Mitosis and Meiosis (Alberts et al.)
The cell cycle is an orderly sequence of events in which the cell duplicates its contents
and then divides in two. The M-phase consists of mitosis and cytokinesis. During the Sphase (S = synthesis), the cell replicates its nuclear DNA. The S-phase is flanked by two
phases where the cell continues to grow, the G1-phase is the interval between M- and Sphase (G=gap) and the G2-phase is the interval between the end of S-phase and the
beginning of M-phase.
http://teachline.ls.huji.ac.il/72373/substance_x/cell-cycle2.jpe
Mitosis is a process of cell division which results in the production of two daughter cells
from a single parent cell. The daughter cells are identical to one another and to the
original parent cell.
Stages of Mitosis:
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Interphase: (not strictly part of mitosis) The cell increases in size, the DNA of
the chromosomes is replicated and the centrosome (organelle that acts as the
microtubule organizing centre) is duplicated
Prophase: The replicated chromosomes, each consisting of two sister chromatids,
condense. Outside the nucleus, the mitotic spindle assembles between the two
centrosomes which have replicated and moved apart.
Prometaphase: the nuclear envelope breaks down and the chromosomes attach
to spindle microtubules via their kinetochores , and undergo active movement.
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Metaphase: the chromosomes are aligned at the equator of the spindle, midway
between the spindle poles. The paired kinetochore microtubules on each
chromosome attach to opposite poles of the spindle
Anaphase: The centromeres divide and the paired chromatids separate to form
two daughter chromosomes. Each is pulled towards the pole it faces.
Telophase: Daughter chromosomes arrive at the poles and the microtubules
disappear. The condensed chromatin expands and the nuclear envelope reappears
creating two daughter nuclei. The cytoplasm begins to divide and the cell wall
between the two pinches inwards
Cytokinesis: the cytoplasm is divided in two by a contractile ring of actin and
myosin with pinches the cell to create two daughter cells each with its own
nucleus.
Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced.
Meiosis involves a reduction in the amount of genetic material.
Meiosis comprises two successive nuclear divisions with only one round of DNA
replication.
Four stages can be described for each nuclear division.
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Interphase: Before meiosis begins, genetic material is duplicated.
First division of meiosis
o Prophase 1: Duplicated chromatin condenses. Each chromosome consists
of two, closely associated sister chromatids. Crossing-over (two
chromosomes of a homologous pair may exchange segments producing
genetic variations in germ cells) can occur during the latter part of this
stage.
o Metaphase 1: Homologous chromosomes align at the equatorial plate.
o Anaphase 1: Homologous pairs separate with sister chromatids remaining
together.
o Telophase 1: Two daughter cells are formed with each daughter
containing only one chromosome of the homologous pair.
Second division of meiosis: Gamete formation
o Prophase 2: DNA does not replicate.
o Metaphase 2: Chromosomes align at the equatorial plate.
o Anaphase 2: Centromeres divide and sister chromatids migrate separately
to each pole.
o Telophase 2: Cell division is complete. Four haploid daughter cells are
obtained.
One parent cell produces four daughter cells. Daughter cells have half the number of
chromosomes found in the original parent cell and with crossing over, are genetically
different.
Meiosis differs from mitosis primarily because there are two cell divisions in meiosis,
resulting in cells with a haploid number of chromosomes.
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Diploid cells have two homologous copies of each chromosome, usually one
from the mother and one from the father.
Haploid: A set of chromosomes containing only one member of each
chromosome pair. The sperm and egg are haploid and, in humans, have 23
chromosomes.
Euploidy, or the euploid number is the normal number of chromosomes within a
cell for a species, for example the euploid number of chromosomes in a human
cell is 46.
Aneuploidy is defined as an abnormal number of chromosomes.
Chromosome segregation: The chromosomes are simultaneously separated into
pairs called sister chromatids, once aligned the sisters are pulled apart and two
daughter cells produced each with a full complement of chromosomes.
https://eapbiofield.wikispaces.com/file/view/repro_diagram2.gif
Principle types of chromosomal abnormalities (Haslett et al.):
Numerical Abnormalities:
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Triploidy (3n) and Tetraploidy (4n) - result in spontaneous abortion
Parthenogenesis: 2n from same parent – spontaneous abortion
Sex chromosome aneuploidies:
o Phenotypically male
 47, XXY – Klinefelter’s syndrome
 47, XYY – usually asymptomatic
o Phenotypically female
 47, XXX – trisomy X – usually asymptomatic
 45, X0 – Turner’s syndrome
Aneuploidy:
o trisomy 21: Down’s Syndrome
o trisomy 18: Edward’s syndrome
o trisomy 13: Patau’s syndrome
Structural Chromosome Aberrations:
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Inherited
o 46, XY, del(5p) – Cri du chat syndromw
o 45, XY, t(14;21) – fusion of 14 and 21, normal but carrier
o 46, XY, t(14;21) trisomy 21 – Down’s syndrome
Acquired
o 46, XY, t(9,22) Philadelphia chromosome (chromic myeloid leukemia)
o 46, XY,t(2,8) or t(8,14) or t(8,22) – Burkitt’s Lymphoma
References:
1. Essential Cell Biology, Alberts et al., Garland Publishing, Inc., 1998. ISBN: 08153-2045-0
2. Davidson’s Principles and Practice of Medicine, 18th Ed, Haslett et al., Churchill
Livingstone,1999. ISBN:0443 059446