Cell division is incredibly tightly regulated (cancer is described as an unregulated cell growth).In respect tothe cell division,there is a machinery that is a transitory state and this machinery is renewed every cell cycle.
Key components of this machinery are:
1) Spindle: is made of microtubules or cytoskeleton, centrioles, kinetochores and contractile ring.
2) And this machinery is controlled by a control system which integrates signals from growth and division.
These signals are:
a) Cell division kinases (CDK)
b) Each kinase has an activity that is controlled by a cycline (regulatory subunit of CDK) and the activity
of these 2 systems is controlled by a third system which is;
c) regulated destruction mechanism.
Cell division is highly regulated:
The cells are dividing by 8 minutes (but neurons by 100 years) but the typical cells (eg liver cells) divides
1/year, and the cells of gut are dividing 1/day .
1) Cell division requires partitioning of organelles for this there is 3 stratégies :
A) Abundant organelles like ribosomes peroxisomes ( 1/2 goes in one side and 1/2 to the other side)
B) Mitochondria chloroplast: these are ancient captured bacterias and conserves their own mechanism
of division .So chloroplast and mitochondria split during cell division .
C) Golgi and endoplastic reticulum becomes small vesicles and then after cell division they come back
together and reform Golgi and ER
D) Centrosome = centriole is the poorly understood in eukaryotes cells.
Centriole template his own replication so we need a centriole to make a new centriole (centriole play a key
role in mitotic spindle apparatus and centrioles are structures, that DNA contemplate his replication.
G1 and G0: G1 is the phase in which the cell ask herself “am I big enough to divide” are there enough food?
There are also the + or – factors from other cells or the environment.And if the final decision is not
to divide the cell goes back to G0 phase where there is no division.
S phase : the phase in which nuclear DNA replication and mitochondrial DNA replication occurs .
G2: another decision making phase is G2: cell verify if every single genome has been replicated or not. If it has not, there is a mechanism that ask the cell to halt in G2 until the last nucleotide could be replicated. G2 will be prolonged until the cell has all needed to go to M.Another control point is to see if each kinetochore archived bipolar spindle attachment.
M: and finally mitosis happens:
Chromosomes consist of one DNA molecule and the proteins associated with that and we must not underestimate the importance of the proteins and the mass of proteins associated with the DNA (their mass exceeds the mass of DNA).
There are critical landmarks on the chromosomes and the most important is the centromere. The centromere is the point at which the chromosome attaches to the spindle apparatus to allow chromosome segregation that is it’s only function; but at this point there is a very complete protein structure (fifty different proteins called connect cord. So, the connect cord is the structure that assembles at this point to archive protein segregation).
We know that there are the origin of replication on DNA, result in eukaryote cells there are multiple origins of replication.
At the very end of chromosomes, there are specialized structures called telomerase.
Telomerase the enzyme which intervene in the replication of telomere.
Chromosomes have lots of genes collectively our chromosomes.
How 22250 genes which make proteins as well as RNA
Ploidy = number of sets of chromosomes
Haploid = organisms who have one set of chromosomes
Diploid = organisms who have two sets of chromosomes
Triploid = organism who have three sets of chromosomes
Tetraploid = organism who have four sets of chromosomes
Hexaploid = organism who have six sets of chromosomes
Octaploid = organism who have eight sets of chromosomes
The letter N is used to refer to number of chromosome haploid cell.
So, for humans and potatoes N=23 and as we are diploid, so we have 2 x 23 chromosomes
C value refers to the contact of DNA (amount of DNA in the cell)
N is a discontinuous functions but the C value is the continuous function.
So, the amount of DNA in a diploid nucleons is 2 C in G1 and the amount of C in G2=4C
But in G2 the number of chromosomes dose not change.
How do we measure DNA content?
We can do that because of amazing machine
FACS= fluorescent activated cell sorter or cell scanner
The fluorescent dye that steins DNA but it stains DNA proportionately to the DNA reserve