Lecture 1 Flashcards
(24 cards)
Describe purines
6 membered pyrimidine ring and a 5 membered imidazole ring
E.g. adenine and guanine
Describe pyrimidines
6 membered pyrimidine ring.
E.g. cytosine, thymine, uracil (mRNA only)
How are nucleotides joined?
A condensation reaction between adjacent deoxyribose sugars through a link created as the phosphate back bone. Phosphate and water are eliminated
Phosphodiester linkages
Chargaff’s rule
A=T
C=G
the amount of G and C varies a lot from organism to organism
Hydrogen bonds between bases
Adenine and Thymine form 2
Cytosine and guanine form 3
Describe the structure of DNA
DNA is composed of 4 nucleotides: adenine, guanine, thymine and cytosine
DNA is a double stranded helix with antiparallel strands.
Nucleotides in each strand are linked by 5’-3’ phosphodiester bonds.
Bases on opposite strands are linked by hydrogen bonds.
The double helix is made yp of antiparallel strands that run in the 5’ to 3’ direction
Why does each strand act as a template for building a new strand in replication?
The two strands are complementary to each other.
Describe the secondary structures of DNA
The most common structure of DNA is B-DNA as this is favoured in aqueous solution.
Dehydrated DNA forms A-DNA and is found in nucleic acid structures in vivo.
Z-DNA is a left handed double helix
Triple stranded DNA
A-DNA
Right handed helix. Short and wide. Base pairs tilted away from perpendicular to the helix axis.
Open structure where the base pairs do not form across the centre of the helix.
Structure adopted by double stranded RNA and some DNA-RNA hybrid double strands
What structure can RNA not adopt?
The B-DNA as it has an extra hydroxyl group which is too large to fit causing steric clashes with nearby atoms.
B-DNA
Two spiral grooves run up the helix. A wider major groove and a narrower minor groove. This occurs as the glycosidic bonds to the bases do not lie diametrically opposiite each other.
Structure is stabilised in aqueous environments by a row of water molecules that fit into the minor grooves to form hydrogen bonds with the atoms and bases.
The major grooves allow macromolecules (enzymes) to gain access to base pairs and recognise specific sequences
Describe EcoR 1
Restriction enzyme that recognises a specific DNA sequence. The protein fits into the major groove of the DNA double helix.
Describe the TATA Box- bonding protein
Eukaryotic transcription factor that recognises promoter sequences and leads to initiation of transcription for most genes.
The C terminal domain of the protein interacts with A-T regions of DNA through the minor groove and the binding causes the helix to unwind.
What denatures DNA?
Strands separate to give single stranded DNA or RNA.
Heat, extreme pH, hydrogen bond breaking agents (urea)
Describe why DNA denatures
Hydrogen bonds are broken resulting in the separation of the strands in the double helix.
What determines the temperature at which DNA will denature?
The amount of cytosine/guanine
The number of bases
Briefly describe DNA replication
Separation of the two strands.
Each strand acts as a template, nucleotides added following base pairing rules.
Nucleotides connected to form the sugar-phosphate backbone.
Replication is semi-conservative
What were the three models hypothesised for DNA replication?
Semiconservative
Conservative
Dispersive
Secondary structure of RNA
Two complementary strands of RNA can form double helices, but most secondary RNA structures form a stem-loop.
A stem loop is formed from a single strand by base pairing between internally complementary sequences.
What are the three main types of RNA and what is a similarity?
rRNA, tRNA, mRNA
all single stranded
Role of mRNA
copied from the DNA acting as a template for protein synthesis- it has a limited secondary structure
Role of tRNA
Protein synthesis as the molecules that fit between the amino acids and mRNA
Ribosomal RNA
Components of the ribosome
Catalytic RNAs
involved in RNA processing
Most important in the early stages of the evolution of life
