SAT Biology E/M Subject Test

Part II: Subject Review

Chapter 7 Mitosis and Meiosis

Cell division is a very important topic in the study of molecular biology for the SAT Biology E/M Subject Test. This chapter will introduce you to the topics of mitosis (cell division) and meiosis (the formation of gametes), as well as to what occurs in the cell before, during, and after these processes.

HOW A WHOLE CELL REPRODUCES ITSELF: MITOSIS

For the SAT Biology E/M Subject Test, you need to know about cell division, which is also called mitosis. Before we talk about mitosis itself, let’s talk about what happens before mitosis occurs. To make things easy, we’ll talk about a human cell.

Most human cells (all, in fact, except for sperm and ova) have 46 chromosomes in their nuclei. The chromosomes are found in pairs, so we can say that the nuclei of the cells have 23 pairs of homologous chromosomes. This should be no surprise to you, because we’ve already talked about it. Before a cell undergoes mitosis, every single chromosome in its nucleus replicates. In a human cell, all 46 chromosomes have to replicate. We’ve already discussed how that’s done.

Interphase

Interphase is the time during which chromosomes replicate, but a lot of other things happen during interphase; for instance, the cell carries out all of its normal activities. Interphase is sometimes called the resting stage of the cell—not because the cell is taking it easy, but because the cell is not actively dividing.


The S Phase

The chromosomes replicate during a portion of interphase called the S phase. “S” stands for “synthesis”—in this case synthesis (or replication) of DNA.


Once interphase is over, the cell has replicated every one of its 46 chromosomes. How many chromosomes does it have now? Well, the answer would seem to be 46 × 2 = 92. When a cell has finished interphase, you’d think it has 92 chromosomes, and more or less, you’d be right. But the terminology can get confusing.

Watch Out for This Word: Chromatid

After interphase, each chromosome and the duplicate piece of DNA that was just made are held together at their center by a region called a centromere. The two chromosomes and the centromere make one united physical structure.

We look at the entire structure—the two chromosomes joined by a centromere—and call the whole thing a chromosome. The word chromatid is used to describe each of the individual chromosomes.

When interphase is over, all of the cell’s 46 chromosomes have doubled. We might want to say the cell has 92 chromosomes, but that’s not the way it is described. Instead, we say the cell still has 46 chromosomes, each now consisting of two chromatids.

Quick Quiz #1

Fill in the blanks and check the appropriate boxes:

  1. A human cell, after interphase, has a total of _______________ chromosomes, each made up of two _________________________.

  2. During the stage called interphase, [  all  only some ] of the cell’s chromosomes replicate.

  3. According to current biological terminology, a human cell, after interphase, has in its nucleus a total of [  92  46 ] chromosomes, each chromosome having at its center a ___________________________ that joins the chromatids together.

  4. DNA replication is [  the only process  one of many processes ] that takes place during interphase.

Correct answers can be found in Chapter 15.

Mitosis Happens in Four Steps

After interphase, mitosis begins. Step 1 is called prophase. In prophase, the centrioles move away from each other to opposite sides of the cell. They form a bunch of fibers called the mitotic spindle. These fibers attach to the chromosomes at their centromeres and help to push and pull them around during mitosis. The chromosomes condense (coil up even tighter) and we can see them (under the microscope, of course). The nuclear membrane begins to break up, too.

The Cell at Prophase of Mitosis

Note that in the drawing above, and in the ones that follow, the cell has only four chromosomes. That’s okay. To simplify things, we have left out the other 42 chromosomes. But everything that’s happening to these four chromosomes is happening to the other 42. You’ll just have to imagine them. Also notice that one set of the chromosomes is light and one is dark. This is just to clarify that each chromosome has a homologous partner. The long light-colored chromosome and the long dark-colored chromosome are a homologous pair. Likewise, the shorter light-colored chromosome and the shorter dark-colored chromosome are a homologous pair.

Step 2 is called metaphase. During metaphase, the chromosomes line up—pushed and pulled by the spindle fibers—at the equator of the cell. The equator of the cell is known as the metaphase plate.

The Cell at Metaphase of Mitosis

Step 3 is called anaphase. In anaphase, the centromere that joins each pair of chromatids splits in two so that each chromatid separates from its partner. And guess what? Now each chromatid is once again called a chromosome. So once the centromeres split, you have to admit that the cell briefly has 92 chromosomes. The newly separated chromosomes move toward opposite poles of the cell with the help of the spindle fibers.

