Uploaded by Maryam El-Guindy

The Cell Cycle

The Cell Cycle
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represents about 90% of the cell cycle
Cells perform their specific tasks while in interphase
Cells must have access to their DNA so the DNA needs to be in chromatin form (spread out)
G1 phase (Growth phase)
● Cell recovers from cell division
● Cell becomes larger
● Organelles must be replicated
Cells are most functional directly after G1 phase
G1 checkpoint checks for​:
1.​Cell Adhesion
Is the cell attached to the extracellular matrix?
Are we around the cells we’re supposed to be around
2.​Cell Density
Is there enough room for the cell to divide
3. ​DNA Damage
If a cell fails the G1 checkpoint it is either destroyed or goes to the G0 phase where the cell
cycle is paused until the errors are corrected
The G0 phase is also entered if the body sends a signal to stop producing cells
Cancer Cells usually do not go through the G1 checkpoint
S phase synthesis phase
● DNA is copied
● Producing DNA is called DNA synthesis
G2 phase growth phase
● Cells grows in order to get ready to divide
● Cell becomes slightly larger
● Extra copies of important organelles (eg mitochondria) are produced
It is very important that the cell leaves G2 phase because if not it will produce double the
substances it should produce
G2 checkpoint
● Checks for errors in DNA replication
If cell doesn’t pass the errors are either fixed or the cell is destroyed
M Phase
● A single parent cells produces two genetically identical daughter cells
● Produces Somatic Cells (normal cells)
A single parent cell produces four genetically unique cells with half as many
Go from diploid to haploid
Produces Gametes in Animals (sperm and egg)
Actual division of the cell that occurs in telophase
Telophase ends when cytokines completes
DNA organization
Law Of Independent Assortment
In Prophase I, when tetrads form, the orientation of one tetrad has no effect on the orientation of
the rest of the tetrads
Law Of Crossing Over
Failure of chromosomes to separate during anaphase I or Anaphase II of meiosis
Nondisjunction in single chromosomes
Non disjunction for all chromosomes
Results in extra sets of homologous chromosomes (4n 8n 12n)
An abnormal number of chromosomes of that type
Only one chromosome of that type
Three chromosomes of that type
Down syndrome
Trisomy of the chromosome 21
Sex chromosomes trisomy are far less lethal than normal cell trisomy
Part of the chromosome is removed
This doesn’t cause problems if the persons other chromosomes have the deleted part
because usually the other chromosome can produce enough for both
Inbreeding is a problem because the other person is more likely to have the deleted part
of the chromosome as well
Multiple copies of the same gene on a single chromosome
Overproduction occurs
One portion of a chromosome is moved to a non homologous chromosome
Immediate result
● Some cells will produce things they shouldn’t produce
● Some cells will not produce what they should produce
Long term Results
● Offspring will have either an overproduction or underproduction of certain genes
Two non homologous chromosomes join together
● Length of the chromosome changes
● Shape of the chromosome changes
● 2 centromeres with 3 telomeres
Genes switch places on a chromosome
● non immediate effects
● During crossing over both deleting and duplication will happen
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