cell division, cell diversity and cellular organisation

cell cycle

the regulated sequence of events that occurs between one cell division and the next

what are the three phases of the cell cycle?

interphase, mitosis, cytokinesis

what does the length of the cell cycle depend on?

environmental conditions
cell type
organism

what is movement from one phase to another triggered by?

chemical signals called cyclins

what happens during interphase?

the cell increases in mass and size and carries out its normal functions

what are the three phases of interphase?

G1, S, G2

G1 phase

- signal is received telling the cell to divide
- cells make the RNA, enzymes and other proteins required for growth

S phase

DNA replicates \= each chromosome has two identical chromatids

G2 phase

- cell continues to grow
- new DNA is checked and any errors are repaired
- preparations for cell division are made

cytokinesis

after mitosis - the whole cell divides and one nucleus moves into each cell \= 2 genetically identical daughter cells

draw out the cell cycle

what is important about the new DNA?

it is accurate \= new cells can carry out their function

what are there in the cell cycle to check DNA for errors?

checkpoints that check the genetic information within the replicated DNA

what is the process of this?

specific proof-reading enzymes and repair enzymes are involved
--\> will repair the error where possible

what might the cell do if errors can't be repaired?

destroy itself to prevent passing on harmful mutations

how many checkpoints are there in the cell cycle?

four

where do they occur?

during G1, during S, during G2 and during metaphase

checkpoint during G1

chromosomes are checked for damage
--\> if damage is detected then the cell doesn't advance into S phase until repairs have occurred

checkpoint during S

chromosomes are checked to ensure they've been replicated
--\> if all chromosomes haven't been replicated then the cell cycle stops

checkpoint during G2

check for DNA damage
--\> cell cycle will be delayed until necessary repairs have been made

checkpoint during metaphase

determines whether chromosomes are correctly attached to the spindle fibres

mitosis

the process of nuclear division by which two genetically identical daughter nuclei are produced (also genetically identical to parent cell)

what are the stages of mitosis?

prophase, metaphase, anaphase, telophase

prophase

- chromosomes condense and become visible
- chromosomes are made of 2 sister chromatids joined at the centromere
- centrosomes move towards opposite poles
- spindle fibres begin to emerge from the centrosomes
- nuclear envelope breaks down into small vesicles
- nucleolus disappears

diagram of prophase

microscope image of prophase

metaphase

- centrosomes reach opposite poles
- spindle fibres continue to extend from centrosomes
- chromosomes line up at the equator of the spindle
- spindle fibres reach the chromosomes and attach to the centromeres

diagram of metaphase

microscope image of metaphase

anaphase

- sister chromatids separate at the centromere
- spindle fibres begin to shorten
- separated chromatids \= chromosomes - pulled to opposite poles by spindle fibres

diagram of anaphase

microscope image of anaphase

telophase

- chromosomes arrive at opposite poles and decondense
- nuclear envelopes begin to reform around each set of chromosomes
- spindle fibres break down
- new nucleoli form in each nucleus

diagram of telophase

microscope image of telophase

cytokinesis

- division of the cytoplasm
- cell organelles are distributed between the 2 cells
- cell surface membrane pinches inwards \= cleavage furrow in middle of cell \= contracts and divides cytoplasm
\= 2 identical daughter cells

how many chromosomes do human diploid cells have?

46

what is mitosis important for?

growth, repair, asexual reproduction

mitosis - growth

enables unicellular zygotes to grow into multicellular organisms

mitosis - repair

- damaged tissue can be repaired by mitosis followed by cell division
- cells are constantly dying \= need to be continually replaced by genetically identical cells

mitosis - asexual reproduction

- unicellular organisms \= cell division results in the reproduction of a genetically identical offspring
- multicellular organisms \= new individuals grow from the parent organism by cell division and then detach in different ways

meiosis

a form of nuclear division that results in the production of haploid cells from diploid cells
--\> produces gametes

haploid cells

a cell containing only one set of chromosomes - gametes

what is the difference between mitosis and meiosis?

meiosis has 2 divisions

how many stages does meiosis have and what are they?

8 - each division has a prophase, metaphase, anaphase and telophase

meiosis - prophase 1

- DNA condenses and becomes visible as chromosomes
- each chromosome \= 2 sister chromatids joined by a centromere
- chromosomes are arranged side by side in homologous pairs (bivalents)
- centrioles migrate to opposite poles and spindle is formed
- nuclear envelope breaks down and nucleolus disintegrates

what may happen with the chromatids in prophase 1?

crossing over of non-sister chromatids at the chiasmata

meiosis - metaphase 1

- bivalents line up along equator with spindle fibres attached to centromeres
- independent assortment \= maternal and paternal chromosomes in each pair position themselves independently of the others

meiosis - anaphase 1

- homologous pairs are separated
--\> microtubules pull whole chromosomes to opposite ends

meiosis - telophase 1

- chromosomes arrive at opposite poles
- spindle fibres start to break down
- nuclear envelopes form around the 2 groups of chromosomes and nucleoli reform

what happens between telophase 1 and prophase 2?

cytokinesis
no DNA replication

meiosis - prophase 2

- nuclear envelope breaks down
- chromosomes condense
- spindle forms at a right angle to the old one

meiosis - metaphase 2

chromosomes line up along the equator

meiosis - anaphase 2

centromeres divide and individual chromatids are pulled to opposite poles
\= 4 groups of chromosomes with half the number of chromosomes of a parent cell

meiosis - telophase 2

nuclear membranes form around each group of chromosomes

meiosis - cytokinesis

cytoplasm divides and forms new cell surface membranes \= 4 cells formed each with 23 chromosomes (gametes)

number of chromatids in each cell in each division of meiosis

before meiosis \= 92 (due to replication in interphase)
after first division \= 46 (normal cell)
after second division \= 23 (gamete)

stages of meiosis diagram

does meiosis produce identical cells?

