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Cell Cycle & Division
Cell Division:
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All cells are derived from preexisting cells (Cell Theory)
Cell division is the process by which cells produce new cells
Cell division differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, &
animals)
Some tissues must be repaired often such as the lining of gut, white blood cells, skin cells
with a short lifespan
Other cells do not divide at all after birth such as muscle & nerve
Controlled by proteins called cyclins & enzymes that attach to
them
Reasons for Cell Division:
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Cell growth
Repair & replacement of damaged cell parts
Reproduction of the species
Copying DNA:
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Since the instructions for making cell parts are encoded in the
DNA, each new cell must get a complete set of the DNA
molecules
This requires that the DNA be copied (replicated, duplicated)
before cell division
Chromosomes & Their Structure:
The plans for making cells are coded in DNA
DNA, deoxyribose nucleic acid, is a long thin molecule that stores genetic
information
 DNA in a human cell is estimated to consist of six billion pairs of nucleotides
 DNA is organized into giant molecules called chromosomes
Chromosomes are made of protein & a long, single, tightly-coiled DNA molecule visible only
when the cell divides
When a cell is not dividing the DNA is less visible & is called chromatin
DNA in eukaryotic cells wraps tightly around proteins to help pack the DNA during cell
division
Proteins help control the activity of specific DNA genes
 Proteins bind to centromere and attach chromosome to the
spindle in mitosis
 Centromeres hold duplicated chromosomes together before
they are separated in mitosis
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When DNA makes copies of itself before cell division, each half of the chromosome is called
a sister chromatid
DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell
membrane - plasmid
Chromosome Numbers:
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Humans somatic or body cells have 23 pairs of chromosomes or 46
chromosomes (diploid or 2n number)
The 2 chromatids of a chromosome pair are called homologues
(have genes for the same trait at the
same location)
Organism
Human
Fruit fly
Lettuce
Goldfish
Chromosome
Number (2n)
46
8
14
94
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Human reproductive cells or gametes
(sperms & eggs) have one set or 23 chromosomes (haploid or n
number)
Every organism has a specific chromosome number
Cell Cycle:
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Cells go through phases or a cell cycle during their life before
they divide to form new cells
The cell cycle includes 2 main parts --- interphase and cell
division
Cell division includes mitosis (nuclear division) and
cytokinesis (division of the cytoplasm)
Interphase is the longest part of a cell's life cycle and is
called the "resting stage" because the cell isn't dividing
Cells grow, develop, & carry on all their normal metabolic
functions during interphase
Interphase consists of 3 parts --- G1, S, & G2 phases
Interphase:
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G1 or 1st Growth Phase occurs after a cell has undergone cell division
Cells mature & increase in size by making more cytoplasm & organelles while carrying
normal metabolic activities in G1
S or Synthesis Phase follows G1 and the genetic material of the cell (DNA)
is copied or replicated
G2 or 2nd Growth Phase occurs after S Phase and the cell makes all the
structures needed to divide
Cell division in Prokaryotes:
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Prokaryotes such as bacteria do not have a nucleus
Prokaryotes divide into two identical new cells by the process of binary
fission
Binary fission is an asexual method of reproduction
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In binary fission, the chromosome, attached to cell membrane, makes a copy of itself and
the cell grows to about twice its normal size
Next, a cell wall forms between the chromosomes & the parent cell splits into 2 new
identical daughter cells (clones)
Cell Division in Eukaryotes:
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Eukaryotes have a nucleus & membrane-bound organelles which must be copied exactly so
the 2 new cells formed from division will be exactly alike
The original parent cell & 2 new daughter cells must have identical chromosomes
DNA is copied in the S phase of the cell cycle & organelles, found in the cytoplasm, are
copied in the Growth phases
Both the nucleus (mitosis) and the cytoplasm (cytokinesis) must be divided during cell
division in eukaryotes
Stages of Mitosis:
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Division of the nucleus or mitosis occurs first
Mitosis is an asexual method of reproduction
Mitosis consists of 4 stages --- Prophase, Metaphase, Anaphase, & Telophase
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Prophase:
o Chromosomes become visible when they condense into sister chromatids
o Sister chromatids attach to each other by the centromere
o Centrioles in animal cells move to opposite ends of cell
o Spindle forms from centriole (animals) or microtubules (plants)
o Nuclear membrane dissolves
o Nucleolus disintegrates
Metaphase:
o Chromosomes line up in center or equator of the cell attached to fibers of the spindle
Anaphase:
o Spindle fibers attached to the centromere pull the sister chromatids apart
o Chromosomes move toward opposite ends of cell
Telophase:
o Nuclear membrane forms at each end of the cell around the chromosomes
o Nucleolus reform
o Chromosomes become less tightly coiled & appear as chromatin again
o Cytokinesis begins
Cytokinesis:
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daughter cells
Cytoplasm of the cell and its organelles separate into 2 new
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In animals, a groove called the cleavage furrow forms pinching the parent cell in two
In plants, a cell
plate forms down the
middle of the
cell where the new
cell wall will be
Summary of Mitosis:
Interphase
1. Cell matures & carries on normal
activities
2. DNA copied & appears as chromatin
3. Nucleolus is visible
Early Prophase
1. Chromosomes condense &
become visible
2. Centrioles separate & spindle
starts forming
Late Prophase
1. Spindle forms with aster at each
pole
2. Nuclear membrane & nucleolus
disintegrate
3. Centromere of chromosomes
attaches to spindle fibers
Metaphase
Chromosomes line up at the equator
of the cell attached to fibers of
spindle
Anaphase
1. Centromeres split apart
2. Homologs move to opposite poles
of the cell
Telophase/Cytokinesis
1. Nuclear membrane &
nucleolus reform
2. Cell pinches into 2 cells in
animals
3. In plants, a cell plate
separates the 2 new cells
Cancer is Uncontrolled Mitosis:
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Mitosis must be controlled, otherwise growth will occur without limit (cancer)
Control is by special proteins produced by specialized genes
Mutations in control proteins can cause cancer