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Histone core
of nucleosome
Linker DNA
(a)
30 nm
Nucleosomes are Packed into successively higher-order structures
Two chromatids
(2 x 10 coils)
One coil
(30 rosettes)
One rosette
(6 loops)
One loop
(50 x 106 bp)
30 nm Fiber
“Beads-ona-string” form
of chromatin
DNA
Nuclear Scaffold
Prokaryotic and eukaryotic cells share three common features
regarding daughter cell production:
1) Replication of DNA occurs
2) Separation of replicated chromosomes
3) Cytokinesis
Binary Fission is exclusive to prokaryotes; whereas mitosis
and meiosis are exclusive to eukaryotes.
Binary Fission:
1) Replication of single molecule of DNA
2) Each copy attaches to a different area of the cell
membrane
3) Replicated chromosomes are segregated into daughter
cells following cytokinesis
p53 gene is a tumor-suppressor gene
The p53 protein product functions during
the regulation of apoptosis at the G1/S
checkpoint in the cell cycle.
Cell Cycle regulation
(by two classes of proteins)
1) Cyclins
2) Cyclin-dependent kinases [CDKs (ex. CDC
Kinase); phosphorylation activity], ex. CDC
Kinase
The above two classes of proteins combine to form a
complex
termed Maturation Promoting Factor (MPF)
Cyclin
Interphase
Other
Proteins
Mitosis Interphase
Mitosis
Interphase
Oscillations in the level of the protein cyclin were identified in sea
urchin eggs, where it rose during interphase and fell during mitosis.
The discovery that cyclin was the only protein that fluctuated during
the cell cycle suggested that it might help control the onset of mitosis.
Cyclin is indeed a regulator: it influences the activity of the cdc2
protein, which together with cyclin guides cells into mitosis.
s
CELL CYCLE
M-phase
“Mitosis”
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“IPMAT”
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N = Number of chromosomes in one chromosome set; or allele complement number
C = Amount (mass) complement of DNA
Mitosis
Equational division (No pairing of homologs)
1 X (2N, 4C; G2)
2 X (2N, 2C; G1)
Mitosis
In somatic cells
One cell division,
resulting in two
daughter cells
2n
Chromosome number
per nucleus maintained
(e.g., for a diploid cell)
One premeiotic
S phase per cell
division (e.g., for
a diploid cell)
Normally, no
pairing of
homologs
Daughter
cells
Parental cell
2n
2n
4
3
2
1
G1 S
G2
M
G1
Normally, no
pairing of
homologs
Normally, no
crossovers
Centromeres
divide at
anaphase
Conservative process:
daughter cells’ genotypes identical to parental cell’s genotype
Cell undergoing mitosis can be diploid or haploid