growth stops
Nuclear division = PMAT
Cytokinesis
Gap 0 phase
A resting phase triggered during early G1 at the restriction point by a checkpoint chemical
Some cells e.g. epithelial cells in the gut don’t have this phase
Events within cell during Gap 0 phase
Cells may undergo apoptosis, differentiation or senescence
Some cells e.g. neurones remain in this phase almost indefinitely
Gap 1 phase
A G1 checkpoint control mechanism ensures the cell is ready to enter S phase and begin DNA synthesis
Events within the cell during Gap 1 phase
Cells grow
Cells inc. in size
Transcription occurs
Organelles duplicate
Biosynthesis e.g protein synthesis
Enzymes made which are needed for DNA replication in the S phase
P53 helps control this phase
S phase of interphase
Every molecule of DNA replicated (chromosomes unwound, DNA is diffuse)
Specific sequence for replication (housekeeping first and normally inactive genes replicated last)
Events within the cell during S phase
Now committed to cell cycle
DNA replicates
Chromosomes consist of identical sister chromatids
Rapid phase to reduce chance of spontaneous mutations happening
G2 phase of interphase
Special chemicals ensure the cell is ready for Mitosis by stimulating proteins that will be involved in making chromosomes condense and in the formation of spindle
Events within the cell during G2 phase
The cell grows
What is Mitosis used for?
Asexual reproduction
Growth
Tissue repair
Prophase during Mitosis
Chromosomes (consisting of 2 identical sister chromatids) shorten and thicken - DNA supercoils
Nuclear envelope breaks down
Centriole in animal cells breaks down - 2 new daughter centrioles move to opposite poles
Spindle forms
Metaphase during Mitosis
The pairs of chromatids attach to the spindle threads - attach by their centromeres
Chromosomes @equator
Anaphase during Mitosis
Centromere of each pair of chromatids splits
Motor proteins along the tublin threads pull each sister chromatid in a pair in opposite directions
Chromosomes assume a V shape
Telophase during Mitosis
Separated chromosomes reach the poles
New nuclear envelopes form around the sets of chromosomes
Cells contain 2 nuclei each
Genetically identical to each other and the parent
Cytokinesis in animals
The plasma membrane folds inwards and “nips in” the cytoplasm
Cytokinesis in plants
An end plate forms where the equator of the spindle was, and new plasma membrane and cellulose cell wall material are laid down on either side along this end plate
Meiosis produces
Haploid gametes
Prophase 1 of Meiosis
Chromatin condensed
Each chromosome supercoils
Nuclear envelope breaks down
Spindle threads of tubulin protein form from the centrioles (in animal cells)
Chromosomes in homologous pairs
Crossing over - alleles shuffled
Metaphase 1 in Meiosis
Crossing over
Homologous chromosomes attach to the spindle threads by the centromere
Independent assortment
Anaphase 1 in Meiosis
Homologous chromosomes pulled apart by motor proteins that drag them along the tubulin threads of spindle
Centromeres DON’T divide
Each chromosome consists of 2 chromatids
Alleles shuffled
Telophase 1 in Meiosis
In most animal cells:
2 new nuclear envelopes form around each set of chromosomes then cytokinesis and a short interphase whilst the chromosomes uncoil
In most plant cells:
The cell goes straight from anaphase 1 to prophase 2
Prophase 2 in Meiosis
Nuclear envelopes break down
Chromosomes coil and condense
Each chromosome consists of 2 chromatids
Chromatids NOT identical
Spindle forms
Metaphase 2 of Meiosis
Chromosomes attach by their centromere to the spindle
Chromatids randomly arranged
Anaphase 2 of Meiosis
Centromeres divide
Chromatids pulled apart by motor proteins that drag them along the tubulin threads of spindle to opposite poles
Chromatids randomly segregated
Telophase 2 of Meiosis
Nuclear envelopes form around each of the 4 haploid nuclei
In animals, 2 cells divide to give 4 haploid cells
In plants, a tetrad of 4 haploid cells is formed
How Meiosis produces genetic variation
Crossing over during prophase 1 shuffles alleles
Independent assortment of chromatids in anaphase 1 - random distribution of maternal and paternal chromosomes
Independent assortment of chromatids in anaphase 2
Haploid gametes produced - can undergo random fusion with gametes derived from another organism of the same species
Why multicellular organisms need specialised cells
Large
Smaller SA/V ratio
Most cells not in contact with external environment
Needs specialised cells to carry out particular functions
Genome=
Genetic material within an individual
Gene pool=
All the genetic material within a population
🌟Erythrocytes (RBCs) adaptations
No nucleus and most organelles absent - max. space for haemoglobin to inc. oxygen carrying capacity
V small
Large SA/V ratio - O2 can diffuse easily
Biconcave disc shape - inc. SA/V ratio for oxygen exchange = more efficient uptake of oxygen into RBCs
No nucleus and organelles = flexible - can travel through v narrow capillaries
Filled w haemoglobin - can bind to oxygen to form oxyhaemoglobin to transport it to aerobically respiring cells
🌟Neutrophils (phagocytes) adaptations
Travel towards infected sites by chemotaxis
Ingest bacteria and some fungi by phagocytosis
Contain a lot of lysosomes containing lysin enzymes to digest pathogens
Multi-lobed nucleus - can fit between gaps in endothelial cells of capillaries to leave blood
Contain many mitochondria - to move lysosomes and phagosomes through the cell along microtubules
🌟Spermatozoa adaptations
Many mitochondria - aerobic respiration, ATP so flagella/undulipodium can move
Haploid nucleus - when it fertilises the egg the zygote will be diploid
Small, long and thin - can move easily
Acrosome - enzymes digest the outer protective covering of the ovum so the spermatozoon can penetrate the egg so it can fertilise it
Epithelial cells adaptations
Flattened - short diffusion distance
May have cilia - can waft substances
Palisade cells adaptations
Adapted for photosynthesis because:
Long, cylindrical, closely packed but small air spaces for CO2 to diffuse into cells
Large vacuole - chloroplasts nearer the periphery of the cells, reduces diffusion distance for CO2
Many chloroplasts- for photosynthesis
Contain cytoskeleton threads and motor proteins to move chloroplasts (up in low sunlight, down in high sunlight)
Guard cells: How they work
Light energy used to produce ATP
ATP actively transports potassium ions from surrounding epidermal cells into the guard cells
Water potential lowered
Water enters guard cells from neighbouring epidermal cells via osmosis
Guard cells swell
Tips of cellulose cell wall - more flexible
Thicker = more rigid
Tips bulge, stoma enlarges
Gas exchange can occur
CO2 diffuses in for photosynthesis
Steep concentration gradient maintained
O2 can diffuse out
🌟Root hair cells adaptations
Hair like projections - inc. surface area/larger surface area for osmosis and mineral uptake (active transport) into the roots
Mineral ions actively transported in - water potential lowered, water moves in via osmosis down the water pot. grad.
