Genome, DNA, chromosome, homologous, diploid, haploid, genes, alleles, chromatid - the field
of genetics uses a whole bunch of words specific to this area of biology. Overwhelmed? Well,
lets take a guided tour to understand what they all mean and how they fit together!
First, the genome. This is all of the genetic material of an organism. The human genome
holds many clues as to how we look and behave, but there is so much information there that
we still don’t understand most of it. It is a huge area of ongoing research!
For most organisms this genetic material is made up of DNA. DNA (deoxyribonucleic acid)
is the storage structure for our biological information. It is made of smaller units called
bases (these are the famous A,T,C and G), three billion of them strung together on a
backbone of sugar and phosphate. The DNA is organized in two strands that fit together
in a double helix. The genome of humans is divided into several very long DNA double
helices, each of which is called a chromosome. The DNA of different humans are incredibly
similar, but not identical (except for identical twins) and these differences make each of
us unique.
Each living organism has a characteristic number of chromosomes. Humans, for example,
have 22 autosomal chromosomes, each of them unique in size and DNA sequence, and two types
of sex chromosomes - X and Y. Most of our cells – apart from sperm or eggs - have
pairs of similar chromosomes, this makes us overall diploid organisms. These pairs of
chromosomes, which are called homologous chromosomes, are very similar, but not identical, as one
is from Mom and one is from Dad. The human sperm and egg, which are called gametes, are
different in that they have only one copy of each autosomal chromosome and carry only
one sex chromosome. Having just one copy makes gametes haploid. When a sperm fertilizes an
egg, there are now 23 pairs of chromosomes, or 46 in total. The fertilized egg then divides
making the many cells of the human body.
Now lets look at more closely at this process of cell division, which is also called mitosis.
As a cell prepares to divide, each DNA molecule is replicated, resulting in two identical
copies of each chromosome. This means that the 46 chromosomes temporarily doubles to
92 chromosomes in each cell. The two EXACT copies of each chromosome are called sister
chromatids. Initially the sister chromatids are connected to each other at a central point,
which is often depicted as the “X” shaped drawing of a chromosome. At the end of mitosis,
the two sister chromatids detach and are pulled apart in opposite directions. Two daughter
cells are created, each with 46 chromosomes and the exact same genome.
So, let’s look closer at the structure of a chromosome. Within the long DNA strand,
there are regions called genes that contain the instructions to make proteins. There are
around 20,000 protein-coding genes in humans. The proteins that genes code for make us who
we are. They can give us ginger hair, or be blood type O. These traits are also called
phenotypes in the field of genetics. Zooming back out to the chromosome level once again,
we can start to understand where variation in human phenotypes begins.
As humans have pairs of homologous chromosomes, it follows that they also have pairs of genes.
A pair of genes may be identical to one another. Or they may differ subtly in their DNA sequences,
in which case they are called alleles. Sometimes different alleles still give rise to the same
phenotype. If the two genes on homologous chromosomes produce the same phenotype, for
example both causing blue eyes, the person is a homozygote for that phenotype,. However,
sometimes the two alleles produce different phenotypes, for example blue eyes and brown
eyes. This makes the person a heterozygote. The actual outcome in this person (i.e. what
eye color they end up with) depends on whether one allele is dominant over the other. For
example, the allele for brown eyes is dominant over blue, so the person will have brown eyes.
The blue allele is recessive, because it has no effect on the phenotype of the heterozygote.
Most phenotypes, however, are not as simply explained and understood as eye color. Height,
intelligence, and susceptibility to different diseases, for example, are complex blends
of numerous genes and their many alleles.
Whew! Now that we’ve covered all of these different terms involving DNA, we can delve
into all of the amazing things that DNA does, and understand how the study of genetics is
revolutionizing medicine and our advancing our knowledge of life!