Section 1 - Biological molecules: 2. Nucleic acids Flashcards
What is a nucleic acid
A polymer of nucleotides which store genetic information in biological systems (ie. DNA and RNA)
What is the structure of a nucleotide (3 components)
- Pentose sugar (with 5 carbon atoms)
- A phosphate group
- A nitrogen-containing organic base
Components covalently bonded together in condensation reactions
What is the name of the bond between nucleotides in a polynucleotide
Phosphodiester bonds
What are the 4 bases in DNA
- Adenine
- Thymine
- Cytosine
- Guanine
What are the 4 bases in RNA
- Adenine
- Uracil
- Cytosine
- Guanine
What are the complementary base pairs in the double stranded helix of DNA
Adenine bonds with Thymine (double hydrogen bond)
Cytosine bonds with Guanine (triple hydrogen bond)
How does the structure of DNA relate to it’s function
- Stable (phosphodiester backbone and hydrogen bonds): allows genetic info to be passed on without change
- Two strands joined by hydrogen bonds: allows for separation during replication
- Large molecule: hold more genetic info
- Helix structure: protects the bases from being corrupted by outside forces or chemicals
- Complementary base pairing: allows for the formation of m-RNA
What were the stages of discovering DNA
- 1869: Johann Friedrich discovered nucleic acids
- 1919: Phoebus Levene described the nucleotides within them
- 1944: Oswald Avery discovered that DNA passes on genetic info down generations
- 1950: Erwin Chargaff discovered the complementary base pairing
- 1950: Maurice Wilkins and Raymond Gosling began to collect x-ray diffraction patterns of DNA
- 1952: Rosalind Franklin used improved techniques to produce ‘Photograph 51’, and began evaluating the structure
- 1953: James Watson and Frances Crick published accurate descriptions of the structure of DNA, using Franklin’s work
What was the experiment used to determine the hereditary nature of material in organisms
- Mice injected with living, safe form of bacteria = healthy
- Mice injected with dead, harmful form of bacteria = healthy
- Mice injected with both = infected
This suggests that the dead harmful bacteria can pass on the information needed to make the toxins.
This hereditary material was proven to be DNA, by removing all parts from the living harmful bacteria and adding the individually to living safe bacteria. Only the DNA caused to trait to be passed on.
What was the experiment used to determine that DNA is responsible for passing genetic info down generations.
Viruses infect bacteria to produce more viruses
Viruses only contain DNA and proteins, so one of those must be responsible.
The proteins and DNA in the viruses were labelled with different radioactive elements, and the viruses produced from them only showed signs of radioactive DNA
Where is the 3’ carbon atom located on a pentose sugar
Attached to the hydroxyl group (bottom left when sugar is oriented upright)
Where is the 5’ carbon atom located on a pentose sugar
Attached to the phosphate group (top left when the sugar is oriented upright)
Why can nucleic acids only be synthesised ‘in vivo’
They can only be synthesised in the 5’-3’ direction, as DNA polymerase can only attach nucleotides to the OH group on the 3’ carbon atom
What is semi-conservative replication
The process of DNA replication where one strand of the double helix is conserved, with a new strand attached to it
Describe the process of semi-conservative DNA replication
- The double helix is unzipped by DNA helicase, forming a replication fork
- On the leading strand, leaving the fork with the 3’ end first, the enzyme primase forms a small section of bases attached to it, called a primer
- DNA polymerase binds to this primer, and forms the new strand in the 5’ to 3’ direction as the original strand is passed through in the opposite direction.
- On the lagging strand, leaving the fork with the 5’ end first, the enzyme primase forms multiple primers between which DNA polymerase forms the new strand in the same direction that the original is being passed out, from the 5’ to 3’ end
- These sections of strands formed on the lagging strand are called Okazaki fragments
- The enzyme exonuclease removes all the primers and they are replaced with DNA by DNA polymerase
What is the enzyme that unzips DNA for replication
DNA helicase
What is enzyme that forms primers to begin DNA replication
Primase
What is the enzyme the forms the new strand of DNA replication
DNA polymerase
What is the name of the sections of DNA formed individually on the lagging strand of DNA replication
Okazaki fragments
What direction does the leading strand leave the replication fork in DNA replication
3’ to 5’
What direction does the lagging strand leave the replication fork in DNA replication
5’ to 3’
What is the enzyme that removes the primers after DNA replication
Exonuclease
What were the alternative models of DNA replication instead of semi-conservative replication
- Conservative: original DNA remains intact, and complete new copy is formed
- Dispersive: new and old DNA randomly alternates in both strands
What is the experiment that supports the model of semi-conservative replication (‘the most beautiful experiment’)
- Bacteria grown in 14-N medium
- Bacteria moved and grown in 15-N medium for many generations and when DNA is extracted and centrifuged, all DNA is the heavy, containing 15-N
- After then being grown in 14-N medium for one generation, when extracted and centrifuged, all DNA is in the medium weight band, containing both 14-N and 15-N (disproving conservative replication)
- After 2 generations in 14-N medium, half the DNA was light containing only 14-N, half was medium containing both (disproving dispersive replication)
- After 3 generations, 75% is light and 25% is medium, proving semi-conservative replication, as there is no heavy containing only 15-N, but the proportion containing the original strands halves each generation