Many carrier proteins in the membrane - for active transport of mineral ions
Thin wall = short diffusion path
Many mitochondria = energy for active transport of minerals
Many channel proteins - for water uptake via osmosis
Xylem vessel adaptations
Continuous hollow tubes (no contents) - less resistance to water flow, more space
Walls impregnated with lignin - strengthens walls (prevents collapse), waterproofs wall (reduces lateral movement of water), Increases adhesion - increases capillarity
Spiral pattern of lignin - flexibility
Bordered pits - allows lateral movement of water to get around a blockage
Narrow lumen - more effective capillary action
Phloem adaptations
Small cytoplasm + most organelle absent - less resistance, more space
Sieve plates - allows sucrose through
Joined end to end to form tube - continuous transport
Bi-directional flow - sucrose can go up and down
Living - active processes can take place
Companion cells adaptations
Lots of mitochondria - lots of respiration, allows active processes to occur e.g. active loading of sucrose into sieve tubes
Nucleus - controls companion cell and sieve tube element
Plasmodesmata - allows continuation of cytoplasm between companion cell and sieve tube element
4 main tissue types
Epithelial
Connective
Muscle
Nervous
Meristematic tissue
Thin walls - little cellulose
No chloroplasts
No large vacuole
Can undergo Mitosis to differentiate
How cambium cells differentiate into xylem vessels
Lignification
Ends of cells break down
Continuous column formed
How cambium cells differentiate into sieve tubes or companion cells
Sieve tubes: most organelles lost ,sieve plates develop
Companion cells: retain organelles, continue metabolic processes to provide ATP for active loading of assimilates into the sieve tubes
Stem cells
Undifferentiated
Pluripotent
Can express all their genes
Can divide by Mitosis to provide more cells that can differentiate into specialised cells for growth and repair
Sources of stem cells
Embryonic stem cells
In umbilical cord blood
Adult stem cells in infants and children e.g in the blood, brain, muscle, bone, adipose tissue and skin
Induced pluripotent stem cells (iPS cells) - developed in labs
Potential uses of stem cells
Bone marrow transplants
Cancer treatment
Drug research
Developmental biology
Potential to repairing damaged tissues/replace lost tissues E.g. Alzheimer’s, Parkinsons, diabetes, burns, hearing loss, arthritis
🌟Organisation of cells in multicellular organisms
Cells differentiate
Groups of similar specialised cells working together to perform a common function = tissues
Groups of tissues working together = organs
Groups of organs working together = organ systems
🌟The cell cycle
Interphase:
- G1, S and G2 phases
- G1 - cell grows, respires, proteins made, organelles replicated
- S - DNA replication occurs, chromosomes become sister chromatids joined by a centromere
- G2 - DNA replication is checked for mistakes, organelles replicated
Mitosis:
- Prophase - sister chromatids supercoil, nuclear envelope breaks down, spindle fibres form
- Metaphase - sister chromatids line up on the equator, spindle fibres attach to centromere
- Anaphase - spindle fibres shorten + pull sister chromatids apart towards opposite poles
- Telophase - chromosomes uncoil, nuclear envelope reforms
Cytokinesis:
- Cytoplasm cleaves down furrow to split cytoplasm
- 2 genetically identical daughter cells produces (identical to each other and the parent cell)
🌟Mitosis
Prophase:
- Sister chromatids supercoil and shorten and thicken
- Sister chromatids consist of sister chromatids joined by a centromere
- Now visible under a light microscope
- Nuclear envelope breaks down
- Centriole divides in 2 - each daughter centriole goes to a pole
- Spindle fibres (microtubules) begin to form
Metaphase:
- Sister chromatids line up along the equator
- Spindle fibres attach to the centromeres
Anaphase:
- Centromere splits
- Chromatids separate
- Spindle fibres shorten
- Pull identical chromatids to opposite poles w centromeres leading
Telophase:
- Chromosomes uncoil
- Nuclear envelope reforms
- Spindle fibres break down
🌟Mitosis compared w meiosis
- Mitosis produces 2 genetically identical diploid daughter cells used for growth and repair. Occurs in all body cells and involves only one division
- Meiosis produces 4 genetically different haploid daughter cells and produces gametes. Only occurs in the ovaries and testes and involves two divisions
🌟Cell division and budding of yeast cells
- Nuclei divide by mitosis
- Bulge in surface of the cell
- Nucleus moves into bulge
- Bulge pinches off
- Uneven cytoplasm distribution between the two cells
