2:1:6 Cell Division, Cell Diversity and Cellular Organisation Flashcards
What is the cell cycle
The regulated sequence of events that occurs between one cell division and the next, in three phases: interphase, mitosis, cytokinesis
What happens during interphase (mitosis)
The cell increases in mass and size whilst undergoing normal cellular functions to prepare for mitosis in three stages: G1, S, G2
Outline the the 3 stages in interphase (mitosis)
- G1 - the first growth phase, where protein synthesis and respiration occur
- S - the synthesis phase, where DNA is replicated in the nucleus
- G2 - the second growth phase, where the cell continues to increase in size
What happens during mitotic division (mitosis)
The mitotic phase (M) follows interphase, and cell growth stops and mitosis occurs
What is G0 phase in the cell cycle
An inactive stage that occurs when cells exit (fully specialised and finished dividing) or when it enters (before G1 is triggered)
What is cytokinesis
Follows M phase, phase where the whole cell divides to create two genetically identical daughter cells
What are sister chromatids
The identical DNA molecules in the replicated chromosome
How are chromosomes packaged
DNA is wrapped around histones (chromatin) which in order to replicate comes together compactly (euchromatin) before cell division
What is the centromere
Centre point of chromosomes which holds the sister chromatids together, and where the spindles attatch
What are telomeres
Areas of repetitive DNA at the ends of a chromosome which protest it and preserve genetic information. Each time the cell divides they shorten until the cell dies
Why is the cell cycle regulated
DNA must be accurate to undergo its function, and during S phase errors can occur. Proof reading and repair enzymes with find the cells and repair them, or apoptosis occurs
What are the 4 checkpoints in the cell cycle
- G1 - checks cell has enough nutrients, is growing properly, and DNA isn’t damaged before advancing to S phase
- S - checks for successful replication of chromosomes
- G2 - checks the cell is big enough to divide and the DNA has been replicated correctly
- Metaphase - checks for chromosome spindle attachment before anaphase
What are cyclins
Proteins that controls movement in stages of the cell cycle by increasing in concentration to trigger a specific process in the cell
How do cyclins work
- Cyclins bind to cyclic dependant kinase enzymes (CDK) and activates the kinases
- The kinases attach phosphate groups to specific process proteins in the cell (phosphorylation) activating the proteins
- The CDK targets the activated protein and the cyclin is destroyed
Explain this graph
Cyclin D - increase causes G0 to G1, and G1 to S
Cyclin E - increase causes entry to S
Cyclin A - activates DNA replication
Cyclin B - promotes formation of mitotic spindle
What is mitosis
The process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus: 1 diploid to 2 diploid cells
How many chromosomes in a human diploid cell
46
What are the stages of mitosis in order
Prophase
Metaphase
Anaphase
Telophase
What are autosomes
The non-sex chromosomes
What is the female sex chromosome
XX
What is the male sexchromosome
XY
What is a homologous pair of chromosomes
Pairs of chromosomes replicated from the paternal and maternal chromatids
What happens in prophase (mitosis)
- Chromatin condense by super coiling into chromosomes
- Centrosomes move towards opposite poles
- Microtubule organising centre starts to form spindles at centrosomes
- Nuclear envelope disappears
- Nucleolus breaks down
What happens in metaphase (mitosis)
- Chromosomes line up on the equator
- Centrioles arrange spindles at poles
- Spindle fibres attach to centromeres of chromosomes involving kinetochore proteins
- Spindle fibres shorten to ensure everything is aligned and attached
What happens in anaphase (mitosis)
- Centromeres split in two
- Spindle fibres shorten and pull chromatids to opposite poles
What happens in telophase (mitosis)
- Chromosomes arrive at opposite poles and start to decondense
- Nuclear envelopes reform around the sets of chromatin
- Sprindle fibres break down
- New nucleoli form within the nucleus
What happens in cytokinesis
- A contractile ring pinches the cytoplasm between the nuclei
- A cleavage furrow forms pulling together the cytoplasm, splitting it
Labelled diagram of mitosis
Identify the mitosis stage
Anaphase
Identify the mitosis stage
Metaphase
Identify the mitosis stage
Telophase
Identify the mistosis stage
Prophase
What are the 4 uses of mitosis
- Tissue repair/replacement: damages/old cells replaced by healthy ones
- Organismal growth: deriving new cells via mitosis
- Asexual reproduction (eukaryotes): plants
- Development of embryos: zygotes undergo mitosis and differentiate to form embryos
Why is mitosis important in asexual reproduction
