Download 2 Types of Cell Division

Chapter 8:
Cellular Reproduction
1. The Cell Cycle
2. Mitosis
3. Meiosis
2 Types of Cell Division
2n
1n
Mitosis:
• occurs in somatic cells (almost all cells of the body)
• generates cells identical to original
Meiosis:
• occurs in gamete production (sperm & egg)
• ½ the normal chromosome content (haploid or “1n”)
Chromosome
Content
• somatic cells
are diploid (2n)
• 2 of each
chromosome
• i.e., 1 from each
parent
• gametes are
haploid (1n)
• 1 of each
chromosome
23 pairs of human chromosomes
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1. The Cell Cycle
The Cell
Division
Cycle
M
G1
G2
S
• all eukaryotic
cells progress
through the 4
phases of the
cell cycle with
each division
Stages of the Cell Cycle
G1:
• preparation for DNA replication
• non-dividing cells are arrested at this stage of
the cell cycle (referred to as G0)
S phase:
• replication of genetic material (DNA Synthesis)
G2:
• preparation for cell division
M phase:
• cell division (Mitosis or Meiosis)
***G1, S & G2 collectively make up “Interphase”***
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2. Mitosis
4 Stages of Mitosis
1) Prophase
2) Metaphase
3) Anaphase
4) Telophase & Cytokinesis
Interphase
G1, S phase & G2
• all events in preparation
for cell division
• DNA replication
• duplication of organelles
• preparation of “mitotic
apparatus”
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Prophase
1) duplicated
chromosomes
condense, remain
attached at the
centromere
2) break down of
nuclear envelope
3) centrosomes move
to opposite poles
4) spindle fibers form
Duplicated Chromosomes
• in prophase, condensed duplicated chromosomes
remain attached at the centromere
• each “half” of duplicated chromosome referred to
as sister chromatids
Still considered as
1 chromosome!
Metaphase
• chromosomes
align along the
metaphase plate
of the cell
• mediated by spindle
fibers connecting
each centromere to
a centrosome at ea
“pole” of the cell
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Anaphase
• sister chromatids
are pulled to
opposite poles of
the cell
• separate at the
centromere, each
chromatid still
attached to a
spindle fiber
• spindle fibers
shorten, pulling
each chromatid to
opposite poles
Telophase
Opposite of prophase:
• nuclear envelope
reforms around
chromosomes at
each pole
• spindle fibers break
down
• chromosomes
decondense
Cytokinesis begins…
Cytokinesis
Division of the
cytoplasm
completes the
process:
• the 2 daughter cells
are genetically
identical to the
original parent cell
• each daughter cell
is now in G1 (the
beginning of
interphase)
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3. Meiosis
Meiosis Produces 1n Gametes
Meiosis requires 2 rounds of cell division:
Meiosis I
• differs from mitosis in prophase, metaphase, anaphase
Meiosis II
• basically the same process as mitosis
(chromosome #)
1n
2n
1n
(DNA content)
4n
2n
Meiosis I
Differs from mitosis in several key ways:
prophase I
metaphase I
anaphase I
Prophase I: homologous chromosomes pair, exchange
genetic material by “crossing over”
Metaphase I: homologous chromosomes align as pairs
Anaphase I: homologous chromosome pairs separate
(sister chromatids are NOT separated)
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Coat-color
genes
C
“Crossing Over”
in Prophase I
Eye-color
genes
E
Tetrad (homologous pair of
chromosomes in synapsis)
e
c
1
Breakage of homologous chromatids
C
E
c
e
2
• an important source of
new genetic combinations
(genetic variability)
Joining of homologous chromatids
C
E
c
e
Chiasma
3
Separation of homologous
chromosomes at anaphase I
C
E
C
e
E
c
e
c
4
Separation of chromatids at anaphase
II and completion of meiosis
C
E
C
e
c
E
c
e
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
Independent Assortment of Chromosomes
due to alignment in Metaphase I
• an additional source of genetic variability
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
2n
possibilities
(n = # of
homologous
pairs, e.g. 23)
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
Meiosis II
prophase II
metaphase II
anaphase II
telophase II/cytokinesis
• essentially the same as mitosis
• results in four haploid (1n) gametes
• each is genetically unique due to 1) “crossing over” and
2) independent assortment of homologous chromosomes
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Key Terms for Chapter 8
• cell cycle, mitosis, meiosis
• haploid, diploid, somatic cell, gamete
• interphase, prophase, metaphase, anaphase
telophase, cytokinesis
• centromere, centrosome, spindle fibers
• sister chromatids vs non-sister chromatids
• homologous chromosomes
• crossing over, independent assortment
Relevant Review Questions:
1-6, 8, 9, 13, 17, 20
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