Download CELL DIVISION

CELL DIVISION
Cells divide. This makes
cells small.
Why do cells divide?
Why must cells be small?
PURPOSES OF CELL DIVISION
1. Growth- increase in size of the organism
(by dividing cells, not by increasing the
size of the cells)
2. Repair – needed because of worn out or
injured cells (your skin cells are replaced
every 28 days; your stomach every 7)
3. Reproduction (2 types)
• Asexual – one parent. Offspring
identical to parent - mitosis
• Sexual – combination of genetic
material from two parents - meiosis
CELLS ARE SMALL
Surface area to volume ratio must
remain high for the cell:
• To take in sufficient nutrients and oxygen
to maintain life
• To remove waste
• To move molecules across the cell
efficiently
When cells in an organism divide,
growth and repair result.
High Surface Area to Volume
Ratio?????
Surface area = 6 x height x width = 6
Volume = l x w x h = 1
Ratio = 6:1 = 6
Surface area = 6 x height x width = 150
Volume = l x w x h = 125
Ratio = 150:125 or 1.2
6 is higher than 1.2!
Asexual reproduction – passes on
chromosomes through mitosis to make a
clone (like binary fission in bacteria…more
later…)
Sexual reproduction – chromosomes are
separated in meiosis (more later…) and
then combine to make a new organism (like
egg & sperm fusing to make a
zygote…more later…)
Vocabulary
• Chromosome – structure found in
the nucleus of eukaryotic cells
that contains the genetic material;
made of chromatin
• Chromatin – Strands of DNA found
in the nucleus; makes up
chromosomes when condensed
around proteins
Vocabulary
• Chromatids—one of the two strands
of a chromosome that become visible
during mitosis or meiosis
• Centromere—the
region of the
chromosome that
holds the two
sister chromatids
together
Vocabulary
• Mitosis – Process of nuclear
division; karyokinesis
• Cytokinesis - Process of
division of the cytoplasm
The Cell Cycle
The cell cycle is a continuous process that occurs
in SOMATIC CELLS (body cells ex. Skin). It is
an ordered set of events of cell growth and
division resulting in two daughter cells, which
then start the process again.
2 main parts:
I. Growth & preparation
•
Interphase (90% of the cell’s life)
–
G1
–
S
–
G2
II. Cell division
•
•
Mitosis – division of the nucleus
Cytokinesis – division of the cytoplasm; usually follows mitosis,
but sometimes doesn’t occur
–
What would this result in?
STAGES OF CELL CYCLE
I. Growth & Preparation
(Must occur before mitosis)
INTERPHASE- getting ready stage (happens
before mitosis, can often see nucleolus, DNA
threadlike chromatin)
INTERPHASE
• 90% of the time, the cell is in
this phase
• Grows
• Performs operations unique to
the type of cell
INTERPHASE
• GROWTH 1 STAGE – G1
– Decides whether or not the cell will divide
– Makes its structural proteins and
enzymes to perform its functions
• A pancreas cell will produce and secrete
insulin
• Salivary gland will produce and secrete
enzymes in the mouth to aid in digestion
– Each chromosome is a single molecule of
DNA and associated proteins
INTERPHASE
S Synthesis (DNA Replication)
– Each of the chromosomes is copied
(in humans this makes 92 chromatids
held by 46 centromeres)
INTERPHASE
GROWTH 2 PHASE – G2
– DNA replication is checked by DNA repair
enzymes
– Cell prepares for mitosis
– Proteins organize themselves to form a series
of fibers called the spindles
• Involved in chromosome movement during mitosis
• Spindle fibers composed of microtubules
INTERPHASE IN AN ANIMAL CELL
INTERPHASE IN A PLANT CELL
II. Cell Division - MITOSIS
• Continuous process
• 4 main parts
– prophase, metaphase, anaphase, telophase
– P-MAT
PROPHASE
ANAPHASE
METAPHASE
TELOPHASE
MITOSIS
• PROPHASE
– Condensing of 2 chromatids to form
chromosome hinged by a centromere
• Coil up
• Become visible
– Centrioles begin to migrate to opposite sides of
the cell
– Nuclear envelope dis-assembles
MITOSIS
• METAPHASE
– Spindle fibers align the chromosomes
along the middle of the cell nucleus.
• This line is referred to as the metaphase
plate.
– This organization helps to ensure that in
the next phase, when the chromosomes
are separated, each new nucleus will
receive one copy of each chromosome
MITOSIS
• ANAPHASE
– Chromatids move apart from one another
– Each chromosome is attached to a
spindle which moves it toward one pole
– Results in equal separation and
distribution of chromosomes
MITOSIS
• TELOPHASE
– Chromatids arrive at opposite poles of cell
– New membranes form around the daughter nuclei.
– The chromosomes disperse (uncoil) and are no longer
visible under the light microscope.
– The spindle fibers continue to dis-assemble
– Cytokinesis may also begin during this stage.
– This phase reverses many of the processes of
prophase
Tissue sample showing cells in
multiple phases of mitosis
CYTOKINESIS
• Process in which the cytoplasm divides and
two separate cells form.
• In animals, it begins with the formation of a
cleavage furrow
• Microfilaments (actin fibers) contract
during cleavage and assist the division of
the cell into two daughter cells
– Think of a string being pulled tight around a
cube of jello (gelatin)
CYTOKINESIS
• In plant cells, cleavage does not occur
• New cell wall is formed in the center of
the cell by vesicles from the Golgi
• As the vesicles join, they form a double
membrane called the cell plate
– Forms in middle and moves outward
– Separates the daughter cells
Animal cell cleavage
Plant cell plate
MITOSIS—Summary
Animation:
http://www.hybridmedicalanimation.com/work/animation/the-stagesof-mitosis/
PROPHASE- chromosomes evident, nuclear
membrane disappearing (P for Phat (fat),
chromosomes condense/fatten and become
visible)
METAPHASE- sister chromatids lined up in the
middle/equator (M for middle, chromosomes
lined up in the middle of cell)
ANAPHASE- sister chromatids pulled apart (A
for Apart or Away because the chromatids
pull apart and move away from center)
TELOPHASE- chromosomes are at ends of
cell, cells prepare to separate (T for Two
new nuclear envelopes are forming)
Cleavage
furrow
Cell plate
Not all cells reproduce…
• Some leave the cell cycle here and
do not undergo cell division
– Red Blood Cells – which “kick out”
their nucleus to make room for the
hemoglobin and therefore can’t divide
– Brain and spinal cord cells – rarely if
ever divide; called G0 (pronounced G
naught)
Other cells can’t stop dividing…
Uncontrolled cell
growth is known as
cancer. Read how this
can occur on pp. 126-7
& 138-9 in your text.
Websites
• Cell Mitosis Lab Practice
http://www.biology.arizona.edu/cell_bio/activities/cell_cyc
le/activity_description.html
• Mitosis pictures & Video
http://www.iknow.net/CDROMs/cell_cdrom/cell3.ht
ml#mitosis
• Cell Cycle Interactive Game
http://nobelprize.org/medicine/educational/2001/c
ellcycle.html