Ch 8
BIO-100
CELL REPRODUCTION: Cell Division
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Cell and living things grow and reproduce
It is done through cell division
What is cell division?
– The process in which one cell gives rise to 2 or more cells, daughter cells
• There are 3 types of cell division:
– Binary fussion
– Mitotic cell division
– Mitotic cell division
Cell Division
• Required for growth and development - mitosis
• Required for asexual reproduction – single cell parent organisms divides by binary
fusion or mitosis to produce another (paramecium)
• Required for sexual reproduction – in meiotic cell division in fusion of gametes (eggs &
sperm).
2 Types of Cell Division in Eukaryotic Cells
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Mitosis = cellular division in which the daughter cells receive the exact chromosomes
and genetic makeup of the parent cell; occurs during growth and repair
Meiosis = nuclear division that occurs as part of sexual reproduction in which daughter
cells receive the haploids (equal) number of chromosomes
Cell Increase and Decrease
Increase and decrease of cell numbers
• Cell division increases number of somatic (body) cells
• Mitosis - division of nucleus of cell
• Cytokinesis - division of cytoplasm
• Occurs throughout life, growth, & development
• Cell repair
• Binary fusion – “splitting in two” in prokaryotes
• Apoptosis - decreases cell number by programmed cell death
• Occurs throughout life
• Prevents abnormal cells from proliferating
The cell cycle
The cell cycle
• The cell cycle has sets of events that occur between the time a cell divides and the time
the resulting daughter cells divide
• Stages of interphase –longest phase of the cycle
 Normal cell functions occur & preparation for division
 G1 stage – cells double in organelles, accumulates materials needed for DNA
synthesis and cell division
 S stage - DNA replication
 G2 stage - synthesis of proteins needed for division
Cell Increase and Decrease
• Mitotic stage
– Follows interphase
– Includes mitosis and cytokinesis
• Control of cell cycle (by internal/external signals)
– The protein cyclin (signals) must be present for stages to progress
– G2 checkpoint - stops cycle if DNA is not done replicating or is damaged
– M checkpoint - stops if chromosomes not aligned for accurate distribution to daughter
cells
– G1 checkpoint - protein p53 stops cycle if DNA damaged
Control of the cell cycle - Cell Decrease
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Apoptosis
– Progressive series of events resulting in cell destruction
– Cells rounds up, and loses contact with surrounding cells
– Nucleus breaks up and cell undergoes fragmentation
– Cells have 2 sets of enzymes called Caspases
– One set initiates the apoptosis events
– The other set activates enzymes that digest the cell
Apoptosis: is a sequencial event that results in fragmented cells that are phagocytized by
WBC
Maintaining the Chromosome Number
• Chromosome number- terminology
– Chromatin - tangled mass of threadlike DNA in nondividing cell
– Chromosomes - condensed rod-shaped DNA molecules during division
– Diploid (2N) number - characteristic chromosome number, chromosomes in pairs
– Haploid (N) number - half the diploid number, found in gametes
Maintaining the Chromosome Number
• DNA is condensed & compacted into a cell
• If stretched out- 6 ft. long
• The degree of compaction condensation varies at each stage of the cell cycle
• Most of the time (in cell life-cycle) its extended for accessible chromosomal
transcription.
• Individual chromosomes consist of a single DNA & exist as chromatin
• During cell division, the chromosomes sort out and condense
• They thicken and wind up onto spools during cell division
• Overview of mitosis
– Nuclear division in which chromosome number remain constant
– DNA replication produces duplicated chromosomes
– Each duplicated chromosome is composed of 2 sister chromatids held together by a
centromere
– Sister chromatids are genetically identical
– During mitosis, the centromere divides and each chromatid becomes a daughter
chromosome
– Occurs when tissues grow or repair occurs
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Mitosis overview
DNA Replication during Interphase
Chromosomes are duplicated into 2 sister chromatids
Centromeres divide
Chromatids separate
Become daughter chromosomes
Before nuclear division, the DNA replicates
Chromosomes duplicate – 2 sister chromatids
Held together by the centromere
• Sister chromatids are genetically identical
• By the end of mitosis, each chromosome
divides into a single chromatid
Maintaining the Chromosome Number
• Mitosis in detail-animal cells
– Prophase-nuclear membrane disappears, centrosomes migrate, spindle fibers
appear
– Metaphase-chromosomes line up at equator, associated with spindle fibers
– Anaphase-centromeres divide, sister chromatids migrate to opposite poles,
cytokinesis begins
– Telophase-nuclear membranes form, spindle disappears, cytokinesis occurs
Phases of Animal mitosis
Late Interphase: the chromatin condenses into chromosomes and the centromesomes
duplicate in preparation for mitosis
Early Prophase
• Visible duplicated chromosomes
• Centrosomes begin to move apart
• Nuclear envelop is fragmenting
• Nucleolus disappears
Late Prophase
• Spindle formation and centromeres of Chromosomes become attached to centromeric
spindle fibers
Metaphase
• Chromosomes are lined up at the equator
• Each chromosome consists of 2 sister
Anaphase
• Centromeres divide
• Daughter chromatids towards poles of the spindle
Telophase