Also during anaphase, the cell physically begins splitting in two. The area where it pinches inward is called the cleavage furrow. The cleavage furrow occurs in animal cells as seen below.

The Cell at Anaphase of Mitosis

Step 4 is called telophase. A nuclear membrane forms in each new cell and two daughter cells result, each of which has 46 chromosomes. The cytoplasm then divides during a process called cytokinesis.

The Cell at Telophase of Mitosis

Then, of course, the two new daughter cells enter interphase.


The Order of Mitosis

So let’s review the order:

•   Before mitosis, interphase

•   Mitosis:

      1. Prophase

      2. Metaphase

      3. Anaphase

      4. Telophase


Note that mitosis in animal cells occurs throughout the organism, but in plant cells it only occurs in certain growing regions called meristems. There is the apical meristem that allows for growth in length and can be found at the tip of the stem and tip of the root. This is why biology classes often look at root tip slides of Allium (the onion) when they study mitosis. The lateral meristem allows for growth in width. An example is the vascular cambium in woody stems. Cancer is an example of uncontrolled cell division that relates to problems stemming from the cell cycle.

Quick Quiz #2

Fill in the blanks and check the appropriate boxes:

  1. The cell’s chromosomes become visible during a stage called ________________________.

  2. The spindle apparatus forms during a stage called [  anaphase  prophase  metaphase ].

  3. The division of the cell’s cytoplasm is known as ________________, and this occurs during [  prophase  anaphase  telophase ].

  4. Duplicate chromosomes (the chromatids) separate from each other and move to opposite poles of the cell during a stage called _________________________.

  5. During a stage called _________________________, all of a cell’s chromosomes replicate.

  6. The centromeres divide during a stage called ___________________.

  7. During prophase, the _________________________ move away from one another toward opposite sides of the cell.

Correct answers can be found in Chapter 15.

GENES, PROTEINS, AND CHROMOSOMES

It used to be said that each gene in a eukaryotic cell corresponds to a single protein. We now know that one gene can create several proteins or other gene products, but the old one-gene-one protein theory is still the simplest definition of a gene and that the definition keeps changing as we learn more. The one-gene-one protein theory means that when we say “gene,” we’re talking about some portion of a chromosome that gives rise, ultimately, to one protein molecule. A gene is any part of any chromosome that is responsible for the creation of one protein molecule.

A chromosome is just a long piece of DNA, and one chromosome contains many, many genes. It takes three nucleotides to make one mRNA codon, and even though one codon codes for only one amino acid, and a single protein is a long, long chain of amino acids, a single chromosome is so very, very long that it may give rise to hundreds and hundreds of proteins. That may be difficult to imagine, but it’s true.


Remember Why Chromosomes Are Important: They Contain Genes

•   Chromosomes are very long strands of DNA.

•   DNA is a chain of nucleotides.

•   A strand of DNA can direct the production of a molecule of mRNA, which is also a chain of nucleotides.

•   mRNA travels from the nucleus to the cytoplasm and binds to a ribosome.

•   A series of three mRNA nucleotides is a codon, which codes for a particular amino acid.

•   Amino acids (carried by tRNA) bind to the ribosome according to the order of the mRNA molecule’s codons.

•   Peptide bonds are formed between the amino acids and a polypeptide (a protein) is formed.

This process is known as protein synthesis, or translation.

•   Not all of the DNA in a chromosome is used to make mRNA.

•   The portions of the chromosome that are transcribed to mRNA, and ultimately translated to protein, are called genes.


So remember: When we say “gene,” we’re talking about one portion of one chromosome. What portion? A portion that ultimately produces—via mRNA and ribosomes—one protein. Also remember that one chromosome contains enough nucleotides to bring about the production of many different proteins. This is another way of saying that one chromosome contains a large number of genes.

One last thing about genes is the idea of gene expression. When a gene creates a protein it is said that gene is “being expressed.” For the SAT Biology subject test, you should definitely be familiar with the concept of gene expression within genetics.

But Where Did These Chromosomes and Their Genes COME From?

They came from your mom and dad. Remember, we said that all human cells (except for sperm and ova) have 46 chromosomes, and the chromosomes were found as two sets of 23 chromosomes each. One set of 23 chromosomes came from your mom in an ovum, and one set of 23 chromosomes came from your dad in a sperm cell. So that means that sperm and ova (sex cells, or gametes) have only 23 chromosomes, half the number of normal (non-sex, or somatic) cells. Cells that have two complete sets of chromosomes are described as being diploid, and cells that have only one set of chromosomes are described as being haploid. So sperm and ova are haploid cells.

