Transcript Chapter 6: Chromosomes and Cell Reproduction

Chapter 6:
Chromosomes and
Cell Reproduction
Biology II
Cell Division

Cell division occurs at different times in
an organism’s life
– Growth and development
– Repair
– Reproduction
 Gametes
– reproductive cells (egg/sperm)
– When a cell divides, DNA is first copied
and then distributed so
that each cell ends up
with a copy of DNA
Prokaryotic Cell Division
Prokaryotes have single circular DNA
molecule
 Reproduce by binary fission

– Form of asexual reproduction that produces
identical offspring
– Occurs in 2 stages
 DNA
is copied
 Cell divides by adding a new cell membrane to a
point on membrane between the 2 DNA copies
Genes

Information encoded in DNA is organized into
units called genes
– Segment of DNA that codes for a protein or RNA
molecule

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Single DNA molecule
has 1000s of genes
linked together
Genes play important role
in determining how an
organism’s body develops
and functions
When genes are being used, the DNA is
stretched out so that its information can be
used to direct protein production
Eukaryotic Cell
Division

As eukaryotic cell prepares to divide,
chromosomes become visible
– DNA and proteins associated with DNA

Before DNA coils, DNA
is copied, forming
chromatids
– 2 exact copies of DNA
that make up each chromosome
– Attached at a point called a centromere

Chromatids separate during cell division and
are placed into each new cell, ensuring that
each new cell will have same genetic
information as the original cell
Homologous
Chromosomes

Each human somatic (body) cell has 2
copies of 23 chromosomes (total of 46
chromosomes)
– Each pair is made up of
homologous chromosomes
 Similar
in size, shape, and
genetic content
 Each homologue comes from
one of the 2 parents
– Differ in size, shape, and sets of genes
– Each contains 1000s of genes
Sets of Chromosomes

All cells in the body, other than
gametes, are somatic cells
– Said to be diploid (2n), since they contain
2 sets of chromosomes
 Symbol
“n” is used to represent 1 set of
chromosomes (human diploid #: 2n = 46)
– Gametes are said to be haploid (n), since
they contain only one set of chromosomes

Fusion of 2 haploid gametes,
known as fertilization, forms a
diploid zygote
– Fertilized egg cell
Numbers of
Chromosomes

Each organism has a characteristic
number of chromosomes
– Number of
chromosomes
remain
constant
within each
species
Sex Chromosomes

Humans have 22 pairs of autosomes and 1
pair of sex chromosomes
– Autosomes – chromosomes that are not directly
involved in determine the gender of an individual
– Sex chromosomes – contain genes that will
determine sex of individual
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Referred to as X and Y chromosomes in humans and
many other organisms
Sex of offspring is determined by male
– XX: Female
– XY: Male
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In some insects (grasshoppers), females are
XX and males are XO (O = absence of
chromosome)
In birds, moths, and butterflies, male is XX and
female is XO
Karyotypes

Presence of normal number of
chromosomes is necessary for normal
development and function
– Abnormalities in chromosome number
can be detected by analyzing a
karyotype
 Photo
of the
chromosomes in a
dividing cell that
shows the
chromosomes
arranged by size
Trisomy and
Nondisjunction

Trisomy is a condition in which humans have
more than 2 copies of a chromosomes
– Occurs if one or more chromosomes fail to separate
properly, an event known as nondisjunction
– Trisomy 21 is known as Down’s syndrome


Characterized by short stature, round
face with upper eyelids that cover inner
corners of eyes, and varying degrees
of mental retardation
More likely in pregnancies of older
women because eggs can accumulate
increasing amount of damage over time
Change in Chromosome
Structure

Changes in organism’s chromosome
structure are called mutations
– Breakage of chromosomes can lead to 4
types of mutations
mutation – piece of chromosome
breaks off completely
 Duplication mutation – chromosome fragment
attaches to its homologous chromosome
 Inversion mutation – chromosome piece
reattaches to original chromosome but in
reverse orientation
 Translocation mutation – chromosome piece
reattaches to a nonhomologous chromosome
 Deletion
Section 6-2
The Cell Cycle
The Cell Cycle

Cell division more complicated in
eukaryotes because it involves dividing
chromosomes, cytoplasm, and other
organelles
– Cell cycle: repeating
sequence of cellular
growth and division
during life of an organism
 90%
of cell’s time is
spent in first of 3 phases,
collectively called
interphase
 Cell enters last 2 phases
only if it is about to divide
Stages of Interphase

