Cell Growth and Division
Ch 10.1, 10.2 & 11.4
Cell Growth
• 2 things limit the size of cells as they grow:
– Access to DNA…if cell is too big, too hard to get to
DNA for instructions
– Access to materials…if cell is too big, cell
membrane is not big enough to move things in
and out (surface area changes by factor of 2,
volume by factor of 3)
• So when cells grow too big, they divide
Asexual Reproduction
• Sometimes cells divide and make
exact copies of themselves
• No change in DNA/genetic info.
Cells are genetically identical
• e.g. bacteria, many single celled
organisms, plants (can)
Sexual Reproduction
• Sometimes organisms make gametes
and exchange genetic information
• gametes (egg and sperm, egg and
pollen)
• new cells are genetically different from
originals
• new cells have only ½ the genetic info
of original
DNA/chromosomes
• DNA : DeoxyriboNucleic Acid
– Chemical that holds information on cell
functions
– Has different shapes/forms during life of cell
• Chromatin: long thin strands of DNA, spends
most of its time in this form
• Chromosome: Rod- shaped structures, DNA
and protein (histones), only in this form
during cell division
• Chromatid: two halves of a chromosome
• Centromere: the point where two chromatids
are joined
DNA/chromosomes
DNA/chromosomes
•
Different kinds/shapes of chromosomes
–
–
•
homologous chromosomes
–
–
•
Same shape/genes
1 from Mom and 1 from Dad
If a cell has the “normal” number of
chromosomes, it is called diploid.
That means it will have 2 of each
chromosome.
–
–
•
sex chromosomes: X and Y chromosomes, XX is
female, XY is male.
autosomes: Other chromosomes besides the sex
chromosomes
Body cells are diploid, 2n = 46 for humans
22 pairs autosomes, 1 pair sex chromosomes
Some cells are haploid, have ½ the
number of chromosomes
–
Gametes are diploid n = 23 for humans
Cell cycle
• Events of a cell’s life
• 4 stages
–
–
–
–
G1: growth: Cells grow to mature size
S: synthesis: DNA is copied
G2: growth: Cell prepares for division
M: mitosis: cell divides
• http://www.cellsalive.com/cell_cyc
le.htm (ISH)
• Interphase:
– G1, S and G2 together are known as
interphase … “between phases” … and
not part of mitosis.
• Also a G0 phase. Cells do not copy
DNA or prepare for mitosis,
but are still alive (e.g. nervous
system)
Regulating the cell cycle
• After each phase (G1, S, G2)
of the cell cycle, the cell
checks itself to make sure
no mistakes have been
made. These are called
checkpoints.
• Sometimes something goes
wrong (a mutation) and
the cell does not check
itself and the cell cycle
just keeps growing and
dividing rapidly
• We call that cancer. Mutant
cells divide out of control
and form a tumor.
Mitosis (prophase, metaphase)
• What happens during
prophase:
– Centrioles, spindles
appear
– Chromosomes become
visible (Chromatin tightens
and coils)
– Nuclear membrane breaks
down and disappears
• What happens during
metaphase:
– Chromosomes move to
middle/equator of cell
– Randomly assorted
(doesn’t matter what order)
Mitosis (anaphase, telophase)
• What happens during
anaphase:
– Chromosomes split at
centromere and
daughter chromatids
move to ends of cell
• What happens during
telophase:
– Cell starts to pinch in
– Nucleus starts to
reform
– Chromosomes are at
opposite ends, unwind
back into chromatin
Mitosis (Cytokinesis)
• Mitosis technically
refers to just division
of the nucleus
• Division of the rest of
the cell is called
cytokinesis
•
•
•
http://www.millerandlevine.com/chapter/1
0/ (vid)
http://www.cellsalive.com/mitosis.htm
(ISH)
Pick the cell stage:
http://www.classzone.com/books/ml_sci_li
fe/page_build.cfm?id=resour_ch3&u=1##
– Cytoplasm and contents
(other organelles) divide
– Cleavage furrow forms in
animals where cell
membrane pinches in
– in plants, cell membrane
can’t pinch in so new cell
wall forms in middle,
called cell plate.
Meiosis (prophase I)
• Go through 2 cell divisions, 4
daughter cells
• start with diploid number of
chromosomes and end up
with haploid number of
chromosomes
• Prophase I
• similar to mitosis:
– Nuclear membrane breaks down
– Chromatin forms chromosomes
– Centrioles move to ends
• What’s different:
– Line up by shape (homologous),
process is called synapsis.
Forms a tetrad.
– Crossing over can occur: part of
1 chromosome changes with
same part of its homolog.
Meiosis (anaphase I ,metaphase I)
•
•
Metaphase I
What’s similar to mitosis:
– Chromosomes line up in the
middle
– Independent assortment: which
homolog is on which side is
random
•
What’s different:
– Line up according to shape
(tetrads line up, rather
than individual
chromosomes)
•
•
Anaphase I
What’s similar to mitosis:
– Chromosomes (tetrads) split
apart and move to end of cell
•
What’s different:
– homologs split apart
– centromeres don’t split
– * cells will now be haploid*
Meiosis (telophase I)
• Telophase I
• What’s similar to mitosis:
– 2 cells are formed
– chromosomes are at the
ends of the cell
– nucleus reforms
– cytokinesis happens next
• What’s different:
– haploid cells with 2 copies
of the same chromosome,
not 1 copy of different
chromosomes
• Meiosis I is called reduction
division, because
you’ve reduced the cell
from diploid to haploid
Meiosis II (prophase II &
metaphase II)
•
•
•
The two daughter cells then
undergo a second division
with the same
stages/events.
Prophase II
What’s similar to mitosis:
– Spindles form
– Chromosomes move towards center
– Nucleus breaks down
•
What’s different:
– Nothing. It’s exactly the same,
except that the cell is haploid.
•
•
Metaphase II
What’s similar to mitosis:
– Chromosomes line up in the middle
•
What’s different:
– Nothing. It’s exactly the same,
except that the cell is haploid.
Meiosis II (anaphase II &
telophase II)
•
•
Anaphase II
What’s similar to mitosis:
– Individual chromatids are moved
towards opposite ends of the cells
– centromere splits
•
What’s different:
– Nothing. It’s exactly the same,
except that the cell is haploid.
•
•
Telophase II
What’s similar to mitosis:
– Nucleus reforms around the
chromatids
– Cytokinesis (cell membrane pinches
in, organelles divide and cells
separate)
•
What’s different:
– 4 cells are formed (total)
– Nothing. It’s exactly the same,
except that the cell is haploid.
•
http://www.cellsalive.com/meio
sis.htm
Genetic Variations
• Two things that can happen
during meiosis that can
recombine DNA for future
generations:
– Crossing over: During prophase I:
In the tetrad, pieces of chromatids
break off and switch places. Genes
exchanged between Mom’s and Dad’s
chromosomes
– Independent assortment: Homologous
chromosomes randomly are separated in
Metaphase I. Genes passed on are randomly
picked.
• These will be important ideas when we get to
talking about genetics in a couple of
units.
Comparing Mitosis and Meiosis
• Each starts with
a diploid cell.
• Mitosis ends up
with 2 diploid
cells exactly
identical
• Meiosis ends up
with 4 haploid
cells not
identical
Cell division in Prokaryotes (bacteria)
• Prokaryotic cells and
DNA are much
simpler
• Called binary fission …
literally, splitting in
two.
• DNA is circular and
attached to inside of
cell membrane.
• DNA copies itself
• Cell divides into two