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Cell cycle and mitosis

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Jean Battinieri

on 8 November 2018

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Transcript of Cell cycle and mitosis

sex chromosomes:
determine the sex of an organism and development of sexual characteristics - most often XX is female, and XY is male
2 different types
homologous chromosomes:
same shape, size, and same type of trait
one homolog from each parent
chromosomes not involved in determination of sex
diploid or 2n: two sets of chromosomes
one set from each parent
haploid: one set of chromsomes; chromosome number has been reduced by half in the formation of sex cells
Division of the nucleus;
significant because it allows for
genetic material - DNA-
to be passed on to new cells
and DNA is important because...
1) chromosomes line up in the middle
2) kinetochore fibers - a form of microtubules-
are attached at the centromere
1) spindle fibers pull chromosomes apart at the centromere
2) chromatids move to opposite sides of the cells
1) nuclear membrane reforms
2) nucleolus reforms
3) chromosomes return to chromatin
4) spindle fibers disappear
plants cells from a cell plate which will eventually become part of the cell wall
animal cells form a cleavage furrow; a pinch of the cytoplasm
1) the final split of the cytoplasm
2) this stage actually begins towards the end of anaphase and continues until after telephase
Binary Fission
The Cell Cycle
The stages in the life of a eukaryotic cell
Binary fission
a form of prokaryotic cell reproduction
M phase
Cell division
genome - a cell's DNA its genetic information
Prokaryote's genome usually one long DNA molecule
Eukaryote's genome usually many DNA molecules
a human cell has about 3 meters of DNA - 300,000 times bigger than the cell's diameter
before a cell can divide DNA must be copied and separated so that each daughter cell has a complete genome
every eukaryotic species has a set number of chromosomes
somatic cells aka all body cells but not sex cells contain a diploid chromosome number
gametes aka sex cells have a haploid number of chromosomes
in each eukaryotic chromosome is one very long, linear DNA molecule representing hundreds or thousands of genes
the DNA complex called chromatin is organized into a long, thin fiber
after DNA replication takes place and DNA prepares for division the chromatin becomes tightly coiled
each duplicated chromosome has 2 sister chromatids that contain identical copies of the chromosome's DNA molecule
the centromere joins the sister chromatids
Cell division
sister chromatids are pulled apart and repackaged as complete chromosome sets in two new nuclei
MITOSIS - the division of the nucleus - body cells use this AKA somatic cells
cytokinesis - the division of the cytoplasm - immediately follows mitosis with the end result of 2 genetically equal cells
mitosis and cytokinesis are part of the process that make body cells
a second variation of cell division MEIOSIS reduces chromosome number in half during the formation of sex cells AKS gametes
meiosis reduces chromosome number so that when fertilization occurs chromosome number for that species is retained
Mitotic (M) phase
includes both mitosis and cytokinesis
shortest part of the cell cycle
accounts for 90% of the cell cycle
divided into 3 subphases
G1 phase - cell growth
S phase - DNA replication and cell growth
G2 phase -preparation for division and cell growth
Late interphase
well defined nucleus with one or more nucleoli
outside the nucleus there are 2 centrosomes
microtubules extend from centrosomes
centrosomes are the organizing center for the microtubules
in animal cells centrioles are found in the centrosomes
microtubules extend from centrosomes in radial arrays called asters.
centrosomes w/ centrioles
Chromatin - duplicated
Nuclear envelope
Plasma membrane
1)nuclear membrane breaks down
2) nucleolus dissappears
3) chromosomes become visible
4)spindle fibers form
5)***centrioles move towards opposite sides of the cell this occurs only in ANIMAL CELLS
Binary fission
prokaryotes - bacteria - reproduce by a type of cell division called binary fission which means division in half
most bacterial genes are found on a single chromosome consisting of a circular DNA molecule and proteins
there are NO mitotic spindles
once DNA of the chromosome begin to replicate, the copies of the first replicated region - called the origin of replication - move apart rapidly
the two copies separate by an unknown mechanism
one copy of the origin is now at each end of the cell
the chromosome continues replicating the cell continues to grow
replication is complete - the cell membrane grows inward and a new cell wall is deposited dividing the parent cell into two daughter cells
The cell cycle control system
a set of molecules in the cell that both triggers and coordinates key events in the cell cycle
it has external and internal regulators
checkpoint in the cell cycle is a critical control point where stop and go-ahead signals can regulate the cycle
3 major checkpoints found in G1, G2, and M phases
regulatory proteins such as protein kinases - enzymes that activate or inactivate other proteins by phosphorylation - drive the cycle
cyclins - proteins that bond with kinases regulate the cell cycle but scientist are still learning about these
