Cell Reproduction and Genetics
Dividing to Conquer
Intro
Cells divide in order to grow, repair tissues, and
reproduce themselves. Only living things have
the ability to pass on genetic information and
replicate themselves. In fact, an important
theory of cell biology called the Cell Theory
says that all cells come from pre-existing cells.
Reproduction: Keep On Keepin’ On
• When cells replicate, they make copies of all
their parts, including their DNA, and then
divide themselves to make new cells
• If a cell makes an exact copy of itself, it’s
engaging in asexual reproduction
• Single-celled prokaryotes reproduce asexually
by binary fission (some in as little as 10
minutes)
• Some single-celled eukaryotes and individual
cells within a multi-cellular eukaryote
reproduce asexually using a process called
mitosis
• If a cell produces a new cell that contains only
half of its genetic information, that cell has
engaged in sexual reproduction.
• A special type of cell division known as meiosis
is responsible for all sexual reproduction
• Cells divide for 3 important reasons
– To make copies of cells for growth
• When you watch plants grow taller, or baby animals
grow into adults, your seeing mitosis at work
– To make copies of cells for repair
• You constantly shed skin cells from the surface of your
body. Next you get cut, watch the process as it heals.
New skin is formed from the division of skin cells that
surround the cut.
– To carry on the species
• During asexual reproduction, organisms make exacts
copies of themselves
• During sexual reproduction, gametes (containing half
the genetic information of their parent cells) to make
new individuals
Drifting Apart: Binary Fission
• Bacteria have a simple process of copying
their cells called binary fission that involves
the following steps:
– The bacterial cell makes copies of its
chromosomes
– The bacterial cell gets larger as it makes copies of
the ribosomes and molecules in the cytoplasm
– New plasma membrane and cell wall are built to
divide the cell into two
• Some cells reproduce in
as little as 10 minutes.
Reproducing from 1 cell
to thousands of cells in
just a few hours. Think
about that fact the next
time you leave food out
on the kitchen counter.
Red Light, Green Light: The Cell Cycle
• Eukaryotic cells divide
at different rates.
• Single-celled eukaryotes
divide rapidly when
food is available (THINK
YEAST)
• In multicellular organisms, some cells divide
frequently, while others rarely divide
• Different types human cells have different
behaviors.
Some cells divide all the time
• Cells on surface like skin and mucous membranes are
constantly being shed and replaced
Some cells divide when signaled to divide
• Cells in organs like the liver don’t normally divide, but may
be triggered to divide if the organ is damaged
Some cells don’t usually divide
• Most cells in the nervous tissue of humans don’t divide. If
you have an injury that involves nerve damage in the spine,
the nerves can’t be repaired
• The dividing phase of
eukaryotic cells is called
mitosis, and the
nondividing phase is
called interphase.
• The alternating cycle of
mitosis and interphase
is called the cell cycle
Interphase contains 3 subphases: G1, S, and G2
• G1 (gap) phase: During this phase, the longest of
the cell cycle, cells are active functioning cells.
They grow and copy all the cell contents except
for DNA.
• Mature cells, such as nerve cells, that won’t divide remain at
rest in this phase at a point call G0
• Cells that are going to divide must pass a test, called a
checkpoint, before they can exit G1 and enter the next phase
of interphase
• Checkpoints are points in the cell cycle where cells check to
make sure that everything is proceeding normally
• If cells can’t pass a checkpoint, repairs will be made, if
possible
• If not, the cell may be signaled to commit suicide, called
apoptosis
• In order for cells to pass
the G1 checkpoint,
several conditions must
be met:
– Signals tell the cell to
divide
– Cells must have plenty of
nutrients
– The DNA must be in
good condition
– Cells must be large
enough to divide
• S (synthesis) phase: This phase is when the
cells copy their DNA by DNA replication
– Every DNA molecule is copied exactly, forming
sister chromotids (a pair of identicle DNA
molecules) that are held together by a centromere
• G2 (gap) phase: During this phase, the cell is
getting ready to divide, making the cytoskeletal
proteins it needs to move the chromosomes
around
• The cytoskeletal proteins look like thin threads
called spindle fibers that play a role in sorting
chromosomes during mitosis
• Also during G2 cells check the work they did
during S phase at the G2 checkpoint.
• Again, if cells can’t meet the conditions of
checkpoint 2, they will be stuck in G2 and
programmed for apoptosis
• Before cells can proceed
out of G2 and into
mitosis, several
conditions are checked:
– The DNA isn't damaged
– The cell copied all the
chromosomes
– Signals tell the cell to
proceed into mitosis
Mitosis: One for you, and one for you
• Cells that enter mitosis have successfully
copied their DNA and the rest of the contents
of their cell.
• They’re ready to divide into two new cells,
each with a complete copy of everything.
