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Cell Division
Ch 10 & 11
Sections 10-2, 10-3 & 11-4
Reproduction of Prokaryotes
 Prokaryotes reproduce through a process
called binary fission
 The single, circular chromosome (which is
attached to the plasma membrane) replicates
 The cell grows & elongates, separating the
two chromosomes
 The plasma membrane and cell wall pinch
inward, eventually forming two daughter cells
Eukaryotic Chromosomes and Cell
Division
 Almost all of a eukaryotic cell’s genes are
found in the cell’s nucleus, and are grouped
into multiple chromosomes
 Chromosomes are only visible when a cell is
in the process of dividing
 The rest of the time, the DNA + protein
threads exist as a diffuse mass called
chromatin
Chromosomes
 Chromosomes: Found in eukaryotic cells
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Pass genetic information from one generation
to the next
Made up of DNA
The cells of every organism have a specific #
of chromosomes EX: Fruit flies= 8
chromosomes, Humans= 46 chromosomes,
carrot cells= 18 chromosomes
Each chromosome is composed of two
chromatids
Eukaryotic Cell Division
 Before dividing, the eukaryotic cell
replicates its chromosomes
 The two copies of the chromosome
are called chromatids, and they are
connected to each other at a region
called the centromere
 The pair of chromatids is still only
one chromosome, and the
chromatids are not considered
chromosomes until they separate
from one another
Chromatids
 Chromatids: 2 identical strands that make up
1 chromosome
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During cell division, sister chromatids separate
from each other
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One chromatid then goes to each of the
2 new cells
The Cell Cycle
 The Cell cycle= A series of events that cells
go through as the grow and divide
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Cells grow
Prepare for division
Divide to form 2 daughter cells
Each daughter cell then begins the cycle
again
Events of the Cell Cycle
 Events of the cell cycle can be divided into
two major stages:
1. Interphase= Can be quite long. Divided into 3
parts
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G1= Cells grow and increase their size; producing
proteins and organelles
S= Chromosomes replicate, DNA is synthesized
G2= A time of metabolic activity; proteins needed
for cell division are produced
Events of the Cell Cycle
 Mitosis Phase (M Phase)
 Consists of two processes: mitosis and cytokinesis
 During mitosis, the chromosomes divide and are
distributed into two daughter nuclei
 During cytokinesis, the cytoplasm is divided into two
 These two processes result in the production of two
genetically identical daughter cells
Mitosis
 Mitosis= Process of cellular division.
Divided into 4 phases:
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
Prophase
 Prophase= longest phase of mitosis (50-60%) of
total time required to complete mitosis
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Chromosomes become visible
Centrioles (2 tiny structures located in the cytoplasm)
separate and take opposite sides of nucleus
Condensed chromosomes attach to the spindle
Each chromosome has two chromatids joined at the
centromere
Chromosomes coil more tightly
Nuclear envelope breaks down & nucleolus disappears
The chromosomes begin moving toward the center of the
cell
Metaphase
 Metaphase= Lasts only a few minutes
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The spindle is fully formed
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All the centromeres of chromosomes line up in
center of the cell
Microtubules connect centromere of each
chromosome to the 2 poles of the spindle
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Anaphase
 Anaphase
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The centromeres of the chromosome come
apart and the sister chromatids split &
separate becoming individual chromosomes
(called daughter chromosomes)
Chromosomes move into two groups near
poles of the spindle
The cell elongates
Anaphase ends when chromosomes stop
moving
Telophase
 Telophase- (basically the reverse of Prophase)
 Cell elongation that began in anaphase continues
 Chromosomes begin to disappear into tangle of
dense material (Chromatin fibers uncoil)
 Nuclear envelope reforms around each cluster of
chromosomes
 Spindles break apart and disappear
 Nucleolus becomes visible in each daughter
nucleus
 Mitosis is complete
**CELL DIVISION HOWEVER IS NOT
COMPLETE**
Cytokinesis
 Cytokinesis- Division of the cytoplasm itself
and is the final phase of cellular division
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Usually occurs during the same time as
telophase
In animal cells= cytoplasm is drawn inward
and pinched into two equal parts
In plant cells= Cell plate forms causing a cell
wall to form between the 2 divided nuclei
Regulating the Cell Cycle
 Controls on Cell Division: Cell growth can be
turned on and off depending on the situation
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EX: if cells growing on a Petri dish are
removed, the cells surrounding that now
empty space will start dividing, filling in the
area where the cells were removed
Uncontrolled Cell Growth
 The consequences of uncontrolled cell
growth in multicellular organisms can be very
severe.