Production of new genetically identical individuals by a single parent e.g. unicellular cell division in amoeba or multicellular cell division in buds (hydra/yeast/runners)
How is mitosis different in plant cells
- Plants have a cell wall
- Vesicles assemble along the cell equator
- Vesicles fuse with each other and the plasma membrane to divide the cell
- Vesicles deposit cellulose and pectin by exocytosis between the two membranes to form the cell walls
What is meiosis
Nuclear division that produced four haploid cells from a diploid cells, which are gametes to be used in sexual reproduction
What happens in prophase I (meiosis)
- DNA has already replicated, homologous pairs form bivalents by crossing over (chiasma)
- Chromosomes condense
- Centrioles migrate to opposite poles
- Nuclear envelope breaks down
- Nucleolus disintegrates
What happens in metaphase I (meiosis)
- Bivalents line up along the equator
- Spindle fibres attach to the centromeres
- The maternal and paternal chromosomes in the pairs position themselves independently (independent assortment) so there may not be equal maternal/paternal chromosomes on either side
What happens in anaphase I (meiosis)
- Homologous pairs of chromosomes are separated as microtubule spindles pull the whole chromosome to opposite poles
- Centromeres don’t divide
What happens in telophase I (meiosis)
- Chromosomes arrive at opposite poles
- Spindle fibres break down
- Nuclear envelopes may form around two groups of chromosomes
- Nucleoli reforms
- Cytokinesis occurs
Cytokinesis I (meiosis)
- Division of the cytoplasm occurs
- Cell organelles get distributed between the two developing cells
- cell surface membrane pinches and creates a cleavage furrow, allowing the contractile ring to divide the cytoplasm, forming two haploid cells
Why is there a second division of meiosis
DNA replication isn’t necessary so there is no interphase, and the second stage is necessary for producing 4 haploid cells
Prophase II
- Nuclear envelope breaks down and chromosomes condense
- A spindle forms at the right angle of the old one
Metaphase II
- Chromosomes line up in single file along the equator of the spindle
Anaphase II
- Centromeres divide and individual chromatids are pulled to opposite poles
- Creating 4 groups of chromosomes with half the number of chromosomes than the parent cell
Telophase II
- Nuclear membranes form around the 4 groups of chromosomes
Cytokinesis II
- Cytoplasm divides as cell membranes form, leaving 4 haploid cells
What is crossing over
The process where non-sister chromatids in close proximity exchange alleles during meiosis I
Why is crossing over of chromatids advantageous
It creates a new combination of alleles (recombinant chromosomes) which increases genetic variation of offspring
What is independent assortment
Random alignment of homologous pairs along the equator of the cell during metaphase I, producing different combinations of alleles
Why is independent assortment advantageous
Different combinations of chromosomes in daughter cells increases the genetic variation in gametes
What is the formula for working out the different possible chromosome combinations during independent assortment
2^n (e.g. in humans 2^23 = 8324608)
What is random fusion of gametes and why is it advantageous
Gametes each have different allele, and during fertilisation the male and female gamete fuse randomly which creates genetic variation between zygotes
What are specialised cells
Eukaryotic cells can become specialised with adaptations for a specific function
How and why are Erythrocytes (RBC) specialised
- Biconcave shape to increase surface area to absorb oxygen
- Concentrated in haemoglobin to readily bind to oxygen
- No nucleus, so there is more space for maximum oxygen carrying capacity
- Elastic membrane allows the cell to be flexible and squeeze through capillaries
How and why are neutrophils specialised
- Flexible shape to squeeze through cell junctions and form pseudopodia to engulf organisms
- Concentrated in lysosomes to digest and destroy pathogens
- Lobed nucleus and grainy cytoplasm
How and why are sperm cells specialised
- Head contains nucleus with half the genetic information
- Acrosome in head contains digestive enzymes which break the outer layer of egg cells so the sperm nucleus can fuse with the egg nucleus
- Mid piece concentrated with mitochondria to release energy for tail movement
- Flagellum (tail) rotates to propel the cell
How and why are root hair cells specialised
- Root hair increases surface area to increase uptake of water
- Thin cell walls (short diffusion distance) for water to diffuse
- Permanent vacuole contains cell sap concentrated to maintain a water potential gradient
- Mitochondria for energy to actively transport mineral ions
How and why are ciliated epithelium cells specialised