• Daughter cells form
• Spindles disappear
• Nuclear envelopes and nucleoli appear
• Chromosomes become indistinct chromatin
• Cytokinesis takes place
Early Prophase
Prophase
Late Prophase
Metaphase
Maintaining the Chromosome Number
• Mitosis in plant cells
– Occurs in meristematic tissues
– Undergoes same phases as animal cells
– Plant cells do not have centrioles or asters
• Cytokinesis in plant cells
– Flattened, small disk appears between daughter cells
– Golgi apparatus produces vesicles which move to disk
– Release molecules which build new cell walls
– Vesicle membranes complete plasma membranes
• Cytokinesis in some animal cells
• Cleavage furrow forms between daughter nuclei
• Contractile ring contracts deepening the furrow
• Continues until separation is complete
• Cell division in prokaryotes
– Prokaryotes have a single chromosome
– Divide by the process of binary fission
– Division (fission) produces 2 (binary) daughter cells
– Chromosomal replication occurs before division
– Cell begins to elongate to twice its length
– Cell membrane grows inward until division is complete
– For of asexual reproduction (single parent)
Reducing the chromosome number
• Overview of meiosis
– 2 divisions, 4 daughter cells
– Cells are diploid at beginning of meiosis
– Pairs of chromosomes are called homologues
– Meiosis I
• Homologues line up side by side at equator-synapsis
• When pairs separate, each daughter cell receives one member of the pair
• Cells are now haploid
Phases of meiosis I
Prophase I
• Synapsis occurs, nuclear membrane breaks down
• Homologues line up side by side and crossing over occurs
Metaphase I - Homologous pairs line up at equator such that maternal or paternal
member may be oriented toward either pole
Telophase I - Cytokinesis produces 2 daughter cells which are haploid
Anaphase I
• Homologous chromosomes (ea. consists of 2 chromatids) undergo independent
assortment into daughter cells
• DNA replication occurs, chromosome is
duplicated
• Meiosis I – homologous chromosomes
pair up and then separate
• Meiosis II – centromeres divide, sister
chromatids separate and become daughter
chromosomes
• Overview of meiosis II: Reducing Chromosomes
– Meiosis II
• No replication of DNA occurs
in this division
• Centromeres divide and sister
chromatids migrate to opposite
poles to become individual
chromosomes
• Each of the four daughter cells
produced has the haploid chromosome
number and each chromosome is
composed of one chromatid
• Interkinesis - period between meiosis I and
meiosis II
• Phases of meiosis II
– Prophase II - Cells have 1 member of
each homologous pair
– Metaphase II - Chromosomes line up at
the equator
– Anaphase II
• Centromeres divide and daughter chromosomes migrate
– Telophase II
• Nuclei form, cytokinesis
Meiosis II in animal cells
Reducing the chromosome number
• Nondisjunction - causes various syndromes which result from abnormal chromosome
numbers
– Failure of homologous chromosomes to separate during anaphase
– Failure of sister chromatids to separate during anaphase II
– Ex: Down syndrome results from nondisjunction of chromosome 21
Comparison of Meiosis and Mitosis
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Meiosis
DNA replication occurs only once prior to both
Meiosis requires 2 divisions
Meiosis produces 4 daughter cells
Daughter cells from meiosis are haploid
Daughter cells from meiosis are genetically variable,
Mitosis
DNA replication occurs only once prior to both
Mitosis requires only 1 division
Mitosis produces 2 daughter cells
Daughter cells from mitosis are diploids
– Daughter cells from mitosis are genetically identical
The Human Life Cycle
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Requires both mitosis and meiosis
– In females meiosis is part of the process of oogenesis
– In males meiosis is part of spermatogenesis
– At fertilization, the zygote divides by mitosis for the processes of growth and
development
– Mitosis is used for repair throughout life
– Mitosis continues after birth until individual reaches maturity.
The Human Life Cycle
• Spermatogenesis
– The production of sperm in males
– Begins at puberty and continues throughout life
– Occurs in seminiferous tubules of testes
– Primary spermatocytes (diploids) divide in meiosis I to form 2 secondary
spermatocytes
– Secondary spermatocytes (haploids) divide in meiosis II to produce 4 sperm
• Oogenesis
– Production of eggs in females
– Occurs in the ovaries
– Primary oocyte (diploid) divides in meiosis I to produce 1 secondary oocyte (haploid)
and 1 polar body
– Secondary oocyte begins meiosis II but stops at metaphase II; polar body may also
divide
– At puberty, after ovulation, secondary oocyte is activated if fertilized to complete
division
– Meiosis II produces 1 ovum and 1 polar body
The Human Life Cycle
• Oogenesis
– Products of oogenesis are 1 large ovum and up to 3 small polar bodies
– Ovum receives nearly all cytoplasm and organelles and half the chromosomes
– Polar body gets the remaining half of the chromosomes
– Allows ovum to have all the cellular “machinery” it needs for embryonic development
Summary
– Spermatogenesis and oogenesis both utilize meiosis
– Spermatogenesis begins at puberty and continues throughout life
– Spermatogenesis produces 4 sperm per primary spermatocyte - results in production
of many sperm
– Oogenesis results in 1 oocyte and up to 3 polar bodies per primary oocyte
– Oogenesis begins prior to birth, stops until puberty, then resumes in a cyclic pattern
– Cyclic release of oocytes continues until menopause when the process stops