Mitosis

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Mitosis
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What is a genome?
It is all the genetic information of a cell in
the form of DNA, which is packaged into
chromosomes
Eukaryotic organisms have a characteristic number
of chromosomes, all of which are linear
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All these chromosomes are found in the nucleus of
the cell
When the cell is not dividing, the DNA and its
associated proteins exist in the form of chromatin
Chromatin begins to condense into chromosomes
when the cell is ready to divide
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What is Mitosis?
Technically, mitosis is the division of the
nucleus of a somatic cell
A somatic cell is a body cell, as opposed to a
gamete, which is a sex cell
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The Mitotic Cell Cycle
The mitotic phase alternates with interphase in
the cell cycle
Mitosis is just one part of the cell cycleThe
mitotic (M) phase, which includes both mitosis
and cytokinesis, is usually the shortest part of
the cell cycle
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Mitotic cell division alternates with a much
longer interphase, which often accounts for about
90 of the cell cycle
It is during interphase that the cell grows and
copies its chromosomes in preparation for cell
division
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Interphase can be divided into sub-phases
These sub-phases are
G1 Phase (First Gap)
S Phase (Synthesis)
G2 Phase (Second Gap)
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During all three sub-phases, the cell grows by
producing proteins and cytoplasmic organelles
Chromosomes are duplicated only during the S phase
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Why must the chromosomes duplicate?
Each daughter cell must have a complete
complement of chromosomes (2n, diploid) to
survive (46 in humans)
Only the gametes have reduced chromosome number
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Some Background
What is the difference between homologous
chromosomes and sister chromatids?
The two chromosomes of a homologous pair are
individual chromosomes that were inherited from
different parents
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These are sister chromatids
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Set of 46 homologous chromosomes of the human male
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Mitosis is conventionally broken down into five
different sub-phases
Prophase Prometaphase Metaphase Anaphase Telophase
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Cytokinesis in animal and plant cells
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Mitosis in an onion root
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Meiosis
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What is Meiosis?
Meiosis is the process by which gametes, or sex
cells, are created
During mitosis, the chromosome number is
maintained During meiosis, the chromosome number
is halved
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Whereas mitosis occurs in all parts of the body,
involving all types of somatic cells, meiosis
takes place only in the gonads Ovaries Testes
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A normal complement of chromosomes in humans is
referred to as 2n
This is because we have two copies of each
somatic chromosome, one from each parent (besides
X and Y)
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After meiosis, the normal complement of
chromosomes is cut in half We end up with (½)2n
n
This is the haploid state. At fertilization
(fusion of gametes) the 2n diploid number is
restored
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Meiosis, like mitosis, is preceded by the
replication of chromosomes
Unlike mitosis, during meiosis, this replication
is followed by two consecutive cell divisions
instead of just one
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These consecutive cell divisions are
called Meiosis I and Meiosis II
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Metaphase during mitosis
Homologous pair at metaphase plate
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2n
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Metaphase during meiosis I
Homologous pair at metaphase plate
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We are now haploid
n
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Metaphase Plate in Meiosis II
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Four haploid daughter cells
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Remember these concepts! We will be seeing
meiosis again when we study Mendelian genetics!