1.6 cell division

Question 1

what is the cell cycle?

Answer 1

ordered set of events which culminates in the division of a cell into two daughter cells

Question 2

what are the 2 stages of the cell cycle?

Answer 2

INTERPHASE: stage in the development of a cell between 2 successive divisions

• continuum of 3 distinct stages:
  1. G1: 1st intermediate gap stage where cell grows and prepares for dna replication
  2. S: synthesis stage in which dna is replicated
  3. G2: 2nd intermediate gap stage in which cell finishes growing and prepares for cell division

M (MITOTIC) PHASE: period of cell cycle in which cell and contents divide

• comprised of 2 distinct stages:
  1. mitosis: nuclear division, where dna (as condensed chromosomes) is separated into 2 identical nuclei
  2. cytokinesis: cytoplasmic division, where cellular contents are segregated and cell splits into two

Question 3

what is interphase?

Answer 3

• portions of cell cycle not visibly involved in cell division
• active period in the cell cycle when many metabolic reactions occur in nucleus and cytoplasm to prepare cell for successful division
• majority of time
• 4 main activities:
  1. metabolic reactions: chemical reactions necessary for life (e.g. cellular respiration for the production of atp)
  2. protein synthesis: production of proteins (like hormones) and enzymes for cell growth and function
  3. organelle numbers increase: in anticipation for cytokinesis
  4. dna replication: double dna quantity before mitosis
• more detailed: doctor mnemonic
  Dna replication: dna copied during S phase of interphase
  Organelle duplication : must be duplicated for twin daughter cells
  Cell growth: cytoplasmic volume must increase prior to division
  Transcription / translation: key proteins and enzymes must be synthesised
  Obtain nutrients: vital cellular materials must be present before division
  Respiration (cellular): atp production needed to drive division process

Question 4

what is mitosis?

Answer 4

• division of NUCLEUS [not cell] into 2 genetically identical daughter nuclei, where duplicated dna molecules are arranged into 2 separate nuclei
• consists of 4 stages: prophase, metaphase, anaphase, telophase [division of cell in 2 (cytokinesis) occurs concurrently with telophase]
• 2 key objectives:
  1. double nuclear count (leading to cell division into 2 daughter cells)
  2. genetic stability

Question 5

compare chromatin to chromsomes

Answer 5

CHROMATIN:

• dna usually loosely packed within nucleus as unravelled chromatin
• in this unravelled form, dna is accessible to transcriptional machinery so genetic material can be translated
• dna organised as chromatin in all non-dividing cells and throughout process of interphase

CHROMOSOME: structure containing genetic material responsible for storage of genetic information

• dna temporarily packaged into tightly wound and condensed chromosomes prior to division (via supercoiling)
• in this condensed form, dna able to be easily segregated however is inaccessible to transcriptional machinery
• dna organised as chromosome during mitosis (condense in prophase, decondense in telophase)
• EXTRA:
• eukaryotic chromosomes consist of dna associated w special proteins known as histones (chromosomes visible during mitosis after prophase; condensed from chromatin)
• prokaryotic chromosomes not associated w proteins and known as naked dna

Question 6

what are chromatids?

Answer 6

• chromosomes is condensed form of dna which is visible during mitosis via microscopy
• as dna is replicated during S phase of interphase, chromosome initially contains 2 genetically identical dna strands: sister chromatids
• sister chromatids held together by central region called centromere
• when they separate during mitosis, they become independent chromosomes, each made of single dna strand

Question 7

detail the process of mitosis including premitosis (end of interphase)

Answer 7

BEFORE MITOSIS
INTERPHASE
- dna present as uncondensed chromatin (not visible under microscope)
- dna contained within clearly defined nucleus
- centrosomes and other organelles duplicated
- cell enlarged in preparation for division

