Transcript CELL DIVISION - Mrs. Cobbs' Biology 2010

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. 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
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