2.1.6 - Cell Division Flashcards
The Cell Cycle
The cell cycle is a sequence of events that takes place in a cell, resulting in division of the nucleus and the formation of two genetically identical daughter cells
Stages of cell cycle
Interphase : - G1 - S - G2 Mitotic phase - Mitosis - Cytokinesis
Interphase
- DNA replicated and checked for errors
- Protein synthesis
- Organelles grow and divide e.g. Mitochondria
- Normal metabolic processes occur e.g. Respiration
G1
- First growth phase
- Protein synthesis
- Organelles are synthesised
- Cell increases in size
S
- Synthesis phase
- DNA replicated
G2
- Second growth phase
- Energy stores are increased
- DNA checked for errors
G0
- G0 is when a cell leaves the cell cycle
Reasons for going to G0 - Differentiation - To become specialised
- Damaged DNA - It can no longer replicate (senescent)
- Age - The older you are the more senescent cells you have (linked with cancer and arthritis)
G1 checkpoint
- Cell size
- Nutrients
- Growth factors
- DNA damage
Metaphase checkpoint
- Chromosomes aligned
- Chromosomes attached to spindles
G2 checkpoint
- Cell size
- DNA replication
- DNA damage
Checkpoints
If the conditions are not satisfied in each checkpoint the cell enters G0
Cancer
- Caused by a mutation of genes that regulate the cell cycle
- Uncontrolled cell growth
- Forms a tumour
Stages of mitosis
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
- Chromosomes condense and coil, so they shorten and thicken
- Chromosomes become visible
- Nuclear envelope and nucleolus disappear
- Centrioles move to opposite poles of the cell
Early prophase
- Chromosomes become visible
Late prophase
- Nuclear envelope and nucleolus disappear
- Centrioles migrate
Metaphase
- Chromosomes align at equator/ metaphase plate
- Attach to spindle by centromere
Anaphase
- Spindle fibres contract and shorten
- Chromatids separate and move to opposite poles
Telophase
- Nuclear envelope and nucleolus reform
- Chromosomes uncoil and become long and thin
Cytokinesis (Animal cells)
Cleavage furrow forms in the middle of the cell. The cell surface membrane is pulled inwards by the cytoplasm until it’s close enough to fuse around the middle, forming two cells.
Cytokinesis (Plant cells)
- Cell wall prevents cleavage
- Vesicles from the Golgi apparatus line up along the metaphase plate. Vesicles fuse with each other and the cell surface membrane dividing the cell into two.
Importance of mitosis
- Growth and repair
- Replace cells
- Asexual reproduction
- Maintain chromosome number in all cells
Homologous chromosomes
Matching pair of chromosomes, one inherited from each parent
Allele
Different versions of the same gene
Locus
The position of a gene on a chromosome
Gamete
Sex cell
Haploid
Half the number of chromosomes
Diploid
Normal chromosome number
Two of each chromosome
46 chromosomes
Significance of meiosis
- Produces haploid cells
- Creates genetic variation through independent assortment and crossing over
Independent assortment
- Metaphase 1
- Bivalents line up on equator independent of each other. Oriented randomly
- Metaphase 2
- Independent assortment of chromatids
Crossing over
- Prophase 1
- Homologous chromosomes pair up bivalents form
- Crossing over : Non-sister chromatids entangle
- Alleles are exchanged
Suggests the importance of the creation of different allele combinations in populations
Creating different allele pairs during meiosis is an important source of genetic variation. Genetic variation is important for the process of natural selection, giving individuals in a population characteristics that might be advantageous in changing environments. If there was no genetic variation the entire population would be vulnerable to an external factor
Meiosis
- Meiosis 1 : The first division is the reduction division when the pair of homologous chromosomes separate into two cells
- Meiosis 2 : The pairs of chromatids are separated, forming two more cells. Four haploid cells are produced
Prophase 1
- Chromosomes condense, nuclear envelope and nucleolus disappear. Homologous chromosomes pair up forming bivalents.