How do sperm and ova come to have only 23 chromosomes? They undergo a special type of cell division called meiosis.

By the Numbers

Diploid number refers
to the number of chromosomes
a cell has when
it’s in a diploid state. For
a human cell, the diploid
number is 46. Haploid
number
 means the number
of chromosomes a cell
has when it’s in a haploid
state. Naturally, the
haploid number is always
one-half the diploid
number. For a human
being, the haploid
number is 23.

THE FORMATION OF GAMETES: MEIOSIS

The gametes—the sperm and ova—are the only human cells that are haploid. Each has 23 chromosomes. When a sperm and an ovum get together—that is, when the sperm fertilizes the ovum—the chromosomes from the sperm join with the chromosomes in the ovum. The newly formed cell—the zygote—is diploid. The diploid zygote then undergoes mitosis to begin the new human’s development. We’ll look at the specifics of how a sperm is formed and how an ovum is formed in just a little while. But first let’s go over the basics of meiosis.


During Meiosis

1.   The cell undergoes DNA replication during interphase, just as it would if it were about to go through ordinary mitosis. All of the chromosomes replicate, and we’re left with a cell that still has 46 chromosomes, each made up of two chromatids joined by a centromere.

2.   The replicated chromosomes are split up in the course of two sets of divisions: prophase I, metaphase I, anaphase I, telophase I, and prophase II, metaphase II, anaphase II, and telophase II.

3.   The differences between mitosis and meiosis are all found during the first set of divisions: prophase I, metaphase I, anaphase I, and telophase I.


Meiosis I

Meiosis I consists of four phases: prophase I, metaphase I, anaphase I, and telophase I. Remember that the chromosomes have already replicated and are found as two chromatids held together at the centromere. The biggest difference between these four phases and the four phases of mitosis is that at the very beginning, the homologous chromosomes pair up in a process called synapsis. This changes everything.

Prophase I

Synapsis occurs during prophase I. All the chromosomes have to find their homologous partner and pair up. Chromosome 1-A has to find chromosome 1-B, chromosome 2-A has to find chromosome 2-B, and so on. It takes a while, and prophase I is the longest phase of meiosis. When synapsis is complete, all the chromosomes are paired up with their partners. So instead of finding 46 replicated chromosomes floating around, we find 23 pairs of replicated chromosomes. Because each pair consists of four chromatids (two chromatids per replicated chromosome, and two replicated chromosomes), this pair is also known as a tetrad (tetra = four). Notice that, in the drawings below, only four pairs of the 23 pairs are shown.

All of the other normal events that occur in prophase still happen. The spindle is formed, the chromosomes condense, and the nuclear membrane disintegrates. After synapsis occurs, an event called crossing over takes place. Basically, this means that like segments on homologous chromosomes are exchanged.

Metaphase I

During metaphase of mitosis, the chromosomes line up on the equator of the cell. During metaphase of meiosis, the chromosomes also line up on the equator of the cells.

Crossing Over

Note that for simplicity in
the drawings for meiosis,
we are not showing
crossing over. However,
you can imagine that each
one of the black chromosomes
has a little bit of
grey, and each of the grey
chromosomes has a
corresponding little bit of
black. Also, the
chromosomes do not
necessarily line up with all
the black on the left and
all the grey on the right.
It could be mixed up, with
some of the black on the
right and some of the grey
on the left.


In meiosis, chromosomes stay in their homologous pairs. So instead of 46 individual chromosomes lining up, there are 23 pairs of chromosomes.


For the SAT Biology Subject Test, you should be familiar with the concept of Independent Assortment of Chromosomes. The way that the chromosomes line up during metaphase affects that outcome of the genetic information in the gametes that form. This is because the genes on non-homologous chromosome pairs are inherited independently of one another. For example, if n = the haploid number, and n = 4, then, 2n = 16. This means that there are 16 possible combinations for the chromosomes in the gametes. If genes are on the same chromosome they are called linked genes and are inherited together. Crossing over may separate linked genes.

Anaphase I

During anaphase of mitosis, the 46 replicated chromosomes split at their centromeres, and one chromatid goes to each of the opposite poles of the cell. In anaphase I of meiosis, the centromeres DO NOT divide. Instead, the homologous pairs separate, with one entire replicated chromosome (a pair of chromatids and a centromere) moving to each of the opposite poles of the cell.


During Anaphase

•   In mitosis, the chromatids of each chromosome separate.

•   In meiosis, the homologous pairs separate.


