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Chapter 6 Chromosomes and Cell Reproduction
Section 1 Chromosomes
Section 2 The Cell Cycle
Section 3 Mitosis and Cytokinesis
Prokaryotic vs. Eukaryotic
Prokaryotic Cell Reproduction Bacteria reproduce asexually by binary fission, a process that produces
identical offspring.
Eukaryotic Cell Reproduction Before eukaryotic cell division, DNA coils tightly around proteins and
forms chromosomes. At cell division, each chromosome consists of two chromatids attached at the
centromere.
Gene is a segment of DNA that codes for a protein or RNA molecule.
A single molecule of DNA has thousands of genes lined up like train cars.
Before a eukaryotic cell prepares to divide, the chromatids become visible. Chromatids then duplicate
into two chromosomes which are attached at a point in the center called a centromere.
The Number of Chromosomes is Constant Within a Species
Each human “somatic cell” has two copies of 23 different chromosomes for a total of 46 chromosomes.
Chromosomes contain thousands of genes which is essential for human survival.
Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content.
Homologous Chromosomes
Dad
Mom
All of the cells in the body, other than the gametes, are somatic cells.
When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid.
When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid.
Haploid
n = 23
Diploid
2n = 46
A Zygote is a fertilized egg cell, which is the first of a new individual.
Sex Chromosomes
Of the 23 pairs of chromosomes in human somatic cells, 22 pairs are called Autosomes.
Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an
individual.
Sex Chromosomes are one of the 23 pairs of chromosomes in humans, contain genes that will determine
the sex of the individual.
X Chromosome
Y Chromosome
Any individual with a Y chromosome is Male.
Any individual without a Y chromosome is Female.
Because a female can donate only an X chromosome to her offspring, the sex of an offspring is
determined by the male, who can donate either an X or a Y chromosome.
Male is (XY)
Female (XX)
? Offspring X
Change in Chromosome Number
Karyotype is a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by
size.
Mutations can occur in the chromosomes themselves. Some of these mutations include: deletion,
duplication, inversion, and translocation.
Deletion
Duplication
Inversion
Translocation
Section 2 The Cell Cycle
The Life of a Eukaryotic Cell
The Cell Cycle
The cell cycle is a repeating sequence of cellular growth and division during the life of an organism.
The eukaryotic cell cycle consists of phases of growth, DNA replication, preparation for cell division, and
division of the nucleus and cytoplasm.
Interphase: 90 percent of its time consists of the first three phases of the cell cycle.
First growth (G1) Phase: a cell grows rapidly and carries out its routine functions. Most cells that are
not dividing remain in the G1 phase.
Synthesis (S) Phase: A cells DNA is copied during this phase. At the end each chromosome consists of
two chromatids attached at the centromere.
Second growth (G2) Phase: preparations are made for the nucleus to divide. Microtubules are
rearranged so the cell can prepare for mitosis.
Mitosis: this process is the actual cell division in which the nucleus of a cell is divided into two nuclei
called mitosis. Each nucleus ends up with the same number of chromosomes as the original cell.
Cytokinesis: is the process during cell division in which the cytoplasm divides.
Control of the Cell Cycle
Cells have a set of “red light-green light” switches that tells the cell when to divide.
These checkpoints allows the cell to communicate like traffic lights when to start dividing and when to
stop.
1. Cell growth (G1) checkpoint: makes the decision of whether the cell will
divide.
If the cell is large enough and healthy enough certain proteins will stimulate the cell to begin the
synthesis (S) phase. The cell will copy its DNA.
Muscle cells and nerve cells remain in this stage permanently.
1. DNA synthesis (G2) checkpoint: DNA replication is checked at this point by DNA
repair enzymes. This stage prepares to go into mitosis.
2. Mitosis checkpoint: Once the cell is completed with mitosis the beginning of the
G1 phase begins.
Cancer is the uncontrolled growth of cells
and the failure of the cell cycle checkpoints
to pause and stop developing new cells.
Section 3 Chromatid Separation in Mitosis
Every second about 2 million new red blood cells are produced in your body by cell divisions occurring in
the bone marrow.
During mitosis, the chromatids on each chromosome are physically moved to opposite sides of the
dividing cell with the help of the spindle.
Spindles are cell structures made up of both centrioles and individual microtubules fibers that are
involved in moving chromosomes.
Interphase G1, S, G2
Centrosome divides and the chromatids reproduce.
Prophase
Chromosomes coil up and become visible during prophase. The nuclear envelope dissolves and a
spindle forms.
Metaphase
During the metaphase the chromosomes move to the center of the cell and line up along the equator.
Spindle fibers link the chromatids of each chromosome to opposite poles.
Anaphase
Centromere divide during anaphase. The two chromatids (now called daughter chromosomes) move
toward opposite poles as the spindle fibers attached to them shorten.
Telophase
A nuclear envelope forms around the chromosomes at each pole. Chromosomes, now at opposite
poles, uncoil and the spindle dissolves. The spindle fibers break down and disappear. Mitosis is
complete.
Cytokinesis
At the end of mitosis, cytokinesis begins
The cytoplasm of the cell is divided in half, and the cell membrane grows to enclose each cell forming
two separate cells.
In animal cells and other cells that lack cell walls, the cell is pinched in half by threads.
In plant cells the Golgi apparatus fuse at the midline of the dividing cell which forms the cell plate.
A new cell wall then is formed separating the plant cell into two new plant cells.