The Continuity of Life: Cellular Reproduction

– In sexual reproduction
• Fertilization of sperm and egg produces offspring

– In asexual reproduction
• Offspring are produced by a single parent, without the participation of sperm and egg
Like begets like, more or less
• Some organisms reproduce asexually
• And their offspring are genetic copies of the parent and of each other

Other organisms reproduce sexually
• Creating a variety of offspring

Cells arise only from preexisting cells

– Cell division is at the heart of the reproduction of cells and organisms

• Because cells come only from preexisting cells
Division Mechanisms
Prokaryotic organisms
– Binary fission

Eukaryotic organisms
– Mitosis
– Meiosis

• Prokaryotes reproduce by binary fission
– Prokaryotic cells
• Reproduce asexually by cell division

– As the cell
replicates its
single chromosome,
the copies move apart
• And the growing
membrane then
divides the cells

The large, complex chromosomes of eukaryotes duplicate with each cell division

– A eukaryotic cell has many more genes than a prokaryotic cell

• And they are grouped into multiple chromosomes in the nucleus

– Individual chromosomes contain a very long DNA molecule associated with proteins
• And are visible only when the cell is in the
process of dividing
– If a cell is not undergoing division
• Chromosomes
occur in the
form of thin,
loosely packed
chromatin fibers

Before a cell starts dividing, the chromosomes
replicate
• Producing sister chromatids joined together at the centromere

– Cell division involves the separation of sister chromatids
• And results in two
daughter cells,
each containing
a complete and
identical set of
chromosomes

The cell cycle multiplies cells
– The cell cycle consists of two major phases

During interphase
• Chromosomes duplicate and cell parts are made

During the mitotic phase
• Duplicated chromosomes are evenly distributed into two daughter nuclei

The cell cycle multiplies cells
– The cell cycle consists of two major phases
Interphase – G1
• Interval (“Gap”) of cell growth


The cell cycle multiplies cells
– The cell cycle consists of two major phases
Interphase - S
• DNA replication (“Synthesis”)
• Chromosomes duplicated

The cell cycle multiplies cells
– The cell cycle consists of two major phases
Interphase – G2
• Second interval (“Gap”)
• Preparation for division
Control of the Cycle
• Once S begins, the cycle automatically runs through G2 and mitosis
• The cycle has a built-in molecular brake in G1
• Cancer involves a loss of control over the cycle, malfunction of the “brakes”
Stopping the Cycle
• Some cells normally stop in interphase
– Neurons in human brain
– Arrested cells do not divide
• Adverse conditions can stop cycle
– Nutrient-deprived amoebas get stuck in interphase

During interphase
• Chromosomes duplicate and cell parts are made

During the mitotic phase
• Duplicated chromosomes are evenly distributed into two daughter nuclei

The cell cycle multiplies cells
– The cell cycle consists of two major phases
Mitosis
• Period of nuclear division
• Usually followed by cytoplasmic division
– cytokinesis
• Four stages
Stages of Mitosis
Prophase
Metaphase
Anaphase
Telophase

PMAT
Early Prophase -
Mitosis Begins
Duplicated chromosomes begin to condense
Late Prophase
• New microtubules are assembled
• One centriole pair is moved toward opposite pole of spindle
• Nuclear envelope starts to break up
Transition to Metaphase
Metaphase
• All chromosomes are lined up at the spindle equator

• Chromosomes are maximally condensed
Anaphase
• Sister chromatids of each chromosome are pulled apart

• Once separated, each chromatid is a chromosome
The Spindle Apparatus
• Consists of two distinct sets of microtubules
– Each set extends from one of the cell poles
– Two sets overlap at spindle equator
• Moves chromosomes during mitosis
Spindle Apparatus

Telophase
• Chromosomes decondense

• Two nuclear membranes form, one around each set of unduplicated chromosomes
Cytoplasmic Division
• Usually occurs between late anaphase and end of telophase
• Two mechanisms
– Cell plate formation (plants)
– Cleavage (animals)

Cytokinesis differs for plant and animal cells
– In animals
• Cytokinesis occurs
by a constriction of
the cell (cleavage)
Animal Cell Division

– In plants
• A membranous cell
plate splits the cell
in two
Cell Plate Formation
Results of Mitosis
• Two daughter nuclei
• Each with same chromosome number as parent cell
• Chromosomes in unduplicated form



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