Anaphase I of Meiosis

Telophase I

Telophase I of meiosis is very similar to telophase of mitosis. The two cells finish dividing their cytoplasm (cytokinesis), and nuclear membranes reform around the chromosomes. But this leaves us with a strange situation. The two new cells DO NOT have 23 homologous pairs of chromosomes (46 total chromosomes); they have 23 replicated chromosomes (each chromosome is made of two identical chromatids).


Because there are no homologous pairs, the cells are considered haploid by Telophase I.


Telophase I of Meiosis

Meiosis II

Meiosis II is virtually identical to mitosis, in terms of how the chromosomes are moved and how they are split. However, because we’re starting with the two cells formed in meiosis I, they have only half the number of chromosomes that a cell would have when undergoing mitosis. Remember that, in mitosis, the cell starts with 46 replicated chromosomes. The cells we’re starting out with in meiosis II, because of meiosis I, have only 23 replicated chromosomes. But the phases and the chromosome movements are identical to those of mitosis. During prophase II the spindle forms, the nuclear membrane disintegrates, and the DNA condenses (of course, there is no pairing of chromosomes this time, because there is nothing to pair up with—the homologous partners were separated during anaphase I). During metaphase II, the chromosomes line up individually along the equator and, during anaphase II, the centromere splits and the chromatids divide. Then the chromatids are called chromosomes again. During telophase II, a nuclear membrane forms around the newly split chromosomes, and we are left with four haploid cells.

The four haploid cells do not replicate any further unless fertilization triggers new cell cycles.

Quick Quiz #3

Fill in the blanks and check the appropriate boxes:

  1. If, for a particular organism, the diploid number of chromosomes is 10, then the haploid number is _________________________.

  2. The first metaphase of meiosis (metaphase I) differs from metaphase of mitosis in that a _________________________ of chromosomes lines up on each spindle fiber.

  3. The four cells resulting from meiosis are ______________________.

  4. Crossing over occurs after _________________________.

  5. The word [  haploid  diploid ] refers to a cell for which each chromosome does NOT have a homologous partner.

  6. The first anaphase of meiosis (anaphase I) differs from anaphase of mitosis in that centromeres [  do  do not ] divide.

  7. Prophase I of meiosis [  is  is not ] similar to prophase of mitosis.

Correct answers can be found in Chapter 15.

Meiosis and the Formation of Sperm Cells: Spermatogenesis

Because sperm and ova are the gametes, the formation of sperm and ova is called gametogenesis. When we talk specifically about the formation of sperm, we call it spermatogenesis. Spermatogenesis requires meiosis, (not mitosis).


Spermatogenesis

1.   The spermatogonium replicates all of its chromosomes during interphase. It now has 46 chromosomes, and each chromosome is made of two chromatids joined by a centromere.

2.   The cell undergoes prophase I, the homologous chromosomes pair up (synapsis), and crossing over occurs.

3.   The cell undergoes metaphase I in which the paired chromosomes line up on spindles at the equator. We see two centromeres on each spindle fiber.

4.   The cell undergoes anaphase I, but the centromeres don’t divide. Instead, the homologous chromosome pairs separate.

5.   The cell finishes dividing during telophase I, and we now have two cells. Each cell has 23 chromosomes, and each chromosome is made up of two chromatids, still joined by a centromere. These cells are considered haploid.

6.   Each of these two cells then goes through prophase, metaphase, anaphase, and telophase II. This second set of divisions DOES resemble mitosis. Chromosomes condense (but do not pair up) during prophase II; they line up individually along spindle fibers during metaphase II, and the centromeres divide during anaphase II. The cells finish dividing during telophase II, and at the end we have four cells, each of which have 23 unreplicated chromosomes (they’re still haploid).


We start with a diploid cell called a spermatogonium (it’s gonna become a sperm cell). Spermatogonia live in tiny tubules called seminiferous tubules, located in the testes. The testes are the male gonads. One spermatogonium, which is diploid, undergoes meiosis and produces four sperm cells, which are haploid. The difference between oogenesis and spermatogenesis is that only one gamete forms in oogenesis and becomes the ovum, whereas in spermatogenesis all four gametes become functional sperm.

Meiosis and the Formation of Ova: Oogenesis

When we say oogenesis, we are talking about the formation of female egg cells, also known as ova (singular = ovum). We deal, again, with meiosis.

Oogenesis is similar to spermatogenesis: A diploid cell forms haploid cells through meiosis.


