1.5 Nucleic acids are important information-carrying molecules

Describe the basic functions of DNA and RNA in all living cells

DNA - holds genetic information which codes for polypeptides (proteins)

RNA - transfers genetic information from DNA to ribosomes

Name the two types of molecules from which a ribosome is made

RNA and proteins

Draw and label a DNA nucleotide and an RNA nucleotide

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 can 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-containg organic base

• phosphodiester bonds join adjacent nucleotides

• 2 polynucleotide chains held together by weak 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

• linear (single stranded)

Compare and contrast the structure of DNA and (messenger) RNA

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 → stable / strong molecule

• double helix with sugar phosphate backbone → protects bases / hydrogen bonds

• long molecule → stores a lot of genetic information (that codes for polypeptides)

• double helix (coiled) → compact

Suggest how you can use incomplete information about the frequency of bases on DNA strands to find the frequency of other bases

1. % of adenine in strand 1 = % of thymine in strand 2 (and vice versa)

2. % of guanine in strand 1 = % of cytosine in strand 2 (and vice versa)

Because of specific complementary base pairing between 2 strands

Why is semi-conservative replication important?

ensure 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

semi-conservative - each new DNA molecule consists of one original / template strand and one new strand

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 (phosphate end of developing strand)

Name the two scientists who proposed models of the chemical structure of DNA and RNA replication

Watson and Crick

Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication

1. bacteria grown in medium containing heavy nitrogen (15N) so nitrogen is incorporated into DNA bases

• DNA extracted and centrifuged → settles near bottom, as all DNA molecules contain 2 ‘heavy’ strands

2. bacteria transferred to medium containing light nitrogen (14N) and allowed to divide once

• DNA extracted and centrifuged → settles in middle, as all DNA molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand

3. bacteria in light nitrogen (14N) allowed to divide again

• DNA extracted and centrifuged → half settles in middle, as contains 1 original ‘heavy’ strand and 1 new ‘light’ strand; half settles near top, as contains 2 ‘light’ strands 

other models (e.g. dispersive, conservative) not supported - bands would be in different places