Chapter 8
The Cellular Basis of Reproduction and Inheritance
Overview
In this chapter we will begin our exploration of the process of cell division, which is a precursor to later
discussions on patterns of inheritance and genetics. First we will examine the structure of the chromosome and
the importance of chromosome number. Second we will look at the life cycle of the cell and the key stages
within this cycle.
Mitosis, or nuclear division, is a fluid process, but in order to study it we will first have to break it down into
stages. In each stage we will learn what is occurring in the cell. The major stages that we will examine are:
prophase, metaphase, anaphase & telophase. Then we will examine the process of cytokinesis, or cytoplasmic
division and the differences between plant and animal cells.
The process of mitosis produces two identical daughter cells, a useful process when variation is not needed.
However, in order to respond to changes in the environment, new genetic combinations are often required. The
process of meiosis, or "reduction division", provides a method of generating new unique genetic combinations.
The process of meiosis in simply two consecutive mitosis events without an intervening interphase. However,
the chromosomes align differently in meiosis to allow for new genetic combinations to be formed. There are
three mechanisms by which genetic variation in generated: crossing-over, independent assortment and random
fertilization of gametes. We will examine each of these in this unit.
Assigned Reading
Text, Pages 125-148
PowerPoint Presentation
Chapter Review, Page 149-150
Testing Your Knowledge, Page 150-151
Key Terms
 Chromosome
 Binary fission
 Chromatin
 Sister chromatids
 Centromere, kinetochore
 Mitosis, meiosis
 Interphase (with G1, S, and G2 phases)
 Prophase, metaphase, anaphase, Telophase
 Cytokinesis
 Diploid, haploid
 Homologous chromosome
 Centrosomes (Centrioles)
 Spindle fibers (microtubules)
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Growth factors, density-dependent inhibition, anchorage dependence
Metastasis
Somatic cells, gametes
Synapsis
Crossing over
Allele
Karyotyping
Nondisjunction, deletion, duplication, inversion, translocation
Trisomy, Down syndrome
Klinefelter syndrome, Turner syndrome
Introduction
In the initial chapters of our course we described living organisms as having the capability for reproduction. In
this chapter we will examine the process by which the eukaryotic cells divide. A few key points to keep in mind
while you begin this chapter. First, each new cell must have an exact copy of the genetic material, and second
each cell must have sufficient metabolic machinery to function after division.
As you read the second part of the chapter you will quickly notice that the process of meiosis is very similar to
that of mitosis. In fact, we will see that meiosis is two consecutive divisions without an intervening interphase.
Why change the process? Mitosis produced 2 identical daughter cells, yet in the beginning of the course we
mentioned that in the process of natural selection that there is variation among offspring. If mitosis did not
produce the variation, then we must have a process which does.
In this chapter you need to focus on the methods of producing variation. Steps of meiosis are very similar to
those found in mitosis.
It is important that you understand the process of mitosis thoroughly since the next few chapters will rely upon
this knowledge.
Chromosomes
Before we begin examining the process of cell division, we need to
examine the structure of a chromosome. Chromosomes are
protein—DNA complexes which serve to organize the DNA. We each
have 2 copies of each chromosome: one from our mother and one
from our father. The term for this condition is diploid, while cells
that only have one copy are called haploid. After formation of the
zygote, cell division (for most organisms) ensures that all of the
following cells (called daughter cells) are also diploid. In order for
this to happen we must duplicate the genetic material.
Examine Figure 8.4B carefully.
Notice that when a single
chromosome is duplicated, the replicate remains joined to the
original at a site called a centromere. These chromatids are identical
(for our purposes) and thus are called sister chromatids. Remember
that you would have two copies of each chromosome (a chromosme 7 from mom and a chromosome 7 from
dad). These chromosomes are not identical, since your mother and father are different individuals, but they are
similar in the instructions that each contains. We call these chromosomes homologous chromosomes, meaning
similar.
Cell Cycle
Cells have a life cycle associated with them.
