Cell cycle and mitosis: Cell cycle stages Flashcards
what is cell division?
The cell cycle is the orderly sequence of events required for the duplication of a eukaryotic cell into two genetically identical daughter cells.
what processes do cells undergo to proliferate
The cell undergoes nuclear division (mitosis) and a cytoplasmic division (cytokinesis)
why is cell division required?
Is required: to replace dead/injured cells from wear & tear, stress, chemical damage etc
and adds new ones in tissue growth
A human being goes from 1 cell to around 75 trillion
Do all body (somatic) cells undergo division?
not all but most.
read blood cells and adult brain cells do not undergo mitosis
Apoptosis
1) When the unnecessary enzymes start activating in the cell, they eat up the proteins due to which cell starts becoming round.
2) DNA present inside the nucleus starts separating and eventually it shrinks down.
3) There is a nuclear membrane around the nucleus, when the apoptosis starts, it degrades and cell’s nucleus becomes without the outer layer.
4) Due to the absence of the nuclear membrane, the DNA molecule starts rupturing into small fragments. These fragments are not in a particular size.
5) As nucleus is no longer protected that is why it breaks down into many pieces along with the uneven pieces of DNA molecule.
6) Due to the breakage inside the cell, cell itself starts degrading through the process of blebbing.
7) Blebbing converts the cell into mall pieces which are eaten by other small cells known as phagocytes.
Necrosis
small blebs form; the structure of the nucleus changes
the blebs fuse and become larger;no organells are located in the blebs
the cell membrane rupturs and releaes the cell’s content; the organelles are not functional.
how are cells controlled?
Cells have a finite number of divisions
Controlled by activation of ‘suicide genes’
what prevents apoptosis
bcl-2 prevents apoptosis unless disrupted
Between 50 and 70 billion cells die each day due to apoptosis in the average human adult
what are the main parts of the cell cycle?
- Cells must first replicate all their homologous chromosomes
- Cell replication has 2 main stages
- Interphase (when the cell is not dividing)
- The mitotic (M) phase when a cell is dividing
what is Interpahse
It is the phase between two successive mitotic divisions
• During this phase the cell grows and prepares itself for division
Most cells only spend a small amount of time dividing
During interphase cells are carrying out normal functions & preparing to divide if needs be
what are the subdivisions of Interphase?
It may be subdivided into
– G1 phase
– S phase and
– G2 phase
what occurs in G0
An interphase cell in G0 is not dividing or preparing to divide e.g. most muscle cells and neurones
describe G1 phase
Cells don’t pass G1 without growth factors
Lasts for about 8-10 hours of a 24 hour cycle (can last weeks)
High rate of metabolism
Protein synthesis
Vigorous growth
Duplicates most organelles
Centrosome replication begins
S phase
Lasts for about 8 hours
DNA replicates.
Synthesis of new histones.
Assembly of new chromatin.
what enzyme catalyses DNA replication
describe the process of DNA replication
Why is DNA replication Semi-conservative
DNA replication
• It is catalysed by enzyme DNA polymerase.
• Energy for the process comes from hydrolysis of ATP.
• The DNA uncoils due to breaking of hydrogen bonds between nitrogen bases
• The original strands function as templates for the synthesis of new strands.
• Each new strand contains bases complementary to the original strand.
• Hydrogen bonds are formed between the bases of the original and the new strands creating two daughter molecules
• The method is termed as semi-conservative as each daughter
molecule contains only one newly produced strand
