TOPICS
A. The Cell
B. The Chromosome Structure
C. Cell Division
1. Mitosis 2. Meiosis
D. Life Cycles
1. Terminal or Gametic Meiosis
2. Initial or Zygotic Meiosis
3. Intermediary or Sporic Meiosis
Components of a Nucleosome :
Histone octomer: H2A, H2B, H3, H4 (2 copies each) 146 bp of superhelical DNA
ASPECT/FEATURE MITOSIS MEIOSIS
Number of divisions
1 2
Number of cells produced per division
2 4
Type of cells produced
Diploid (2N) Haploid (N)
Genetically identical to parent cell
Different from parent cell;
varied
Type of cells involved
Somatic cells Sex cells
MITOSIS
- nuclear division of a cell which produces two daughter nuclei that are genetically identical to the parent nucleus. - cytoplasmic division usually follows nuclear division.
MITOSIS
P
R
O
P
H
A
S
E
Chromosomes become more coiled and thicker, the 2 sister chromatids being connected by their CENTROMERES.
Nucleolus disappears towards the later part.
The 2 pairs of centrioles move apart and spindle fibers start to form between them.
Nuclear
envelope
Condensed
chromosomes
Mitotic spindle
Separating centrosomes
microtubules
Centrosome with 2 centrioles
MITOSIS P
R
O
M
E
T
A
P
H
A
S
E
Breakdown of nuclear envelope
Kinetochore MTs attach to kinetochores
Polar MTs push against each other, moving centrosomes apart
MITOSIS
M
E
T
A
P
H
A
S
E
Centrosomes now at opposite poles of the cell.
Chromosomes, now maximally condensed , are lined up at the metaphase plate (equatorial plate). They are held in place by spindle fibers coming from the centrosomes (kinetochore MTs).
centrosome
Kinetochore
MT
Polar MT
Astral MT
Metaphase plate
MITOSIS
A
N
A
P
H
A
S
E
Separation of sister chromatids and their migration to opposite poles
At the end of this stage, a complete set of chromosomes has assembled at each pole of the cell.
Single-strand daughter
chromosomes
Increasing separation of spindle
poles
MITOSIS
T
E
L
O
P
H
A
S
E
Chromosomes assemble in sets at the 2 poles. They uncoil and eventually look like they did at interphase. Nuclear envelope
reforms around each set of chromosomes.
Spindle fibers
disappear.
Nucleolus reforms.
Nuclear division by mitosis is now complete.
MITOSIS
C
Y
T
O
K
I
N
E
S
I
S
Animal cytokinesis
Plant cytokinesis
In animal cells, a CLEAVAGE FURROW forms.
In plant cells, a CELL PLATE forms.
centrosome
Complete nuclear
envelope
No variation in chromosome number
No genetic variation
Formation of 2 identical diploid
daughter cells
Consequences of MITOSIS:
MEIOSIS
- two successive divisions of a diploid (2N) eukaryotic cell of a sexually reproducing organism that result in four haploid (N) progeny cells, each with half of the genetic material of the original cell.
The 2 sets of chromosomes complete their migration to opposite poles.
Nuclear envelope reforms around each chromosome set.
Cytoplasm divides.
How meiosis generates GENETIC VARIABILITY:
the exchange of genetic material between homologous chromosomes during Meiosis I
the random alignment and orientation of maternal and paternal chromosomes in Meiosis I
the random alignment of the sister chromatids at Meiosis II
Initial or zygotic
Ex: most fungi & protists
Often a dominant, multicellular haploid stage (n), transitory diploid stage (2n)
Zygotic and gametic life cycles are called HAPLOBIONTIC.
Intermediary or sporic
Alternation of generations: both haploid (n) and diploid (2n) are multicellular.
has two mitosis events (DIPLOBIONTIC)
Ex: algae and plants