no - produces genetically different cells \= genetically different offspring

what are the mechanisms in meiosis that increase the genetic diversity of gametes?

crossing over
independent assortment
random fusion of gametes

what do these mechanisms result in?

different combinations of alleles in gametes

crossing over

non-sister chromatids exchange alleles
- homologous chromosomes pair up and are in close proximity
- non-sister chromatids can cross over and get entangled at points called chiasmata
- entanglement places stress on DNA molecules \= section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
\= new combination of alleles on the two chromosomes

independent assortment

production of different combinations of alleles in daughter cells due to random alignment of homologous pairs along the equator

independent assortment process

- homologous chromosomes pair up and are pulled towards equator
- each pair can be arranged with either chromosome on top
- orientation of one homologous pair is independent of any other pair
- homologous chromosomes are then separated and pulled apart

random fusion of gametes

in fertilisation, any male gamete can fuse with any female gamete to form a zygote \= genetic variation between zygotes (each has a unique combination of alleles)

stem cells

undifferentiated cells \= not specialised
--\> divide by mitosis and undergo differentiation \= become specialised for a particular function

specialised cells

cells that are adapted to carry out a particular function

erythrocytes - function

transport oxygen around the body and carbon dioxide to the lungs

what are erythrocytes formed from and why?

stem cells in bone marrow - mature erythrocytes have no nucleus \= can't divide \= new cells formed from stem cells

what is the process of this?

- stem cell is multipotent
- divides to form proerythrocytes
- changes occur and cell can only form erythrocytes
- haemoglobin builds up in cytoplasm
- nucleus is ejected
- further changes \= mature erythrocyte

erythrocytes - adaptations

- biconcave \= increases surface area over which oxygen can be absorbed
- high amounts of haemoglobin \= readily binds to oxygen
- no nucleus \= more space in the cell for haemoglobin \= maximises oxygen capacity
- elastic membrane \= cell is flexible and can change shape to get through narrow capillaries

neutrophils - function

destroy pathogens by phagocytosis and the secretion of enzymes

neutrophils - adaptations

- flexible shape \= allows them to squeeze through cell junctions in the capillary wall, and enables them to form pseudopodia that engulf microorganisms
- lot of lysosomes \= digestive enzymes to digest invading cells
- flexible nuclear membrane \= cell can penetrate cell junctions and creates lobed nucleus

how are the xylem and phloem formed?

stem cells in the tissue between them (cambium) divides to form phloem cells on outside and xylem cells on inside
--\> stimulated by hormones

what is cambium an example of?

meristem - undifferentiated plant tissue that creates new cells

how do xylem cells differentiate?

lose their cytoplasm and deposit lignin in their cell walls
end cell walls may be lost

how do phloem cells differentiate?

some loss of cytoplasm and organelles
development of sieve plates at ends of cells

sperm cells - function

reproduction - to fuse with an egg and initiate the development of an embryo

sperm cells - adaptations

- nucleus in head \= contains half the normal number of chromosomes
- acrosome in head \= digestive enzymes that break down the outer layer of the egg so that the nuclei can fuse
- lots of mitochondria in mid-piece \= release energy for tail movement
- tail rotates \= propels the sperm cell forwards and allows it to move towards egg

root hair cells - function

absorption of water and mineral ions from soil

root hair cells - adaptations

- root hair \= increase SA \= rate of osmosis is greater
- thinner walls \=

what are the 3 different types of stem cells?

totipotent, pluripotent, multipotent

totipotent

can differentiate into any type of body cell
can form extra-embryonic cells

pluripotent

can form any type of body cell but not extra-embryonic cells

multipotent

can differentiate into several cell types but not all

what does this mean stem cells can be used for?

allows a certain type of cell to be grown and used to repair damaged ones

why is there a lesser chance of rejection?

uses individual's own stem cells

what conditions are researchers trying to treat with stem cells?

- Alzheimer's - by growing stem cells into nerve cells in brain
- Parkinson's - to replace the dead dopamine-producing cells
- spinal injuries
- blood diseases
- heart attacks - repairing damaged heart tissue

what else are stem cells valuable for?

study of developmental biology - see how body develops from a fertilised egg

what type of stem cells are particularly useful in these studies?

embryonic - due to wider range of cell types they can form

why is their use controversial?

embryos used are waste from IVF but have potential to develop into new individuals \= people object to their use

tissue

group of similar cells that perform a particular function

epithelium

sheet of cells that covers a body surface or lines a body cavity

squamous epithelium

- any type of epithelium made of a layer of flattened cells
- sits on a basement membrane
- form a surface covering \= thin cross section reduces the distance that substances have to travel to pass through

ciliated epithelium

- layer of cells with cilia on their surfaces
- found where something has to be moved across the surface
--\> movement of cilia shifts material along surface \= specialised

cartilage

- connective tissue
- made of specialised cells \= chondrocytes \= produce extracellular matrix of collagen fibres (strengthen) and elastin fibres (flexibility)
- protects and strengthens e.g. in noses, ears etc.

muscle

- moves parts of the body
- organ made of muscle tissue, nerve tissue, blood and connective tissue

what are the 3 types of muscle?

skeletal (voluntary), cardiac, smooth (involuntary)

skeletal muscle cells

- highly specialised and multinucleate
- also called muscle fibres - groups of fibres form a fascicle and groups of fascicles form a muscle
- muscle fibres are made of myofilaments arranged into myofibrils

xylem

- plant tissue
- transports water and minerals up plant stem
- strengthens stem
- cells are all dead and have no cytoplasm