2.1.6 - Cell Division, Cell Diversity and Cellular Organisation Flashcards
What are the 3 key stages of the cell cycle?
- Interphase (G1, S, G2)
- Nuclear division (mitosis/meiosis)
- Cytokinesis
What happens during G1?
- Protein synthesis occurs to make proteins involved in synthesising organelles
- Organelles replicate
What happens during S?
DNA is replicated resulting in a doubling of the mass of DNA in the cell
What happens during G2?
- Cell continues to grow
- Energy stores increase
What happens during mitosis?
Nucleus divides into 2
What happens during cytokinesis?
Cytoplasm splits into 2 genetically identical daughter cells
How does cytokinesis occur in animals?
A cleavage furrow forms in the middle of the cell and the cytoskeleton causes the cell membrane to draw inwards until the cell splits into two.
How does cytokinesis occur in plants?
The cell membrane splits into two new cells due to the fusing of vesicles from the Golgi apparatus. The cell wall forms new sections around the membrane to complete the division into two cells.
How is the cell cycle regulated?
Checkpoints are made throughout the cell cycle to ensure that each stage is fully completed before progressing to the next one.
What do the different checkpoints check for?
- G1 checkpoint checks for cell size, nutrients, growth factors and DNA damage
- G2 checkpoint checks for cell size, DNA replication and DNA damage
- Metaphase checkpoint checks for chromosome attachment to spindle
What does mitosis result in?
Two identical diploid cells
What are the 4 key stages of mitosis?
- Prophase
- Metaphase
- Anaphase
- Telophase
What happens during prophase?
- Chromosomes shorten and thicken by supercoiling and become visible under a microscope when stained
- Nuclear envelope disappears
- Centrioles move to the poles of the cell producing a network of spindle fibres between them
What happens during metaphase?
- Chromosomes move to the equator of the cell
- Each chromosome becomes attached to a spindle fibre by its centromere
What happens during anaphase?
- Spindle fibres contract which separates the sister chromatids
- Spindle fibres pull the chromatids towards opposite poles of the cell centromere first
What happens during telophase?
- As the two sets of chromosomes reach each of the cell poles a nuclear envelope forms around each one to form two new nuclei
- Chromosomes start to uncoil
- Spindle fibres break down and disappear
- Nuclear membrane reforms
How could mitosis be observed under a light microscope?
- Take a thin slice of the root tip of an onion or garlic and place it on a microscope slide
- Break down the slice down with a needle
- Add a stain to make the chromosomes visible
- Push the cover slip down to squash the tip so there is only a single layer of cells and light can pass through
What is mitosis needed for?
- Growth
- Tissue repair
- Asexual reproduction
What does meiosis result in?
Four genetically different haploid daughter cells
How are the genetic differences introduced in meiosis?
- Independent assortment of homologous chromosomes
- Crossing over
What is crossing over?
- During prophase 1 the homologous chromosomes pair to form bivalents
- Crossing over genetic material can occur between the non sister chromatids of bivalents at locations called chiasmata
- Breaks can occur in the genetic material where the chromatids cross over and part of the chromatids are exchanged between the homologous pairs
- This results in new combinations of alleles in the resulting gamete
What is independent assortment?
- During metaphase 1 the homologous pairs of chromosomes line up opposite each other on either side of the equator
- The side of the equator that the paternal and maternal chromosome of each aligns on is random
- During metaphase 2 there is also random distribution and segregation of the sister chromatids
- As a result each gamete receives different combinations of the maternal and paternal chromosome
What does the term homologous chromosomes mean?
A pair of chromosomes that have the same genes at the same loci
What happens during prophase 1?
- Chromatids condense
- Homologous chromosomes form bivalents
- Crossing over occurs
What happens during metaphase 1?
- Bivalents line up at the equator
- Independent assortment occurs
What happens during anaphase 1?
- Spindle fibres pull homologous chromosomes to opposite poles of cell
What happens during telophase 1?
- Nuclear envelope form around new nuclei
What happens during prophase 2?
