
Two types of division of cells:

________________- Reduction division
◦ Found in reproductive cells (egg & sperm). End up with
half of the number of chromsomes.

______________- exact replication of cell.
 Found in somatic cells. Divide into 2 equal daughter cells.
◦ Life cycle has been divided into two major periods:
 _______________
 When cell is growing, maturing, and differentiating.
 Cells spend majority of time in this phase.
 ________________ phase
 When cell is actively dividing.
Life Cycle of the Cell

Period between cell divisions (“resting phase”, but
there is no resting!)

Nucleus and nucleoli are visible and chromatin is
arranged loosely throughout the nucleus.

Three subphases:
◦ ___________ 1 (G1)- intense metabolic activity and cell
growth; cell size and number of organelles doubles
◦ ___________ (S)- DNA replication
◦ ___________ 2 (G2)- very brief; synthesis of enzymes
and proteins needed for mitosis
Interphase

Occurs during S phase of Interphase

DNA must be replicated before
the cell can divide – a copy of
DNA is made to be passed to the
daughter cell

STEPS:

Chromatin uncoils, histones
separate from DNA, and a helicase
enzyme separates the helix into 2
nucleotide chains, breaking the
hydrogen bonds

A replisome (made of primase and
replicase) makes a short chain of
RNA, called a primer, to match the
uncoiled DNA template
DNA
Replication

After the RNA primer is made, DNA
polymerase III places free floating
nucleotides with the exposed, unpaired
nucleotides on the DNA chain
◦ If the original strand is “GATTAG”, the
complementary one is “CTAATC”.

When the pairing is complete, DNA
Polymerase I replaces the RNA primer
with DNA nucleotides.

Finishing touches are put on the 2 new
identical DNA helixes. Histones wrap
around them, before they supercoil into
chromatids held together by a
centromere.

G1
◦ Growth

S
◦ Growth and DNA replication

G2
◦ Growth and making of enzymes/proteins needed for cell
division

Mitosis:
◦
◦
◦
◦

Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
◦ division of cytoplasm
Cell cycle
◦ Finishing touches put on chromosomes
(2 chromatids held together by a
centromere)
◦ Cytoplasm becomes more viscous
◦ Microtubules disassemble, cell
becomes round
◦ Spindle apparatus appears
 Centrioles have just finished replicating
and each pair moves from being near the
nucleus (centrosome) to being at opposite
poles of the cell. Microtubules called
spindle fibers extend from centrioles to
the center of the cell
Mitosis:
Prophase
◦ Nuclear envelope disintegrates
◦ Chromosomes are lined up in
center of spindle (cell) called
the equator to form the
metaphase plate.
◦ Centromere of each
chromosome is attached to a
spindle fiber.
Mitosis:
Metaphase
◦ Centromeres split apart and
each chromatid becomes its
own chromosome.
◦ Sister chromatids separate as
the spindle fiber pulls them
away from each other toward
the centrioles.
Mitosis:
Anaphase
◦ Cell elongates and cytoplasm
constricts along metaphase
plate.
Mitosis:
Telophase

Final stage of mitosis

Begins when chromosomal movement
stops

Chromosomes reach poles and begin to
unravel into chromatin.

New nuclear envelope appears as well
nucleoli.

Microtubules that were spindle fibers
disassemble

Cytokinesis (division of cytoplasm) ends
telophase.

New daughter cells enter interphase.

Cell division is important not only for growing animals, but
also for adults that need their older cells replaced.

FACTORS INFLUENCING CELL DIVISION:
 Normal cells stop dividing when they come into
contact with surrounding cells (CONTACT
INHIBITION)
 Cells may release growth-inhibiting signals
once their numbers are sufficient.
 Cells have checkpoints (proteins) that control
whether or not they enter into the mitotic phase
Control of Cell Division
-Begins in nucleus (location of DNA “instruction manual”)
-DNA does not leave the nucleus
◦ a messenger (mRNA) must carry the instructions from the nucleus to the
cytoplasm
TRANSCRIPTION-
process of making mRNA
◦ RNA polymerase separates the double helix into single
strands of DNA
 RNA polymerase connects free floating RNA nucleotides to their
coordinating DNA nucleotides beginning with the promoter and
ending with the terminator.
◦ Each group of 3 RNA nucleotides is termed a codon. Each codon
represents a different amino acid.
◦ Once the terminator is reached, transcription ends. RNA polymerase
detaches, mRNA is released, and the DNA forms a double helix again.
Protein Synthesis

Some editing is done to the mRNA.
◦ Extra “nonsense” codons called introns are removed from the
strand by assembly lines called spliceosomes.
◦ The remaining exons are connected and the mRNA can leave the
nucleus through nuclear pores.

TRANSLATION- process of building new protein
◦ Information on mRNA is translated from nucleotides to
amino acids.
 The two subunits of a ribosome wrap around a strain of mRNA in the
area of the initial codon.
 tRNA can fit into the active site of an mRNA. Every tRNA is linked to
an amino acid.
 as each codon is read, tRNA with a corresponding anticodon brings
the appropriate amino acid to the mRNA/rRNA
 a peptide chain is formed as amino acids link together and is
eventually released
Protein Synthesis

Errors can occur in the genetic code during the many replications
of DNA causing an alteration in the genetic information.

Chromosomes become altered which may be passed on to future
generations.

May occur spontaneously or due to an identifiable mutagens.

Examples of mutations:
◦ sections of DNA may be left out
◦ nucleotide mismatching

Mutations may be so severe that cell dies, but may also cause no
issues whatsoever.

Mutations in an unborn fetus are more severe than those in
adults.

Some mutations can be repaired by enzymes.
Genetic Mutations

Differentiation- The progressive acquisition of
individual characteristics by cells to enable them to
perform different functions.
◦ The temporary or permanent inhibition of genes that may be
active in other cells.

No one cell can contain all of the metabolic and
structural machinery needed to perform the
secretion, absorption, contraction, conduction,
storage, and elimination processes that are required
for homeostasis in the body.

The genetic material tells the cell what type of protein
to make and what functions to perform.
◦ The proteins that a cell makes are key to its specialization
Cell Differentiation and Development