Cell Reproduction
Mitosis and Meiosis
Two types of Reproduction
Sexual Reproduction
 the production of new
living organisms by
combining genetic
information from two
individuals of different
types (sexes).
Asexual Reproduction
 is a mode
of reproduction by which
offspring arise from a
single organism, and
inherit the genes of that
parent only.
Sexual Reproduction
 Every cell in your body has 46 chromosomes (or 23
sets) this is called a diploid cell.
 Sexual reproduction includes two sex cells called a
egg and sperm.
 Sex cells (gametes) have 23 chromosomes; this is
called haploid cell. (half the number of
chromosomes)
 The sex cells combine creating a full set of
chromosomes for an animal or plant. This is called
fertilization.
 Sexual reproduction introduces
new gene combinations in a population
through genetic recombination.
Sexual Reproduction in a Flowering plant.
Eggs in the ovary are fertilized through the pollen
tube forming a zygote which in turn forms a seed.
Pollen are the sperm cells which will fertilize the eggs.
Variations in Genes
Plants and animals receive genes from both the
mother and the father producing offspring with a
different genetic combination.
Combination of Genes
Combination of Genes
different results.
Asexual Reproduction
Binary Fission
Prokaryotic fission,
which is binary
fission, is a form of
asexual
reproduction and
cell division used by
all prokaryotes,
(bacteria and
archaebacterial),
and some
organelles within
eukaryotic
organisms (e.g.,
mitochondria).
Budding
Budding is a form of
asexual reproduction
in which a new
organism develops
from an outgrowth or
bud due to cell
division at one
particular site. The new
organism remains
attached as it grows,
separating from the
parent organism only
when it is mature,
leaving behind scar
tissue.
Vegetation
Reproduction
Vegetative
reproduction is a
type of asexual
reproduction
found in plants
where new
independent
individuals are
formed without
the production of
seeds or spores.
Fragmentation/
Regeneration
Fragmentation
also referred to as
regeneration is a
form of asexual
reproduction
where a new
organism grows
from a fragment of
the parent. Each
fragment develops
into a mature, fully
grown individual.
Spore Formation
parent organism
produces spores:
single cells that
can develop into
new individuals by
repeated mitosis
The Great Divide
05/04
To Be Answered…
THINK:
 How many cells are you composed of?
 When an organism grows bigger do you
get more cells or just bigger cells or both?
 When do your cells divide the fastest?
Slowest?
 Do cells ever stop dividing?
 Are all cells capable of division and
replacement?
Why Would a Cell Divide?
 As cells absorb nutrients and get larger, the
volume of the cell increases faster than the
surface area
 This means that a cell can no longer absorb
nutrients and get rid of wastes fast enough to
support its demands (volume)
 So what’s a cell to do?
 Solution: divide in 2!
Surface area
for exchange
not great
enough to
support cell’s
needs
When Would a Cell Divide?
 Growth
 Repair or Replacement
 Cancer
Different cells divide at different rates:
 Most mammalian cells = 12-24 hours
 Some bacterial cells = 20-30 minutes
Getting Older…
 All cells are only allowed to complete a
certain number of divisions
 Then they die (programmed cell death)
How does cell division change over a lifetime?
 Childhood = cell division > cell death
 Adulthood = cell division = cell death
 The Later Years = cell division < cell death
The Cell Cycle
Stages of the Cell Cycle
 2 stages = interphase (growth & replication of
DNA) & mitotic phase (division of cell into 2
daughter cells)
 Cell spends
about 90% of
the time in
interphase
Cell Cycle Tidbits
How long is one cell cycle?
 Depends on the cell- skin cells = ~24
hours, nerve cells = never after
maturity, cancer cells = very short
 Remember: every cell only has a
certain # of divisions it can undergo,
then it dies = apoptosis (programmed
cell death)
Interphase
 Divided into 3 phases:
 G1 (1st gap) = small cell is absorbing nutrients,
growing & doing its job (i.e. making proteins)
 S (synthesis) = cell is continuing to grow &
duplicates its DNA (i.e. chromosomes) in
preparation for making duplicate cells during
mitosis
 G2 (2nd gap) = cell keeps growing & doing its
job (i.e. making proteins); it grows too
big…solution = divide in 2
Mitosis: A Closer Look
Interphase
 Prior to entering the mitotic
phase, the cell has just
come out of interphase
 Replicated DNA during S
(synthesis)
 2 complete sets of
chromosomes that must
be distributed equally
between 2 cells = mitosis
The Mitotic Phase
 Equal distribution of 2 sets of chromosomes (DNA)
into 2 identical daughter cells
 Divided into 4 stages of Mitosis:
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cytokinesis
Prophase
 Chromatin condenses
visible chromosomes
 Appear as sister
chromatids held together
by centromere
 Nuclear membrane
dissolves
 The centrioles migrate to
opposite poles & spindle
fibers form between them
Metaphase
 Chromosomes
line-up on the
metaphase
plate
 Centromeres
are attached to
spindle fibers
Anaphase
 Centromeres divide
 Spindle fibers contract
 Result = sister
chromatids are pulled
away from one another
towards the poles
Telophase
 The chromosomes
reach the poles
 Nuclear
membranes form
around the 2 new
nuclei
Cytokinesis
 The cytoplasm
distributed equally
between the 2 new
cells
 In animals, a
cleavage furrow
forms from outside
 In plants, a cell
plate forms from
inside out
Animal
Plant
What Mitosis Actually Looks Like
Interphase
Metaphase
Prophase
Anaphase
Telophase
http://www.sci.sdsu.edu/multimedia/mitosis/mitosis_gif2.html
http://science.nhmccd.edu/biol/bio1int.htm
Summary of Mitosis
Plant Cell Mitosis
What Happens After Mitosis?
