Transcript Meiosis

Cellular Reproduction
Chapter 9
Limits of Cell Growth:
The larger the cell, the
more difficult it is
for materials to enter
and leave it.
Rates of Cell Growth:
E. coli can double its volume in = 30 minutes.
In one day, a single cell would grow into a 14 kg mass of
bacteria. In 3 days = mass of the earth
Controls on Cell Growth: cells of the heart and
nervous system rarely grow. Cells of the skin and
digestive tract grow and divide rapidly throughout
Life; old, worn out, and damaged cells. Cells will
grow until they come into contact with each other.
Uncontrolled Cell Growth:
Cancer cells grow out of control
until they have used up all of their nutrients
Chromosomes: Made up of chromatin (DNA and
protein)
Total length of DNA is = 10,000 x length of the
chromosome.
In 1973, Don and Ada Olins and Christopher
Woodcock discovered the chromosome’s DNA
was coiled around special proteins called
histones.
DNA and histone molecules form beadlike
structures called nucleosomes.
Nucleosomes form a thick fiber, shortened by a
system of loops and coils.
Chromosome Structure
 Composed
of 2 sister chromatids and a
centromere.
Binary Fission of a Prokaryote
The Cell Cycle
 Is
the period from the beginning of one
mitosis to the beginning of the next
 Consists of interphase and M phase
Metaphase
Prophase
Anaphase
Telophase
Interphase
Controls On The Cell Cycle
Interphase
Interphase
 Is
a period of intense activity
 Proteins are made
 DNA copied
 ATP is made and utilized
 Nucleus is synthesizing mRNA to direct
cellular activities
Interphase
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Consists of G1, S, and G2
Cells do not move through the cycle at the same
rate
G1 consists of cell growth and development;
increase in supply of proteins, and synthesis of
some organelles
S stands for synthesis: DNA replication occurs
here
G2 synthesis of more organelles and the cell
prepares for mitosis (shortest of the phases)
Mitosis
 Division
of the nucleus
 Occurs in somatic or
body cells
 Can last from a few
minutes to several
days
Mitosis Diagram
Prophase (longest phase)
 Chromosomes
become visible as they
shorten and thicken
 Disintegration of the nucleolus and the
nuclear envelope
 Centrioles move apart
 Formation of the mitotic spindle fibers
 Takes up about 50-60%
of mitosis
Metaphase (shortest phase)
 Centrioles
have reached the opposite
poles
 Chromosomes line up along the equator or
middle of the cell
 Centromere of each chromosome is
attached to a spindle fiber
Anaphase (3rd phase)
 Centromeres
split
 Spindle fibers draw chromatids to poles of
cells
 Chromatids are now individual
chromosomes
Telophase
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Chromosomes have reached opposite poles
Chromosomes uncoil and become less distinct
Nuclear membrane begins to reform around
chromosomes
Spindle begins to break apart
Nucleolus reappears
Cells begin to divide by formation of cell plate in
plant cells and cleavage furrow in animal cells
Formation of two new daughter cells
Telophase (final phase)
Cytokinesis
 Division
of the cytoplasm
 Occurs at the end of telophase
 Results in two identical daughter cells
Review of Mitotic Stages
Meiosis
 The
process of gamete formation in which
the chromosome number is reduced to
half the normal number
 Requires two nuclear divisions called
Meiosis I and Meiosis II
Meiosis I

Interphase-the cell replicates its chromosomes
 Prophase I-each pair of homologous chromosomes
comes together to form a four-part structure called tetrad
 Crossing Over-homologous chromosomes exchange
genetic material
 Metaphase I-homologous chromosomes line up in
tetrads independently of each other
 Anaphase I-centromeres do not split, this ensures that
each new cell will receive only one chromosome from
each homologous pair
 Telophase I-each new cell has one half of the genetic
information
Crossing over
Pairing of
homologous
chromosomes
Chiasma
formation
Chromosomes
Breakage
and joining
Recombinant
chromosomes
Crossing Over Animated
 www.contexo.info/.../images/mimovchange
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Prophase I
Metaphase I
Telophase I
Anaphase I
Prophase II
Meiosis II
Prophase II
Anaphase II
Metaphase II
Telophase II
Result of Meiosis
4 Haploid cells Gametes
Meiosis begins by replicating
the chromosomes
Centromeres Do Not Split at Meiosis I
Centromeres Split at Meiosis II
Oogenesis
 Process
by which egg cells are made
 Results in four haploid gametes
 One functional ovum capable of
fertilization
 Three polar bodies disintegrate
Spermatogenesis
 Process
by which sperm cells are
produced
 Results in four functional sperm cells
MITOSIS
1 cell division:
2 daughter cells
2n
MEIOSIS
2 cell divisions:
4 product of meiosis
2n
Somatic cells
2n
Cells in sexual cycle
n
n
n
n
2n
n=chromosome number
Final result
Mitosis :
two genetically identical diploid cells
from one diploid parent cell
Diploid
organism: two sets of chromosomes (one
from each parent); 2n
Mitosis
Conservative process:
Daughter cells genotypes identical
to parental genotypes
 Meiosis
:
 Four genetically different haploid cells;
from one diploid parent cell
Haploid
organism: one set of chromosomes; n
Meiosis
Promotes variation among the product
of meiosis (crossing over)
Mitosis vs Meiosis
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Alike
Both have the stages
PMAT
Both go through
Cytokinesis
Both divide and make
daughter cells
Both are a form of
reproduction
Both take place inside
the cell
Chromosomes
duplicate and DNA
replication occurs in
each process
Different
 Mitosis produces two
identical daughter cell,
meiosis produces four
genetically different cells
 Crossing over and tetrad
formation (synapsis) occur
in prophase I of Meiosis
 The centromeres split in
Anaphase II
 Mitosis has one division,
Meiosis has two
 Mitosis occurs in somatic
or body cells
 Meiosis occurs in
reproductive organs;
producing gametes
 Mitosis is asexual
reproduction
 Meiosis ensures sexual
reproduction