Chapter 7: Mitosis and Meiosis

Cell division is also called mitosis. 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. Before a cell undergoes mitosis, every single chromosome in its nucleus replicates. In a human cell, all 46 chromosomes have to replicate.

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. Longest phase of the cell cycle. This is a period of cell growth and metabolism.

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.

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

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.

A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.

Each of the two threadlike strands into which a chromosome divides longitudinally during cell division. Each contains a double helix of DNA.

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.

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.

After interphase.

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.

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.

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.

Chromosomes reach opposite poles of the cell, the reversal of many prophase events occurs, chromosomes uncoil and decondense to become threadlike chromatin, and a nuclear envelope forms around the chromosomes .Two daughter cells result, each of which has 46 chromosomes. The cytoplasm then divides during a process called cytokinesis.

So let's review the order:
• Before mitosis, interphase
• Mitosis:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
PMAT

meristems

Allows for growth in length and can be found at the tip of the stem and tip of the root

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.

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.

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.

• 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.

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.

The appearance in a phenotype of a characteristic or effect attributed to a particular gene. The process by which possession of a gene leads to the appearance in the phenotype of the corresponding character.

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.

A mature haploid male or female germ cell that is able to unite with another of the opposite sex in sexual reproduction to form a zygote.

Any cell of a living organism other than the reproductive cells.

Cells that have two complete sets of chromosomes are described as being diploid (46 chromosomes).

Cells that have only one set of chromosomes are described as being haploid (23 chromosomes).

haploid cells.

They undergo a special type of cell division called 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.

A diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum.

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 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.

The fusion of chromosome pairs at the start of meiosis.

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).

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.

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.

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

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.

• In mitosis, the chromatids of each chromosome separate.
• In meiosis, the homologous pairs separate.

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.

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.

Between cell divisions, the loosely packed chromosomes replicate. The cell grows and performs normal cellular activity.

No difference.

Chromosomes begin to condense and become visible. These chromosomes are made of two identical pieces of DNA, and each piece of DNA is called a chromatid.

The same events occur as in mitosis. Additionally, the chromosomes pair with their homologous partners and exchange DNA (crossing over).

The centrioles are now at opposite ends of the cell, and the spindle fibers from the centrioles attach to the individual chromosomes.

The only difference is that the spindle fibers from each centriole attach to one chromosome of a matching homologous protein pair.

The chromosomes line up along the metaphase plate (an imaginary line that divides the cell). The spindle fibers begin to rug each chromatid toward the opposite ends of the cell.

The only difference is that the chromosome pairs line up on the sides of the metaphase plate.

The fibers pull the chromatids towards opposite each ends of the cell.

The chromosomes pairs separate. Both halves of the chromosomes move toward the other end of the bells. The sister chromatids do not separate like they do in mitosis.

The chromatids which have now turned back into chromosomes, are at the ends of the cell. A new nuclear membrane forms.

The division in a male sperm cell is equally divided. In a female, most of the cell's cytoplasm will be concentrated in one of the two cells. The larger will divide again and the smaller cell will degenerate.

The rest of the cell divides.

No difference.

gametogenesis.

When we talk specifically about the formation of sperm, we call it spermatogenesis. Spermatogenesis requires meiosis, (not mitosis).

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.

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

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.

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

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.

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).

The testes are the male gonads.

diploid; haploid

that only one gamete forms in oogenesis and becomes the ovum, whereas in spermatogenesis all four gametes become functional sperm.

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.

a primary oocyte. Primary oocytes are found in ovaries in the female reproductive system. The ovary is the female gonad.

an ovum.

polar bodies. This is very different from spermatogenesis, in which four mature sperm are produced from a single spermatogonium.

A single ovum is produced per month.

1. Independent assortment of chromosomes during metaphase I of meiosis
2. Crossing over during synapsis in Prophase I of meiosis
3. Random fertilization