Video Transcript
Which statement about chromosomes
is true? (A) Chromosomes are always visible
in the cell nucleus. (B) Somatic cells are haploid,
which means they contain one set of chromosomes. (C) Members of different species
usually have the same number of chromosomes. (D) Members of the same species
usually have the same number of chromosomes. Or (E) reproductive cells are
diploid, which means they contain two sets of chromosomes.
Let’s review what we know about
chromosomes. A chromosome is a long molecule of
DNA that is coiled around proteins. The coiling of DNA around proteins
is a way of condensing the DNA or packing it up and making it smaller. Chromosomes become particularly
visible when they are in the form of highly condensed DNA. DNA is condensed or highly coiled
when not in use, but when it is in use, it needs to be loose so that the
complementary strands can separate from each other and the DNA base sequence on an
individual strand can be used as a template in the process of replication or of
building proteins.
When cells are preparing to divide,
they first replicate all of their DNA and then all of the DNA is coiled into its
most condensed state in preparation for the actual process of cell division, which
will include moving the chromosomes around the cell and separating the replicated
DNA. When DNA is highly coiled into its
most condensed state, we can, with the help of a microscope, visibly see the highly
condensed chromosomes inside of a cell. We’re most likely to be able to
visibly see the chromosomes like this during the process of cell division when all
of the cell’s DNA has been highly coiled into these highly condensed visible
chromosomes and the cell’s nuclear membrane is dissolved, allowing the chromosomes
to be moved around the cell.
Replicated chromosomes, the kind we
usually see at the beginning of cell division, typically look like x’s because they
consist of a chromatid or a highly coiled and condensed strand of DNA and its
replicated copy sister chromatid. And these chromatids are joined
together in the middle by a centromere. In humans and many other organisms,
most cells in the body, which we call somatic cells, are diploid, meaning that there
are two copies of each chromosome. And the two copies together create
what we call a chromosomal set.
Within a set of chromosomes, each
chromosome contains the same genes, although there may be different alleles or
versions of each gene on each of the chromosomes. Where did these two separate
chromosomes with the same genes come from? Well, one came from that
individual’s mother, and the other came from that individual’s father. So in order to create a diploid
offspring, each parent has to contribute one chromosome from each set. That’s why gametes, or sperm and
egg, contain only one chromosome of each set, which is called being haploid. So somatic cells are diploid with
two copies of each chromosome, while reproductive cells are haploid with just one
copy of each chromosome.
For the purposes of cell division,
it’s important that cells have the correct number of chromosomes. For example, if an offspring that
is inheriting chromosomes from each parent receives a different number of
chromosomes from each parent, that may cause difficulties in cell division in the
developing embryo so that it may just essentially stop developing and not
survive. For this reason, while different
species have different numbers of chromosomes — for example, humans have 46,
domestic cats have 38, and goldfish have 50 — within each species, each individual
will nearly always have the same number of chromosomes as other individuals of that
species.
So let’s look again at the answer
choices and determine which of these statements is true. (A) Chromosomes are always visible
in the cell nucleus. We know that this is not true
because when DNA is in use, it’s loose and not highly coiled and condensed into a
visible chromosome. Also during cell division, the
nuclear membrane dissolves. So at that point, you could see
chromosomes outside of the nucleus.
(B) Somatic cells are haploid,
which means they contain one set of chromosomes. Haploid does mean containing one
chromosome of each set, but somatic cells or body cells are diploid. So this statement is not correct
either.
(C) Members of different species
usually have the same number of chromosomes. We just mentioned three different
species that have three different numbers of chromosomes. So this statement is not correct
either.
(D) Members of the same species
usually have the same number of chromosomes. This is correct. For example, humans usually have 46
chromosomes and cats usually have 38.
Let’s look at the last answer
choice. (E) Reproductive cells are diploid,
which means they contain two sets of chromosomes. Diploid does mean having two
chromosomes for each set, but it’s somatic cells that are typically diploid, while
reproductive cells are typically haploid. So this last statement is not
correct either.
The correct statement and correct
answer is that members of the same species usually have the same number of
chromosomes.
