HOLT BIOLOGY
UNIT 1-PRINCIPLES OF CELL BIOLOGY
CHAPTER 06
CHROMOSOMES AND CELL REPRODUCTION
I. Chromosomes
Objectives
• Identify four examples of cell division in eukaryotes and one example in prokaryotes.
• Differentiate between a gene, a DNA molecule, a chromosome, and a chromatid.
• Differentiate between homologous chromosomes, autosomes, and sex chromosomes.
• Compare haploid and diploid cells.
• Predict how changes in chromosome number or structure can affect development.
A. Formation of New Cells by Cell Division
1. Introduction
a. Cell division, also called cell reproduction, occurs in humans and other organisms at different
times in their life.
b. The formation of gametes involves yet a special type of cell division.
c. Gametes are an organism’s reproductive cells, such as sperm or egg cells.
d. When a cell divides, the DNA is first copied and then distributed.
2. Prokaryotic Cell Reproduction
a. Prokaryotes reproduce by a type of cell division called binary fission.
b. Binary fission is a form of asexual reproduction that produces identical offspring.
c. In asexual reproduction, a single parent passes exact copies of all of its DNA to its offspring.
d. 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.
e. Eventually the dividing prokaryote is pinched into two independent cells.
3. A gene is a segment of DNA that codes for a protein or RNA molecule.
a. 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.
b. As a eukaryotic cell prepares to divide, the DNA and the proteins associated with the DNA coil
into a structure called a chromosome.
4. The two exact copies of DNA that make up each chromosome are called chromatids.
a. The two chromatids of a chromosome are attached at a point called a centromere.
b. The chromatids, which become separated during cell division and placed into each new cell,
ensure that each new cell will have the same genetic information as the original cell.
B. How Chromosome Number and Structure Affect Development
1. Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content.
a. Each homologue in a pair of homologous chromosomes comes from one of the two parents.
b. The 46 chromosomes in human somatic cells are actually two sets of 23 chromosomes.
c. When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid.
d. When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid.
2. The fusion of two haploid gametes—a process called fertilization—forms a diploid zygote.
a. A zygote is a fertilized egg cell.
b. Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an
individual.
c. The sex chromosomes, one of the 23 pairs of chromosomes in humans, contain genes that will
determine the sex of the individual.
d. In humans and many other organisms, the two sex chromosomes are referred to as the X and Y
chromosomes.
3. Humans who are missing even one of the 46 chromosomes do not survive.
a. Humans with more than two copies of a chromosome, a condition called trisomy, will not
develop properly.
b. 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.
4. Changes in an organism’s chromosome structure are called mutations.
5. Breakage of a chromosome can lead to four types of mutations:
i. deletion mutation
ii. duplication mutation
iii. inversion mutation
iv. translocation mutation
II. The Cell Cycle
Objectives
• Identify the major events that characterize each of the five phases of the cell cycle.
• Describe how the cell cycle is controlled in eukaryotic cells.
• Relate the role of the cell cycle to the onset of cancer.
A. The Life of a Eukaryotic Cell
1. The Cell Cycle
a. The cell cycle is a repeating sequence of cellular growth and division during the life of an
organism.
b. A cell spends 90 percent of its time in the first three phases of the cycle, which are collectively
called interphase.
2. The five phases of the cell cycle are:
i. First growth (G1) phase During the G1 phase, a cell grows rapidly and carries out its routine
functions.
ii. Synthesis (S) phase A cell’s DNA is copied during this phase.
iii. Second growth (G2) phase In the G2 phase, preparations are made for the nucleus to divide.
iv. Mitosis The process during cell division in which the nucleus of a cell is divided into two
nuclei is called mitosis.
v. Cytokinesis The process during cell division in which the cytoplasm divides is called
cytokinesis.
B. Control of the Cell Cycle
1. 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).
2. Other feedback signals can delay the next phase to allow for completion of the current phase (yellow
or red light).
3. Control occurs at three principal checkpoints:
a. Cell growth (G1) checkpoint This checkpoint makes the decision of whether the cell will divide.
b. DNA synthesis (G2) checkpoint DNA replication is checked at this point by DNA repair
enzymes.
c. Mitosis checkpoint This checkpoint triggers the exit from mitosis.
4. When Control Is Lost: Cancer
a. Certain genes contain the information necessary to make the proteins that regulate cell growth
and division.
b. If one of these genes is mutated, the protein may not function, and regulation of cell growth and
division can be disrupted.
c. Cancer, the uncontrolled growth of cells, may result.
III. Mitosis and Cytokinesis
Objectives
•
•
•
Describe the structure and function of the spindle during mitosis.
Summarize the events of the four stages of mitosis.
Differentiate cytokinesis in animal and plant cells.
A. Chromatid Separation in Mitosis
1. Introduction.
a. During mitosis, the chromatids on each chromosome are physically moved to opposite sides of
the dividing cell with the help of the spindle.
b. Spindles are cell structures made up of both centrioles and individual microtubule fibers that are
involved in moving chromosomes during cell division.
2. Forming the Spindle
a. When a cell enters the mitotic phase, the centriole pairs start to separate, moving toward opposite
poles of the cell.
b. As the centrioles move apart, the spindle begins to form.
3. Separation of Chromatids by Attaching Spindle Fibers
a. 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.
b. When the microtubule “fishing line” is “reeled in,” the chromatids are dragged to opposite poles.
c. As soon as the chromatids separate from each other they are called chromosomes.
B. Mitosis and Cytokinesis
1. Mitosis
a. Step 1 Prophase The nuclear envelope dissolves and a spindle forms.
b. Step 2 Metaphase During metaphase the chromosomes move to the center of the cell and line up
along the equator.
c. Step 3 Anaphase Centromeres divide during anaphase.
d. Step 4 Telophase A nuclear envelope forms around the chromosomes at each pole. Mitosis is
complete
2. Cytokinesis
a. As mitosis ends, cytokinesis begins.
b. 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.
c. The end result of mitosis and cytokinesis is two genetically identical cells where only one cell
existed before.