 The cell
returns to
interphase
 Chromosomes
uncoil back
into chromatin
 The cycle
repeats itself
over & over…
At What Stage Are Our Cells At In The Cell Cycle?
 Different cells can
be in different
stages
 Interphase
 Mitosis:
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cytokinesis
Can You Identify the Stages of Mitosis?
Put the
following
mitosis
stages in the
correct
sequence
The Guarantee
 The product of
mitosis is 2 cells
 The daughter cells
are identical to
each other & to the
mother cell
Why is this so
important?
Mother
cell
Identical
daughter
cells
The Daughter Cells
 In humans, the 2
daughter cells will have
46 chromosomes (23
pairs)
 1 chromosome
originally from mom & 1
from dad
 Each chromosome is
said to have the same
gene sequence
Identical
daughter
cells
Review of Asexual Reproduction
 Mitosis is a
form of
asexual
reproduction
 New
individuals are
produced by 1
parent & thus,
are identical
to their parent
Mother
cell
Runners produces by
strawberries
Identical
Budding by hydra & yeast
daughter
cells
Cuttings from plants
https://www.youtube.c
om/watch?v=JcZQkm
ooyPk
Meiosis – A Source of Distinction
Why do you share some but not all characters of each parent?
What are the rules of this sharing game?
At one level, the
answers lie in
meiosis.
Meiosis does two things 1) Meiosis takes a cell with two copies of every
chromosome (diploid) and makes cells with a
single copy of every chromosome (haploid).
This is a good idea if you’re going to combine
two cells to make a new organism. This trick
is accomplished by halving chromosome
number.
In meiosis, one diploid cells produces four
haploid cells.
Why do we need meiosis?
• Meiosis is necessary to halve the number
of chromosomes going into the sex cells
Why halve the chromosomes in gametes?
• At fertilization the male and female sex
cells will provide ½ of the chromosomes
each – so the offspring has genes from
both parents
2) Meiosis scrambles the specific
forms of each gene that each sex
cell (egg or sperm) receives.
This makes for a lot of genetic
diversity. This trick is accomplished
through independent assortment and
crossing-over.
Genetic diversity is important for the
evolution of populations and species.
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Fertilization
Meiosis
• Sex cells divide to produce gametes (sperm or egg).
• Gametes have half the # of chromosomes.
• Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
• Meiosis is similar to mitosis with some chromosomal
differences.
Spermatogenesis
n=23
human
sex cell
sperm
n=46
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=46
n=23
meiosis I
meiosis II
Meiosis I (four phases)
• Cell division that reduces the chromosome
number by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
• Centriole pairs also replicate.
Interphase I
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
• Homologous pairs (tetrads) carry genes controlling
the same inherited traits.
• Each locus (position of a gene) is in the same
position on homologues.
• Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Karyotype
• A method of organizing the chromosomes of a
cell in relation to number, size, and type.
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologue
Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans the
“Autosomes”
are sets 1 - 22
21 trisomy – Downs
Syndrome
Can you see the
extra 21st
chromosome?
Is this person
male or female?
Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
In Humans the
“Sex
Chromosomes”
are the 23rd set
XX chromosome - female
XY chromosome - male
Boy or Girl? The Y Chromosome “Decides”
Y chromosome
X chromosome
Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
• Chiasmata (chiasma) are the sites of crossing
over.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Another Way Meiosis Makes Lots of
Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of
different gamete types produced by independent
assortment.
Sex Chromosomes
XX chromosome - female
XY chromosome - male
Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes
come together to form a tetrad.
• Tetrad is two chromosomes or four
chromatids (sister and nonsister chromatids).
Prophase I
spindle fiber
aster
fibers
centrioles
Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
Metaphase I
OR
metaphase plate
metaphase plate
Question:
1. In terms of Independent Assortment
-how many different combinations
of sperm could a human male
produce?
Answer
• Formula: 2n
• Human chromosomes:
2n = 46
n = 23
• 223 = ~8 million combinations
Anaphase I
• Homologous chromosomes separate and
move towards the poles.
• Sister chromatids remain attached at their
centromeres.
Anaphase I
Telophase I
• Each pole now has haploid set of
chromosomes.
• Cytokinesis occurs and two haploid daughter
cells are formed.
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
metaphase plate
metaphase plate
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
https://www.youtube.com/watch?v=zGVBAHAsjJM
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember:
four haploid daughter cells
produced.
gametes = sperm or egg
Variation
• Important to population as the raw
material for natural selection.
• Question:
2. What are the three sexual sources of
genetic variation?
Answer:
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!
Question:
3. A cell containing 20 chromosomes
(diploid) at the beginning of meiosis would,
at its completion, produce cells containing
how many chromosomes?
Answer:
• 10 chromosomes (haploid)
Question:
4. A cell containing 40 chromatids at the
beginning of meiosis would, at its completion,
produce cells containing how many
chromosomes?
Answer:
• 10 chromosomes
Sources
• www.ursulinehs.org/powerpoint/meiosi
s.ppt
• www.biology4teachers.com/Cell%20Divi
sion/MEIOSIS.ppt
• http://www.iteachbio.com/Life%20Scien
ce/LifeFunctionsandTheCell/Meiosis.ppt