Video Transcript
Which of the following is correct
about the somatic cells, body cells, of an organism and the gametes, sex cells, that
the organism produces? (A) Gametes contain double the
genetic material of a somatic cell. (B) Gametes contain half the
genetic material of a somatic cell. (C) Gametes contain around 75
percent of the DNA that a somatic cell contains. (D) The genetic material of a
somatic cell is DNA, whereas the genetic material of a gamete is RNA. (E) The genetic material of a
somatic cell is RNA, whereas the genetic material of a gamete is DNA.
Before we answer our question,
let’s go over some of the terms introduced here, starting with DNA. DNA is a biological molecule that
stores and transmits genetic information. DNA is also typically transferred
to offspring during sexual reproduction by condensing into chromosomes. Sexual reproduction involves two
parents, and the offspring inherits DNA from both parents. In sexual reproduction, we want the
new offspring to have the same amount of DNA as the parents. So each parent only pass on half of
their DNA to each offspring. Notice that both the parent’s cells
and the new offspring contain a full set of DNA tightly packed into chromosomes. These cells with two sets of DNA
are called somatic cells.
This brings us to more important
vocabulary to review. Somatic cells are cells with two
sets of chromosomes. Cells that contain two sets of
chromosomes are known as diploid. Somatic cells are also called body
cells because they make up everything in the body except for cells used for sexual
reproduction. So basically, somatic cells are
everything in your body except for sperm or egg cells, which are the gametes. We’ll talk about those later. Here, we have an example of a human
somatic cell and its nucleus, shown in yellow. Let’s take a closer look at the
nucleus.
In the human somatic nucleus, we
can see that we have 23 chromosomes from the biological mother, shown in pink, and
23 chromosomes from the biological father, shown in blue. All somatic cells have two sets of
DNA, one from each parent. The 23 chromosomes from the
biological mother and the 23 chromosomes from the biological father combine to give
us the full 46 chromosomes that a human cell needs. When making an offspring, we only
want half of the DNA from each parent. This is where the gametes come
in.
Gametes are the sex cells, which in
humans are the sperm cell or egg cell, depending on if you’re male or female,
respectively. Gametes have only one set of
chromosomes, which is known as haploid. Gametes are produced during meiosis
with the intent of fusing with another gamete to produce a new individual during
sexual reproduction. In humans, the human egg cell has
23 chromosomes, and the human sperm cell has 23 chromosomes. They fuse together during sexual
reproduction, and this gives human cells the total of 46 chromosomes for our somatic
cells. All organisms work this way where
their gamete chromosome number should be half of the somatic chromosome number.
We already discussed humans, which
have 23 chromosomes in their gametes and 46 chromosomes in their somatic cells. Mice have 20 chromosomes in their
gamete. Therefore, they have 40 chromosomes
in their somatic cells. Now you try. If a flatworm has eight chromosomes
in their gamete, how many chromosomes would we expect in a somatic cell of a
flatworm? Because the somatic cell should be
double that of the gamete, the flatworm should have 16 chromosomes in its somatic
cells. Let’s try that a different way. If corn has 20 chromosomes in its
somatic cell, how many chromosomes should there be in the corn’s gamete? You should have found that the corn
would have 10 chromosomes in its gamete.
Based on what we’ve learned, we
should be able to answer our question. Our question asked, which of the
following is correct about somatic cells of an organism and the gametes that the
organism produces? The chromosomes in both the somatic
cell and a gamete are made of DNA. Therefore, we can eliminate answer
choices (D) and (E). We know that gametes are not double
the amount of genetic material of the somatic cell, so we can eliminate answer
choice (A). We also know that gametes are not
75 percent of the DNA that a somatic cell contains. So we can therefore eliminate
answer choice (C). Therefore, the correct answer is
answer choice (B). Gametes contain half the genetic
material of a somatic cell.
