chromosomes

Chromosomes

First of all, look again at the previous brief discussion of chromosomes to be found near the
beginning of this chapter. In actual fact, the chromosome does not consist of just DNA. Instead,
the nuclear DNA (also known as nucleic acid) of eukaryotes is combined with protein molecules
known as histones. Note that a eukaryote is any organism whose cells contain a nucleus and
other organelles enclosed within membranes (see Chapter 2 for more about the cell).
The DNA and histones together make up the nucleosomes contained within the cell nucleus.
This nucleic acid–histone complex is known as chromatin.
Now we run into a problem: if we unravelled all the nucleic acid from every cell in a human
adult body it would stretch to the Moon and back about 8000 times. So how do we manage to
package that number of DNA and histone molecules into our rather small bodies? The answer,
of course, is that we have to fold them so that they fit into each cell of the body – just like having
to fold clothes to ensure that they fit into a suitcase when going on holiday. And just as clothes
often will only fit in the suitcase if they are neatly folded, the same applies to the chromatin in
our cells. It cannot just be pushed in haphazardly – it would never fit and there would be a great
possibility of things going wrong.
So, in order to fit within our cells, the chromosomes twist on one another, then twist into
loops, before finally assuming the shape that is commonly recognised as a chromosome – the X
shape which is easily seen in a human cell (Figure 3.3) (Jorde et al., 2009).
Let us look in more detail at chromosomes. Each chromosome is made up of two chromatids
joined by a centromere. Looking at Figure 3.3, you can see that one half of the chromosome is
a chromatid, and where they join near the top of the X, that is the centromere.
In most humans, each nucleated cell (i.e. each cell with a nucleus) within the body has
46 chromosomes, arranged in 23 pairs (Figure 3.4). Of those 23 pairs, one pair determines
the gender of the person.
•• Females have a matched homologous (means ‘the same’) pair of X chromosomes.
•• Males have an unmatched heterologous (means ‘different’) pair – one X and one Y
chromosome.
•• The remaining 22 pairs of chromosomes are known as autosomes. In biology the word
‘some’ means body, so autosome means ‘self body’. Thus, ‘autosome’ can be defined as the
chromosomes that determine physical/body characteristics – in other words, all the
characteristics of a person that are not connected with gender.
The position a gene occupies on a chromosome is called a locus, and there are different loci
for colour, height, hair, and so on (‘loci’ is the plural of ‘locus’). Think of the locus as the address of
that particular gene on Chromosome Street – just like your address signifies that that is where
you live.Genes that occupy corresponding loci are called alleles. So, the gene for the same characteristic
on each of the two chromatids is an allele. Alleles are found at the same place in each of the two
corresponding chromatids, and an allele determines an alternative form of the same characteristic.
Remembering that one of your chromatids comes from your mother and the other corresponding
chromatid comes from your father may be of help in understanding this. As an example, think of
the colour of eyes. There is one particular gene that determines eye colour and it is found at the
same place on each of the two chromatids of one chromosome. One gene will come from the
father and the other from the mother. If parents of a child have different coloured eyes from each
other, perhaps the mother has green eyes and the father brown eyes, then the child may have
green or brown eyes, depending upon factors that will be discussed later in this chapter.
So each of these particular genes at that same point (or locus) on each chromatid determines
eye colour. This applies to every one of a person’s characteristics. A person with a pair of identical
alleles for a particular gene locus is said to be homozygous for that gene, while someone with
a dissimilar pair is said to be heterozygous for that gene