Mitosis

W/ few exceptions, each cell of an organism has same DNA both quantitatively and qualitatively and can produce new organism

Most cells diploid - regenerate whole organism via:

• Vegetative propagation in plants
• Nuclear transplantation
• Cloning
• Mitosis

Cell division occurs by binary fission

Prokaryotic chromosome = single DNA mol that replicates then attatches each copy to different part of cell membrane

When cell begins to pull apart - replication and original chromsomes seperated

Cytokinesis occurs = 2 cells of identical genetic composition (except w/ mutations)

Single-celled organism disadvantages:

• Limited life
• No cell replacement
• Limits size (SA:V)
• Limited specialisation

Increase in complexity or size = more genes/DNA = new mechanisms to cope w/ cell division = mitosis

Human haploid DNA has 3 x10^9 nucleotides - 1m long

Human diploid chromosome number = 46

Number of different reasons:

• Growth
• Wound healing
• Cell/tissue replacement
• Asexual reproduction
• Cancer

Mitosis dangerous period of time - DNA packed tightly into chromosomes so cannot transcribe and create proteins = dormant

Body as whole may not react to changing conditions

Interphase

• Nucleus replicates DNA and centrosomes

Interphase-Prophase transition

• Chromatin coils
• Centrosomes move to cell poles

Prophase -

• Chromatin coils and supercoils (compact)
• = chromosomes - identical, paired chromatids)

Prometaphase

• Nuclear envelope breaks down
• Kinetochore MT appear and interact w/ polar MT of spindle = movement of chromosomes
• Protozoa/fungi - nuclear membrane intact (less DNA - seperate w/in nucleus confines)

Metaphase

• Duplicated centromere regions connecting paired chromatids become aligned at plane at cell's equator

Anaphase

• Centromeres divide
• New chromosomes (1 member of each pair) moves towards poles

Telophase

• Seperating chromosomes reach poles

Next interphase begins

• Nuclear envelope/nucleoli re-form 
• Chromatin diffuse (spread out)
• Cytoplasm divides (cytokinesis)

Number, size and shape pattern of chromosomes constant for a single species - differs between species 

Somatic cell chromosomes present themselves as homologous pairs - 1 each contain either maternal and paternal sister chromatids which pair up during meiosis (same genes, different alleles)

Karyotype - number and visual appearance of chromosomes in nuclei of organism/species

3 different types of chromosomes:

• Metacentric - centromere in centre of chromsome
• Acrocentric - centromere close to one end of chromsome
• Telocentric - centromere right at end of chromsome

Spindle fibre attatchment can be localised (kinetochores) or non-localised (polycentromeres/holocentromeres)

Kinetochore - attatchment point on centromere for microtubules allowing chromosome movement

Polycentromeres (Ascarid nematodes) - spindle fibres attach along centromere in discreet areas

Holocentromeres (Homoptera, Hemiptera) - no discreet attatchment areas, spinde fibres attatch aong whole chromosome

Centrioles - pair of minute cylindrical organelles near nucleus, made up of protein MTs

• Replicate by assembly
• Originally thought to produce mitotic spindle, but not required for spindle function
• Plants have no centrioles
• If centrioles lasered out in animal cells - spindle still forms
• Function unknown - associated w/ cell motility

Centrosome - electron dense cloud near nucleus, associated w/ centrioles in animal cells (microtubule organising centre)

Spindle pole body - found on nuclear membrane in fungi (remains intact in mitosis)

Microtubules made of tubulin - hollow tubes ~24nm diameter

Two non-identical polypeptide chains called A and B tubulin

Microtubules lengthened or shortened using GTP and dynamic instability

Function:

• Creation of spindle
• Metaphase plate alignment (chromsomes independent or pairing)
• Orientation of chromsome for centromere splitting (independent assortment)
• Chromsome movement to poles

3 types of microtubules:

• Kinetochore - attatch to kinetochores
• Polar - connection between polar fibres from each end
• Astral - anchor spindle poles to cell membrane

Exhibit dynamic instability - primarily at + end (- end capped at centrosome)

Polar MTs and kinetochore MTs exhibit additional microtubule flux - net addition of tubulin heterodimers at + ends near kinetochores and net loss of tubulin subunits at -ve ends near centrosomes

= MTs remain attatched to centromere/MOC while depolymerising

Therefore + end can both polymerise and depolymerise

Microtubules grow out by polymerising at + end (- end also occurs as process is dynamic)

Microtubules attatch to kinetochore and pull chromosome to centre

Centromere splits due to tension caused by depolymerisation of kinetochore MT

Two main microtubule-dependant motor proteins for spindle assembly and fuction:

• Kinesin - usually towards + end
• Dyneins move towards - end

Kinetochore fibres pull - move by depolymerisation at kinetochore spindle pole - promoted by low levels of colchicine

• Kinesin-4,10 binds to chromsome and spindle and walks towards + end

Polar fibres push and pull and move by polymerisation at equator

• Kinesin-5 pushes and pulls at same time, both to + ends = reaches middle and pushes poles apart
• Kinesin-14 pulls towards - end and pulls poles together

Astal fibres pull - attach to cell membrane region. Friction used to pull fibres as traction, otherwise fibres floating about w/ no real movement

• Dynein attaches to plasma mebrane and astal fibres and pulls towards MOC