Download Cell division

CELL DIVISION
Mitosis
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Why do cells divide?
…to grow
…to reproduce
…to replace lost cells
…to repair damaged cells
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Chromosomes in Eukaryotes
• Long threads of DNA wrap around
proteins called histones
• When a cell is not dividing the
DNA is loose & unwound; called
chromatin
• When the cell divides the chromatin
condenses into tightly coiled
chromosomes
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Chromosomes
• Somatic cells – cells of the body
• Chromosome number in each somatic cell is the
same throughout the body and is species-specific.
– humans: 46
dog:78
mouse:40
• “n” is the number of different
types of chromosomes
• Haploid = n
Diploid = 2n
• Homologous chromosomes are
matching pairs of chromosomes
Figure 8.12
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Homologous Chromosomes
• A human somatic cell - has 46 chromosomes (23
pairs)
– In humans n=23, but we are diploid organisms so
2n=46
– 22 pairs of homologous
(matching) chromosomes,
called autosomes
– 1 pair of sex chromosomes,
XY or XX
A karyotype is an orderly
arrangement of chromosomes
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Chromosomes
• Before a cell divides, it must duplicates all of its chromosomes
so that each new cell gets a complete copy of DNA
• A duplicated chromosome consists of 2 sister chromatids,
which are identical molecules of DNA
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Phases of Cell Division
• The cell cycle consists of two phases:
– Interphase – cell carrying out daily functions or
preparing to divide
– Mitosis- division of the nucleus
• Cytokinesis – division
of the cytoplasm
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Phases of Interphase
• G1 phase – cell spends 90-95% of its time in this phase
growing and carrying out its everyday functions
• S-phase - DNA is replicated
• G2 phase - final “check point”: make sure everything is
is ready for mitosis
Interphase
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Prophase
• Chromatin condenses into chromosomes
• Centrioles migrate to opposite poles
• Nuclear membrane breaks down
• Mitotic spindles form
Early Prophase
Late Prophase
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Metaphase
 Sister chromatids are aligned in the center
of the mitotic spindle
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Anaphase
• Mitotic spindles shorten pulling chromosomes to
opposite poles
• Sister chromatids separate becoming daughter
chromosomes
• The cell begins to elongate
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Telophase
• Chromosomes uncoil into chromatin
• Spindle breaks down
• Nuclear membrane reforms
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Cytokinesis
• Original cytoplasmic mass
divides resulting in two
identical daughter cells
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Review mitosis stages (animal cells)
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Cell division out of control!!!!
• Cell division is usually under strict control in order
to ensure appropriate proportions
• When cell cycle regulation fails, cells start dividing
uncontrollably and result in abnormal masses of
dividing cells called tumors
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Tumors
• Benign – does not invade adjacent tissues,
encapsulated
• Malignant – invades adjacent tissues
– Metastasize – cells break away from primary tumor
and travel to other areas of the body
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
• Normal Cells
– Stop dividing after a certain # of divisions
– Have contact inhibition
– Differentiated (have a job)
– Undergo apoptosis if DNA is damaged
• Cancer cells
– Immortal, divide indefinitely
– No contact inhibition, pile up on each other
– Non differentiated (no job)
– Large abnormal nuclei or chromosome numbers
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Factors that control cell division
• External signals from neighboring cells
– Proto-oncogenes – produce proteins that turn on cell
division
– Mutations in proto-oncogenes (now called oncogenes) may
cause these genes to stay turned on and produce excess
growth stimulating proteins resulting in uncontrolled cell
division
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Factors that control cell division
• External signals from neighboring cells
– Tumor suppressor genes – code for proteins that
receive stop signals from neighboring cells
• If mutated they cannot suppress cell division
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings
Factors that control cell division
• Apoptosis – programmed cell death, back up system
for cells with damaged DNA
• Telomeres – regions at the end of chromosomes that
become shorter each time the cell divides
– Keeps cells from becoming immortal
– Cancer cells have enzyme telomerase which repairs
telomeres
If you would like more information on factors that can lead to
cancer read the article “How Cancer Arises” posted under
assignments on blackboard. This is optional.
Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings