Download Chapter 8 – Cell Division

How do cells reproduce?
• Cell division is at the heart of reproduction
• Multicellular organisms originate from a rapidly
dividing fertilized egg (cell); eggs and sperm are
themselves created from a special type of cell
division
• Cell division replaces worn-out or damaged cells,
keeping the total number of cells relatively
constant
• There are two types of cellular division: mitosis
and meiosis
Cell division and reproduction
• Asexual reproduction involves the
creation of genetically-identical
offspring from a single parent; no eggs
or sperm are involved
• Involves replication of
chromosomes, the structures
containing the organism’s DNA
• Bacteria, yeast, protists, and
certain plants and animals
Asexual Reproduction
• Asexual reproduction is a very efficient means
of reproduction
– Faster than sexual reproduction
– Increases numbers of organisms quickly
– Ability to reproduce in absence of mate (male
doesn’t need female and vice versa)
• Genetic diversity, however, is
sacrificed
Sexual Reproduction
• The ability for an organism to form gametes,
or sex cells (eggs and sperm), results in the
formation of similar, but not identical,
offspring
• In sexual reproduction, the resulting offspring
are genetically similar, but not
identical to either parent;
offspring inherits a combination
of genes from each parent
Cells arise from pre-existing cells
• Cell division allows an embryo to develop into
an adult, and is the basis of egg and sperm
formation
• It also ensures the continuity of life from one
generation to the next
• In the case of unicellular organisms, cell
division can reproduce an entire organism
Binary fission
• Prokaryotes (Bacteria and Archaea) reproduce
by a type of cell division called binary fission;
“dividing in half”
• These cells possess a single chromosome,
which is replicated prior to the cell dividing
into 2
What about eukaryotic cells?
• A bacteria contains ~3,000 genes; human cells
contain ~25,000 which are grouped into
multiple chromosomes located in the nucleus
• Each chromosome consists of 1 long DNA
strand, with hundreds or thousands of genes
• Integrated into this chromosome are
proteins!, which help maintain its structure
and control the activity of its genes
Chromosomes
• Human cells have 46
chromosomes
• Before a eukaryotic cell can
divide, it must replicate its
chromosomes
• The DNA molecule of each
chromosome is copied and new
proteins attach as needed
A duplicated chromosome
Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to
daughter
cells
The cell cycle
• The process of cell division is a key component
of the cell cycle, an ordered sequence of
events beginning with the ‘birth’ of the cell
from a dividing parent and ending with its
own division into 2 cells
• The cell cycle consists of a growing stage
called interphase, and the actual cell division,
called the mitotic phase
The Cell Cycle
• Most of the cell cycle in spent in interphase
• During this time, the cell performs its various
functions within the organism
• Additionally, the cell acquires a rich supply of
proteins, creates more organelles such as
mitochondria and ribosomes, and grows
during this time
• Chromosomes are replicated during
interphase
The Cell Cycle
• Interphase is divided into 3 stooges, er,
stages…
– The G1 phase: cell grows
– The S phase: cell grows, chromosomes replicated
– The G2 phase: cell grows
G stands for “gap” (first and second gap)
S stands for “synthesis” (DNA)
INTERPHASE
S
(DNA synthesis)
G1
G2
The Cell Cycle
• During interphase, the cell grows (G1),
continues to grow while DNA is replicated (S),
and then grows more as it completes
preparations for cell division (G2)
• Cell division occurs in the mitotic phase (also
called the M phase)
– Accounts for only 10 of the total time required for
the cell cycle
The Cell Cycle
• Like interphase, the mitotic phase is divided
into (2) stages
– Mitosis: the nucleus (and all its contents,
including the duplicated chromomes) divide and
are evenly distributed to the ‘daughter’ cells
– Cytokinesis: the cytoplasm is divided into 2
Mitosis and cytokinesis produces 2 genetically
identical cells, each with a single nucleus,
surrounding cytoplasm and plasma membrane
Mitosis
• Mitosis (the division of nuclear material) is
subdivided into 5 main stages:
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase
Mitosis
• During mitosis, chromosome movement is
dependent on the mitotic spindle, a football (go
Giants! Go Jets!) shaped structure of
microtubules that guides the separation of the 2
sets of separating chromosomes
• During interphase, chromosomes are not
distinguishable because they exist as loose fibers
of chromatin; chromatin becomes more tightly
packed and visible as mitosis ensues, allowing
easy tracking of each step of mitosis
Prophase
• The mitotic spindle forms during the first
stage, prophase
• The chromatin fibers containing DNA become
more tightly coiled and folded forming
discrete chromosomes that can be seen with a
light microscope
• Remember, there are 2 pairs of chromosomes
at this stage as they were replicated during
the S phase of interphase
Prophase
Visible
chromosomes;
nuclear
envelope still
present
Early mitotic
spindle
present
Prometaphase
• During the second stage of mitosis,
prometaphase, the nuclear envelope breaks
away
• Proteins embedded in the chromatin attach to
microtubules of the spindle, and move the
chromosomes towards the center of the cell
Prometaphase
Dissolution of
nuclear
envelope;
chromosomes
moved
towards the
center of the
cell
Mitotic
spindle extend
‘pole’ to ‘pole’
Metaphase and Anaphase
• During metaphase, the mitotic spindle spreads
across the entire cell, with the chromosomes
aligned perpendicularly at its center
(remember each chromosome has been
replicated into 2 prior to mitosis)
• In Anaphase, the sister chromatids of each
chromosome separate and move away from
each other (toward opposing poles)
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Daughter
chromosomes
During mitosis,
each
chromosome has
been replicated
consisting of 2
sister chromatids;
these
chromosomes
align and
separate during
metaphase and
anaphase,
respectively
Telophase
• During the fifth (and final) stage of mitosis
called telophase, nuclear envelopes form
around the 2 copies of separated
chromosomes; the chromatin fiber uncoils
and the mitotic spindle disappears
• Sort of a reverse prophase!