- Chromosomes condense
- Spindle reforms
- Nuclear envelope breaks down again
What happens during metaphase 2?
- Chromosomes randomly arrange themselves on spindle fibres at equator by centromeres
- Independent assortment occurs
What happens during anaphase 2?
- Chromatids are pulled apart by contracting spindle fibres to poles of cell
What happens during telophase 2?
- Nuclear envelope forms around new haploid nuclei
How are erythrocytes specialised for their function?
- Contain haemoglobin to transport oxygen from lungs to body tissues
- Biconcave shape to increase surface area for diffusion and to increase cell flexibility to fit through narrow capillaries
- Have no nucleus so there is more space for haemoglobin
How are neutrophils specialised for their function?
- Cells are flexible so they can surround pathogens and engulf them
- Contain lysosomes filled with the hydrolytic enzyme lysozyme
- Have a lobed nucleus and granular cytoplasm
How are squamous epithelial cells specialised for their function?
- Flat thin shape to facilitate diffusion of materials across it
- Only a single layer of cells which provides a short diffusion distance
How are ciliated epithelial cells specialised for their function?
- Have tiny extensions called cilia to move mucus across mucous membranes or ova along the fallopian tubes
- Contain goblet cells which release mucus to trap molecules
How are sperm cells specialised for their function?
- Have flagellum to swim to ovum
- Contain many mitochondria to release energy from ATP for locomotion towards egg cell
- Acrosome in head of the cell contains digestive enzymes to digest the wall of the egg cell so the sperm can penetrate and fertilise the egg cell
How are palisade cells specialised for their function?
- Long and thin so that the many chloroplasts can absorb maximum sunlight for photosynthesis
- Have thin cell walls to reduce the diffusion distance of carbon dioxide
How are root hair cells specialised for their function?
- Long hair like projection maximises surface area in contact with soil for uptake of water and mineral ions
How are guard cells specialised for their function?
- Have flexible walls more so on one side which results in the cells bending when turgid to open stomata and closing when flaccid which helps to control water loss by transpiration
How are multicellular organisms organised?
Cells - Tissue - Organ - Organ system - Organism
What is a tissue?
A collection of similar cells working together with a common function. Examples are cartilage, muscle, xylem, phloem, squamous epithelia and ciliated epithelia.
What is an organ?
A collection of tissues working together to perform a particular function
What are stem cells?
Undifferentiated cells that can self renew and become specialised
What are the 4 types of stem cells?
- Totipotent
- Pluripotent
- Multipotent
- Unipotent
What are totipotent stem cells?
- Occur only for a limited time in early mammalian embryos
- Can differentiate to produce any type of body cell including placental cells
What are pluripotent stem cells?
- Found in embryos
- Can differentiate into all tissue types except placental cells
What are multipotent stem cells?
- Found in many tissues at any post embryonic life stage
- Can differentiate to form a limited number of different cell types
What are unipotent stem cells?
- Found in many tissues at any post embryonic life stage
- Can only differentiate into one type of cell
How are erythrocytes formed?
Stem cells in bone marrow differentiate into erythrocytes by losing their nucleus, RER, mitochondria and Golgi apparatus. They become packed with haemoglobin and form a biconcave disc shape.
How are neutrophils formed?
Stem cells in bone marrow differentiate into neutrophils by retaining their nucleus which becomes lobed and their cytoplasm fills with lysosomes.
How are xylem vessels formed?
Meristem cells differentiate into xylem vessels by elongating and depositing lignin in the cell walls to strengthen and waterproof them. The cell wall dies and the end cell walls break down to form long continuous tubes.
How are phloem vessels formed?
Meristem cells differentiate into phloem vessels by forming companion cells and sieve tube elements.
What are the potential uses of stem cells in research and medicine?
- Repair of damaged tissues such as breaks in spinal cords causing paralysis
- Treatment of neurological conditions such as Alzheimer’s and Parkinson’s
- Research into developmental biology to help scientists understand how a fertilised egg develops into a multicellular organism and how and why the process can go wrong