Oogenesis

1.   The initial cell is a primary oocyte.

2.   Primary oocytes are found in ovaries in the female reproductive system. The ovary is the female gonad.

3.   The final cell is called an ovum.

4.   Oogenesis results in the production of a single ovum from a single primary oocyte. Two of the daughter cells resulting from meiotic division simply disintegrate. They get no cytoplasm and no organelles. These cells are called polar bodies. This is very different from spermatogenesis, in which four mature sperm are produced from a single spermatogonium.

5.   Oogenesis occurs on a monthly basis, beginning at puberty and ending at menopause (the end of regular menstrual cycles, an event that usually occurs between the ages of 46 and 54). A single ovum is produced per month.


Other than the differences in names of the initial and final cell, and the absorption of daughter cells in oogenesis, the overall process is pretty much the same.

For the exam, you should also be familiar with the idea of genetic variation. This refers to genetic differences both within and among populations. There are three causes of variation among offspring. All of these lead to genetic variation within populations and are therefore important in the process of evolution. These three causes of variation among offspring are:

1.   Independent assortment of chromosomes during metaphase I of meiosis

2.   Crossing over during synapsis in Prophase I of meiosis

3.   Random fertilization

Quick Quiz #4

Fill in the blanks and check the appropriate boxes:

  1. The cells produced at the end of telophase I are considered to be [  haploid  diploid ].

  2. Spermatogenesis begins at _________________________ and lasts ______________________________.

  3. _________________________ mature sperm are produced from a single spermatogonium.

  4. The female gonad is the _________________________.

  5. Oogenesis begins at ________________________ and ends at ________________________.

  6. [  Four  One ] mature ova (ovum) are (is) produced from a single primary oocyte.

  7. Spermatogenesis takes place on a [  daily  monthly ] basis, whereas oogenesis takes place on a [  daily  monthly ] basis.

  8. The cells that disintegrate during oogenesis are called ___________________________________________.

Correct answers can be found in Chapter 15.

Key Words

mitosis

interphase

centromere

chromosome

chromatid

prophase

mitotic spindle

metaphase

metaphase plate

anaphase

cleavage furrow

telophase

cytokinesis

meristems

apical meristem

cancer

one-gene-one-protein

gene expression

gametes

somatic

diploid

haploid

diploid number

haploid number

meiosis

zygote

synapsis

prophase I

tetrad

crossing over

metaphase I

Independent Assortment of Chromosomes

linked genes

anaphase I

telophase I

meiosis II

gametogenesis

spermatogenesis

spermatogonium

seminiferous tubules

oogenesis

ova/ovum

primary oocyte

polar bodies

Summary

•   Somatic cells undergo a process called mitosis. This is the way that these cells divide.

•   Chromosomes replicate during the S phase of interphase. At the end of interphase, all 46 chromosomes in a cell have been replicated.

•   The first stage of mitosis is prophase. The centrioles move away from each other to opposite sides of the cell, and the spindle is formed. The chromosones condense, and the nuclear membrane disintegrates.

•   During metaphase, the second stage of mitosis, the chromosomes line up at the equator of the cell.

•   During anaphase, the third stage, the centromere splits, separating each chromatid from its partner. The newly seperated chromosomes are pulled to opposite sides of the cell.

•   In the last stage, telophase, two daughter cells form, with 46 chromosomes each.

•   Gametes form during a process called meiosis.

•   In meiosis, as in mitosis, chromosomes are replicated during interphase. At the end of interphase, all 46 chromosomes in a cell have been replicated.

•   The replicated chromosomes are split up in the course of two sets of divisions: meiosis I and meiosis II.

•   As in mitosis, there are four stages to each set of meiotic division: prophase, metaphase, anaphase, and telophase.

•   A main difference between mitosis and meiosis is that at the beginning of meiosis, homologous chromosomes pair up during a process called synapsis.

•   Another important difference between mitosis and meiosis is that the daughter cells in mitosis are genetically identical to the original cell. In meiosis the resulting gametes have one half the number of chromosomes of the original cell and are not genetically identical.

•   The centromeres do not divide in anaphase I; the homologous pairs separate into two cells.

•   In anaphase II of meiosis, the centromeres do divide, resulting in four haploid cells with 23 chromosomes each.

•   Spermatogenesis is the formation, through meiosis, of 4 haploid sperm cells from a single spermatogonium.

•   Oogenesis is the formation, through meiosis, of a single haploid ovum from a single primary oocyte.

Now that you know how sperm and ova come to have only 23 chromosomes each …