1.6 cell division Flashcards
1
Q
what is the cell cycle?
A
ordered set of events which culminates in the division of a cell into two daughter cells
2
Q
what are the 2 stages of the cell cycle?
A
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
3
Q
what is interphase?
A
- 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
4
Q
what is mitosis?
A
- 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
5
Q
compare chromatin to chromsomes
A
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
6
Q
what are chromatids?
A
- 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
7
Q
detail the process of mitosis including premitosis (end of interphase)
A
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
- 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 - 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) - 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 - 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
8
Q
compare cytokinesis in animals and plants
A
- 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
9
Q
what is supercoiling?
A
- 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
10
Q
what is the mitotic index? and how do you calculate it?
A
- 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)
11
Q
what are cyclins?
A
- 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
12
Q
what are the cyclin expression patterns?
A
- 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
13
Q
what occurs at the G1 checkpoint in interphase?
A
- 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
14
Q
what are tumours?
A
- 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
15
Q
what are mutagens?
A
- 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)
16
Q
what are oncogenes?
A
- 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
17
Q
how do cancer cells occur?
A
- 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
18
Q
what is metastasis?
A
- 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)
19
Q
what is the relationship between smoking and cancer?
A
- 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