Internal regulators
In anaphase there is a checkpoint
anaphase doesn't start until all chromosomes are properly attached to the spindle in metaphase - this ensures that daughter cells don't have missing or extra chromosomes
kinetochores not attached to centromere send a delay signal - when all spindles are attached - activates the anaphase pathway
when activated it will breakdown cyclin and inactivate the proteins holding the sister chromatids together
External Regulators
proteins that respond to events outside the cell
growth factor - a protein released by certain body cells that stimulate other cells to divide
density dependent inhibition - during cell division in which crowded cells stop dividing
cells signal other cells
when a cell population reaches a certain density, the amount of required growth factors and nutrients available to each cell becomes insufficient to allow continued growth
anchorage dependence - animal cells must be attached to an underlying support - involves plasma membrane proteins and cytoskeleton linked to it
Cancer cells
do not respond normally to the body's control mechanisms
they divide excessively and invade other tissues
they can kill the organism
they do not heed normal signals that regulate the cell cycle
they do not exhibit density dependent inhibition
they do not stop growing when growth factors are depleted
this may be caused by an abnormality in the signaling pathway
they may make their own growth factor
IF cancer cells stop dividing they do so at random points in the cycle rather than at the normal checkpoints
they can go on dividing indefinitely with a supply of nutrients
Cancer cell formation
begins when a single cell goes through transformation - process that converts a normal cell to a cancer cell
due to the alteration of genes that control the cell cycle
immune system should recognize it and destroy it but it evades destruction
it then goes on to create a tumor - a mass of abnormal cells
if the mass of cells remains at the original site the tumor is benign
malignant tumors become invasive enough to impair the functions of one or more organs - this is what they call CANCER
CANCER CELLS are abnormal:
may have unusual number of chromosomes
metabolism doesn't work correctly
can separate from other cells and enter the blood stream and spread to new parts of the body
Limiting Factors of cell growth
depletion of nutrients
changes in temperature
changes in pH
density - dependent inhibition
1. Synthesis of cyclin begins in late S phase - G2 - accumulation
2. Cyclin combines with Cdk producing MPF when enough is made passes G2 checkpoint
3. MPF promotes mitosis through phosphorylating proteins - MPF activity peaks during Metaphase
4. During Anaphase, the cyclin component of MPF is degraded causing M phase to end and returning to G1
5. G1 the degradation of cyclin continues and the Cdk component of MPF is recycled
Cdk - cyclin dependent kinase
MPF-maturation-promoting factor or M-phase promoting factor
Normal cells 20-50 times then they age and die
Cancer cells evade the controls that make them go through apoptosis
When a cell evades and makes more forms tumor, abnormal cells found in normal tissue
if they remain at original spot - benign tumor
cells whose genetic and cellular changes enable them to spread to new tissues and impair of the functions of one or more organs - malignant tumor AKA cancer
the spread of cancer cells from the original site is metastasis
Genes associated with cancer
genes that regulate cell growth and division includes genes for growth factors, their receptors, and the intracellular molecules of signaling pathways
mutations to any of the genes named above in somatic cells can lead to cancer
mutations can also be due to environmental influences, chemical carcinogens
Normal versions of the cellular genes, called proto-oncogenes, stimulate normal cell growth and division
Cancer causing genes called oncogenes
form from a genetic change that leads to an increase in the amount of the normal gene's product or increase in the activity of that protein
Oncogenes fall into three main categories:
movement of DNA within the genome (translocation)
amplification of the proto-oncogene
point mutation in a control element or in the proto-oncogene itself

Tumor-Suppressor Genes
genes whose normal products inhibit cell division; the proteins that they encode help prevent uncontrolled cell growth
can allow cancer to occur due to reduction of the tumor supressor protein

Defects in 2 key genes can cause cancer
1. the ras proto-oncogene
G protein that relays a signal from a growth factor receptor on the plasma membrane to a cascade of protein kinases
it makes a protein that stimulates the cell cycle - usually needs to be triggered by a growth factor - increases cell division
hyperactive ras proteins can cause excessive cell division

2. p53 gene
normal tumor suppressor gene- produces transcription factor that "inhibits" the cell cycle
the "guardian angel of the genome"
can activate p21 gene which makes a product that halts cell cycle by binding to cyclin dependent kinases allowing the cell time to repair the damaged DNA
can turn on genes directly involved in DNA repair
if DNA is not repairable will turn on genes that cause cell death
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