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Cell Division
Chapter 8
http://www.youtube.com/watch?v=Q6ucKWIIFmg
Cell Division
 Doubling organelles and proteins
 DNA replication
 Nuclear division
 Cytoplasmic division

Cell Increase and Decrease
Maintain homeostasis
Cell numbers kept in check by this
mechanism
Through cell division of somatic cells and cell
death
Cell division – interphase, mitosis and cytokinesis
Cell death – apoptosis
Cell Increase and Decrease
 Somatic cells
 Asexual reproduction
 increase in number of
somatic cells
 Increase in number
unicellular organisms
 Germ cells
 Sexual reproduction
 requires the production
of eggs and sperm
Chromosomes
Mitosis verse Meiosis??
Mitosis
Division of somatic
cell nuclei
Meiosis
Division of gamete
nuclei
Cell Cycle – Somatic cells
 Set of stages that
involves cell growth
and nuclear division
 Consists of:
Interphase
 G1
S
 G2
Meiotic stage
 Mitosis and
Cytokinesis
http://www.cellsalive.com/cell_cycle.htm
Interphase
 When the cell carries
on its usual functions
 Main stages:
G1
 Gap before DNA
synthesis begins
S
 Time when DNA
duplicated***************
G2
 Gap between time
DNA duplication ends
and mitosis begins
Interphase in Meiosis?
 Meiosis is a "oneway" process
Cannot be said to
engage in a cell cycle
as mitosis does
 Preparatory steps that
lead up to meiosis are
identical in pattern
and name to the
interphase of the
mitotic cell cycle
Cytokinesis in Animal Cells
 Cytoplasmic cleavage
 Accompanies mitosis
Separate process
 Cleavage furrow forms
between daughter
nuclei
 Contractile ring
contracts deepening
the furrow
 Continues until
separation is complete
Human DNA in somatic cells
 22-23 pairs of
homologous
chromosomes
Difference?
Autosomes (1-22)
Sex chromosomes (23)
 Somatic cells have
46 chromosomes
Diploid
2n
Human DNA in gametes
 Due to reductional
division
 Halves the diploid
number (2n) to a
haploid number (n)
 23 total chromosomes
DNA Replication
 Duplicated chromosome
 Composed of 2 sister
chromatids
 held together by a
centromere
 Sister chromatids
 Genetically identical
 When separate, each
daughter nucleus gets a
chromosome
DNA
copied
DNA
divided
Division of the Nucleus
 Nucleus must be
divided
Parent cell’s DNA into 2
nuclei
 2 ways nucleus can
divide:
Mitosis
Meiosis
Mitosis
Maintaining the Chromosome Number
Mitosis
Mitosis
4 main stages:
Prophase
Metaphase
Anaphase
Telophase
The Cell Cycle
 1. Prophase –
Mitosis begins!
Threadlike form
Spindle fibers appear
DNA start to condense
Aster formed
Nuclear envelope starts
to break apart
Centrioles move to
opposite sides of the
cell
The Cell Cycle
 2. Metaphase
Duplicated chromosomes
aligned midway between the
poles
Associated with spindle fibers
The Cell Cycle
 3. Anaphase
Sister chromatids
separate from each other
and move to opposite
poles
Become daughter
chromosomes
The Cell Cycle
 4. Telophase
Return to threadlike form
as in prophase
New nuclear envelope
separates each
chromosome cluster
2 new nuclei!!!!!
http://www.cellsalive.com/mitosis.htm
Meiosis
Reducing the Chromosome Number
Meiosis
Mechanism for dividing the nucleus of
germ cells
Oogonia and spermatogonia
2n
Meiosis must take place prior to formation
of gametes
Sperm and eggs
n
First stage in sexual reproduction
Meiosis Overview
 Occurs in the life cycle of sexually reproducing
organisms
 Reduces the chromosome number
 2 divisions, 4 daughter cells
 Cells are diploid at beginning of meiosis
 TWO consecutive divisions
Result is 4 haploid nuclei
 Divided into:
Meiosis I
Meiosis II
Comparisons between males and females
 Spermatogenesis
Begins at puberty and
continues throughout
life
 Oogenesis
 Begins in the fetus
 Primary oocytes are
arrested in prophase I
 At puberty, one primary
oocyte continues the
process of meiosis during
each menstrual cycle
Reducing the Chromosome Number
 Genetic Recombination
Promotes genetic
variability
Occurs in Two Ways
 Crossing Over
 Independent Assortment of
paired chromosomes
How Plant Cells Divide
 Occurs in meristematic tissues
 Same phases as animal cells
 Plant cells do not have centrioles or asters
Plant Cells
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
Cell Division in Prokaryotes
 Binary Fission
 Prokaryotes have a single
chromosome
 Chromosomal replication
occurs before division
 Cell elongates to twice its
length
 Cell membrane grows
inward until division is
complete
Comparisons
 Mitosis
DNA replication occurs
only once
Requires only one
division
Produces two daughter
cells
Diploid daughter cells
 2n
Genetically identical
cells produced
Occurs all the time
 Meiosis
DNA replication occurs
only once
Requires two divisions
Produces four daughter
cells
Haploid daughter cells
n
Genetically variable
cells produced
Occurs only at certain
times
Comparisons
Overview of the Life Cycle of Humans
Mitosis and Meiosis Gone Awry
Cancer
Chromosomal Abnormalities
Cancer
disease of the cell cycle
Cancer cells
do not respond normally to the cell cycle control
system
can form tumors
if malignant, can spread to other parts of the body
Important Terms
Tumor
Benign
Malignant
Metastasis
Cancer
Carcinoma
Sarcoma
Lymphoma
Cancer Treatment
Cancer treatment can involve
Radiation therapy
which damages DNA and disrupts cell division
Chemotherapy
which uses drugs that disrupt cell division
Chromosomal Abnormalities
 Nondisjunction
 members of a
chromosome pair fail to
separate during anaphase
 produces gametes with an
incorrect number of
chromosomes
 If normal sperm fertilizes
an egg with an extra
chromosome
 result is a zygote with a
total of 2n + 1
chromosomes
Chromosomal Abnormalities
 Down Syndrome:
Trisomy 21
condition in which an
individual has an extra
chromosome 21
Affects about one out of
every 700 children
Chromosomal Abnormalities
 Nondisjunction can also affect the sex
chromosomes
Klinefelter's Syndrome
 XXY males
Turner Syndrome
 XO female