• Cytokinesis follows this final stage of mitosis,
pinching the cell into 2
Telophase and Cytokinesis
Got all that?
• The eukaryotic cell cycle consists of:
– Interphase (G1, S, G2) – growth & DNA replication
– Mitosis
• Prophase – mitotic spindle forms, chromatin condenses
• Prometaphase – nuclear envelope dissolves, chromosomes
attach to spindle
• Metaphase – mitotic spindle spreads pole to pole with
chromosomes aligned at center
• Anaphase – each sister chromatid of replicatec
chromosome separates
• Telophase – nuclear envelope reforms, chromatin uncoils
– Cytokinesis – cell divides into 2
Cell Division
• The timing of cell division must be regulated
in order to grow and develop normally
• Skin cells and stomach cells are replaced
regularly as they are constantly abraded and
sloughed off
• Other cells, such as liver cells, do not divide
unless damaged; In this way, cell division
repairs wounds and heals
Cell Division
• Proteins regulate cell division by stimulating
cells to divide in their presence
• For example, injury to the skin causes blood
platelets to release a protein which promotes
rapid growth of connective tissue cells that
help seal the wound
• Proteins control each cycle of mitosis and each
stage does not occur until triggered to do so
by these proteins
Cell Division
• Proteins serve as a control system for each
stage of the cell cycle
• Want a job? Research on controls over the cell
cycle is one of the hottest areas in biology
today. Why?
• Without check points, cells will continue to
divide unregulated….. = cancer
Cancer
• Cancer is a disease of the cell cycle
• Cancer cells divide uncontrollably and
do not respond normally to the cell
cycle control system
• Cancer begins when a single cell
undergoes transformation from a
normal cell to a cancer cell
• Cancer cells may proliferate into a
tumor, an abnormally growing mass of
body cells
Cancer
• Benign tumors remain at the site and can
usually be removed easily with surgery
• Malignant tumors spread into neighboring
tissues and other parts of the body,
interrupting organ function as it goes
• Cancer cells may secrete molecules that cause
blood vessels to spread toward the tumor, and
allow proliferation of the cancer cells via the
circulatory system (metastasis)
Cancer
• Radiation damages DNA in cancer cells
moreso than it does in normal cells and can be
used as a cancer treatment
• Chemotherapy is used to treat metastatic or
widespread tumors; involves the use of drugs
that disrupt cell division (some drugs prevent
the mitotic spindle from forming in the first
place); however side effects are seen in
normal, rapidly-dividing cells
Meiosis
• Meiosis is the process of cell division in which
the number of chromosome is cut in half
• Unlike mitosis, which results in a ‘daughter’
cell containing the exact number of
chromosomes as the ‘parent’ cell
• Meiosis takes place in reproductive organs
and produces gametes, sex cells, such as eggs,
sperm, and pollen (plants)
Meiosis
Homologous pair of
chromosomes
Sister chromatids
One duplicated
chromosome
• Human cells have 46
chromosomes, made
up of 23 pairs of
homologous
chromosomes
• Cells with 2 sets of
chromosomes are
considered diploid
Meiosis
• The two chromosomes composing a pair are
called homologous because they both carry
genes controlling the same inherited
characteristics
• One exception are the sex chromosomes, X
and Y
– Females have a homologous pair (XX), while males
have 1 X and 1 Y
– The other 22 chromosomes are called autosomes
Meiosis
Homologous pair of
chromosomes
From Mother
From Father
• For both sex
chromosomes and
autosomes, we inherit
one chromosome of
each pair from our
mother and the other
from our father
Meiosis
• The 46 chromosomes in the human cell
consists of 23 pairs of homologous
chromosomes
• Homologous chromosomes are similar, but
not identical; they may carry different versions
of the same genetic information
• For example, one chromosome may code for
blond hair, while the other codes for dark hair;
or both may contain the same gene (ex. Blue
eyes)
Meiosis
• Human cells contain 22 pairs of autosomes,
and 1 pair of sex chromosomes (X and/or Y)
Chromosome
1 is the
largest;
containing
8000 genes
Chromosome
21 is the
smallest;
containing
only 300
genes
Sex
chromosomes
http://www.sciencemuseum.org.uk/exhibitions/genes/153.asp
Meiosis
• Meiosis is a special type of cell division that
will produce cells containing half the number
of chromosomes
• Cells containing half the number of
chromosomes are sex cells, or gametes
• Gametes contain a single set of chromosomes
and are considered haploid (half)
• All other cells containing 2 homologous sets of
chromosomes is said to be diploid
Meiosis
• For humans, the diploid number is 46
• Nearly all of our cells are diploid; the
exceptions are the gametes!
