Mitosis & Meiosis

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Function of Cell Division (Mitosis & Meiosis)

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Growth, repair, and reproduction

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Mitosis

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Produces 2 genetically identical daughter cells.
Diploid (2n)
Chromosome number remains the same through division.
Used for growth and repair.

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Four phases of Mitosis

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Prophase, metaphase, anaphase, telophase

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Meiosis

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Produces gametes in reproductive structures called gonads.
Gametes are haploid (n)

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Two divisions of Meiosis

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Meiosis 1 & Meiosis 2

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Meiosis 1

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a.k.a. Reduction division
-Synapsis and crossing over occur

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Meiosis 2

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Like mitosis (mechanics)
Sister chromatids separate

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Kinetichore

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Disk-shaped protein that attaches chromatid to mitotic spindle.

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Centromere

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Specialized region holding sister chromatids together.

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Sister chromatids

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2 strands of DNA--one is an exact copy of the other

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Chromosome structure

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Kinetichore, Centromere, Sister chromatids

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Cell cycle

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Interphase, Mitosis, Cytokinesis

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Interphase (cell cycle)

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G1--period of rapid growth
S--synthesis or replication of DNA
G2--continued cell growth (preparation for cell division)
Most of cell's life spent in this phase.

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Mitosis (cell cycle)

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Division of the nucleus

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Cytokinesis (cell cycle)

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Division of the cytoplasm

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Cell Cycle: A Closer Look

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Built in checkpoints in G1, G2, and Mitosis will either stop, or trigger cell division

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Restriction Point

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Most important checkpoint in the cell cycle. Without the "go-ahead" signal, the cell becomes non-dividing.
Arrested in G0: Muscle and nerve cells.

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Timing in Cell Cycle

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Controlled by cyclins and cyclin-dependent kinases (CDK's)

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Prophase

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Nuclear membrane disintegrates.
Strands of chromosomes condense and become visible.
Nucleolus disappears.
Longest part of the mitotic phases.

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Metaphase

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Chromosomes line up along the metaphase plate.
Centrioles move to poles of the cell.
Spindle fibers run from centrioles to the kinetichores, forming the mitotic spindle.

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Anaphase

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Sister chromatids separate as spindle fibers pull them apart.
Shortest mitotic phase.

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Telophase

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Chromosomes cluster at poles of the cell.
Nuclear membrane re-forms.
Chromosomes uncoil; become invisible.

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Crossing Over

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Exchange of genetic material between non-sister chromatids.
Increases genetic variation in offspring.
Produces recominant chromosomes.
--Consists of combined genes from two parents.
Occurs during synapsis.

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Synapsis

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Occurs during prophase 1 in Meiosis 1.
Homologous chromosomes pair up and form tetrads.

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Chiasmata

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Site of crossing over between non-sister chromatids.

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Spermatogenesis

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Gamete formation in males.
Produces 4 viable haploid sperm cells.
Swimming nucleus.

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Oogenesis

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Gamete formation in females.
Produces 4 haploid cells.
Cytokinesis is unequal.
Eventually produces one viable egg (ovum) and 3 polar bodies.
Ovum has more cytoplasm and therefore more nutrients for nourishment once fertilization occurs.
Almost all mitochondria inherited via female.

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Asexual Reproduction

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Single parent.
Produces a clone.
Genetically identical to parent.

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Sexual Reproduction

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2 parents.
Haploid cells join.
Offspring have traits of both parents.

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Types of asexual reproduction

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Fission--bacteria
Fragmentation--sea stars
Budding--new individuals split off from existing ones (i.e. sponges)

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Advantages of Asexual Reproduction

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Simplest and most primitive.
Produces many offspring quickly.
No need to search for mate.

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Disadvantages of Asexual Reproduction

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DNA varies little--can't adapt to changes
Requires stable environment

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Advantages of Sexual Reproduction

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Different combinations increases diversity
Allows for adaptation

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Disadvantages of Sexual Reproduction

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Requires large expenditure of energy

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Evolution of Sexual Reproduction

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As organisms increased in size--multicellular breaks become more frequent; need to repair.
Only diploid cells can repair chromosome breaks

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Life Cycle

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Span of life from one generation to the next.

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Eukaryotic Life Cycles

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Haploid life cycle, diploid life cycle, or alternation of generations

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Haploid Life Cycle

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Simplest
Major portion of life cycle is as a haploid organism
Zygote is only diploid cells--divides meiotically (i.e. chlamydomonas)

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Diploid Life Cycle

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Most of life cycle is spent as a diploid organism
Gametes are the only haploid cells in the cycle
Zygote--divides mitotically

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Alternation of Generations

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Characteristic of sexual life cycle of plants
Generations alternate between n and 2n
Haploid=gametophyte
Diploid=sporophyte

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Mosses and other Bryophytes

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Haploid generation dominates
Sporophyte is dependent on the gametophyte

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Ferns

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Seedless vascular plants
2 generations are independent, but sporophyte is larger
Fertilization
--Flagellated sperm swims from antheridia to archegonia to fuse with eggs.

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Seed plants--flowering plants

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Sporophyte dominates
Gametophyte is dependent on the sporophyte

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Contact Inhibition

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a.k.a. density-dependent inhibition
Property of normal cells where they stop dividing when they are in contact with other cells.
Cancer cells lack this.
--Cancer=uncontrolled cell growth