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Cell Reproduction
Stem Cell Shakes
In The News
Dividing Nuclear Material
• Cells must accurately separate genetic
material during cell reproduction
• Methods
– Mitosis
– Meiosis
Mitosis
• Produces two cells identical to original cell
• Each cell has full DNA complement
• Used for growth and repair of somatic (body)
cells
Meiosis
• Produces 4 cells that are not identical to
parent cell
• Daughter cells have half genetic complement
• Produces gametes (sex cells)
Animal Life Cycle
• Egg = female gamete
• Sperm = male gamete
• Zygote
– Fusion of egg & sperm
– Full complement of
genetic material
Diploid Cells (2N)
• Full genetic complement
• 23 pair of chromosomes in humans
Haploid Cells (N)
• Produced during meiosis
• Contain only one set of chromosomes
• Reduction of chromosomes allows for
combination to form diploid zygote
Fertilization
• One sex cell from each parent joins
• Creates diploid zygote
• Process is called sexual reproduction
Animal Life Cycles
• Diploid phase dominates
• Gametes live hours to days
Plant Life Cycles
• Most have multicellular haploid phase
• Phase names
– Gametophyte = haploid
– Sporophyte = diploid
• Either phase can dominate, depending
on plant type
Single Cell Eukaryote
Reproduction
• Reproduce by mitosis
• Called asexual reproduction
• Produced two identical organisms
Asexual Reproduction
• All single cell eukaryotes
• Some plants
• Some animals
Cell Cycle (Life Cycle)
Interphase
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Most of cell cycle
Cell grows
Organelles replicated
DNA replicated
Readies for mitosis
Condenses DNA
G1 Interphase
• Cell growth-doubles in
size
• Carries out its normal
life functions
S Interphase
• DNA replicated
• At the end of this
phase, the cell
will contain two
identical copies
of heredity info.
G2 Interphase
• DNA condenses into chromosomes
– Strands = sister chromatids
– Chromatids connected by centromere
Stages of Mitosis
• Prophase
• Metaphase
• Anaphase
• Telophase
Prophase
• Chromosomes shorten & thicken
• Nucleolus & nuclear envelope disappears
Prophase
• Spindle fibers formed from microtubules
• Microtubules surround microtubuleorganizing center (centrosome)
• Chromosomes attach to spindle fibers at
kinetochore
Metaphase
• Pairs of sister chromatids align at center
– Forms metaphase plate
Anaphase
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Chromatids separate at centromere
Chromatids pulled in two directions
Chromosomes move toward poles
Equally divides hereditary material
Telophase
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Cell readied for division (cytokinesis)
Spindle fibers disassemble
Nuclear envelope reforms
Nucleolus reappears
Cytokinesis
• Cell division after mitosis
• Cleavage furrow enlarges
Animal Cytokinesis
• Cell pinched in two (lack cell walls)
• Pinching by microfilaments contracting
Plant Cytokinesis
• New cell wall must be laid down
• Cell plate forms (Golgi Complex)
Mitosis
Cancer
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Deregulation of cell cycle
Loss of control of mitosis
Result of mutation
Over 200 types of cancer exists
Number of cancer cases will
double by 2050, as an increasing
proportion of U.S. population
reaches older ages
United States
Cancer Death Rates
Characteristics of Cancer
(no matter what type)
• Uncontrolled cell growth- most significant
characteristic of all cancer cells
• Loss of cell differentiation (specialization)
• Invasion of normal tissues, occupying
space in which normal cells would
otherwise reside
• Metastasis or spread to multiple sites
Stages of Cancer
1. Initiation
2.Promotion
3.Progression
Initiation of Cancer
(Transformation)
• Series of gene mutations damages DNA
• Proto-oncogenes become oncogenes (“on”
switches for tumors or masses)
• Usually additional mutations are needed
– Affect tumor suppressor genes, ”off” switches
– These genes are inactivated, allowing cancerous growth
• Initiation does not directly result in cancer, it
results in a precancerous cell.
• Heredity acts only as an initiator, not a promoter
Promotion of Cancer
• Cells are stimulated to grow & divide when they
normally would not.
• It is a gradual process, unlike initiation
• Carcinogens – agents that both initiate & promote
cancer
– Viruses
– Chemicals
– Radiation
• Benign cells
– Masses of partially transformed cells, confined &
encapsulated
– Cells exhibit dysplasia-growth patterns char. of cancer
cells
Normal Cells
Dysplastic Cells
Progression of Cancer
• Cells become less differentiated
• Cells invade other tissue
• Move to other areas of the body
• Called malignant cells
Meiosis
• Chromosome number halved
– 2N  N
• Required for sexual reproduction
• Starts with diploid parents
Homologues
• Homologous chromosomes
• 2 chromosomes, same linear gene sequence
• corresponding genes on the 2 chromosomes are
called alleles, alternative forms of same gene
Process of Meiosis
Two stages:
Meiosis I
Meiosis II
Results in 4 haploid daughter cells
Meiosis I
• Pairs line up = synapsis
• crossing over begins
• Splitting homologous pairs, tetrads
Meiosis I
• Cross over – non sister chromosomes
may cross over one another
• Chromatids may exchange segments
Stages of Meiosis I
Interkinesis
• Interphase-like period
• Between meiosis I and meiosis II
• No DNA replication
Meiosis II Stages
Meiosis
Importance of Meiotic
Recombination
• Provides variability for offspring
• Essential to process of evolution
End chapter 12