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Chromosomes
& Cell
Division
Cell Division
The growth and splitting
of cells into two new,
identical cells called
daughter cells.
Meiosis
Parent
cell
DNA
replicates
Chromosomes
separate
Mitosis
• Cell division to produce identical
diploid “daughter” cells
• Growth and repair
Chromatids separate
Mitosis
Meiosis
• 2 sequential cell divisions to
produce non-identical haploid cells
(gametes)
• Prepare for sexual reproduction
Parent
cell
DNA
replicates
Chromatids
separate
Chromosomes
• A typical human cell nucleus
contains 46 chromosomes
made of DNA.
• You get 23 from
mom and 23 from
dad.
DNA
When DNA is being used
for daily activities it is
called chromatin.
• Chromatin: the relaxed
fibers of DNA.
DNA “Packing”
When it is time for
mitosis, the DNA is
copied and
“packed” for the
move.
DNA “Packing”
To “pack”, DNA molecules
wrap around proteins called
histones and form a rodshaped structure called a
chromosome.
Chromatin
• Uncoiled DNA
• Not visible under microscope
Histones
• Proteins that help DNA supercoil
Sister Chromatids
• Each half of the chromosome
• Identical copies of DNA
Centromere
• Center point in chromosomes where the sister
chromatids are connected
A Chromosome
Chromatid
Centromere
Sister
Chromatids
Chromosome
Chromosome Segments
Chromosomes are divided
into segments called
Gene
genes.
• A small segment of DNA
that codes for a protein
that results in a
particular trait.
• Alternate forms of a
gene are called alleles.
Chromosome Pairs
• Chromosomes come in pairs.
• Half of the pair came from
your mother, the other from
your father.
• The two chromosomes when
together are called sister
chromosomes or
homologous chromosomes.
A Pair of Chromosomes
Chromosome
from Mother
Chromosome
from Father
Homologous
Chromosomes
Diploid
• 2 of each type of chromosome
• One from father, one from mother
• All body cells
Haploid
• 1 of each type of chromosome
• Only in gametes (reproductive cells)
Aneuploid
• Wrong number of chromosomes
• Extra or missing chromosomes
Karyotype
Cell Cycle
• Life cycle of the cell.
• Made up of the events
from one mitosis to
another.
–Phases are:
interphase, mitosis,
and cytokinesis
Interphase
• DNA is uncoiled and “working”
• Growth, DNA replication,
cell prepares to divide
• 90% of time is interphase
Mitosis
• Division of nucleus
• Produces 2 identical “daughter” nuclei
Cytokinesis
• Division of cytoplasm into 2 identical “daughter” cells
• Time for growth and development.
• Cells spend most of their time in
interphase.
• The end of interphase is spent
preparing for mitosis.
– G1: growth 1, doubles in size and gets
large enough to divide
– S: synthesis phase, copies DNA
(replication)
– G2: growth 2, copies organelles and makes
proteins, enzymes that will be needed for
mitosis.
Mitosis
• Follows interphase
• Divides the genetic material (DNA) only.
• Steps involved:
– Prophase
– Metaphase
– Anaphase
– Telophase
Reasons For Mitosis
• Multicellular Organisms
• Growth
• Repair
• Replacement of dead cells/
Maintenance
• Maturation (full grown)
• Unicellular organisms
• Asexual reproduction
Growth and Maintenance
• Growth: an increase in
size because of the
increase in the number of
cells.
• Maintenance: replacing
cells that were dead or
damaged.
• Chromosomes form
• Nuclear membrane and nucleolus
disappear
• Centrioles move to opposite poles
• Spindle fibers begin to form.
• Fibers made by the centrioles that pull
the chromosome copies apart during
mitosis
• “Middle” phase
• The chromosomes are
pushed and line up on the
equator or the center of the
cell.
• Centromere holding the
chromatids together
splits.
• Spindle fibers shorten
and pull the chromatids
apart.
•
•
•
•
Chromosomes are at opposite poles.
Centrioles and spindle fibers disappear.
The chromosomes begin to uncoil.
The nuclear membrane and nucleolus
reappear.
“Reverse Prophase”
Cytokinesis
• Follows mitosis
• Splits the cytoplasm only.
• Results in two identical daughter cells.
• The beginning of the process is
indicated by a cleavage furrow in an
animal cell and a cell plate in a plant
cell.
Summary of Mitosis
Prophase
Metaphase
• Chromatin supercoils into Chromosomes
• Nuclear membrane disappears
• Chromosomes line up along the middle of the cell
• Spindle fibers begin to pull sister chromatids apart
• Sister chromatids move along spindle fibers toward
opposite ends of cell
• Spindle fibers lengthen and elongate the cell by
Anaphase
pushing the poles further apart
Telophase
• Daughter chromosomes reach opposite poles of cell
• Nuclear membranes form again