Video Transcript
The diagram shows two chromosomes
undergoing crossing-over. What is the advantage of this? (A) It creates completely new
genes. (B) It reduces the risk of
mutation. (C) It increases genetic
variation. Or (D) it increases the likelihood
of fertilization.
The question is asking us to work
out what the advantage of a process called crossing-over is, which is shown to us in
a diagram. In order to answer this question,
we’re going to need to understand a little bit more about how genetic information is
organized in animal cells into structures called chromosomes. The nucleus of animal cells, like
this one, contains the genetic information as DNA. In animals like humans, DNA is
organized into chromosomes, some of which we can see in the nucleus here.
In most human body cells, there are
46 chromosomes in total or two sets of 23. One set of these 23 comes from the
mother, and the other set comes from the father. The chromosomes from the mother can
be said to be homologous to the chromosomes from the father because they’re nearly
identical. As there are two sets of
chromosomes in most body cells, they’re called diploid cells. In gametes, like sperm cells from
the father and egg cells from the mother, there is only one set of chromosomes. So, gametes can be described as
haploid cells. Gametes are made through a process
called meiosis.
Meiosis is a type of cell division
where one parent cell can make four genetically different haploid daughter
cells. The genetic variation in the
gametes produced through meiosis is partly as a result of a process called
crossing-over. This occurs during one of the
stages of meiosis. As shown in this diagram, it
includes swapping of some genetic information from each of the two homologous
chromosomes in a pair.
There are four chromatids in each
homologous pair of chromosomes and two chromatids in each single replicated
chromosome from each parent. As each of these four chromatids
will end up in different daughter cells by the end of meiosis, the swapping of
sections of DNA between homologous chromosomes can increase the genetic variation of
the gametes produced. This does not create new genes as
sections of DNA are simply swapped between homologous chromosomes.
Crossing-over does not affect the
risk of mutation of DNA, nor does it either increase or decrease the likelihood of
fertilization. It simply trades a section of DNA
between homologous chromosomes that may have slight differences. This can happen to many of the 23
chromosome pairs during meiosis and can increase genetic variation in the
gametes. Therefore, we’ve worked out that
crossing-over increases genetic variation.