Examine Figure 8.5 as you read this section.
Notice that the cell cycle is divided into two major
areas:
Interphase and mitosis/cytokinesis.
Interphase is the working phase of the cell, this is
when it performs the functions required by the
organism. Mitosis is cell division.
The cell cycle is a fluid process, and in some cells
may take only 20 minutes or less to complete.
Notice that Interphase consists of 2 growth phases and a period of DNA replication called S phase. In S phase we
manufacture the sister chromatids mentioned above.
Missing from Figure 8.5 is a phase called G-0 phase. Not all cells follow this cycle, some cells are arrested at
certain points in the cycle and leave the normal cell division. These cells are said to have entered a G-0 phase. A
good example of these would be some neuron cells. A significant amount of scientific research is going into
understanding the signals that these cells use. Imagine being able to put cancer cells into G-0 phase or take
neuron cells out of G-0 phase!
Prophase
Remember that mitosis is cellular division and that for eukaryotic cells the genetic material is located within the
nucleus of the cell. The first stage of mitosis is basically a preparation stage. However, several important
processes occur in this first step. Structures known as centrioles exist in the cytoplasm of the cell. Centrioles
direct the operation of microtubules (part of the cytoskeleton) which will be used to divide the sister chromatids
in later stages. In prophase, the centrioles begin to migrate to opposite poles, microtubule spindle fibers begin
to be formed and nuclear envelope dissolves. Inside the nucleus, the DNA, which had existed as long strands,
begins to condense in preparation for division.
Metaphase
Remember that the stages of mitosis are fluid, with no gaps in the process. The second stage of the process is
called metaphase. In metaphase the chromosomes line along along a central imaginary line called a metaphase
plate. Spindle fibers from each of the centrioles had attached to the centromere (central area of the
chromosome) towards the end of prophase. Note that the homologous chromosomes act independently of one
another.
Anaphase
Metaphase is brief. Once the chromosomes
are aligned along the metaphase plate, the
spindle fibers (under the direction of the
centrioles) shorten and the sister chromatids
are separated. One the sister chromatids are
no longer attached, we have entered
anaphase.
Telophase
The last major phase of mitosis occurs when
the sister chromatids ( now called
chromosomes again) reach the opposite
poles of the cell. At this point, the nuclear
envelope reforms around the genetic
material.
Cytokinesis
Recall from the beginning of this lecture that the process of cell division involves not only the division of the
genetic material, but also the division of the cellular components. The process of mitosis divided the genetic
material and now the process of cytokinesis will divide the cellular machinery.
There are two methods by which this can occur depending on whether it is happening in an animal cell (Figure
8.7A) or a plant cell ( Figure 8.7B). Please examine these figures closely and note the differences and similarities.
Sexual vs. Asexual Reproduction
From this point on in the course the terms sexual reproduction should immediately bring to mind the
introduction of variation, which asexual reproduction can be considered cloning (not exactly true, but the
association is helpful). Meiosis is the process of sexual reproduction, and we will see below that it introduces
variation into the cell division in three manners. Meiosis is often called "reduction division" since the end result
is cells with half the original number of chromosomes. In other words, we start with diploid cells and end up
with haploid cells. These haploid cells (as we will see below) are eventually called gametes.
Please take some time now to examine Figure 8.14, it is very important.
First Level of Genetic Variation
One of the first differences between mitosis and meiosis is that in the prophase I of meiosis the homologous
chromosomes pair up with one another. Compare and contrast the diagram below. In mitosis, homologous
chromosomes acted independently, that is, the two copies of chromosome 7 (for example) in the cell did not
exchange any material.
In meiosis, not only do the homologous chromosomes pair up, they actually exchange genetic information. This
is shown very nicely in Figure 8.18B of the text. This process is called crossing over and is much more
complicated then it appears. Crossing over must be exact, since genetic material can't be safely deleted or extra
added without endangering the organism.