• The main purpose of mitosis is to make sure
that the chromosomes are divided up
correctly
• Prophase (before):
– The chromosomes of the
cell get ready to be moved
around by coiling
themselves up into tight
packages
– The nuclear membrane
breaks down
– The Mitotic spindle forms
and attaches to the
chromosomes
– The nucleoli break down
and become invisible
• Metaphase (middle)
– The chromosomes are
tugged by the spindle
fibers until they’re lined
up in the middle of the
cell
• Anaphase (up)
– The replicated
chromosomes separate
so that the two sister
chromatids (identical
halves) from each
replicated chromosome
go to opposite sides
• Telophase (end)
– The cell gets ready to
divide into two
– New nuclear membranes
form around the two sets
of chromosomes
– The chromosomes uncoil
and spread throughout the
nucleus
– The mitotic spindle breaks
down
– The nucleoli reform and
become visible again
Cytokinesis: Seeing how daughter cells go their
own way
• In animals cells a
cleavage furrow forms
and cytoskeletal
proteins contract,
squeezing the cell in
two
• In plant cells a new cell
wall forms at the center
of the cell
How Sexual Reproduction Creates
Genetic Variation
• When living things reproduce sexually, each
parent contributes a cell to make a new
organisms.
• Sperm and egg join together, combining their
genetic information.
• If sperm and egg were like any other cells, this
combination would create a problem because
each new generation would have twice the
genetic information as the generation before.
• So, sperm and egg need to be made a special
way , a type of cell division that cuts the
genetic information of the cell in half
• That way, when sperm and egg combine, the
new organism has the right amount of genetic
information
• The special division that creates sperm and
egg is called meiosis
Meiosis:
• Meiosis is unique
because the resulting
cells have only half of
their parents
chromosomes
• Meiosis is a special type
of cell division that
occurs in gonads of
sexually reproducing
organisms
The Life Cycle of Humans
1. Cells undergo meiosis to produce gametes
•
In humans, meiosis occurs in glands called gonads: testes in males and
ovaries in females. Male gametes are called sperm, and female gametes
are called eggs
2. Sperm and egg join together in fertilization,
creating a first cell, called a zygote
•
The nuclei of the sperm cell joins with the nucleus of the egg cell.
Combining the chromosomes into the nucleus of the zygote
3. The zygote divides by mitosis to create
multicellular organisms
•
Development occurs as cells specialize to create different tissues and
organs
Counting Chromosomes
• Gametes have half the genetic material (half
the number of chromosomes) as somatic cells
• One complete set of chromosomes is called
the haploid number and is represented by the
letter N.
• Gametes have one set of chromosomes,
making them haploid (1N or N)
• Somatic cells have two sets of chromosomes,
making them diploid (2N)
o
o


Human gametes have 23 chromosomes (N)
Human somatic cells have 46 chromosomes (2N)
Pea plant gametes have 7 chromosmes (N)
Pea plant somatic cells have 14 chromosomes
(2N)
Fruit Fly gametes have 4 chromosomes (N)
Fruit Fly somatic cells have 8 chromosomes (2N)
 The number of chromosome sets a cell has is
called its ploidy
Homologous Chromosomes
• Cells have matching pairs of chromosomes
called homologous chromosomes
• Homologous chromosomes
are a pair of chromosomes
that contain the same type
of gene as each other
• Diploid chromosomes get one homologous
chromosome from mom and the other from
dad
You can identify
different types of
chromosomes
when cells are
about to divide and
the chromosomes
condense into tight
coiled bundles by
observing:
•Chromosome
length
•Position of the
centromere
•Staining
pattern
• Biologist sort
the
chromosomes
into homologous
pairs based on
the way they
look
• The sorted pairs
of chromosomes
are displayed as
a chromosome
map called a
karotype
Back to Meiosis
• The ultimate goal of meiosis is to separate the
homologous chromosomes carefully so that every
gamete gets one complete set
• In humans, that means a set that contains 1 of
each of the 23 kinds
• Meiosis has two stages:
–
–
–
–
–
Meiosis I whose purpose is to
separate pairs of homologous
chromosomes
Meiosis II whose purpose is to
separate sister chromotids
The Events of Meiosis I
• During prophase I, several events occur
– The nuclear membrane breaks down
– The nucleoli disappear
– Homologous chromosomes find each other and
pair up. The two replicated chromosomes of each
pair stick together, forming a structure called a
tetrad. Tetrads have four arms because each
replicated chromosome has two sister chromatids
– The chromosomes condense , coil up, and
become visible
– The spindle attaches to the chromosomes
Crossing over during prophase I
• When homologous chromosomes are paired up
during prophase I of meiosis, little bits of DNA are
switched
• Crossing-over involves several steps:
– Homologous chromosomes are attached along their length
– Proteins make small cuts in the DNA backbone of the homologous
chromosomes
– Proteins reseal the breaks in the DNA, attaching one homologous
chromosome to the other
• Crossing-over homologous chromosomes
during prophase I increases genetic variability
among gametes produced by the same
