1.6: Cell Division Flashcards
Cell Cycle
an ordered set of events which culminates in the division of a cell into two daughter cells
two main parts:
-interphase
-M phase
Interphase
G1 – First intermediate gap stage in which the cell grows and prepares for DNA replication
S – Synthesis stage in which DNA is replicated
G2 – Second intermediate gap stage in which the cell finishes growing and prepares for cell division
Many events need to occur in interphase to prepare the cell for successful division
These key processes include:
DNA replication – DNA is copied during the S phase of interphase
Organelle duplication – Organelles 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 is needed to drive the division process
DNA is present as uncondensed chromatin (not visible under microscope)
DNA is contained within a clearly defined nucleus
Centrosomes and other organelles have been duplicated
Cell is enlarged in preparation for division
M phase
The period of the cell cycle in which the cell and contents divide to create two genetically identical daughter cells
This phase is comprised of two distinct stages:
Mitosis – Nuclear division, whereby DNA (as condensed chromosomes) is separated into two identical nuclei (PMAT)
Cytokinesis – Cytoplasmic division, whereby cellular contents are segregated and the cell splits into two
DNA supercoiling
Chromatin:
DNA is usually loosely packed within the nucleus as unravelled chromatin
In this unravelled form, the DNA is accessible to transcriptional machinery and so genetic information can be translated
DNA is organised as chromatin in all non-dividing cells and throughout the process of interphase
Chromosomes:
DNA is temporarily packaged into a tightly wound and condensed chromosome prior to division (via supercoiling)
In this condensed form, the DNA is able to be easily segregated however is inaccessible to transcriptional machinery
DNA is organised as chromosomes during the process of mitosis (condense in prophase, decondense in telophase)
Chromosome versus Chromatid
A chromosome is the condensed form of DNA which is visible during mitosis (via microscopy)
As the DNA is replicated during the S phase of interphase, the chromosome will initially contain two identical DNA strands
These genetically identical strands are called sister chromatids and are held together by a central region called the centromere
When these chromatids separate during mitosis, they become independent chromosomes, each made of a single DNA strand
Mitosis
The process of nuclear division whereby duplicated DNA molecules are arranged into two separate nuclei
Stages of mitosis:
-Prophase
-Metaphase
-Anaphase
-Telophase
Prophase
DNA supercoils and chromosomes condense (becoming visible under a microscope)
Chromosomes are comprised of genetically identical sister chromatids (joined at a centromere)
Paired centrosomes move to the opposite poles of the cell and form microtubule spindle fibres
The nuclear membrane breaks down and the nucleus dissolves
Metaphase
Microtubule spindle fibres from both centrosomes connect to the centromere of each chromosome
Microtubule depolymerisation causes spindle fibres to shorten in length and contract
This causes chromosomes to align along the centre of the cell (equatorial plane or metaphase plate)
Anaphase
Continued contraction of the spindle fibres causes genetically identical sister chromatids to separate
Once the chromatids separate, they are each considered an individual chromosome in their own right
The genetically identical chromosomes move to the opposite poles of the cell
Telophase
Once the two chromosome sets arrive at the poles, spindle fibres dissolve
Chromosomes decondense (no longer visible under light microscope)
Nuclear membranes reform around each chromosome set
Cytokinesis occurs concurrently, splitting the cell into two
Cytokinesis
the process of cytoplasmic division, whereby the cell splits into two identical daughter cells
Animal Cells
After anaphase, microtubule filaments form a concentric ring around the centre of the cell
The microfilaments constrict to form a cleavage furrow, which deepens from the periphery towards the centre
When the furrow meets in the centre, the cell becomes completely pinched off and two cells are formed
Because this separation occurs from the outside and moves towards the centre, it is described as centripetal
Plant Cells
After anaphase, carbohydrate-rich vesicles form in a row at the centre of the cell (equatorial plane)
The vesicles fuse together and an early cell plate begins to form within the middle of the cell
The cell plate extends outwards and fuses with the cell wall, dividing the cell into two distinct daughter cells
Because this separation originates in the centre and moves laterally, it is described as centrifugal
Mitotic Index
P+M+A+T/total number of cells
Cyclins
are a family of regulatory proteins that control the progression of the cell cycle
Cyclins activate cyclin dependent kinases (CDKs), which control cell cycle processes through phosphorylation
When a cyclin and CDK form a complex, the complex will bind to a target protein and modify it via phosphorylation
The phosphorylated target protein will trigger some specific event within the cell cycle (e.g. centrosome duplication, etc.)
After the event has occurred, the cyclin is degraded and the CDK is rendered inactive again
Cancer development
Tumour
abnormal cell growths resulting from uncontrolled cell division and can occur in any tissue or organ
Mutagen
an agent that changes the genetic material of an organism
Oncogene
a gene that has the potential to cause cancer
Metastasis
the spread of cancer from one location (primary tumour) to another, forming a secondary tumour
Smoking and cancer
Cigarette smoke contains over 4,000 chemical compounds, over 60 of which are known to be carcinogenic
There appears to be a strong positive correlation between the frequency of smoking and the development of cancer
The risk of lung cancer is strongly correlated with smoking, with ~90% of lung cancers attributable to tobacco use
Smoking also increases the risk of over a dozen other cancers, including mouth, stomach, liver, pancreas and bowel