• Sexual reproduction allows a haploid sperm
cell to fuse with a haploid egg cell during the
process of fertilization producing a zygote
• The resulting zygote is diploid; it has 2 sets of
homologous chromosomes: 1 from Mom, and
1 from Dad
Meiosis
• Meiosis occurs only in reproductive organs
• During meiosis, a ‘mother’ cell divides and
produces 4 genetically distinct ‘daughter’ cells
which contain half the number of
chromosomes as the ‘other cell
• Why 4? This is because meiosis begins with
mitosis! (insert “UGH!!!”s here….)
Meiosis reduces the chromosome
number from diploid to haploid
• Just as a cell entering mitosis
has duplicated its
chromosomes, so too, does a
cell entering meiosis (resulting
in 92 chromatids)
• During prophase 1 (so called
because it is the first cycle
occuring during Meiosis 1 (out
of 2)) the process of crossing
over occurs
Crossing over
• Crossing over is the process by which aligned
chromatids of homologous chromosomes
exchange genetic segments resulting in a
genetically-new chromatid
• The driving force of genetic diversity and
evolution!
• Independent orientation of chromosomes in
meiosis and random fertilization lead to varied
offspring
Crossing over
Cell division
• In both mitosis and meiosis, the chromosomes
duplicate only once, in the preceding
interphase
• Mitosis replicates cells for growth, tissue
repair and asexual reproduction and produces
daughter cells genetically identical to the
parent cell (diploid)
• Meiosis produces haploid cells that are
genetically distinct from the parent cell
Genetic diversity
• Changes in an organism’s DNA create different
versions of genes (and resulting characteristics)
• Reshuffling of these different versions during
sexual
reproduction
produces
genetic
variation
http://www.duggarfamily.com/
Genetic diversity
• For a human there are 23 chromosomes and
223 combinations of chromosomes that
meiosis can package into gametes
• 223 equals 8 million (possible combinations)!!!
• Each gamete you produce contains 1 of ~8
million possible combinations inherited from
your father and your mother
• The random fusion of egg and sperm will
produce a zygote with any of 64 trillion (8 mil
x 8 mil) combinations of chromosomes!!!
• Occurs at
1 or more
points
along adjacent
chromatids
• Points contact
each other
• DNA is exchanged
http://www.accessexcellence.org/AB/GG/crossing.html
Homologous chromosomes carry
different versions of genes
• A pair of homologous chromosomes can bear
2 different kinds of genetic information for the
same characteristic
Brown coat (C); black eyes (E)
Coat-color
genes
Eye-color
genes
Brown
Black
C
E
C
E
C
E
c
e
c
e
Meiosis
c
White
e
Pink
White coat (c); pink eyes (e)
Chromosomes of
the four gametes
Alterations of chromosome
number and structure
• With 64 trillion possible combinations of
chromosomes, what could possibly go
wrong??!!??
• Chromosome number abnormalities do occur
and are often fatal
• An additional copy of chromosome 21 (the
short one) results in Down’s syndrome (also
known as trisomy 21)
Alterations of chromosome
number and structure
• Individuals with Down’s syndrome exhibit
distinctive features: flattened nose bridge,
short stature, heart defects, and a shortened
life span
• The additional chromosome usually comes
from the mother (with a risk of 1% with
pregnancy after age 40)
• Why?
Alterations of chromosome
number and structure
Abnormal numbers of sex
chromosomes
• Alteration in the number of copies of sex
chromosomes are not lethal
Abnormal numbers of sex
chromosomes
• The Y chromosome is very small and carries
relatively few genes
• What about the X chromosome? It’s big and it
does carry a lot of genes
• In females, the extra X chromosome is
inactivated since the presence of this
additional chromosome would otherwise be
fatal!
Abnormal numbers of sex
chromosomes
• The inactivation of X chromosomes is random;
results in a random expression of genes!
www.flickr.com/photos/mayason/3194660130/
Alterations of chromosome
structure can cause birth defects
• Other errors can occur involving deletion or
duplication of chromosome structure
• Such chromosomal changes present in sperm
and egg can cause congenital disorders
• Such changes in a somatic cell may contribute
to cancer (not inheritable); why damage to
our DNA may cause cancer (radiation, UV, etc.)