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cell division and cellular organization (stem cells and differentiation…
cell division and cellular organization
the cell cycle and mitosis
the cell cycle is the process of cell growth and divsion
starts when the cell has been produced by cell division and ends with the cell dividing to produce 2 identical cells
the cell cycle consists of a period of cell growth and dna replication (interphase) and a period of cell division (m phase)
m phase involves mitosis and cytokinesis
interphase= G1, G2 and s phase
the cell cycle is regulated by checkpoints to ensure there is no errors
phases
G1= cell grows and new organelles and proteins are made, rna transcription, biosynthesis
G2=cell keeps growing and proteins needed for cell division are made
s phase= cell replicates its dna, rapid to reduce risk of mutations
m phase= mitosis and cytokinesis, cell growth stops
G2 checkpoint= cell check whether dna has been replicated with any errors or damage
G1 checkpoint= the cell checks that chemicals needed for replication are present and for any damage to the dna before s phase
G0 phase= cells may under go apoptosis (cell death)or differentiation
mitosis
it is needed for: growth, repairing damaged tissue, asexual reproduction
interphase comes before mitosis in cell cycle
cell carries out normal functions
cell's dna is unravelled and replicated
organelles are replicated
ATP content is increased
1)prophase
chromosomes get shorter and thicker- more distinct
centrioles divide and start moving to opposite poles of cell
nuclear envelope breaks down
nucleolus gets smaller and eventually disappears
2) metaphase
chromosomes line up along the equator of the cell
spindle fibres (produced by centrioles) attach to centromeres of sister chromatids
at metaphase checkpoint cell checks that all the chromosomes are attached to spindle fibres before mitosis can continue
3)anaphase
centromeres divide, separating each pair of sister chromatids
spindle fibres contract/ shorten pulling chromatids towards poles of cell (centromere first), motor proteins walk along tublin threads
4) telophase
chromatids reach opposite poles
they begin to uncoil and become longer- less distinct
a nuclear envelope forms around each group of chromosomes
5)cytokinesis
cytoplasm divides
forms to genetically identical daughter cells to the original
in animal cells a cleavage furrow forms to divide the cell membrane
in plant cells cell wall is laid
staining chromosomes is used to see all the different phases
sexual reproduction and meiosis
meiosis produces gametes for sexual reproduction
in sexual reproduction two gametes (egg and sperm) join together at fertilization to form a zygote. the zygote then divides and develops into a new organism
meiosis involves reduction divison. cells that divide by meiosis have the full number of chromosomes to start with but the cells that are formed fro meiosis have half the number.
cells formed by meiosis are all genetically different because each new cell ends up with a different combination of chromosomes
meiosis involves 2 divisions
prophase 1= chromosomes condense, get shorter and fatter. chromosomes arrange themselves in homologous pairs and crossing over occurs. centrioles start to move to poles and nuclear envelope break down
metaphase 1= homologous pairs line up across equator and spindle fibres attach to centromeres, arrangement is random (independent assortment)
anaphase 1= spindle fibres contract, motor proteins walk along tublin threads separating homologous pairs - one chromosome goes to each end of the cell, crossed over area separates resulting in allele shuffling, centromeres don't divide
telophase 1= a nuclear envelope forms around each group of chromosomes and cytokinesis occurs to produce 2 haploid daughter cells
meiosis 2 = mitosis
in anaphase 2= pairs of sister chromatids are separated each new daughter cell inherits one chromatid from each chromosome to produce 4 haploid daughter cells
chromatids cross over in prophase 1
homologous pairs of chromosomes pair up
the chromatids twist around each other and bits of chromatids swap over
chromatids still contain the same genes but now have a combination of different alleles
meiosis produces cells that are genetically different
crossing over
each of the four daughter cells formed from meiosis contain chromatids with different alleles
independent assortment
each homologous pair of chromosomes in your cell is made up of one chromosomes from each parent
when the homologous pairs line up in metaphase 1 and are separated in anaphase 1, is completely random which chromosome from each pair ends up in the daughter cell
so the four daughter cells have different combinations of paternal and maternal chromosomes
leads to genetic variation in offspring
occurs in anaphase 1 and 2
stem cells and differentiation
stem cells are unspecialised cells
multicellular organisms are made up of many different cell types that are specialised for their function
all specialised cells originate from stem cells
stem cells are found in embryos which can develop into any type of cell, adult stem cells are limited
stem cells differentiate into specialised cells
stem cells divide to become new cells
stem cells become specialised by differentiation
growth and repair
cells in bone marrow
bones are living organs containing nerves and blood vessels
adult stem cells differentiate to replace worn out eyrthrocytes and neutrophils
cells in meristem
differentiate into xylem and phloem
stem cells can develop into different specialised cell types so scientists could use them to replace damaged tissues in a range of diseases
stem cells are also used by scientists researching developmental biology e.g. how organisms grow and develop
cells are specialised for their particular function
animal cells
neutrophils (type of white blood cell)- flexible shape allows them to engulf foreign pathogens or particles. lysosomes in their cytoplasm contain digestive enzymes to break down engulfed particles
erythrocytes (red blood cells)- biconcave disc shape provides a large surface area for gas exchange. no nucleus, more room for haemoglobin (protein that carries oxygen), flexible, very few organelles
epithelial cells- cilia beat to move particles away, squamous epithelia are very thin and allow gas diffusion
sperm cells- have a flagellum to swim to egg, lots of mitochondria to provide energy to swim, digestive enzymes to enable the sperm to penetrate the surface of the egg
plant cells
palisade mesophyll cells- contain lots of chloroplasts so they can absorb a lot of sunlight, thin walls so carbon dioxide easily diffuses into cell, large vacuole so chloroplasts are closer to edge of cell, cytoskeleton to move chloroplasts
root hair cells- large surface area to maximise absorption, thin walls for entry of water and ions, lots of mitochondria to provide energy for active transport, carrier proteins for ion active transport
guard cells- tiny pores for gas exchange, thin outer walls and thickened inner walls force stomata to open allowing gas exchange
tissues, organs and systems
tissue= a group of cells specialised to perform a specific function
animal tissues
squamous epithelium- a single layer of flat cells lining a surface, found in lungs ,cells joined form continuous sheets by tight junctions and desmosomes
ciliated epithelium- a layer of cells covered in cilia, found in trachea
muscle tissue- made up of bundles of elongated muscle fibres. 3 types: smooth, cardiac, skeletal
cartilage- a type of connective tissue found in the joints. formed when chondroblasts secrete an extracellular matrix which they become trapped inside
plant tissues
xylem- contains hollow xylem vessels which are dead and strengthened with lignin
phloem- made up of sieve and companion cells , hols in sieve plates so sap can move through