Video Transcript
During its life cycle, the malarial
parasite Plasmodium infects and multiplies inside human red blood cells, as
shown in the figure. What reproductive process occurs
inside red blood cells? (A) Asexual reproduction to form
sporozoites and merozoites. (B) Asexual reproduction to form
merozoites and gametocytes. (C) Sexual reproduction to form
sporozoites and merozoites. (D) Sexual reproduction to form
merozoites and gametocytes. Or (E) sexual reproduction to form
gametocytes and a zygote.
This question asks us about the
reproductive process in the parasite that causes malaria in humans,
Plasmodium. Let’s review how Plasmodium
infects and multiplies in human red blood cells and how their life cycle is
perpetuated through mosquitoes through alternation of generations. To do this, let’s remove the answer
options and figure for now so we have some more space to work with.
Alternation of generations is an
interesting phenomenon by which an individual organism undergoes both sexual and
asexual reproductive processes within its life cycle. We can think of this life cycle in
Plasmodium as starting with haploid cells called sporozoites, which are
found in the salivary glands of a female Anopheles mosquito. You might recall that haploid cells
contain half the genetic information of most other body cells, which is often
represented as n.
When a mosquito infected by
Plasmodium bites a human, the Plasmodium sporozoites will be
introduced to the human body as mosquitoes feed by piercing the skin of humans. These sporozoites reproduce through
an asexual process called schizogony in the human liver cells, producing many
daughter cells that mature into haploid cells called merozoites. These merozoites are released from
the liver cells and spread throughout the body, infecting the human’s red blood
cells and causing symptoms of malaria infection, like fever, sweating, and
chills.
In the red blood cells, the number
of merozoites increases rapidly. And they can differentiate into
haploid gametocytes that are either male or female. When another female
Anopheles mosquito bites an infected human, these gametocytes are
transferred into that mosquito. The gametocytes soon develop into
mature male and female gametes, or sex cells, once they’ve been ingested by a
mosquito, which allows Plasmodium to reproduce sexually.
This begins the sexual stage of the
Plasmodium life cycle. And the two gametes fuse to form a
diploid zygote in the mosquito’s midgut. The zygote develops and produces
thousands of sporozoites, the haploid cells that will infect the salivary glands of
the mosquito. The sporozoites could then be
transmitted to another human, which initiates the life cycle again.
The question is asking us what
reproductive process occurs inside human red blood cells. And we know that Plasmodium
undergoes asexual reproduction in the human body to first form haploid merozoites,
which infect red blood cells, and then differentiate into haploid gametocytes. If we bring back our answer
choices, we can see that this matches option (B). The reproductive process that
occurs in human red blood cells, and the correct answer to this question, is
therefore (B), asexual reproduction to form merozoites and gametocytes.