organism
• Every time meiosis occurs, crossing-over can
happen a little differently, shuffling the
genetic deck as gametes are made
• This is one reason why siblings can be so
different from each other
• During metaphase I, homologous pairs of
chromosomes are lined up in the middle of
the cell
• During anaphase I, homologous chromosomes
are separated from each other, and one from
each pair goes to opposite sides of the cell
• During telophase I, nuclear membranes form,
creating two nuclei. These nuclei are now
haploid because they only have one of each
type of chromosome. The spindle breaks
down
• Cytokinesis occurs, resulting in the formation
of two cells
• After meiosis is complete, both cells proceed
directly to meioisis II without going through
the stages of interphase
The Events of Meiosis II
• During prophase II, a spindle forms in each cell
and attaches to the chromosomes. If nuclear
membranes formed during telophase I, they
break down again
• During metaphase II, the chromosomes are
lined up in the middle of the cell
• During anaphase II, sister chromotids are
separated and move to opposite sides of the
cell
• During telophase II, several events occur
– Chromosome uncoil
– The spindle breaks down
– Nuclear membranes reform
– Nucleoli reappear
• Cytokinesis occurs in both cells, resulting in
the formation of four cells from the original
cell
• These four cells develop into gametes.
• In females of some species, only one of the
four cells will actually become an egg. The
other three may break down or become tissue
that supports the egg
Causes of Genetic Variation from
Meiosis and Sexual Reproduction
• Sexual reproduction increases genetic
variation in offspring, which in turn increases
the genetic variability in species
• If you look at the children in a large family
each one is unique
• Multiply that by all the families of all the
sexually reproducing organisms on Earth and
you begin to see the genetic impact of sexual
reproduction
Mutations
•
•
•
•
DNA replication occasionally makes uncorrected
mistakes when copying the cell’s genetic information
These changes are called spontaneous mutations and
they introduce change into the genetic code
Exposure of cells to mutagens (environmental agents,
such as X-rays and certain chemicals that cause changes
in DNA) can increase the number of mutations that
occur in cells
When changes occur in a cell that produces gametes,
future generations are affected
Crossing-Over
•
•
•
When homologous chromosomes come together during
prophase 1 of meiosis, they exchange little bits of DNA with
each other
This crossing over results in new gene combinations and new
chances for variety
Crossing-over is one way to explain how a person can have
red hair from his mother’s father and a prominent chin from
his mother’s mother. After crossing-over, these two genes
from different people wound up together on the same
chromosome in the person’s mother and got handed down
together
Independent Assortment
•
•
•
•
Independent assortment occurs when homologous
chromosomes separate during anaphase 1 of meiosis
When homologous pairs of chromosomes line up in
metaphase 1, each pair lines up independently from the
other pairs
So, the way the pairs are oriented during meiosis in one cell
is different from the way they’re oriented in another cell
When the homologous chromosomes separate, many
different combinations (2 23 ) of homologous chromosomes
can travel together toward the same end of the cell.
Fertilization
•
•
•
•
•
Imagine millions of genetically different sperm swimming
toward and egg
Fertilization is random so the sperm that wins the race in
one fertilization event is going to be different than the
sperm that wins the next race
Each egg is genetically different too
Fertilization produces random combinations of genetically
diverse sperm and egg, creating almost unlimited
possibilities for variation
One exception is genetically identical twins that develop
from the same fertilized egg
Nondisjuction
•
•
•
•
Sometimes meiosis doesn’t occur quite right
When chromosomes don’t separate the way they’re supposed to,
that’s called nondisjunction
The purpose of meiosis is to reduce the number of chromosomes
from diploid to haploid, something that normally happens when
homologous chromosomes separate from each other during
anaphase 1.
Occasionally a pair of chromosomes finds it just too hard to
separate, and both members of the pair end up in the same
gamete
Nondisjuction
•
•
•
Two of the final four cells resulting from the meiotic process are
missing a chromosome. This condition usually means the cells are
doomed to die
Each of the other two cells has an additional chromosome. An
extra chromosome is not something to hope for. Many times the
over endowed cells die but sometimes they survive and go on to
become sperm or egg cells
When an abnormal cell goes on to unite with a normal cell, the
resulting zygote (and offspring) has three of one kind of
chromosome. This is known as trisomy
Nondisjuction
•
•
All the cells that develop by mitosis to create the new individual
will be trisomic (meaning they’ll have that extra chromosome)
One possible abnormality occurring from an extra chromosome is
Down syndrome, a condition that often results in some mental
and developmental impairment and premature aging