First growth phase (G1): cell grows rapidly
and carries out routine functions
– Cells remain in this phase until they prepare to
divide
– Some cells (nerve/muscle cells) never divide

Synthesis phase (S): cell’s DNA is copied
– At end of this phase, each chromosome consists
of 2 chromatids attached at a centromere
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Second growth phase (G2): preparations
are made for nucleus to divide and
microtubules are rearranged in preparation
for mitosis
Stages of Cell Division

Mitosis: process during cell division in
which nucleus of cell is divided into 2 nuclei
– Each nucleus ends up with the same number
and kinds of chromosomes as original cell
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Cytokinesis: process during cell division in
which cytoplasm divides
Recall: Mitosis and cytokinesis produce
new cells identical to the
original cells
– Allows organisms to grow,
replace damaged tissues,
or to reproduce asexually
Control of Cell Cycle

Cells know when to divide based on a series
of three main checkpoints at which feedback
signals from cell can trigger or
delay the next phase
– Cell growth (G1) checkpoint: makes
decision whether cell will divide
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Are conditions favorable for division?
Is cell healthy and large enough?
If so, certain proteins will stimulate cell
to begin synthesis phase
– DNA synthesis (G2) checkpoint: DNA replication
is checked by DNA repair enzymes
– Mitosis checkpoint: triggers exit from mitosis and
signals beginning of G1 phase
Cancer

Specific genes contain information to
make proteins that regulate cell growth
and division
– If one of these genes is mutated, protein
may not function and regulation of cell
growth and division may be disrupted
 Cancer:
uncontrolled growth of cells
– Some mutations cause cancer by
overproducing growth-promoting
molecules, speeding up cell cycle
– Others cause cancer by
inactivating control proteins that
normally act to slow or stop cell cycle
Section 6-3
Mitosis and Cytokinesis
Mitosis and the Spindle

During mitosis, the chromatids of each
chromosome are physically moved to
opposite sides of dividing cell
– Structures called spindles are involved in
moving these
chromosomes
 Made
up of
centrioles
and individual
microtubule fibers
Spindle Formation

Organelles that organize the assembly of
the spindle are called centrosomes
– Found at each of the cell’s poles

In animals, a pair of
centrioles is found
inside each centrosome
– Like spindle fibers,
centrioles are made of
microtubules
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Each spindle fiber consists
of an individual microtubule
Each centriole is made of
nine triplets of microtubules arranged in a circle
– Though plant cells do not have centrioles, they
form a spindle almost identical to that of an
animal cell
Separation of Chromatids

As cell prepares to divide, microtubules
in spindle extend out toward opposite poles of
cell
– Once microtubules attach to centromeres
and poles, the 2 chromatids (now called
chromosomes) can be separated

As the paired chromatids separate, they
move toward opposite poles along paths
described by microtubules to which they
are attached
– Chromatids draw closer to poles of cell as spindle
microtubules break down and become shorter
– Once breakdown of the spindle is complete, each
pole has one complete set of chromosomes
Stage 1: Prophase
Stage 1 of mitosis is called prophase
 During this stage:

– Chromosomes coil up,
becoming visible
– Nuclear envelope
dissolves
– Spindle forms
Stage 2: Metaphase
The second stage of mitosis is called
metaphase
 During this stage:

– Chromosomes move
to center of cell and
line up along equator
– Spindle fibers link
chromatids of each
chromosome to
opposite poles
Stage 3: Anaphase
The third stage of mitosis is called
anaphase.
 During this stage:

– Centromeres divide
– Separated chromatids
(now called
chromosomes) move
toward opposite poles
as spindle fibers
attached to them shorten
Stage 4: Telophase
The last stage of mitosis is called
telophase.
 During this stage:

– Nuclear envelope forms
around chromosomes at
each pole
– Chromosomes (now at
opposite poles) uncoil
– Spindle dissolves as
spindle fibers break down
and disappear
Stages of Cell Cycle
Cytokinesis

As mitosis ends, cytokinesis begins.
– Cytoplasm of cell is divided in half
– Cell membrane
grows to enclose
each cell, resulting
in formation of 2
separate but
genetically
identical cells
Cytokinesis:
Plant vs. Animal Cells

Cytokinesis varies
according to cell type:
– In animals (lack cell walls),
cells are pinched in half
by belt of protein threads
– In plants (have cell walls),
vesicles formed by Golgi
apparatus fuse at midline of
dividing cell to form a cell plate

A new cell wall then forms on both sides of cell plate,
separating the plant cell into 2 genetically identical
cells