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Reproduction
Mitosis & Meiosis
Cell Division
• allows for repair &
replacement of worn
out cells
• basis of reproduction in
every organism
• unicellular organisms
– cell division
reproduces entirely
new organisms
• allows organisms to
develop from one
fertilized egg cell into
multicellular organism
of 60 trillion cells
DNA
• body must have way to
ensure that each time a
cell divides information is
maintained & directly
copied
• information is found in
chromosomes
– can only be seen during
cell division
• remainder of the time
exists as mass of very
long fibers -chromatin
• each chromosome = one
long DNA molecule
containing thousands of
genes
Chromosomes
• genes are found on
chromosomes in the
nucleus
• number-specific to a
species
• human cells except
ovum & sperm have
46 chromosomes
• dog cells have 78
Chromosomes
• during cell division genetic
material makes an exact
duplicate of itself
• resulting in a chromosome
containing two identical
copies or sister
chromatids
• joined by a centromere
• when cell divides
chromatids separate
• one goes to one daughter
cell
• other to another daughter
cell
• resulttwo cells with
identical genetic material
Cell Cycle
• ordered sequence
of events that
begins when cell is
formed & continues
until cell divides
• two broad stages
• interphase
– growing stage
• mitotic phase
– cell division stage
Interphase
• 90% of cycle
• normal functions are
performed
• cell prepares for cell
division
• everything in
cytoplasm is doubled
• cell increases in size
• chromosomes
duplicate
Substages of Interphase
•G1 phase
•S phase
•G2 phase
G1 Phase
• mitochondria,
cytoskeletal elements,
ER, ribosomes, Golgi
membranes & cytosol
are made in quantities
for two cells
• continues until G2 stage
• centrioles begin to
replicate
• may last hours, days,
weeks, or months
S Phase
• 6-8 hours
• chromosom
es duplicate
–DNA
replicates
G2 Phase
• 2-5 hours
• last minute protein
synthesis
• completion of
centriole replication
• each chromosome
consists of 2
identical sister
chromatids linked
by centromere
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Mitotic Phase
M phase
cell divides
produces two identical daughters cell
divided into two stages
Mitosis
– nuclear division
– duplicated DNA is separated into 2
nuclei
– sister chromatids separate at
centromere
– one goes into each of two daughter
cells
• Cytokinesis
– cytoplasm divides into two cells
Cytokinesis
• cytoplasm division
• animal cells-cleavage
• first sign-appearance of
cleavage furrow
• microfilaments surround
cell
• pulled tight to divide
cytoplasm
• plant cells
• cell plate or cleavage
plate forms inside cell &
grows outward
• eventually new piece of
wall divides cell into two
Stages of Mitosis
• mitosis is
continuous
• divided into four
main stages
• Prophase
• Metaphase
• Anaphase
• Telophase
Interphase
• mitosis begin after
interphase
Prophase
• begins when
chromosomes coil tightly.
• become visible as
individual structures
• there are 2 copies of
each chromosome
• each termed a sister
chromatid
• connected by
centromere
• as chromosomes
appearnucleoli
disappear
Prometaphase
• nuclear envelope
disappears
• spindle fibers form
among
chromosomes
• kinetochore of each
chromatid attaches
to spindle fiber
• centrioles begin to
move to opposite
poles due to spindle
fibers
Metaphase
• mitotic spindle
fully formed
• chromosomes
line up on
metaphase
plate
Anaphase
• begins when centromere
of each chromosome
comes apart separating
sister chromatids
• kinetochores move
daughter chromosomes
to opposite poles of cell
• ends when complete
collection of
chromosomes has
reached poles of cell
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Telophase
nuclear membrane forms
nuclei enlarge
chromosomes uncoil
chomatin filaments form
while nucleoli reappear
mitosis is completed
cells prepare to return to
interphase
in order to make two
complete cells cytoplasm
must divide
Cytokinesis
usually takes place at
same time as telophase
Identify the Stages of
Mitosis
Meiosis
Reduction Division
Chromosomes
• every nucleus in every
somatic cell carries
genetic blueprint
• 46 chromosomes
• each paired with a like
chromosome
• 23 pairs
• 23 chromosomes came
from our mothers
• 23 from our fathers
Homologous Chromosomes
• pairs of chromosomes
are homologous
• carry same genes
• genes code for a
particular trait
• come in several forms
or alleles
• genes may be alike
– Homozygous
• genes may be unlike
– Heterozygous
Diploid & Haploid
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cells containing 23 pairs of
chromosomes are diploid
abbreviated-2n
2n = 46
all cells in human are diploid with
exception of gametes
– sperm & egg cells
have haploid number
– half number in diploid cell
23 chromosomes
n = 23
during fertilization gametes fuse
producing diploid zygote which
develops into a diploid organism
haploid gametes keep chromosome
number from doubling in each
generation
gametes are made by a special type
of cell division-meiosis or reduction
division
Meiosis
• basis of sexual reproduction
• reduction division
• cells produced contain half
number of chromosomes as
typical body cell
• one diploid cell4 haploid
cells-4 sperm or 1 egg & 3
polar bodies
• occurs in stages
• many resemble stages of
mitosis
• preceded by replication of
chromosomes
• followed by two successive
nuclear divisions: meiosis I
(reduction) & meiosis II
(division)
Phases of Meiosis I
