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Biology
Chapter 6 Notes
Chromosomes and Cell Reproduction
Formation of New Cells by Cell Division
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Cell division, also called cell reproduction, occurs in humans and other
organisms at different times in their life.
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The formation of gametes involves yet a special type of cell division. Gametes
are an organism’s reproductive cells, such as sperm or egg cells.
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When a cell divides, the DNA is first copied and then distributed.
Prokaryotic Cell Reproduction
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Prokaryotes reproduce by a type of cell division called binary fission.
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Binary fission is a form of asexual reproduction that produces identical
offspring.
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In asexual reproduction, a single parent passes exact copies of all of its DNA
to its offspring.
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Binary fission occurs in two stages: first, the DNA is copied (so that each new
cell will have a copy of the genetic information), and then the cell divides.
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Eventually the dividing prokaryote is pinched into two independent cells.
Chapter 6 notes
page #1
Eukaryotic Cell Reproduction
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A gene is a segment of DNA that codes for a protein or RNA molecule.
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When genes are being used, the DNA is stretched out so that the information
it contains can be used to direct the synthesis of proteins.
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As a eukaryotic cell prepares to divide, the DNA and the proteins associated
with the DNA coil into a structure called a chromosome.
How Chromosome Number and Structure Affect Development
Sets of Chromosomes
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Homologous chromosomes are chromosomes that are similar in size, shape,
and genetic content.
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Each homologue in a pair of homologous chromosomes comes from one of
the two parents.
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The 46 chromosomes in human somatic cells are actually two sets of 23
chromosomes.
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When a cell, such as a somatic cell, contains two sets of chromosomes, it is
said to be diploid.
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When a cell, such as a gamete, contains one set of chromosomes, it is said to
be haploid.
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The fusion of two haploid gametes—a process called fertilization—forms a
diploid zygote. A zygote is a fertilized egg cell.
Sex Chromosomes
Chapter 6 notes
page #2
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Autosomes are chromosomes that are not directly involved in determining
the sex (gender) of an individual.
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The sex chromosomes, one of the 23 pairs of chromosomes in humans,
contain genes that will determine the sex of the individual.
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In humans and many other organisms, the two sex chromosomes are referred
to as the X and Y chromosomes.
Change in Chromosome Number
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Humans who are missing even one of the 46 chromosomes do not survive.
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Humans with more than two copies of a chromosome, a condition called
trisomy, will not develop properly. An extra copy of chromosome 21 results
in down syndrome.
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Abnormalities in chromosome number can be detected by analyzing a
karyotype, a photo of the chromosomes in a dividing cell that shows the
chromosomes arranged by size.
Change in Chromosome Structure
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Changes in an organism’s chromosome structure are called mutations.
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Breakage of a chromosome can lead to four types of mutations:
1. deletion mutation – a piece breaks off completely
2. duplication mutation – it carries 2 copies
3. inversion mutation – reattachment is reversed
4. translocation mutation – attaches to the wrong
chromosome
Chapter 6 notes
page #3
Section 2 Notes -- The Life of a Eukaryotic Cell
The Cell Cycle
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The cell cycle is a repeating sequence of cellular growth and division during
the life of an organism.
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A cell spends 90 percent of its time in the first three phases of the cycle,
which are collectively called interphase.
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The five phases of the cell cycle are:
1. First growth (G1) phase During the G1 phase, a
cell grows rapidly and carries out its routine
functions.
2. Synthesis (S) phase A cell’s DNA is copied
during this phase.
3. Second growth (G2) phase In the G2 phase,
preparations are made for the nucleus to divide.
4. Mitosis. The process during cell division in
which the nucleus of a cell is divided into 2 nuclei
is called mitosis.
5. Cytokinesis. The process during cell division in
which the cytoplasm divides is called cytokinesis.
Control of the Cell Cycle
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The cell cycle has key checkpoints (inspection points) at which feedback
signals from the cell can trigger the next phase of the cell cycle (green light).
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Other feedback signals can delay the next phase to allow for completion of
the current phase (yellow or red light).
Chapter 6 notes
page #4
Control occurs at three principal checkpoints:
1. Cell growth (G1) checkpoint This checkpoint
makes the decision of whether the cell will divide.
2. DNA synthesis (G2) checkpoint DNA replication
is checked at this point by DNA repair enzymes.
3. Mitosis checkpoint This checkpoint triggers the
exit from mitosis.
When Control Is Lost: Cancer
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Certain genes contain the information necessary to make the proteins that
regulate cell growth and division.
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If one of these genes is mutated, the protein may not function, and regulation
of cell growth and division can be disrupted.
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Cancer, the uncontrolled growth of cells, may result.
Chapter 6 notes
page #5
Section 3 Notes -- Chromatid Separation in Mitosis
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During mitosis, the chromatids on each chromosome are physically moved to
opposite sides of the dividing cell with the help of the spindle.
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Spindles are cell structures made up of both centrioles and individual
microtubule fibers that are involved in moving chromosomes during cell
division.
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Centrioles and spindle fibers are both made of hollow tubes of
proteins called microtubules
Forming the Spindle
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When a cell enters the mitotic phase, the centriole pairs start to separate,
moving toward opposite poles of the cell.
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As the centrioles move apart, the spindle begins to form.
Separation of Chromatids by Attaching Spindle Fibers
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The chromatids are moved to each pole of the cell in a manner similar to
bringing in a fish with a fishing rod and reel.
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When the microtubule “fishing line” is “reeled in,” the chromatids are
dragged to opposite poles.
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As soon as the chromatids separate from each other they are called
chromosomes.
Chapter 6 notes
page #6
Mitosis and Cytokinesis
Mitosis
Step 1 Prophase The nuclear envelope dissolves and a spindle forms.
Step 2 Metaphase During metaphase the chromosomes move to the center of the cell
and line up along the equator.
Step 3 Anaphase Centromeres divide during anaphase.
Step 4 Telophase A nuclear envelope forms around the chromosomes at each pole.
Mitosis is complete
Cytokinesis
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As mitosis ends, cytokinesis begins.
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During cytokinesis, the cytoplasm of the cell is divided in half, and the cell
membrane grows to enclose each cell, forming two separate cells as a result.
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The end result of mitosis and cytokinesis is two genetically identical cells
where only one cell existed before.
Chapter 6 notes
page #7