- Chromatids entangle (crossing over)
Metaphase 1
- Homologous chromosomes assemble along the metaphase plate
- The orientation of each chromosome is random and independent( independent assortment)
Anaphase 1
- Homologous chromosomes pulled to opposite poles
- Sections of DNA which become entangled during crossing over, break off and rejoin. The point at which the chromatids break and rejoin are called chiasmata
- When exchange occurs this forms recombinant chromatids
Telophase 1
- Nuclear membrane reforms
- Chromosomes uncoil
- Cell undergoes cytokines and divides into two cells
Prophase 2
- Chromosomes condense and become visible
- Nuclear envelope breaks down and spindle formation begins
Metaphase 2
- Individual chromosomes assemble on the metaphase plate.
- There is independent assortment again
Anaphase 2
- Chromatids are pulled to opposite poles
Telophase 2
- Chromosomes uncoil and form chromatin again
- Nuclear envelope and nucleolus reform
- Cytokinesis
- Four haploid cells
Why is reduction division necessary in the production of gametes?
- Gametes are sex cells and two sex cells must combine to produce a diploid offspring
- Therefore gametes must contain only half the number of chromosomes otherwise with each new generation the number of chromosomes would increase
Erythrocytes
- Flattened biconcave shape : Increases the SA:V ratio
- Don’t have a nuclei : Increases space available for haemoglobin
- Flexible : Squeeze through narrow capillaries
Neutrophils
- White blood cells
- Multi-lobed nucleus : Makes it easier for them to squeeze through small gaps
- Lysosomes : Cytoplasm contains many lysosomes to attack pathogens
- Flexible : Can engulf pathogens
Squamous epithelial cells
- Along the respiratory tract
- Flat and thin : Rapid diffusion
Ciliates epithelium
- Hair-like structures(cilia) : Cilia on the surface move in a rhythmic manner. Moves mucus away from the lungs
- Goblet cells: Release mucus to trap any unwanted particles
Sperm cells
- Male gametes
- Flagellum : Move to the ovum
- Many mitochondria : Supply energy to swim
- Acrosome : Contains digestive enzymes, which are released to digest the protective layers around the ovum
Palisade cells
- Chloroplast: Absorb large amount of light for photosynthesis
- Rectangular box shapes : Closely packed to form a continuous layer
- Thin cell walls : Increases rate of diffusion
- Large vacuole : Maintains turgor pressure
Root hair cells
- Long extensions(root hairs) : Increases the surface area and maximises the uptake of water and minerals
Guard cells
- Pairs of guard cells on the surface of leaves form the stomata.
- Guard cells lose water and become less swollen, they change shape and the stoma closes to prevent further water loss
- Cell wall thicker on one side, so the cell does not change shape symmetrically
Specialised cell
- Produced by differentiation
- Differentiation is when a cell becomes specialised to perform a particular function
Tissues
- A collection of differentiated cells that have a specialised function
Organ
Collection of tissues that are adapted to perform a particular function
Organ systems
Organs working together to carry out a major function
Cartilage
- Connective tissue
- Protects and strengthens
- Chondrocytes produce an extra cellular matrix of collagen fibres(strength) and elastin fibres (flexibility)
- Prevents ends of bonds from rubbing together and causing damage
Muscle
- Muscle is a tissue that needs to be able to shorten in length in order to move bones.
- 3 types of muscle :
- > Skeletal(movement)
- > Smooth (involuntary e.g. stomach lining)
- > Cardiac (heart)
- Made up of bundles of elongated cells called muscle fibres. A bundle of muscle fibres is called a fascicle.
Xylem
- Transports water and minerals up the stem
- Xylem cells are dead and have no cytoplasm
- Parenchyma cells fill gaps between the other cells
- Lignin for support
- Vessel elements, which are elongated dead cells. Transports the water, has a wide lumen.
Phloem
- Transports organic nutrients up and down the plants
- Composed of sieve tube cells separated by perforated walls called sieve plates.
- Sieve tube cells : Reduced cytoplasm, few organelles, end walls form sieve plates
- Companion cells - help the sieve cells with their functions using ‘plasmodesmata’(these allow molecules to pass between cells).
Stem cells
Undifferentiated cells
Potency
A stem cell’s ability to differentiate into different cell types is called potency
Totipotent
- These stem cells can differentiate into any type of cell
- E.g. Fertilised egg
Pluripotent
- These stem cells can form all tissue types but not whole organisms.
Multipotent
- Can only form a range of cells within a certain type of tissue.
E.g. Haematopoetic stem cells
