Continuity and Variation
GENE TIC MATE RIAL
Objectives
By the end of this lesson, you should be able
to:
•Distinguish among DNA, Chromosomes, Genes and
Alleles
Nucleic Acids
Just like lipids, carbohydrates and proteins, nucleic
acids are major biomolecules.
They are the basis of all genetic material of all living
organisms and are found in the nucleus.
Nucleic acids occur in one of two forms:
RNA – Ribonucleic acids
DNA – Deoxyribonucleic acids
DNA
.
DNA – is an extremely long chain that is built
by multiple smaller molecules called
nucleotides in various combinations.
A specific sequences of nucleotides in specific
regions of the DNA strand carry a code to make
specific proteins. These regions are called
genes.
The DNA is so long and thin but it is not visible.
It is stored in the nucleus as a fine thread
called chromatin.
DNA strands must be tightly coiled
and wrapped around ball-like
proteins called histones, to become "visible".
At this point they are called chromosomes
Figure 1. How DNA strands are coiled to
form chromosomes.
Chromosomes
Organisms have two sets
of chromosomes – one
from each parent.
The half from either parent
is called the haploid
number, n.
Figure 2. The forty-six (2n or 23 pairs) human
chromosomes.
The complete pair is called
the diploid number, 2n.
Chromosomes form an 'X' shape – the intersection is called a centromere.
Chromosomes continued
The diploid number and the make-up of the chromosomes are unique to
each species.
Humans have a chromosome number of 46 pairs. See figure 2.
Fruit flies have 4 pairs of chromosomes and dogs have 78.
For each chromosome from one parent, there is a matching chromosome
from the other parent. These are called homologous pairs.
This means that the same gene is present on both members of the same
pair of chromosomes at the same location!
Alleles
One gene may come in
different expressions.
These expressions are
called alleles.
One gene may have
more than two alleles
Figure 2b. The gene for rose
colours has more than 2
allelic expressions
Figure 2. Allelic variations of the gene responsible for
flower colour.
P.S. The locus is the position that a gene can be found on a
chromosome for a species.
Distribution of Genetic Material
All body cells – also called somatic or vegetative cells - contain all the genetic
material of the species.
That is, all somatic cells have 2n chromosomes.
However, not all genes may be active in all somatic cells – think about
cell specialization for example.
Also, gametes are haploid. During sexual reproduction they will fuse with the
other haploid gamete from the other parent to join homologous pairs during
fertilization.
Questions?
P R E PA R E D B Y DA VE R I C H A R DS F O R W B S B I OL O G Y D E PA R T M E N T 2 0 24 .