MITOSIS

1. PROPHASE
   - dna supercoils and chromosomes condense (becoming visible under microscope)
   - chromosomes comprised of genetically identical sister chromatids joined at centromere
   - paired centrosomes move to opposite poles of cell and form microtubule spindle fibres [in animal cells; in plant cells no centrioles form and spindle fibres develop independently]
   - nuclear membrane breaks down and nucleus dissolves
   - longest phase
2. METAPHASE
   - microtubule spindle fibres from both centrosomes connect to centromere of each chromosome
   - microtubule depolymerisation causes spindles fibres to shorten in length and contract
   - causes chromosomes to align along the centre of the cell (equatorial plane or metaphase plate)
3. ANAPHASE
   - continued contraction of spindle fibres causes genetically identical sister chromatids to separate (motor proteins)
   - centromeres holding each pair of sister chromatids divide
   - once chromatids separate, they are each considered an individual chromosome in their own right
   - genetically identical chromosomes move to opposite poles of cell
4. TELOPHASE
   - once 2 chromosome sets arrive at poles, spindle fibres dissolve / disassemble
   - chromosomes decondense / uncoil back to chromatin (no longer visible under light microscope)
   - nuclear membranes [envelope] reform around each chromosome set
   - cytokinesis occurs concurrently, splitting cell into 2

Question 8

compare cytokinesis in animals and plants

Answer 8

• process of cytoplasmic division; where cell splits into 2 identical daughter cells
• occurs simultaneously with telophase during mitosis
• different in plant and animal cells

ANIMAL:

• after anaphase, microtubule filaments form concentric ring [of proteins, made up of actin and myosin fibres] around around centre of cell
• microfilaments constrict to form cleavage furrow, which deepens from periphery to centre
• when furrow meets in the centre, cell becomes completely pinched off and 2 cells are formed
• cell splits at equatorial plane
• centripetal: occurs from the outside and moves towards the centre

PLANT:

• after anaphase, carbohydrate-rich vesicles form in a row at the centre of cell on equatorial plane
• vesicles fuse together and early cell plate begins to form within middle of cell
• cell plate extends outwards and fuses with cell wall, dividing cell into 2 distinct daughter cells
• centrifugal: separation originates in the centre and moves laterally

Question 9

what is supercoiling?

Answer 9

• a way to pack dna densely together in compact structure that is capable of moving around without damaging dna within
• strain placed on dna double helix by over-winding and under-winding portions of dna
• dna coils back onto itself to become shorter and wider
• histones present in chromosomes faciliatate

Question 10

what is the mitotic index? and how do you calculate it?

Answer 10

• measure of the proliferation status of a cell population (i.e. the proportion of dividing cells)
• ratio between the number of cells in mitosis and the total number of cells
• can be determined by analysing micrographs and counting the relative number of mitotic cells versus non-dividing cells
• mitotic index = (cells in mitosis) / (total no. of cells)

Question 11

what are cyclins?

Answer 11

• family of regulatory proteins that control progression of cell cycle
• cells cannot progress to next stage of cycle unless specific cyclin reaches its threshold
• bind to enzymes called cyclin-dependent kinases (cdk), which control cell cycle processes through phosphorylation
• when cyclin and cdk form a complex, complex binds to a target protein and modifies it via phosphorylation
• phosphorylated target protein triggers some specific event within cell cycle (e.g. centrosome duplication etc)
• after event has occurred, cyclin is degraded and cdk is rendered inactive again

Question 12

what are the cyclin expression patterns?

Answer 12

• cyclin concentrations need to be tightly regulated in order to ensure the cell cycle progresses in a proper sequence
• different cyclins specifically bind to, and activate, different classes of cyclin dependent kinases
• cyclin levels peak when target protein is required for function and remain at lower levels at all other times
• cyclin a: activates dna replication inside nucleus in S phase [between S and G2]
• cyclin b: promotes assembly of mitotic spindle and other tasks in cytoplasm to prepare [between G2 and M]
• cyclin d: triggers cells to move from G0 to G1 and from G1 to S [between M and G1]
• cyclin e: prepares cell for dna replication in S phase [between G1 and S]
• maturation promoting factor (mpf): cyclin-dependent kinase that helps to trigger passage of cell past G2 checkpoint

Question 13

what occurs at the G1 checkpoint in interphase?