- Hairlike structures (cilia) which shift material along the surface of the cells
- Goblet cells along the basement membrane secrete mucus to trap dust and prevent them from entering vital organs
How and why are squamous epithelium cells specialised
- Squamous epithelium consists of a single layer of flattened cells which has a thin cross section and short diffusion pathway
- Permeable for easy diffusion
How and why are palisade cells specialised
- Cytoplasm concentrated in chloroplasts to maximise light absorption
- Tall and thin shape so light can penetrate deeper so light doesn’t reflect against other cells, and so they can be packed densely
How and why are guard cells specialised
- Thin outer cell walls, and thick inner cell walls allowing the cell to bend whilst turgid
- Cytoplasm concentrated with chloroplasts and mitochondria for energy
What are tissues
A group of cells that work together to perform a particular function (e.g. muscle cells form muscle tissue to move parts of the body)
What are organs
Different tissues working together to form organs
What are organ systems
Groups of organs (e.g. digestive system)
How and why are xylem vessel cells specialised
- No top and bottom walls between cells, so hollow tubes are formed to allow water flow
- Cells are dead (no organelles) to allow water flow
- Outer walls thickened with lignin to strengthen and support the plant
How and why are phloem vessel cells specialised
- Made of living cells supported by companion cells
- Cells are joined with sieve plates and have few subcellular structures to allow easy flow through
How and why are muscle cells specialised
- Three types of muscle (skeletal, smooth, cardiac)
- Have layers of protein filaments which slide to cause contraction
- Cells have high density of mitochondria for energy
- Skeletal muscles cells fuse to form multi nucleated cells which contract in unison
How and why are squamous epithelium cells specialised
- Cilia which beat in coordination to shift material along the surface
- Goblet cells secrete mucus which traps dust and prevents them from entering vital organs
How and why are squamous epithelium cells specialised
- Single layer of flattened cells
- Thin cross section for short diffusion pathway
- Permeable
How and why is cartilage specialised
- Strong and flexible to provide support (e.g. in the trachea)
What is a stem cell
A cell that can divide an unlimited number of times producing cells that can remain as stem cells or differentiate into specialised cells
What is potency
The ability of stem cells to differentiate into more specialised cell types
What are totipotent stem cells (totipotency)
Stems cells that can differentiate into any type of cell found in the embryo including extra-
embryonic cells as well as whole organisms. The embryonic cells and zygotes (up to the 16 cell stage of development) are also totipotent
What are pluripotent stem cells (pluripotency)
Embryonic stem cells that can differentiate into any cell type found in the embryo but can’t differentiate into extra embryonic cells
What are multipotent stem cells (multipotency)
Adult stem cells that have lost the potency associated with embryonic stem cells
Function of multipotent adult stem cells
Remain in tissues to produce new cells for essential processes of growth, cell replacement and tissue repair.
Stem cell diagram
What are stem cells found in bone marrow
Multipotent adult stem cells which can only differentiate into erythrocytes, monocytes, neutrophils and lymphocytes
Why do stem cells form erythrocytes
- RBC’s have no nucleus therefore can divide
- New erythrocytes must be formed from bone marrow stem cells in a process called erythropoiesis
What are meristems
Undifferentiated tissue in a plant which can give rise to new cells, usually located in tips of roots and shoots (e.g. Cambium tissue)
What is the cambium
A meristem in plants where the stem cells at the inner edge differentiate into xylem cells and the stem cells at the outer edge differentiate into phloem cells, and both processes are stimulated by hormones
What are the uses of embryonic stem cells
- They can differentiate widely so can be used for therapeutic treatment of disease
- Research it tightly regulated due to ethical concerns due to the embryos being taken from IVF and having to potential to become humans
- Embryonic cells are totipotent if taken 3-4 days after fertilisation or pluripotent is taken on day 5
What are the uses of multipotent adult stem cells
- Stem cell therapy where cells could be introduced into damaged tissue to treat diseases (e.g. leukaemia/skin burns)
- Less ethical issues as donor can give permission
Table of Uses for Stem Cells treating Alzheimer’s, Parkinson’s, Macular Degeneration, Spinal injuries, Blood diseases, Type 1 Diabetes, Heart attacks
