Chapter 10
The Continuity of Life:
How Cells Reproduce
Why Do Cells Divide?
Prokayotic
•
Binary fission-
reproduction
Eukaryotic
•
Mitosis
– Growth and Development
– Asexual Reproduction
•
Meiosis
–
Sexual Reproduction
What Occurs During the Eukaryotic Cell Cycle?
• There
Are Two Types of Division in Eukaryotic Cells
– Mitotic Cell Division (Mitosis)
– Meiotic Cell Division (Meiosis)
• The
Life Cycles of Eukaryotic Organisms Include Both Mitotic and Meiotic Cell Division
How Is DNA in Eukaryotic Cells Organized into Chromosomes?
• Eukaryotic
Chromosomes Consist of DNA Bound to Proteins
– Chromosomes
Condense During Cell Division
Lots of DNA
•
Stretched out,
the DNA from one human somatic cell would be more than two meters long
•
A single line of
DNA from a salamander cell would extend for ten meters
How Is DNA in Eukaryotic Cells Organized into Chromosomes?
• Duplicated
Chromosomes Separate During Cell Division
How Is DNA in Eukaryotic Cells Organized into Chromosomes?
• Eukaryotic
Chromosomes Usually Occur in Pairs (diploid, 2n)
• Not
All Cells Have Paired Chromosomes (haploid, n)
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
Cell Cycle
•
Cycle starts when
a new cell forms
•
During cycle,
cell increases in mass and duplicates
its chromosomes
•
Cycle ends when
the new cell divides
Interphase
• Usually
longest part of the cycle
• Cell
increases in mass
• Number
of cytoplasmic components doubles
• DNA
is duplicated
Stages of Interphase
• G1
– Interval
or gap after cell division
• S
– Time
of DNA synthesis (replication)
• G2
– Interval
or gap after DNA replication
Mitosis
• Period
of nuclear division
• Usually
followed by cytoplasmic division
•
Four stages:
Prophase
Metaphase
Anaphase
Telophase
Stages of Mitosis
Prophase
Metaphase
Anaphase
Telophase
Early Prophase -
Mitosis Begins
Duplicated chromosomes begin to
condense
Late Prophase
• Spindle
microtubules assemble
•
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
Telophase
• Chromosomes
decondense
•
Two nuclear membranes form, one around each set
of unduplicated chromosomes
Results of Mitosis
• Two
daughter nuclei
• Each
with same chromosome number as parent cell
• Chromosomes
in unduplicated form
How Does Mitotic Cell Division Produce Genetically Identical Daughter
Cells?
•
During Prophase,
the Chromosomes Condense and Are Captured by the Spindle
•
During Metaphase,
the Chromosomes Line Up along the Equator of the Cell
•
During Anaphase,
Sister Chromatids Separate and Move to Opposite Poles of the Cell
•
During Telophase,
Nuclear Envelopes Form Around Both Groups of Chromosomes
•
During
Cytokinesis, the Cytoplasm Is Divided between Two Daughter Cells
Cytoplasmic Division
• Usually
occurs between late anaphase and end of telophase
• Two
mechanisms
– Cell
plate formation (plants)
– Cleavage
(animals)
Human Chromosome Number
•
Two sets of 23
chromosomes each
– One set from father
– One set from mother
•
Mitosis produces
cells with 46 chromosomes--two of each type (diploid)
•
Meiosis produces
cells with 23 chromosomes – one of each type (haploid)
How Does Meiotic Cell Division Produce Haploid Cells?
• Meiosis
Produces Four Haploid Daughter Nuclei
• Meiosis
I Separates Homologous Chromosomes into Two Haploid Daughter Nuclei
• Meiosis
II Separates Sister Chromatids
How Does Meiotic Cell Division Produce
Haploid Cells?
– Paired Homologues Exchange DNA
– During Metaphase I, Paired Homologues Line Up at the
Equator of the Cell
– Attachment of Chromosomes to Spindles Differs between
Mitosis and Meiosis I
– During Anaphase I, Homologous Chromosomes Separate
– After Telophase I and Cytokinesis, There Are Two
Haploid Daughter Cells
•
Meiosis II
Separates Sister Chromatids into Four Haploid Daughter Cells
Meiosis and Sexual Reproduction Produce Genetic Variability in 3 ways
• Shuffling
of Homologues Creates Novel Combinations of Chromosomes: independent assortment
• Crossing
Over Creates Chromosomes with Novel Combinations of Genetic Material
• Fusion
of Gametes Creates Genetically Variable Offspring
Fusion of Gametes Creates Genetically
Variable Offspring
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