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Cancer cells DO NOT respond to the signals
that regulate the growth of most cells
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They divide uncontrollably forming tumors
Some fail to respond to internal regulator proteins
Some fail to respond to external regulator
proteins
Causes of cancer can be: smoking, radiation
exposure, and even some viral infections
The Cell Cycle & Cancer
 Cancer is a disease of the cell cycle
 Systems that control the cell cycle do not
work in cancer cells
 As a result, cancer cells divide excessively
and can invade other body tissues
 The excessive growth can result in an
abnormal mass called a tumor
Tumors
 Benign tumor-abnormal mass of normal cells;
can cause problems if they grow in an organ;
remain in their original site; can usually be
completely removed by surgery
 Malignant tumor-cancerous; capable of
spreading to other body areas; displaces
normal tissue; can form new tumors by
breaking apart and spreading through the
circulatory system (called metastasis)
Types of Cancers
 Carcinomas-originate in the external or
internal coverings of the body (skin)
 Sarcomas-originate in tissues that support
the body (bone and muscle)
 Leukemias and Lymphomas-cancers of blood
forming tissues (bone marrow, spleen, and
lymph nodes)
Treatments for Cancer
 Radiation- parts of the body with cancerous
tumors are exposed to high energy radiation,
which disrupts cell division (damage to
ovaries or testes may cause sterility)
 Chemotherapy-administration of drugs that
disrupt cell division; some interfere with the
formation or functioning of the spindle
The Work of Mendel
 1822 – Gregor Mendel used purebred pea
plants to study the inheritance of traits.
 Fertilization – the process in sexual
reproduction in which male & female
reproductive cells join to form a new cell.
Meiosis
 Essential for the formation of gametes
(ova and sperm) that will be used for
sexual reproduction
 Occurs in two consecutive cell divisions
 Reduces the chromosome number by ½
(for humans, this means creating gametes
that have 23 chromosomes)
Meiosis
 Types of cells that undergo meiosis:
 Gametes (sex cells)
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Egg and sperm
Meiosis: Overview
Parent Cell:
diploid= 2n
After Meiosis I
After
Meiosis II
Daughter Cells:
Haploid= n ; and all genetically different from each other and parent cell
Overview of Mitosis
Parent Cell=2n
(diploid)
Daughter cell=
2n
Daughter cell=
2n
(diploid)
(diploid)
Meiosis Vocabulary
Chromosomes – the number in each cell is cut
in half and are different from each other and
the parent cell.
Gametes – are made during meiosis and are
specialized cells involved in sexual
reproduction.
Tetrad – a structure containing 4 chromatids.
(key to Meiosis) Pg. 276
Crossing-over – the exchange of portions
chromatids during the pairing up of
chromosomes. Pg. 276
Meiosis Vocabulary
Chromosome Number P. 275
 Homologous – chromosomes have a
corresponding chromosome from the
opposite-sex parent. Ex. Adult fruit fly has 8
chromosomes- 4 from male parent & 4 from female
parent
 Diploid - “two sets”; a cell that contains both
sets of homologous chromosomes & genes.
2(N) Ex. Fruit fly has a diploid number of 8 so
2N=8.
 Haploid – “one set”; a cell that contains one
set of chromosomes & genes. (N) Ex. N =4
Human Chromosomes
 Occur in pairs (there are 23 pairs) in somatic
cells (typical body cells); these are called
diploid cells (di=2)
 Occur unpaired (a single set of 23) in the
gametes; these are called haploid cells
 The paired chromosomes in somatic cells are
called homologous chromosomes, because
they have genes for the same traits
Human Chromosomes
 22 of the chromosome pairs are called
autosomes
 The 23rd pair consists of the sex
chromosomes
 Human females have a pair of sex
chromosomes called X chromosomes (XX)
 Human males have one X chromosome and
one Y chromosome (XY)
Inheritance of Human
Chromosomes
 One chromosome from each pair is inherited
from our mothers, and the other chromosome
in the pair is inherited from our fathers
 At the time of fertilization, the two haploid
gametes (sperm and ovum) unite to form a
diploid cell called the zygote
 Fertilization results in the formation of a
diploid cell, thus restoring the normal diploid
number of human chromosomes
 Meiosis occurs in reproductive organs
Meiosis I
 DNA that is in special cells in the reproductive
organs is copied.
 The cells divide and 2 cells are formed which
are different from each other and the parent
cell.
Meiosis I Phases
Pg. 276
Interphase I – DNA replication, forming
duplicated chromosomes.
Prophase I – Each chromosome pairs with its
corresponding homologous chromosome to
form a tetrad.
Metaphase I – Spindle fibers attach to the
chromosomes.
Anaphase I – Fibers pull the homologous
chromosomes toward opposite ends of the
cell.
Telophase I & Cytokinesis – Nuclear
membranes form and the cell separates into
2 cells.
Meiosis II
 Cells divide again, but DNA is not copied first.
 4 daughter cells are produced.
 Each cell contains half of the number of
chromosomes as the original parent cell.
Meiosis II
 Prophase II – Meiosis I results in 2 (N)
haploid daughter cells, each with half the
number of chromosomes as the original cell.
 Metaphase II – Chromosomes line up in a
similar way to metaphase stage in mitosis.
 Anaphase II – sister chromatids separate &
move toward opposite ends of the cell.
 Telophase II & Cytokinesis – Meiosis II
results in 4 (N) haploid daughter cells.
Comparing Mitosis & Meiosis
Mitosis - 2 cells that are exactly alike & like
parent.
Meiosis – 4 cell with each having only half
the # of chromosomes as the parent cell and
they are genetically different form one
another.
What is a karyotype?
 Picture of your
chromosomes
 Arranged from
largest to smallest
 quickly identify
chromosomal
changes