• Created by: rosieevie
  • Created on: 20-05-17 13:07

Importance of Cell Division

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
1 of 13

Prokaryote Cell Division

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)

2 of 13

Development of Multicellular Eukaryotes

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

3 of 13


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

4 of 13

Stages of Mitosis


  • 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)


  • 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)
5 of 13

Stages of Mitosis 2


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


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


  • Seperating chromosomes reach poles

Next interphase begins

  • Nuclear envelope/nucleoli re-form 
  • Chromatin diffuse (spread out)
  • Cytoplasm divides (cytokinesis)
6 of 13

Chromosome Karyogamy

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
7 of 13

Centromeres and Kinetochores

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

8 of 13

Centrioles and Centrosomes

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)

9 of 13

Spindle Fibres and Microtubules

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


  • 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
10 of 13

Microtubule Movement

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

11 of 13

Formation of Kinetochore Microtubules

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

12 of 13

Kinesin and Dynein

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
13 of 13


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Genetics resources »