Nucleic Acids
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What characteristics should genetic material have?
• replication • storage of information • expression of information • variation by mutation
Give some information on bacteria and viruses on the early study of DNA
They are capable of rapid growth and complete life cycle within hours. They can be easily manipulated experimentally. Mutations can be induced early and selected.
What are 2 key experiments for DNA discovery?
Griffith’s experiment Injecting a mouse with active bacteria and it dies and then heating the bacteria and it dies so the mouse does not get infected. Avery transformation experiment Treatment of heat killed virulent strain with proteases had no effect but DNAase removed the transformation effect. Showed that DNA is needed for translation to occur
What were the two sets of evidence that were used to solve the structure of DNA?
• Base composition analysis if hydrolysed samples of DNA (showed it was double stranded and anti-parallel) • Xray diffraction studies
What did Charguff discover?
That A and T were bonded and C and G as there were equal amounts of each pair
What are the 3 parts of a nucleotide?
• Nitrogenous base • Pentose sugar (deoxyribose) • Phosphate (1,2 or 3)
What makes pyrimidines and purines different?
Purines have a double carbon-nitrogen ring and pyrimidines have a single one
In adenine (the base), what number nitrogen is bound to what number carbon?
Nitrogen 9 to carbon 1
What carbon is the phosphate group bound to in deoxyribose?
5 (5 prime bond)
What can the sugar in nucleotides be phosphorylated by? What does this produce?
By specific kinases, producing nucleotides
How many phosphates are there on a nucleotide?
3
What is a nucleoside?
A nucleotide with no phosphate groups on it
Where does a phosphodiester bond form in DNA?
Between the phosphate and the deoxyribose sugar
What gives a DNA strand polarity? How does the phosphate group attach two sugars together?
The phosphodiester bonds It joins to one sugar at carbon 3 and another at carbon 5 (3 to 5 prime)
What part of the nucleotide is facing inwards and what is facing outwards?
Bases face inwards and the phosphate sits on the outside.
Give some information on the DNS molecule being a double helix
• 2 DNA chains which are complementary to each other are coiled around a central helix of symmetry • The chains are paired in an anti-parallel manner, ie.e the 3’ end of one strand is paired with the 5’ end of the other • The deoxyribose-phosphate backbone is on the outside of the molecules and the hydrophobic bases are stacked on the inside • The spatial relationship gives a major (wide) and minor (narrow) groove. The bases are accessible from these grooves (how enzymes enter)
Is DNA a left or right handed helix? How many nucleotides per turn? How long is each turn? How long is the helix diameter?
Right 10 3.4nm 2nm
The base pairs are — to the axis of the helix?
Perpendicular
Why is the GC bond harder to denature than AT?
GC has 3 bonds and AT has 2
Give some information on denaturing DNA?
2 strands separate when the hydrogen bonds between the bases are disrupted. The temperature at which half the helical structure is lost is defined at the melting temperature. Single stranded DNA has a higher relative absorbance at 260nm then double stranded DNA
Give some brief information on the 3 types of DNA
B form - right handed, 10 residues per turn, perpendicular bases to axis, most common A form - dehydrated B form, 11 bases per turn Z form - left handed, 12 base pairs per turn
Where is DNA found in eukaryotes? Bacteria? Plants?
Eukaryotes - In the nucleus and mitochondria Bacteria - cytoplasm Plants - nucleus and chloroplasts
Compare the structure of DNA in eukaryotes and prokaryotes
Eukaryotes - long linear molecules of dsDNA, bound to proteins to found chromatin. They have closed circular DNA molecules in the mitochondria. Prokaryotes - typically contain and single stranded, supercoiled circular chromosome . Their DNA is complexed with proteins to form nucleoids. Bacteria carry small circular extra-chromosomal molecules called plasmids.
What does a genome look like? How many base pairs is the human genome?
DNA tightly packed into chromosomes and is stored at 10,000x shorter than its length achieved by supercoiling. DNA plus proteins (histones) are called chromatin which is packed into nucleosomes which are then condensed into chromosomes. It can be RNA or DNA. 3.5 x 10 ^ 9