The Somatic Cell Cycle
Diploid Zygote
MITOSIS
Diploid Zygote
Maternal
chromosome
Paternal
chromosome
MITOSIS
Maternal
chromosome
h
Diploid organism
Diploid organism
Paternal
chromosome
The Somatic Cell Cycle
St
Stages
off th
the cell
ll cycle:
l
 M Mitosis= pperiod of cell division
Intterphaase
 G1 Gap1=period during cellular interphase from the end of
one telophase to the beginning of DNA replication
 S Synthesis=period of pre-mitotic interphase when DNA is
replicated
 G2 Gap2=period during interphase after completion of DNA
replication and before beginning of prophase
G1 and G2 are called gaps in the cell cycle when no DNA synthesis
occurs
Cytological observations of the cell cycle are divided into two parts,
interphase and mitosis
The Somatic Cell Cycle
Mitosis
G2 Phase
Cell division
Interphase
S Phase
G0 Phase
G1 Phase
The Mitotic cell cycle
y is essential to cell division
and growth of an organism
Gene Expression and Cell Division conflict:
 Gene expression requires that chromosomal DNA be uncoiled to
allow access to enzymes such as RNA polymerase
 Cell division requires that chromosomes be tightly coiled to allow
g
of chromosomes to daughter
g
cells
orderlyy migration
Gene Expression and Cell Division are separated into
different stages
Life cycle of a sexually reproducing organism correlated with
the amount of DNA per cell
Fertilization
Amount of DN
NA/cell C values
S
4
Mitotic divisions
A
S
A
Meiotic divisions
S A1
3
Z
C
C
A2
2
G
G
1
0
Sequence of stages
G= Gamete, Z=Zygote, S=Period of DNA synthesis, A=Mitotic Anaphase,
C=Somatic Cells, A1=First meiotic anaphase, A2= Second meiotic anaphase
Mitosis
A mechanism by which replicated chromosomes are evenly
distributed to two daughter cells. Each of the daughter cells will
have the same number and kind of chromosomes as the parent.
parent
Stages of mitosis:
 Interphase
 Prophase
 Prometaphase
 Metaphase
p
 Anaphase
 Telophase
 Cytokinesis
Stages of Mitosis
Interphase:
 Chromosomes are fine diffuse strands and are difficult to see
with light microscopy
 Chromosomes are duplicated
 The nuclear envelope
p is intact
Prophase
 Chromosomes condense into distinct structures
 Chromosomes become visible with light microscopy
Prometaphase
 Further
F th chromosome
h
condensation
d
ti
 Nuclear envelope disintegrates
 Chromosomes become associated with spindle apparatus
 Sister
Si t chromatids
h
tid remain
i attached
tt h d att the
th centromere,
t
but
b t begin
b i
to separate at other regions
Stages of Mitosis
Metaphase
Sister
Si
t chromatid
h
tid pairs
i are aligned
li d near the
th spindle
i dl with
ith
centromeres still associated
The centromeres orient toward opposite pools and are lined up on
th metaphase
the
t h
plate
l t
Anaphase
Sister chromatid pairs separate with one member of each pair
moving toward the poles
Telophase
Chromatids reach opposite poles and poleward movement ceases
New
New nuclear envelope is formed around each diploid chromosome
set
Chromosomes begin to decondense
Stages of Mitosis
Cytokinesis
During
D
i mitosis
it i doubling
d bli off all
ll the
th cytoplasmic
t l
i organelles
ll also
l
occurs, they are distributed to daughter cells during this stage
Central ring of myosin and actin fibers contract to separate the
d h cells
daughter
ll
In plants the cell plate is laid down in preparation for the rigid cell
wall
Cytokinesis
Interphase
Prophase
Prometaphase
Prometaphase
Metaphase
Anaphase
Anaphase
Telophase
Cytokinesis
Cytokinesis
Phosphates block
active site
Inactive
P
P
Dephosphorylation
P
Phosphoorylat
ion
G2
S
active site
M
G1
Breakdown of
Cyclin B
Cyclin B
Buildup
B
ild off
Cyclin B
Inactive
P34
cdc2
(Protein kinase)
= MPF or maturation-promoting factor or mitosis promoting factor
DNA Endoreduplication During Maize Endosperm Development
Attributes of endoreduplicating nuclei
 Cell cycle consists of alternating G and S phases
 Chromosome condensation does not occur
 Entire
E ti genome replicates
li t
 Results in exact doubling of DNA content from one round to
another
 Leads to polytenization of the chromosomes
 No strand separation
 No karyokinesis
 No cytokinesis
S phase
S phase
G1
G phase
G2
Mitosis
Endoreduplicating
cell cycle
Mitotic Cell
Cycle
polytene
Normal
Endoreduplication in Maize Endosperm
 All l53 lines evaluated had increase DNA content per nucleus
 Some individual nuclei reached levels of 384C (7 rounds of
endoreduplication)
 Mean DNA content per nucleus in the central region of the
endosperm
d
i 90C in
is
i some inbred
i b d lines
li
15dap
15d
 Mean DNA content per nucleus across the entire endosperm
is 12.8C at 15dap
Allele-specific DNA degradation during maize
endosperm development
All l S
Allele
Specific
ifi Loss
L
and
d Methylation
M th l ti
 Test for methylation status indicated that regions undergoing
allele loss were hypermethylated
Hypthesis
 Regions of chromatin no longer needed for transcription are
targeted/marked for degradation, and the resulting
nucleotides form a reserve for use by the developing and
germinating embryo and perhaps the emerging seedling