Medical and Molecular Genetics
Lecture 2 Nucleic Acids: Composition & Structure
1) List the types and principal functions of the nucleic acids. Deoxyribonucleic acid
(DNA) is found in the cell nucleus and in the mitochondria and functions to store
genetic information used for the synthesis of proteins and enzymes. Ribonucleic acid
(RNA) is found in the nucleus, in the cytosol, and in the mitochondria and has many
functions: mRNA carries messages transcribed from DNA to be translated into
protein, hnRNA is immature form of mRNA, tRNA carries activated amino acids,
and rRNA is a structural component of ribosomes.
2) List the principal chemical building blocks that comprise nucleotides.
Nucleotides are composed of a base (purines or pyrimidines), a pentose sugar (ribose
or deoxyribose), and phosphate groups (mono, di, or tri phosphates).
3) List the differences between DNA and RNA. DNA has one less hydroxyl group at
the 2’ position than RNA. DNA normally exists as a double helix with two
antiparallel strands while RNA normally exists as a single polynucleotide and can
fold so that base pairing occurs between complimentary regions thus having singlestranded and double stranded regions. Thymine occurs only in DNA and Uracil
occurs only in RNA. Adenine, guanine, and cytosine occur in both.
4) Define the primary structure of a nucleic acid. Primary structure is defined by the
sequence of nucleotides as they appear only the length of a polynucleotide polymer.
They are joined by phosphodiester bonds connecting oxygens of the 5’ carbon of one
nucleotide to the 3’ carbon of the adjacent nucleotide. The phosphopentose units
comprise the phosphate backbone of a nucleic acid.
5) Describe the secondary structures of DNA and RNA. Complementary base pairing
generates the secondary structure of nucleic acids where purines pair with
pyrimidines. Adenosine pairs with thymine or uracil forming two hydrogen bonds and
guanosine pairs with cytosine forming three hydrogen bonds.
6) Define the terms: Tm, Hyperchromic shift, hybridization, and heteroduplex.
Tm – the melting temp is the temperature at which 50% of the DNA is denatured.
Hyperchromic shift – light wave absorption increases as DNA is heated because heat
breaks the hydrogen bonds that hold the strands together thus unstacking the
bases and exposing them so they absorb more light.
Hybridization – complimentary strands of nucleic acids derived from different
sources anneal together.
Heteroduplex – is a double stranded nucleic acid that forms from hybridization.