90 ❯ STEP 4. Review the Knowledge You Need to Score High
- Cyclins and protein kinases.Cyclinis a protein that accumulates during G 1 , S, and G 2
of the cell cycle. A protein kinaseis a protein that controls other proteins through the
addition of phosphate groups. Cyclin-dependent kinase (CDK) is present at all times
throughout the cell cycle and binds with cyclin to form a complex known as MPF
(maturation or mitosis promoting factor). Early in the cell cycle, because the cyclin
concentration is low, the concentration of MPF is also low. As the concentration of
cyclin reaches a certain threshold level, enough MPF is formed to push the cell into
mitosis. As mitosis proceeds, the level of cyclin declines, decreasing the amount of MPF
present and pulling the cell out of mitosis.
Haploid Versus Diploid Organisms
One thing that is often a major source of confusion for some of my students is the distinc-
tion between being haploid and being diploid. Let’s start with a definition of the terms:
Ahaploid(n) organism is one that has only one copy of each type of chromosome. In
humans, this refers to a cell that has one copy of each type of homologous chromosome.
Adiploid(2n) organism is one that has two copies of each type of chromosome. In humans,
this refers to the pairs of homologous chromosomes.
During the discussion of meiosis below, the terms haploidanddiploidwill be used
often. Whenever we say “2n,” or diploid, we are referring to an organism that contains two
fullsetsof chromosomes. The letter nis used to represent the number of sets of chromo-
somes. So if an organism is said to have 4nchromosomes, this means that it has four com-
plete sets of chromosomes. Humans are diploid, and consist of 2nchromosomes at all times
except as gametes, when they are n. Humans have 23 differentchromosomes; there are two
fullsetsof these 23 chromosomes, one from each parent, for a total of 46 chromosomes.
Human sex cells have 23 chromosomes each.Meiosis
Now that we have armed you with the knowledge of the distinction between haploid and
diploid, it is time to dive into the topic of meiosis, which occurs during the process of
sexual reproduction. A cell destined to undergo meiosis goes through the cell cycle, synthe-
sizing a second copy of DNA just like mitotic cells. But after G 2 , the cell instead enters
meiosis, which consists of twocell divisions, not one. The second cell division exists because
the gametes to be formed from meiosis must be haploid. This is because they are going to
join with another haploid gamete at conception to produce the diploid zygote. Meiosis is
like a two-part made-for-TV miniseries. It has two acts: meiosis I and meiosis II. Each of
these two acts is divided into four steps, reminiscent of mitosis: prophase, metaphase,
anaphase, and telophase.
Homologous chromosomesresemble one another in shape, size, function, and the
genetic information they contain. In humans, the 46 chromosomes are divided into
23 homologous pairs. One member of each pair comes from an individual’s mother, and
the other member comes from the father. Meiosis I is the separation of the homologous
pairs into two separate cells. Meiosis II is the separation of the duplicated sister chromatids
into chromosomes. As a result, a single meiotic cycle produces fourcells from a single cell.
The cells produced during meiosis in the human life cycle are called gametes.
Again, the AP Biology exam is not going to test your mastery of the minute details of
the meiotic process. However, a general understanding of the various steps is important: