DNA structure (M.B+G)

lecture 2

nucleotide contents

phosphate, sugar and base

nucleoside contents

sugar and base NO PHOSPHATE GRP

specific bond between base and a sugar

beta - N - glycosidic bond

nucleotide bases description

aromatic, heterocyclic molecules w/ nitrogen grps

purines vs pyrimidines

purines = 9 membered double -ring structure → adenine and guanine

pyrimidines = 6 membered single ring structure → thymine and cytosine

DNA subunit key points

• Has a hydrogen atom (-H) at the 2' carbon of the pentose ring.

• This makes the DNA strand more stable and less prone to breakage, which is essential for its long-term genetic storage function.

RNA subunit keypoints

• Has a hydroxyl group (-OH) at the 2' carbon of the pentose ring.

• This -OH group makes RNA more reactive and less stable than DNA, which is important for its various functional roles, such as acting as a messenger, adapter, or component of ribosomes in protein synthesis.

RNA vs DNA 

RNA is more sensitive than DNA to hydrolysis due to being single stranded.

Griffith’s Transforming Principle

• transformation of a non-pathogenic pneumococcal bacteria into a virulent strain

mixing up both live harmless mutant R-form with the heat killed S-cells

• since the DNA is heat resistant,the DNA from the deadly S form could replicate and and transform the live mutant form cells. therefore the mice died of

• S-strain contained a heat - resistant 'transforming principle' that can be inherited

• At the time, still wasn't known if the death was down to protein or DNA so further testing was done

Avery,MacLeod & McCarty (1944)

proved DNA is responsible for transformation in Griffith's experiment

heat killed S-cells were incubated in three diff cond.:

• No protein= hereditary information was still present as transformation still occurred and s cells still appeared

• No RNA = hereditary information was still present as transformation still occurred and s cells still appeared

• no DNA = hereditary information was not present as transformation did not occur as S cells did not appear

key features of francis-crick model

1. Right-handed helix.

2. Two strands, running anti-parallel (5’-3’) into

opposite directions.

3. Sugar-phosphate backbone on outside, giving shroud of negative charge.

4. The backbone of polynucleotides are highly charged (1 unit negative charge for each phosphate group; 2 negative charges per base-pair).

5. Bases on the inside, pairing A-T, G-C.

6. Major and minor grooves presence.

7. 10.5 base-pairs per complete helical turn.

8. Stabilised by hydrogen bonds and (mostly) by hydrophobic base pairing and stacking.

different DNA forms

A-DNA: right-handed, adopted in dehydrated cond., protecting DNA during desiccation (extreme dehydration). identified by X-ray diffraction at 75% relative humidity.

B- DNA: right-handed, similar to A. Most common DNA form, is the watson crick model

Z-DNA: left handed. DNA observed underwound. believed to play role in gene replication. major + minor grooves show little width difference.