Your text shows crossing over occurring at the tips of the chromosomes. But actually, crossing over can occur in
any number of locations, and at multiple times, on the same chromosome. Thus, this process produces new
chromatids that have genetic variations which are combinations of the homologous chromosomes. This is the
first (and most influential) mechanisms of producing variation.
Second Level of Genetic Variation
For a brief moment, let us assume that crossing over does not occur (to simplify the picture). As we have noted,
meiosis is unlike mitosis in that the homologous chromosomes pair up during meiosis which in mitosis they act
independently on one another.
Since we have two copies of each chromosome (one we got from mom, the other from dad), and they are
pairing up, then there are two possible ways that each pair can align themselves. Look at figure 8.16. Notice that
since we have 23 pairs of chromosomes; there are 8,388,608 different ways that these chromosomes can align
themselves at metaphase I. Now let us bring the crossing over back into the picture. Crossing over has the
capability of producing endless variation and we can see what happens above at metaphase I. But we are not
done yet.
Third Level of Genetic Variation
There are many similarities in the meiosis process in males and females, and a number of distinct differences.
After the gametes are produced (each with its own unique genetic signature) there is no way of knowing which
single egg will be fertilized by which sperm. So if there are over 8 million different egg genetic combinations
(from metaphase I above) and over 8 million different sperm genetic combinations (same), then each new
zygote caused by the fertilization of an egg by a sperm represents a 1:64 trillion chance ( 8 million x 8 million).
To look at it another way—your parents would need to have over 64 trillion children before producing an exact
replica of you. You could hit the lottery 10,000 times first!
Meiosis & Mitosis Compared
It is very important that you be able to tell the difference between mitosis and meiosis for the next exam.
Figure 8.15 and the review slides of the PowerPoint presentation sum up these processes. If you do not
completely understand the mechanisms of producing variation, please review the material above again and/or
email me.
Chromosome Abnormalities
It really is amazing that during the mixing of genetics more things don’t go wrong. There are many ways the
body protects against this, but it does happen. Many times this is an abnormality during crossing over, when the
homologous chromosomes don’t align properly. Alleles or DNA sequences may be duplicated, deleted, inverted,
or otherwise switched. This has the potential of causing from mild to severe genetic disorders.
Besides crossing over mistakes, there can be disorders of nondisjunction, where the chromosomes do not
separate properly during meiosis. This can cause abnormal chromosome numbers in the gamete, either too
many or too few. The most well-known of these is Down syndrome, which is a result of a nondisjunction leading
to an extra chromosome 21.
It is important to make sure you understand where and how these abnormalities occur, as well as the kinds of
problems that can happen. Be sure to study both your text and the PowerPoint carefully.
Links of Interest
 Cells Alive ! : Link on bacterial reproduction (not mitosis), but includes a nice movie on E. coli
reproduction
 Mitosis pages at Oklahoma State University. Nice graphics and links to additional tutorial sites
 Interactive Mitosis Tutorial. Prepared by Kathleen Fisher and Jeff Sale
 The Biology Project - The University of Arizona: Cell Cycle, Mitosis
 Online Biology Text by MIT: Link to review page on meiosis has some very good diagrams of crossing
over
Concepts
 Understand the importance of mitosis
 Understand the structure of the chromosome and the key terms associated with cell number.
 Know the major events in each stage of the cell cycle.
 Know the stages of mitosis and what is happening in each.
 Recognize the differences between cytokinesis in plants and animals.
 Recognize the differences and similarities between meiosis and mitosis.
 Understand why meiosis is called “reduction division”.
 Know the relationship between genetic variation and meiosis.
 Understand the major stages of meiosis and the chromosome number (haploid/diploid) of each stage.
 Understand the different ways chromosomes can be abnormal and the consequences of each.
Specifically be able to recognize and/or describe duplication, deletion, inversion, translocation, and
nondisjunction.
 Be able to explain how Down syndrome occurs.
 Describe polyploidy in sex chromosomes and what this can cause.
Review Material
MyBiology.com—Study guides and resource for this text. Specifically look at MP3 Tutor and all Web Activities