Practical Lec2/Genetic
Dr.sarab
DNA Structure
— DNA consists of two molecules that are arranged into a ladder-like structure called a
Double Helix.
— A molecule of DNA is made up of millions of tiny subunits called Nucleotides.
— Each nucleotide consists of:
1. Phosphate group
2. Pentose sugar
3. Nitrogenous base
• Deoxyribose Sugar – sugar molecule containing 5 carbons that lose the OH (hydroxyl
group) on its 2’ carbon
• Phosphate group – group of 4 oxygen atoms surrounding a central phosphorus atom
found in the backbone of DNA
• Nitrogenous base – an alkaline, cyclic molecule containing nitrogen
• The source of variation in DNA is found in the nitrogenous bases.
• 4 nitrogenous bases exist:
Purines: Adenine (A) and Guanine (G) – both are double-ringed structures
Pyrimidines: Thymine (T) and Cytosine(C) – single-ringed structures
• RNA is usually single-stranded and generally does not form double helices.
• The carbon atoms in the five-carbon sugar are numbered clockwise, starting with the
carbon atom to the right of the oxygen. The first one is known as the one prime (1’).
• In DNA, the nitrogenous base is attached to the 1’ carbon by a glycosyl bond
• DNA consists of 2 antiparallel strands of nucleotides.
• Antiparallel – parallel, but running in opposite directions; the 5’end of one strand of
DNA aligns with the 3’end of the other strand in a double helix.
• Purines are nitrogenous bases that have 2 rings of carbon: adenine and guanine
• Pyrimidines are nitrogenous bases that have only 1 ring of carbon: thymine and
cytosine.
• One purine must combine with one pyrimidine for the complementary base pairing,
otherwise the DNA structure would look “wonky”.
• Nucleotides are 0.34nm apart
• To complete one turn of the helix, you need 10 nucleotides
• The 2 strands of DNA run anti-parallel. One strand runs in the 5’ to 3’ direction, the
other runs n the 3’ to 5’ direction.
• The 3’ end terminates with the hydroxyl group of the deoxyribose sugar. The 5’ end
terminates with a phosphate group.
• 5’ – ATGCCGTTA – 3’
• 3’ – TACGGCAAT – 5’
— - Each base will only bond with one other specific base.(Hydrogen bound)
— Adenine (A)==Thymine (T)
— Cytosine (C)≡Guanine (G)
— - Because of this complementary base pairing, the order of the bases in one strand
determines the order of the bases in the other strand.
— - To crack the genetic code found in DNA we need to look at the sequence of bases.
— The bases are arranged in triplets called codons.
A G G - C T C - AA G - T C C - TA G
T C C - GAG - TT C -AG G - AT C
- A group of three bases (a triplet) controlsthe production of a particular amino acid in
the cytoplasm of the cell
- The different amino acids and the order in which they are joined up determines the
sort of protein being produced
- This is known as the triplet code
- Each triplet codes for a specific amino acid
- The amino acids are joined together in the correctsequence to make part of a protein
- A gene is a section of DNA that codes for a protein.
- Each unique gene has a unique sequence of bases.
- This unique sequence of bases will code for the production of a unique protein.
— It is these proteins and combination of proteins that give us a unique phenotype.