Cracking the AP Biology Exam

7

Cell Reproduction

A CLOSER LOOK AT MEIOSIS

Meiosis actually involves two rounds of cell division called meiosis I and meiosis II.

Before meiosis begins, the diploid cell goes through interphase. Just as in mitosis, double-stranded chromosomes are formed during this phase.

MEIOSIS I

Meiosis I consists of four stages: prophase I, metaphase I, anaphase I, and telophase I.

Prophase I

Prophase I is a little more complicated than regular prophase. As in mitosis, the nuclear membrane disappears, the chromosomes become visible, and the centrioles move to opposite poles of the nucleus. But that’s where the similarity ends.

The major difference involves the movement of the chromosomes. In meiosis, the chromosomes line up side-by-side with their counterparts (homologues). This event is known as synapsis.

Synapsis involves two sets of chromosomes that come together to form a tetrad (or a bivalent). A tetrad consists of four chromatids. Synapsis is followed by crossing-over, the exchange of segments between homologous chromosomes.

What’s unique in prophase I is that “pieces” of chromosomes are exchanged between the homologous partners. This is one of the ways organisms produce genetic variation. By the end of prophase I, the chromosomes will have exchanged regions containing several alleles, or different forms of the same gene. By the end of prophase, the homologous chromosomes are held together only at specialized regions called chiasmata.

Metaphase I

As in mitosis, the chromosome pairs—now called tetrads—line up at the metaphase plate. By contrast, you’ll recall that in regular metaphase the chromosomes lined up individually.

Anaphase I

During anaphase I, one of each pair of chromosomes within a tetrad separates and moves to opposite poles. Notice that the chromosomes do not separate at the centromere. They separate with their centromeres intact.

The chromosomes now go on to their respective poles.

Telophase I

During telophase I, the nuclear membrane forms around each set of chromosomes.

Finally, the cells undergo cytokinesis, leaving us with two daughter cells. Notice that at this point the nucleus contains the haploid number of chromosomes, but each chromosome is a duplicated chromosome.

MEIOSIS II

The purpose of the second meiotic division is to separate the duplicated chromosomes, and is virtually identical to mitosis. Let’s run through the steps in meiosis II.

After a brief period, the cell undergoes a second round of cell division. During prophase II, the chromosomes once again condense and become visible. In metaphase II, the chromosomes move toward the metaphase plate. This time they line up single file, not as pairs. During anaphase II, the chromatids of each chromosome split at the centromere and are pulled to opposite ends of the cell. At telophase II, a nuclear membrane forms around each set of chromosomes and a total of four haploid cells are produced:

GAMETOGENESIS

Meiosis is also known as gametogenesis. If sperm cells are produced then meiosis is called spermatogenesis. During spermatogenesis, four sperm cells are produced for each diploid cell. If an egg cell or an ovum is produced, this process is called oogenesis.

Oogenesis is a little different from spermatogenesis. Oogenesis produces only one ovum, not four. The other three cells, called polar bodies, get only a tiny amount of cytoplasm and eventually degenerate. Why does oogenesis produce only one ovum? Because the female wants to conserve as much cytoplasm as possible for the surviving gamete, the ovum.

Here’s a summary of the major differences between mitosis and meiosis:

MUTATIONS

Sometimes, a set of chromosomes has an extra or a missing chromosome. This occurs because of nondisjunction—the chromosomes failed to separate properly during meiosis. This error, which produces the wrong number of chromosomes in a cell, results in severe genetic defects. For example, humans typically have 23 pairs of chromosomes, but individuals with Down’s syndrome have three—instead of two—copies of the 21st chromosome.

Chromosomal abnormalities also occur if one or more segments of a chromosome break. The most common example is translocation (a segment of a chromosome moves to another chromosome).

Here’s an example of a translocation:

Translocation involves transposons, DNA segments that have the ability to move around the genome. Sometimes when they move, they leave behind mutations, and they can cause mutations by inserting into a gene. Transposable elements were first identified by Barbara McClintock who noticed variation in corn kernel color as a result of mobile genetic elements.

Fortunately, in most cases, damaged DNA can usually be repaired with special repair enzymes.

GENE REGULATION

What controls gene transcription, and how does an organism express only certain genes? Most of what we know about gene regulation comes from our studies of E. coli. In bacteria, the region of bacterial DNA that regulates gene expression is called an operon. One of the best-understoodoperons is the lac operon, which controls expression of the enzymes that break down lactose.

The operon consists of four major parts: structural genes, the regulatory gene, the promoter gene, and the operator.

  • Structural genes are genes that code for enzymes needed in a chemical reaction. These genes will be transcribed at the same time to produce particular enzymes. In the lac operon, three enzymes (beta galactosidase, galactose permease, and thiogalactoside transacetylase) involved in digesting lactose are coded for.
  • The promoter gene is the region where the RNA polymerase binds to begin transcription.
  • The operator is a region that controls whether transcription will occur.
  • The regulatory gene codes for a specific regulatory protein called the repressor. The repressor is capable of attaching to the operator and blocking transcription. If the repressor binds to the operator, transcription will not occur. On the other hand, if the repressor does not bind to the operator, RNA polymerase moves right along the operator and transcription occurs. In the lac operon, the inducer, lactose, binds to the repressor, causing it to fall off the operator, and “turns on” transcription.

Other operons, such as the trp operon, operate in a similar manner except that this mechanism is continually “turned on” and is only “turned off” in the presence of high levels of the amino acid, tryptophan. Tryptophan is a product of the pathway that codes for the trp operon. When tryptophan combines with the trp repressor protein, it causes the repressor to bind to the operator, which turns the operon “off” thereby blocking transcription. In other words, a high level of tryptophan acts to repress the further synthesis of tryptophan.