3
THE MOLECULAR BASIS OF GENETICS
OBJECTIVES:
By the end of this session the student should be able to:
* Outline the molecular
structure of DNA and RNA
*
Explain the significance
of specific base pairing
*
Define the main
properties of DNA
*
Work out the
sequence on m-RNA from that on the coding and transcribed strands of DNA
*
Define
transcription and translation and state where they occur
*
Distinguish between single- stranded and
double-stranded nucleic acids
*
Explain the
meaning of denaturation and re-annealing of DNA
NUCLEOTIDES
The
molecules of Deoxyribose Nucleic Acid (DNA) and Ribose Nucleic Acid (RNA) are common to all living organisms and form the basis of life. Both molecules consist of long chains of nucleotides.
A nucleotide consists of three main components (Fig 3.1):
a)
a pentose sugar
consisting of a five-sided ring containing 5 carbon atoms, numbered clockwise
from 1 to 5. This sugar is ribose in
RNA or deoxyribose in DNA,
b) a phosphate group (PO4--) attached to C5 of the pentose ring.
c) a nitrogenous base attached to C1 of the pentose ring. The bases present in DNA are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). In RNA Uracil (U) replaces Thymine.
A nucleoside consists of a ribose sugar with an
attached base. Addition of a phosphate
group to the nucleoside forms a nucleotide. Thus there are five nucleotides
that form the building blocks of DNA and RNA - Adenine, guanine, cytosine,
thymine and uracil nucleotides.
A hydroxyl group (OH), situated at C3 of the pentose ring, is the reactive site where one nucleotide interacts with the phosphate group of another nucleotide to form phosphodiester bond (fig. 3.2).
Di-nucleotides and tri-nucleotides consist of two and three nucleotides respectively. Poly-nucleotides, such as DNA and RNA consists of long chains of nucleotides.
A polynucleotide chain has a distinct polarity. It has a PO4 group attached to the C5 at one end - this is termed the 5’ end; and an -OH group attached to C3 at the other end - this is termed the 3’ end. New nucleotides may be added to a nucleotide chain only at the 3’ end. Thus, increase in length of a nucleotide chain is always unidirectional and occurs only in the 5’ to 3’ direction
The bases present in DNA and RNA are divided into two groups (Fig3.3).
1. Adenine and guanine are purines and consist of two nitrogenous rings. They differ in the position of the amino group (NH2).
2. Cytosine, thymine and uracil are pyrimidines and consist of one nitrogenous ring. They differ in the nature and position of the side groups that are NH2 and CH3 respectively.
Specific base pairing occurs between nucleotides on opposite strands of DNA (Fig. 3.4). Thymine on one strand is always linked to adenine on the other strand by two hydrogen bonds. Cytosine on one strand is always linked to Guanine on the other strand by three hydrogen bonds. Specific base pairing is one of the most important features of DNA. It is crucial in the processes of DNA replication and transcription, which are two of the main functions of this molecule.
DNA
The DNA molecule consists of two very long nucleotide chains (or strands) arranged in the form of a double spiral with the following properties:
* the two strands are antiparallel i.e. the 5' end of one strand lies next to the 3' end of the complementary strand.
* the two strands are twisted in the form of a double spiral
*
the bases on one strand are linked to corresponding
bases on the other strand by hydrogen bonds
RNA
RNA is similar to DNA in that it consists of a polynucleotide chain, but differs from it in the following respects: (a) it consists of only one strand; (b) the pentose sugar is ribose; (c) uracil replaces thymine.
DIMENSIONS OF DNA
The double helical molecule of DNA has a diameter of 2 nanometers (nm). One spiral turn covers a length of 3.4 nm and contains 10 base pairs. (Fig. 3.5)
Note: 1 nm = 10-9m. (In old terminology, molecular dimensions were measured in Ǻngstrom Units (Ǻ) where 1 nm=10Ǻ.)
PROPERTIES OF DNA
The unique structure of the DNA
molecule explains its main properties and functions as the basis of life: