Datasets:
Lecture – 3 Cell division : Mitosis & Meiosis by
Dr. Praveen Kumar
Asst. Prof. GPB
MSSSoA, CUTM, Odisha
Cell Division
All cells are derived from
pre-existing cells
New cells are produced for
growth and to replace damaged or old cells Differs in prokaryotes
(bacteria) and eukaryotes (protists, fungi, plants, & animals)
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Original DNA strand
DNA Replication DNA must be copied or replicated before cell division Each new cell will then have an identical copy of
Two new, identical DNA strands
3
Cell Cycle
DNA Copied
Cells prepare for Division
Cells Mature
4
Daughter Cells
Cell Divides into Identical cells
Interphase - G1 Stage 1st growth stage after cell
division
Cells mature by making
more cytoplasm & organelles
Cell carries on its normal
metabolic activities
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Interphase – S Stage
Synthesis stage DNA is copied or replicated
Two identical copies of DNA
Original DNA
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Interphase – G2 Stage
2nd Growth Stage Occurs after DNA has been copied All cell structures needed for
division are made (e.g. centrioles)
Both organelles & proteins are
synthesized
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What’s Happening in Interphase?
What the cell looks like
Animal Cell
What’s occurring
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Mitosis
Division of the
nucleus
Also called karyokinesis Only occurs in
eukaryotes
Has four stages Doesn’t occur in some cells such as brain cells
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Four Mitotic Stages
Prophase Metaphase Anaphase Telophase
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Early Prophase Chromatin in nucleus
condenses to form visible chromosomes
Mitotic spindle forms from fibers in cytoskeleton or centrioles (animal)
Cytoplasm
Nuclear Membrane
Nucleolus
Chromosomes
Late Prophase
Nuclear membrane & nucleolus
are broken down
Chromosomes continue
condensing & are clearly visible
Spindle fibers called
kinetochores attach to the centromere of each chromosome
Spindle finishes forming
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between the poles of the cell
Metaphase
Chromosomes, attached to the
kinetochore fibers, move to the center of the cell
Chromosomes are now lined up at the
Equator of Cell
equator
Pole of the Cell
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Anaphase
Occurs rapidly Sister
chromatids are pulled apart to opposite poles of the cell by kinetochore fibers
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Telophase
Sister chromatids at opposite
poles
Spindle disassembles Nuclear envelope forms around each set of sister chromatids
Nucleolus reappears CYTOKINESIS occurs Chromosomes reappear as
chromatin
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Cytokinesis
Means division of the cytoplasm Division of cell into two, identical
halves called daughter cells
In plant cells, cell plate forms at
the equator to divide cell
In animal cells, cleavage furrow
forms to split cell
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Daughter Cells of Mitosis
Have the same number of
chromosomes as each other and as the parent cell from which they were formed
Identical to each other, but
smaller than parent cell
Must grow in size to become
mature cells (G1 of Interphase)
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Meiosis Formation of Gametes (Eggs & Sperm)
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Facts About Meiosis Preceded by interphase which includes chromosome replication Two meiotic divisions --- Meiosis
I and Meiosis II
Called Reduction- division Original cell is diploid (2n) Four daughter cells produced that
are monoploid (1n)
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Facts About Meiosis
Daughter cells contain half the number of chromosomes as the original cell
Produces gametes (eggs & sperm) Occurs in the testes in males
(Spermatogenesis)
Occurs in the ovaries in females
(Oogenesis)
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Why Do we Need Meiosis? It is the fundamental basis
of sexual reproduction
Two haploid (1n) gametes are
brought together through fertilization to form a diploid (2n) zygote
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Replication of Chromosomes
Occurs in Interphase
Replication is the
process of duplicating a chromosome Occurs prior to
division
Replicated copies are
called sister chromatids
Held together at
centromere
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Meiosis: Two Part Cell Division
Sister chromatids separate
Homologs separate
Meiosis I
Meiosis II
Diploid
Diploid
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Haploid
Meiosis I: Reduction Division
Nucleus
Early Prophase I (Chromosome number doubled)
Spindle fibers
Nuclear envelope
Late Prophase I
Metaphase I
Anaphase I
Telophase I (diploid)
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Prophase I
Early prophase Homologs pair. Crossing over occurs.
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Late prophase Chromosomes condense. Spindle forms. Nuclear envelope fragments.
Tetrads Form in Prophase I
Homologous chromosomes (each with sister chromatids)
Join to form a TETRAD
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Called Synapsis
Crossing-Over
Homologous
chromosomes in a tetrad cross over each other Pieces of
chromosomes or genes are exchanged
Produces Genetic recombination in the offspring
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Metaphase I
Homologous pairs of chromosomes align along the equator of the cell
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Anaphase I
Homologs separate and move to opposite poles.
Sister chromatids remain attached at their centromeres.
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Telophase I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.
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Meiosis II
Gene X
Only one homolog of each
chromosome is present in the cell.
Sister chromatids carry identical genetic information.
Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.
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Meiosis II: Reducing Chromosome Number
Prophase II
Metaphase II
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Telophase II
Anaphase II
4 Identical haploid cells
Prophase II
Nuclear envelope fragments.
Spindle forms.
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Metaphase II
Chromosomes align along equator of cell.
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Anaphase II
Equator
Pole
Sister chromatids separate and move to opposite poles.
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Telophase II
Nuclear envelope assembles.
Chromosomes decondense.
Spindle disappears.
Cytokinesis divides cell into two.
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Results of Meiosis Gametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome
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Comparison of Divisions Meiosis Mitosis
Number of divisions
Number of daughter cells
Genetically identical?
1
2
Yes
2
4
No
Chromosome # Same as parent
Half of parent
Where
Somatic cells
Germ cells
When Throughout life
At sexual maturity
Role 38
Growth and repair
Sexual reproduction