Answer 13

• crucial checkpoint
• if cell receives appropriate go ahead signal as determined by cyclins, it will progress to S phase and then continue to complete mitosis
• for many cells, if it does not receive go ahead signal at G1 checkpoint, it will move to G0 phase where cell adopts non-dividing state

Question 14

what are tumours?

Answer 14

• abnormal cell growths resulting from uncontrolled cell division and can occur in any tissue or organ
• cancer: malignant tumour / diseases caused by growth of tumours

Question 15

what are mutagens?

Answer 15

• agent that changes genetic material of an organism (acts on dna / the replicative machinery); results in mutation
• 3 different types:
  1. physical: sources of radiation including x-rays (ionising), ultraviolet (uv) light and radioactive decay
  2. chemical: dna interacting substances including reactive oxygen species (ros) and metals (e.g. arsenic) [chemical mutagens that can cause cancer known as carcinogens]
  3. biological: viruses, certain bacteria and mobile genetic elements (transposons)

Question 16

what are oncogenes?

Answer 16

• gene that has the potential to cause cancer
• most cancers caused by mutations to 2 basic classes of genes – proto-oncogenes and tumour suppressor genes
  1. proto-oncogenes: code for proteins that stimulate cell cycle and promote cell growth and proliferation [gas of the car] [when mutated becomes cancerous oncogenes]
  2. tumour suppressor genes: code for proteins that repress cell cycle progression and promote apoptosis [breaks of the car] [when mutated becomes inactivated]
• lead to uncontrolled cell division

mutation:

• e.g. mutant kinases like a mutant Ras gene that is hyperactive and phosphorylate proteins in an unregulated manner leading to no checkpoint controls in the cell cycle and continuous mitotic cycles
• tumour suppressing genes that repair damaged DNA or inhibit the progress of the cell cycle
• mutant apoptosis genes that no longer are able to signal for the programmed cell death of a cell leading to immortal cells

Question 17

how do cancer cells occur?

Answer 17

• several mutations must occur in order for the cell to turn into a tumour causing cell
• makes probability of producing a tumour causing cell to be small
• however, once cell picks up a few relevant mutations, the chance of more mutations increases exponentially
• such cells have a high rate of cell division, increasing the chance of errors during DNA replication
• high rate of growth also means that there are more cells carrying the mutations that can be further mutated
• cells may also acquire mutations that inhibit cell death, prolonging lifespan of cell and increasing the time span that the cell can pick up more mutations without dying

Question 18

what is metastasis?

Answer 18

• spread of cancer from 1 location (primary tumour) to another, forming a secondary tumour
• tumour cells may either remain in their original location (benign) or spread and invade neighbouring tissue (malignant)
• cancer cells in tumour can sometimes detach from the primary tumour
  • some of such cells gain ability to penetrate walls of lymphatic or blood vessels and hence able to enter into circulatory system and travel to other parts of body
  • invade and grow rapidly in their new locations, leading to the formation of secondary tumours
• secondary tumours are made up of the same type of cell as the primary tumour – this affects the type of treatment required
• e.g. if breast cancer spread to the liver, the patient has secondary breast cancer of the liver (treat with breast cancer drugs)

Question 19

what is the relationship between smoking and cancer?

Answer 19

• strong link between smoking and the incidence of cancers
• cigarette smoke contains over 4,000 chemical compounds, over 60 of which are known to be carcinogenic
• presence of carcinogens greatly increase the chances of the cells in the mouth, oesophagus, bronchus and the lung to gain mutations
• increases chance that mutations result in formation of oncogenes