Topic 1.5 - Nucleic Acids Flashcards
What is the basic function of DNA?
Holds genetic information which codes for polypeptides (proteins)
What is the basic function of RNA?
Transfers genetic information from DNA to ribosomes
Name the 2 types of molecule from which a ribosome is made
1) RNA
2) Proteins
Describe the 2 key differences between a DNA nucleotide and a RNA nucleotide
DNA nucleotide:
1) Pentose sugar is deoxyribose
2) Base can be thymine
RNA nucleotide:
1) Pentose sugar is ribose
2) Base can be uracil
Why did many scientists initially doubt that DNA carries the genetic code?
The relative simplicity of DNA - chemically simple molecule with few components
Describe the structure of DNA (6)
1) Polynucleotide
2) Each nucleotide formed from deoxyribose, a phosphate group and a nitrogen containing organic base
3) Phosphodiester bonds join adjacent nucleotides
4) 2 polynucleotide chains held together by hydrogen bonds
5) Between specific complementary base pairs - adenine / thymine and cytosine / guanine
6) Double helix
Describe the structure of RNA (5)
1) Polynucleotide
2) Each nucleotide formed from ribose, a phosphate group and a nitrogen containing organic base
3) Bases - uracil, adenine, cytosine, guanine
4) Phosphodiester bonds join adjacent nucleotides
5) Single helix
Compare the 5 key differences between DNA and RNA
DNA nucleotide:
1) Pentose sugar is deoxyribose
2) Has the base thymine
3) Double stranded / helix
4) Long
5) Has hydrogen bonds
RNA nucleotide:
1) Pentose sugar is ribose
2) Has the base uracil
3) Single stranded / helix
4) Short
5) Does not have hydrogen bonds
Suggest how the structure of DNA relates to its functions (7)
1) 2 strands -> both can act as templates for semi-conservative replication
2) Hydrogen bonds between bases are weak -> strands can be separated for replication
3) Complementary base pairing -> accurate replication
4) Many hydrogen bonds between bases -> stable / strong molecule
5) Double helix with sugar phosphate backbone -> protects bases / hydrogen bonds
6) Long molecule -> store lots of genetic information
7) Double helix -> compact
Why is semi-conservative replication important?
Ensures continuity between generations of cells
Describe the process of semi-conservative replication (6)
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
What is semi-conservative replication?
Each new DNA molecule consists of one original / template strand and one new strand
Why does DNA polymerase move in opposite directions along DNA strands? (4)
1) DNA has antiparallel strands
2) So shapes / arrangements of nucleotides on 2 ends are different
3) DNA polymerase is an enzyme with a specific shaped active site
4) So can only bind to substrate with complementary shape
Who were the 2 scientists that proposed the models of chemical structure of DNA and DNA replication?
Watson and Crick
Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication (3)
1) Bacteria grown in medium containing heavy nitrogen and 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 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 allowed to divide again
- DNA extracted and centrifuged -> half settles in middle, as contains 1 original heavy and 1 new light strand, half settles near top, as contains 2 light strands
