Unit 7- Cell Cycle, DNA, and Protein
Synthesis
7a- Cell Cycle
Bellwork
• Get out poster worksheet, we are going to
discuss correct answers
Bellwork
• Remember, Photosynthesis Poster Due
Tmrw.
Bellwork
• What are the three tenants of cell theory (try
to remember!)
• Why is cell division important for life?
Cell Review
• Cell Theory:
– All living things are composed of cells
– The cell is the basic unit of life
– All cells come from preexisting cells
• Cell Reproduction- aka cell division; process
by which cells produce offspring cells
– Differs in prokaryotes and eukaryotes
Important Vocabulary
• Chromosomes: rod-shaped structures made of DNA &
proteins.
– Each chromosome is a single DNA molecule associated with
proteins
– Eukaryotic DNA wraps tightly around proteins called histones
• Histones help maintain the shape of the chromosome
Important Vocabulary
• Chromatid- form as the DNA makes a copy of itself
before cell division
• Chromatin- the less tightly coiled DNA-protein
complex; substance that composes chromosomes
• Diploid Cells- cells with two sets of chromosomes
• Haploid cells- cells with one set of chromosomes
Cell Division in Eukaryotes
• Both the cytoplasm and the nucleus divide
• Two kinds:
– Mitosis: results in new cells that are identical to the
original cell
• Occurs in organisms undergoing growth, development, repair,
or asexual reproduction
– Meiosis: occurs during the formation of gametes
(reproductive cells)
• Reduces the chromosome number by half
The Cell Cycle
• Cell Cycle: the repeated pattern of growth and division
that occurs in eukaryotic cells
• Most of the cell’s life is spent in the growth phase
known as interphase
– made up of three phases: G1, S, and G2
• The shortest phase in the cycle is the cell division phase
known as mitosis & cytokinesis.
G1 – GAP 1 – Chromosomes are not visible (chromatin)
Cell is rapidly growing and synthesizing proteins for daily functions
(see diagram on page 228)
The Cell Cycle
Cytokinesis- Division of
cytoplasm
Mitosis – Division of the
nucleus
G2 – Gap 2 - Cell is growing &
producing proteins needed for
division
S Stage - Synthesis
Cell’s DNA is copied.
Mitosis
• The purpose of mitosis is the division of the nucleus
– Produces two identical nuclei, each with the same number
of chromosomes
• All somatic cells (cells other than the sex cells that
make eggs and sperm) undergo mitosis.
• Four phases of mitosis:
–
–
–
–
Prophase
Metaphase
Anaphase
Telophase
Prophase
• First & longest phase in mitosis
• Chromatin coils to become visible
chromosomes
– The two identical halves of the doubled
structure are called sister chromatids.
– Held together by the centromere
• Nuclear envelope breaks down &
disappears
• By the end of prophase, the
centrosomes (organelles that
produce spindle fibers) have
separated and have moved to
opposite poles of the cell.
– Start to form spindle fibers
Metaphase
• 2nd & shortest phase in mitosis
• Spindle fibers attach to the
centromere
• Chromosomes are pulled to the
middle of the cell & line up on
the midline or equator, held in
place by the spindle fibers
• Sister chromatids point to
opposite poles
Anaphase
• The centromeres split &
the sister chromatids are
pulled to opposite poles
of the cell.
• After the chromatids
separate, they are
considered to be
individual chromosomes.
Telophase
• Spindle fibers break down and
dissolve
• Chromosomes uncoil
(chromatin)
• A nuclear envelope forms
around each set of
chromosomes
• Cytokinesis begins
Cytokinesis
• Cytoplasm is split forming two
daughter cells each with its own
nucleus & organelles
– In animals: the cell membrane forms a
cleavage furrow that eventually pinches
the cell into two nearly equal parts.
– In plants: a structure known as a cell
plate forms midway between the
divided nuclei, which gradually
develops into a separating membrane.
• The cell wall forms in the cell plate
Cell Plate
Name the Phase
Prophase
Metaphase
Prophase
Anaphase
Metaphase
Telophase
Telophase
Anaphase
Controlling the Cell Cycle
• The cell cycle is driven by a chemical control system
telling the cell when to turn on and off cell division
– Internal signals – cell senses the presence of enzymes
produced within the cell
– External signals – cell senses the presence of chemicals (such
as growth factors) produced by other specialized cells
• Cells also respond to physical signals
– When cells are packed in too closely, division
is turned off
– When cells are not in contact with other cells,
division is turned on
Controlling the Cell Cycle
• Proteins regulate the progress of cell
division at certain checkpoints
–Checkpoint- a critical control point
where stop & go signals can regulate the
cycle
–The cell division mechanism in most
animal cells is in the “off” position when
there is no stimulus present. Specific
stimuli are required to start the
processes.
Controlling the Cell Cycle
• 3 checkpoints:
– Cell Growth (G1) Checkpoint –
makes sure conditions are
favorable & cell is big enough
for division
– DNA synthesis (G2) Checkpoint
– DNA repair enzymes check
the results of DNA replication
– Mitosis Checkpoint – cell makes
sure chromosomes & spindle
are arranged properly to exit
mitosis.
• If a cell passes this checkpoint, it
will enter the G1 phase.
Mitosis Out of Control
• Cancer cells are an example of
cells that do not respond
normally to the body’s control
mechanisms.
• Cancer cells keep dividing even
when they are densely packed
and/or if the proteins that
regulate cell division are not
functioning properly due to a
mutation.
Mitosis Out of Control
• Cancer begins when a single cell is transformed into a
cancer cell- one that does not listen to the regulation
mechanism.
• This cell is normally found & destroyed by the body’s
immune system. If not, this cell will continue to divide by
mitosis and each daughter cell will be a cancer cell.
Mitosis Out of Control
• A mass of these cells that invades & impairs the
functions of 1 or more organs is called a malignant
tumor.
• A benign tumor is a mass of abnormal cells that
remains at the original site.
• Cancer cells may also separate from the original
tumor, enter the blood vessels of the circulatory
systems, & invade other parts of the body, where
they grow to form new tumors.