5 Steps to a 5 AP Biology, 2014-2015 Edition

92 ❯ STEP 4. Review the Knowledge You Need to Score High

In humans, the process of gamete formation is different in women and men. In men, sper-

matogenesis leads to the production of four haploid sperm during each meiotic

cycle. In women, the process is called oogenesis.It is a trickier process than spermatogen-

esis, and each complete meiotic cycle leads to the production of a single ovum, or egg. After

meiosis I in females, one cell receives half the genetic information and the majority of the

cytoplasm of the parent cell. The other cell, the polar body,simply receives half of the

genetic information and is cast away. During meiosis II, the remaining cell divides a second

time, and forms a polar body that is cast away, and a single haploid ovum that contains half

the genetic information and nearly all the cytoplasm of the original parent cell. The excess

cytoplasm is required for proper growth of the embryo after fertilization. Thus, the process

of oogenesis produces two polar bodies and a single haploid ovum.

To review, why is it important to produce haploid gametes during meiosis? During fer-

tilization, a sperm (n) will meet up with an egg (n), to produce a diploid zygote (2n). If

either the sperm or the egg were diploid, then the offspring produced during sexual repro-

duction would contain more chromosomes than the parent organism. Meiosis circumvents

this problem by producing gametes that are haploid and consist of one copy of each type

of chromosome. During fertilization between two gametes, each copy will match up with

another copy of each type of chromosome to form the diploid zygote.

Before moving on, there are a few important distinctions between meiosis and mitosis

that should be emphasized.

MITOSIS MEIOSIS

Resulting daughter cells Two diploid (2n) daughter cells Four haploid (n) daughter cells

Crossover? No Yes—prophase I

Types of cells in which it All cells of the body other than Cells of gonads to produce gametes

occurs for humans the cells of the gonads

Crossover

Chromosome A Chromosome B Genetically recombined chromosomes

Figure 9.5 Crossover.

KEY IDEA

In meiosis during prophase I, the homologous pairs join together. This matching of chro-

mosomes into homologous pairs does not occur in mitosis. In mitosis, the 46 chromosomes

simply align along the metaphase plate alone.

An event of major importance that occurs during meiosis that does not occur during

mitosis is known as crossover(also known as crossing over) (Figure 9.5). When the homol-

ogous pairs match up during prophase I of meiosis, complementary pieces from the two

homologous chromosomes wrap around each other and are exchanged between the chro-

mosomes. Imagine that chromosome A is the homologous partner for chromosome B.

When they pair up during prophase I, a piece of chromosome A containing a certain stretch

KEY IDEA