MCAT Biology Review

Chapter 2: Reproduction

2.2 Meiosis

Whereas mitosis occurs in somatic tissue and results in two identical daughter cells, meiosis occurs in gametocytes (germ cells) and results in up to four nonidentical sex cells (gametes). Meiosis shares some similarities with mitosis. In both processes, for instance, genetic material must be duplicated, chromatin is condensed to form chromosomes, and microtubules emanating from centrioles are involved in dividing genetic material. However, the MCAT tends to ask about differences between these two processes.

In contrast to mitosis, which consists of one round each of replication and division, meiosis consists of one round of replication followed by two rounds of division, as shown in Figure 2.5. Meiosis I results in homologous chromosomes being separated, generating haploid daughter cells; this is known as reductional divisionMeiosis II is similar to mitosis, in that it results in the separation of sister chromatids, and is known as equational division.

Figure 2.5. Meiosis Meiosis results in up to four nonidentical daughter cells.

MEIOSIS I

The human genome is composed of 23 homologous pairs of chromosomes (homologues), each of which contains one chromosome inherited from each parent. This brings up an important note about terminology: whereas homologous pairs are considered separate chromosomes (such as maternal chromosome 15 and paternal chromosome 15), sister chromatids are identical strands of DNA connected at the centromere. Thus, after S phase, there are 92 chromatids organized into 46 chromosomes, which are organized into 23 homologous pairs.

Prophase I

During prophase I, the chromatin condenses into chromosomes, the spindle apparatus forms, and the nucleoli and nuclear membrane disappear. The first major difference between meiosis and mitosis occurs at this point: homologous chromosomes come together and intertwine in a process called synapsis. At this point, each chromosome consists of two sister chromatids, so each synaptic pair contains four chromatids and is referred to as a tetrad. Chromatids of homologous chromosomes may break at the point of synapsis, called the chiasma (plural: chiasmata) and exchange equivalent pieces of DNA, as shown in Figure 2.6. This process is called crossing over. Note that crossing over occurs between homologous chromosomes and not between sister chromatids of the same chromosome—the latter are identical, so crossing over would not produce any change. Those chromatids involved are left with an altered but structurally complete set of genes. Such genetic recombination can unlink linked genes, thereby increasing the variety of genetic combinations that can be produced via gametogenesis. Linkage refers to the tendency for genes to be inherited together; genes that are located further from each other physically are less likely to be inherited together, and more likely to undergo crossing over relative to each other. Thus, as opposed to asexual reproduction, which produces identical offspring, sexual reproduction provides the advantage of great genetic diversity, which is believed to increase the ability of a species to evolve and adapt to a changing environment.

Figure 2.6. Synapsis During prophase I, homologous chromosomes can exchange genetic material via crossing over.

REAL WORLD

The rate of gene unlinking is used to map differences between two genes on the same chromosome. The farther apart two genes are, the more likely they are to become unlinked during crossing over. These statistics can then be used to determine the distance between genes on the chromosome, measured in units called centimorgans.

Because of crossing over, each daughter cell will have a unique pool of alleles (genes coding for alternative forms of a given trait) from a random mixture of maternal and paternal origin. In classical genetics, crossing over explains Mendel’s second law (of independent assortment), which states that the inheritance of one allele has no effect on the likelihood of inheriting certain alleles for other genes.

Metaphase I

During metaphase I, homologous pairs (tetrads) align at the metaphase plate, and each pair attaches to a separate spindle fiber by its kinetochore. Note the difference from mitosis: in mitosis, each chromosome is lined up on the metaphase plate by two spindle fibers (one from each pole); in meiosis, homologous chromosomes are lined up across from each other at the metaphase plate and are held by one spindle fiber.

Anaphase I

During anaphase I, homologous pairs separate and are pulled to opposite poles of the cell. This process is called disjunction, and it accounts for Mendel’s first law (of segregation). During disjunction, each chromosome of paternal origin separates (or disjoins) from its homologue of maternal origin, and either chromosome can end up in either daughter cell. Thus, the distribution of homologous chromosomes to the two intermediate daughter cells is random with respect to parental origin. This separating of the two homologous chromosomes is referred to as segregation.

KEY CONCEPT

It is critical to understand how meiosis I is different from mitosis. The chromosome number is halved (reductional division) in meiosis I, and the daughter cells have the haploid number of chromosomes (23 in humans). Meiosis II is similar to mitosis in that sister chromatids are separated from one another; therefore, no change in ploidy is observed.

Telophase I

During telophase I, a nuclear membrane forms around each new nucleus. At this point, each chromosome still consists of two sister chromatids joined at the centromere. The cells are now haploid; once homologous chromosomes separate, only n chromosomes are found in each daughter cell (23 in humans). The cell divides into two daughter cells by cytokinesis. Between cell divisions, there may be a short rest period, or interkinesis, during which the chromosomes partially uncoil.

REAL WORLD

If, during anaphase I or II of meiosis, homologous chromosomes (anaphase I) or sister chromatids (anaphase II) fail to separate, one of the resulting gametes will have two copies of a particular chromosome and the other gamete will have none. Subsequently, during fertilization, the resulting zygote may have too many or too few copies of that chromosome. Nondisjunction can affect both autosomal chromosomes (such as trisomy 21, resulting in Down syndrome) and the sex chromosomes (such as Klinefelter’s and Turner syndromes).

MEIOSIS II

Meiosis II is very similar to mitosis in that sister chromatids—rather than homologues—are separated from each other.

Prophase II

During prophase II, the nuclear envelope dissolves, nucleoli disappear, the centrioles migrate to opposite poles, and the spindle apparatus begins to form.

Metaphase II

During metaphase II, the chromosomes line up on the metaphase plate.

Anaphase II

During anaphase II, the centromeres divide, separating the chromosomes into sister chromatids. These chromatids are pulled to opposite poles by spindle fibers.

Telophase II

During telophase II, a nuclear membrane forms around each new nucleus. Cytokinesis follows, and two daughter cells are formed. Thus, by completion of meiosis II, up to four haploid daughter cells are produced per gametocyte. We use the phrase up to because oogenesis, discussed later in this chapter, may result in fewer than four cells if an egg remains unfertilized after ovulation.

KEY CONCEPT

Mitosis

Meiosis

2n → 2n

2n → n

Occurs in all dividing cells

Occurs in sex cells only

Homologous chromosomes do not pair

Homologous chromosomes align on opposite sides of the metaphase plate

No crossing over

Crossing over can occur

MCAT Concept Check 2.2:

Before you move on, assess your understanding of the material with these questions.

1.    What is the ploidy of the daughter cells produced from meiosis I? From meiosis II?

·        Meiosis I:

·        Meiosis II:

2.    What is the difference between homologous chromosomes and sister chromatids?

·        Homologous chromosomes:

·        Sister chromatids:

3.    For each phase of meiosis I listed below, what are the differences from the analogous phase of mitosis?

Meiotic Phase

Differences from Mitotic Phase

Prophase I

 

Metaphase I

 

Anaphase I

 

Telophase I