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Hon Bio 2: CELL REPRODUCTION
Process of cell division is highly regulated . . . CELL CYCLE = orderly set
of stages that take place between the time a Eukaryotic cell divides
and the time the resulting daughter cells also divides
STAGES of Cell Cycle:
1.
INTERPHASE } G1 , S and G2
- Longest stage of cell cycle
- Cell performs usual functions for organism
- Time in interphase varies – embryonic  a few hours and
adult cells about 20 hours
G1 Stage :
“growth” – grows in size, increases the number of
organelles, and accumulate materials used for DNA
synthesis
“normal functions” – communicating with other cells,
secreting substances, and carrying out cellular
respiration
Go “arrested” – muscle and nervous cells leave interphase
and enter Go stage, continue normal everyday activity
but no cell division
S Stage:
-DNA synthesis or replication occurs (DNA composed of one
DNA double helix  each chromosome is composed of two
identical DNA double helix
G2 Stage:
- Cell synthesizes proteins that will assist in cell division (ie.
Proteins to form microtubules)
M Stage: (Mitotic)
-cell division stage } Mitosis (nuclear division) and
cytokinesis (division of the cytoplasm)
- when this stage is complete two daughter cells are
present
CONTROL of CELL CYCLE:
- Signals ensure that the cell cycle stages follow one another
in normal sequence
Signal = an agent that influences the activities of a cell
Growth Factors = signaling proteins received at the plasma
membrane
CELL CYCLE CHECKPOINTS 
G1 Checkpoint = (Cell cycle Main checkpoint)
 internal signaling of CYCLINS must be
present for cell to proceed from G1 –S stage
 Primary checkpoint
 p53  protein that stops the cycle at G1
stage if DNA is damaged
G2 Checkpoint= (Mitosis checkpoint)
 prevents the initiation of the M stage
before the S stage
 If DNA is damage, the G2 Checkpoint offers
DNA repair
M Checkpoint= (Spindle assembly checkpoint)
 Mitosis will not continue if chromosomes
are not properly aligned
apoptosis = program cell death
caspases = enzymes that cause apoptosis; always present in cell but are
kept inactive by inhibitors
NOTE: These two processes, cell division & apoptosis, occur constantly
thru/out body in somatic cells (body cells)
EX: Cell division repairs an injury and apoptosis prevents a tumor
from developing
MITOSIS: (nuclear Division)
- Occurs during the M phase of cell cycle
- Four stages} Prophase, Metaphase, Anaphase, Telophase
PROPHASE: (condensation)
 Nucleoli disappears & chromatin condenses into chromosomes
 Nuclear envelope breaks down
 Mitotic spindle is assembled (MTOC s, microtubule organizing
centers, move apart to opposite poles of the nucleus);
microtubules from each MTOC connect to specialized regions in
the centromere  kinetochore
METAPHASE: (alignment)
 Chromosomes align across a metaphase plate, a plane between
the two poles of the spindle
 Metaphase ends when microtubules (attached to kinetochore)
pull chromatids apart (each chromatid is called a chromosome)
ANAPHASE:(separation)
 Begins after the chromosomes are separated into chromatids
 Microtubules pull the chromospmes to opposite poles
 At the end of anaphase, each pole has a complete set of
chromosomes (same as the original cell)
TELOPHASE: (restoration)
 Nuclear envelope is restored around each pole, forming two
nuclei
 Chromosomes disperse into chromatin and nucleoli reappears
CYTOKINESIS = divides the cytoplasm into two cells
PLANT CELLS  vesicles from Golgi bodies migrate to plane between
the two new cells forming nuclei, fusing to form a CELL PLATE
subsequently becomes the plasma membrane
ANIMAL CELLS  Actin filaments form a ring inside the plasma
membrane between the two newly forming nuclei, they pull the plasma
membrane into the center dividing the cells  the groove = cleavage
furrow
DIPLIOD = 2N
HAPLOID = N
somatic cells
gametes (sperm & egg)
EX: Humans  46
23
CENTROMERE attaches sister chromatids
CANCER
 Cellular growth disorder that occurs when cells divide
uncontrollably- loss of control of the cell cycle
 Characteristics of Cancer Cells:
o Lack differentiation = not specialized
o Abnormal nuclei = enlarged & abnormal
nuclei
o Do not undergo apoptosis (programmed
cell death)
o Form tumors = cells that pile on top of each
other
o Undergo Metastasis = spread throughout
body
o Angiogenesis = direct growth of new blood
vessels into new tumor
PROKARYOTIC CELL REPRODUCTION = BINARY FISSION
 Cell division in unicellular prokaryotic organisms
 Asexual reproduction = daughter cells identical to parent cell
BINARY FISSION = division (fission) produces twp (binary) daughter cells
identical to original parent cell
 In about seven hours, a single cell increase to over 1 million
cells!!!
MEIOSIS: (diploid  haploid)
 Nuclear division that reduces the chromosome number from diploid (2N) to
haploid (N)
 Requires two nuclear divisions
 Results in 4 cells each with differ DNA (crossing over)
 Gametes = animal reproductive cells (sperm & egg)
Spores = plant reproductive cells (both haploid & diploid phases=
alternation of generations)
 Haploid cells fuse together to form ZYGOTE (diploid cell)
HOMOLOGOUS CHROMOSOMES = pairs of chromosomes in diploid
body cells; same length & centromere position; contain
for same trait but differ alleles
KARYOTYPE = pictorial display of human chromosomes
genes
FEMALE  XX } sex chromosome
MALE  XY } sex chromsome
ALLELES = alternate forms of a gene (ex: long fingers vs. short fingers)
PHASES of MEIOSIS:
MEIOSIS I
Prophase I = synapsis (homologous pairing up),bivalents (tetrads)
& crossing over
Metaphas I = homologous chromosomes stay together
Anaphase I
Telophase I
INTERKINESIS = similar to interphase but no DNA replication
MEIOSIS II
Prophase II
Metaphse II
Anaphase II
Telophase II
MEIOSIS compared to MITOSIS
See handout p 177 Table 10.1
LIFE CYCLE : (reproductive events from one generation to another)
Animals  individual is always diploid and the meiosis process
produces gametes (haploid phase)
Spermatogenesis (sperm) & Oogenesis (egg)
Plants haploid phase = gametophytes alternates with diploid
phase = sporophyte
Fungi (and algae = Protista)  zygote is only diploid phase (shortlived), therefore mold growing on bread is haploid
EUPLOIDY = correct number of species chromosomes (EX: Humans = 46)
ANEUPLOIDY = change in chromosome number resulting from
nondisjunction during meiosis (EX: monosomy (2N-1) & trisomy
(2N+1))
DOWN’S Syndrome  most common trisomy 21
TURNER’s Syndrome  only one sex chromosome, XO
KLINEFELTER Syndrome two or more sex chromosomes in addition to
a Y chromosome