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interphase
prophase I
metaphase I
anaphase I
telophase I
cytokinesis
Interphase
• chromosomes
duplicate
• end of stage
chromosomes
composed of two
attached, identical
sister chromatids
• centrosomes have
duplicated
Prophase I
• chromatin coils up so
individual chromosomes
become visible
• homologous
chromosomes-each
composed of two
chromatids pair up
• form tetrad
• composed of 2 chromatids
forming thick, 4-strand
structure
• spindle starts to form
between them
Crossing Over
• during prophase I
synapsis forms
(chiasmata)
• crossing over
• chromatids break
• become reattached to
different homologous
chromosomes
– rearranges genetic
information
• important to producing
variability
Metaphase I
• tetrads line up on
metaphase plate
• sister chromatids still
attached by
centromeres
• spindle fibers are
attached to
kinetochores at
centromere region of
each homologous
chromosome pair
Anaphase I
• tetrads separate
• drawn to opposite
poles by spindle fibers
• centromeres remain
intact so each pole
has two chromosomes
attached to
centromere
• only tetrad has
separated
Telophase I
• chromosomes arrive
at poles of cell
• each in duplicate
form
• cytokinesis usually
takes place at same
time
Meiosis II
• essentially same as mitosis
Prophase II & Metaphase II
• Prophase II
• nuclear envelope (if
formed) dissolves
• spindle fibers form
moving chromosomes to
middle of cell
• Metaphase II
• spindles move
chromosomes to
metaphase plate with
kinetochores of sister
chromatids of each
chromosome pointing to
opposite poles
Anaphase II & Telophase II
• anaphase II
• centromeres of sister
chromatids separate
• move toward opposite
poles of cell
• telophase II
• nuclear envelopes form
at the poles
• cytokinesis
• occurs at same time
Genetic Variation
• like begets like
• truer of asexual than sexual
reproduction
• in sexually reproducing species
like does not exactly beget like
• none of you look exactly like your
parents
• none of your siblings look exactly
like you
– unless you are an identical twin
• each offspring inherits a unique
combination of genes from
parents producing unique
combinations of traits
• genetic variability is due to two
factors
Genetic Variation
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half your chromosomes came from your father
half came from your mother
giving you 46
when you produce sperm or egg cells with a haploid number
of chromosomes some gametes got your mom’s
chromosome & some your dad’s
metaphase I-each homologous pair of chromosomes aligns
on metaphase plate
orientation of homologous pair to poles is random
there are 4 possible gametes that could form
this is true if an organism has only two pair of chromosomes
humans have 23 pairs
an independent orientation at metaphase 1
for humans with 23 pairs of chromosomes this works out to 8
million possible chromosome combinations
Genetic Variation
• also due to crossing over
• during prophase I-synapsis
of chromosomes occurs
• genetic information is
exchanged between pairs
of homologous
chromosomes
• results in new genetic
combinations
• offspring inherit gene
combinations totally
different from those
inherited from previous
generations
Genetic Variation
• Fertilization
• contributes to genetic
variability
• any egg may be fertilized
by any sperm
• one egg represents one of
eight million possibilities
being fertilized
• sperm represents one of
eight million possibilities
• resulting zygote has any
one of 64 trillion possible
combinations
Chromosomal Abnormalities
• mistakes can
occur in
meiosis
• detected using
diagnostic tool
• karyotype
Aneuploidy
• aberrant number of chromosomes
• usual cause-non disjunction
• Monosomy
– one chromosome lags behind another
– left out of newly formed cell nucleus resulting in one
daughter cell with a normal chromosome number and
one with a deficiency
– not compatible with life
• polysomy
– too many chromosomes
• autosomal polysomay may result in viable fetus but is
nearly always associated with severely disability
• Advanced maternal age, radiation, viruses & chemicals
• implicated in chromosomal abnormalities
Trisomy 21-Down Syndrome
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extra 21st chromosome
most common chromosomal
disorder
leading cause of mental challenges
occurs in 1/700 live births
first described-1866
mental retardation
protruding tongues
low set ears
poor muscle tone
short stature
epicanthal folds
flat face
often congenital heart deformities
increased susceptibility to
respiratory infections and leukemia
¾ of fetuses with syndrome are still
born or miscarried
Sex Chromosome Disorders
• unusual number of
sex chromosomes
• typically has less
debilitating
symptoms than
extra autosomal
chromosomes
• may be because Y
chromosome
carries few genes
Klinefelter Syndrome
• extra X chromosome-XXY
• abnormal sexual
development
• not diagnosed until puberty
• secondary sex
characteristics do not
develop
• boy lacks testosterone
leading to infertility
• child is tall with long arms &
legs
• female hair distribution,
breast enlargement and high
pitched voice
• testosterone therapy reduces
feminine characteristics
Turner Syndrome
• only one X chromosome
• monosomy disorder
– only one not fatal in
humans
• females have characteristic
appearance-often short
• web of skin between neck &
shoulders
• ovaries do not developsterility
• may be poor development of
secondary sex characteristics
• estrogen can alleviate
symptoms