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3.3 The Cell Cycle
A.Introduction
• Set of stages that take place between the time a
cell divides and the time the daughter cell divides
• Controlled by external and internal signals
• Apoptosis (cell death) occurs at a restriction
checkpoint if the cell did not complete mitosis
and is abnormal
• Some specialized cells no longer go through the
cell cycle - muscle cells and nerve cells
B.Cell Cycle Stages - Interphase
• Cell is not dividing, but is preparing to divide
• The cell carries on regular activities
• Three phases
a.G1 phase – cell doubles number of organelles and
accumulates materials used for DNA synthesis
b.S phase – “synthesis” phase; DNA replication occurs
c. G2 phase – cell synthesizes proteins that will assist cell
division and completes replication of centrioles
The Cell Cycle
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C.Major events during interphase
•
Replication of DNA
a. Before replication, the two strands of DNA are hydrogen
bonded together
b. Parental DNA strands unwind (hydrogen bonds are broken)
c. New complimentary nucleotides pair with nucleotides in the
parental DNA strands and DNA polymerase joins the new
nucleotides
d. When replication is complete, two identical double helix
molecules have been formed
e. Each strand of this double helix is equivalent to a chromatid;
held together by the centromere
Overview of DNA Replication
Ladder configuration & DNA
replication
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2.Protein synthesis
a. DNA also serves as a template for RNA
formation and protein construction
b. Two steps involved in protein synthesis are:
1) Transcription – formation of mRNA
2) Translation – involves mRNA, tRNA, and rRNA;
specifies the order of amino acids in a
polypeptide
Protein Synthesis Process
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D.The Cell Cycle – Mitotic Stage
•
M-stage – nuclear division stage divided into 4 phases
a. Prophase
1)The centrioles near nucleus begin moving towards opposite
ends of nucleus
2)Spindle fibers appear between the centrioles
3)Nuclear envelope begins to fragment
4)Nucleolus begins to disappear
5)Chromosomes appear randomly and attached to spindle
fibers by their centromere
M-stage, cont
b. Metaphase
1)Spindle is fully formed
2)Chromosomes are aligned at the equator
c. Anaphase
1)Centromere splits
2)Sister chromatids separate (now called chromosomes)
3)Chromosomes move toward opposite poles of the
spindle (toward centrioles)
4)Some spindle fibers push the chromosomes apart
while others pull them toward the poles
M-stage, cont
d. Telophase
1) Chromosomes become chromatin
2) Spindle disappears and nucleoli appear
3) Nuclear envelope reassembles and two daughter
cell nuclei can be observed
2.The Cell Cycle - Cytokinesis
a.Cytokinesis – division of the cytoplasm and
organelles
b.Begins in anaphase and completes in telophase
c. Actin filaments form a ring around the equator
that contracts, pinching the cell in half
Mitotic Stage of the Cell Cycle
Mitosis in a whitefish embryo
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3.Importance of mitosis
a. Maintains chromosome number
b. Each cell in our body is genetically identical
c. Each cell type has certain genes turned on and
others turned off to give the different types of
body cells
d. Important to the growth and repair of
multicellular organisms
E.Meiosis
• Known as reduction division because the
chromosome number is cut in half
• The four phase of mitosis are repeated twice
with some changes in the steps that occur
• The gametes (ova and sperm) have half the
chromosomes of normal body cells so that
when they join to form the zygote, the
chromosome number is correct for that
organism.