Nucleic acids

Created by

Hassan Karim

Cards (14)

Describe the basic functions of DNA and RNA in all living cells
DNA = Holds genetic information which codes for polypeptides
RNA = Transfers genetic information from DNA to ribosomes
Name the two types of molecule from which a ribosome is made
RNA and proteins
Describe the differences between a DNA nucleotide and an RNA nucleotide
DNA nucleotide :
● Pentose sugar is deoxyribose
● Base can be thymine
RNA nucleotide :
● Pentose sugar is ribose
● Base can be uracil
Describe how nucleotides join together to form polynucleotides
● Condensation reactions, removing water molecules
● Between phosphate group of one nucleotide and deoxyribose/ribose of another
● Forming phosphodiester bonds
Why did many scientists initially doubt that DNA carried the genetic code?
The relative simplicity of DNA - chemically simple molecule with few components.
Describe the structure of DNA
● Polymer of nucleotides (polynucleotide)
● Each nucleotide formed from deoxyribose, a phosphate group and a
nitrogen-containing organic base
● Phosphodiester bonds join adjacent nucleotides
● 2 polynucleotide chains held together by hydrogen bonds
● Between specific complementary base pairs - adenine / thymine and cytosine / guanine
● Double helix
Describe the structure of (messenger) RNA
● Polymer of nucleotides (polynucleotide)
● Each nucleotide formed from ribose, a phosphate
group and a nitrogen-containing organic base
● Bases - uracil, adenine, cytosine, guanine
● Phosphodiester bonds join adjacent nucleotides
● Single helix
Compare and contrast the structure of DNA and (messenger) RNA
● DNA nucleotide :
Pentose sugar is deoxyribose
Has the base thymine
Double helix
Long (many nucleotides)
Has hydrogen bonds / base pairing
● RNA nucleotide :
pentose sugar is ribose
has the base uracil
single helix
shorter (fewer nucleotides)
no hydrogen bonds
Suggest how the structure of DNA relates to its functions
● Two strands → both can act as templates for semi-conservative replication
● Hydrogen bonds between bases are weak → strands can be separated for replication
● Complementary base pairing → accurate replication
● Many hydrogen bonds between bases → strong molecule
● Double helix with sugar phosphate backbone → protects bases
● Long molecule → store lots of genetic information
● Double helix → compact
Why is semi-conservative replication important?
Ensures genetic continuity between generations of cells.
Describe the process of semi-conservative DNA replication
1.DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix
2. Both strands act as templates
3. Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
4. Hydrogen bonds form between adenine-thymine and guanine-cytosine
5. DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
6. Forming phosphodiester bonds
Use your knowledge of enzyme action to suggest why DNA polymerase
moves in opposite directions along DNA strands
● DNA has antiparallel strands
● So shapes / arrangements of nucleotides on two ends are different
● DNA polymerase is an enzyme with a specific shaped active site
● So can only bind to substrate with complementary shape
Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication
Watson and Crick
Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA rep 
1.Bacteria grown in medium has heavy N (15N) + N is mixed into DNA bases
● DNA extracted & centrifuged → settles at bottom,all DNA has 2 ‘15N’ strands
2.Bacteria transferred to medium has light N (14N),allowed to divide once
● DNA E & C → settles in middle, all DNA Has 1 original ‘15N’ + 1 new ‘14N’ strand
3.Bacteria in 14N allowed to divide again
● DNA E & C → 1/2 settles in middle, as contains 1 original ‘15N’ + 1 new ‘14N’ strand; 1/2 settles near top, as has 2 ‘14N’ strands