Lecture 4 Flashcards
What is the common structure properties of nucleotides like RNA and DNA?
➢ One to three phosphate group(s)
➢ a pentose (5-carbon atom) sugar,
➢ a nitrogenous base (a pyrimidine or purine)
What bases are purines? structure difference?
Adenine & Guanine
- have two cylical shapes (pentagon and hexagon)
What bases are pyrimidines?
structure diff?
Thymine, Uracil and Cytosine
- have ONLY ONE cyclical shape
What sugar and base is ONLY in RNA?
-Ribose (containing two OH group)
- Uracil instead of thymine
What sugar and base is ONLY in DNA?
- Deoxyribose (containing one OH and one H)
-Thymine instead of Uracil
What are nucleotide functions as monomers?
- Building Blocks of Nucleic Acids (DNA & RNA)
-Energy carriers (cell energy currency), carrying high-energy
bonds between phosphate groups (e.g. ATP, GTP)
-Form coenzymes (essential for enzymatic activity)
-Intracellular signalling molecules (e.g. cyclic AMP)
What is the structure of the DNA/RNA strand/ backbone?
In DNA/RNA, nucleotides are covalently linked to form polynucleotide strand (backbone)
▪ via phosphodiester bonds, linking 5’ phosphate of one nucleotide to the 3’ OH of the next
▪ Creating a 5’-to-3’ directionality
▪ Nucleic acids of human cells has a 5’ end and a 3’ end
➢ (e.g., 5’-TCG-3’)
What is the structure of B DNA?
DNA (in B form) has two helical grooves of different widths, providing binding sites
for proteins/drugs
➢ major - wider
➢ minor - narrower
What is the advantage to RNA being single stranded?
▪ dsRNA (double stranded) is possible in nature. Many viruses contain dsRNA genomes.
▪ Immune cells recognise dsRNA as a viral signature, triggering antiviral responses.
What is DNA denaturation?
DNA Denaturation → separation of the DNA double strands into single strands
* DNA renaturation → When conditions are reversed, H bonds can be reformed →
▪ High temperatures (90-100°C) break H-bonds between complementary nucleotides
▪ Higher G-C content increases DNA stability than A-T. (3 H-bonds vs 2) thus more energy needed
▪ Principles of the PCR technique to amplify specific regions of DNA
What is a gene?
A DNA segment that encode specific, inheritable traits.
* Nucleotide sequence in genes determine the specific amino acid sequence of certain
proteins through the GENETIC CODE (DNA → RNA → PROTEINS)– Lec. 4
* Human genome has ≈ 25,000 genes (<1.5% of DNA)
Where is DNA found in eukaryotic cells?
Nuclear DNA in the nucleus
Mitochondrial DNA in the mitochondria
What is DNA replication?
DNA replication is the process by which DNA makes a copy of itself, before cell divisions
▪ Each chromosome will result in 2 identical DNA (sister chromatids)
▪ Copied DNA is segregated into daughter cells
▪ Occurs in the nucleus during the S phase of interphase
What type of process is DNA replication?
DNA replication is a SEMI-conservative process
What are the four stages to DNA Rep?
- Replication Fork Formation
- Initiation
- Polymerase elongation
4.Termination
Describe Replication Fork Formation
- DNA replication starts at a sequences called
origin of replication (rich in A - T) - At each, DNA is denatured by initiator proteins
- Helicase enzymes unwind/unzip the DNA
double strands bidirectionally, by breaking H-
bonds, creating two Y-shaped replication forks
Describe initiation
- DNA polymerase requires an RNA primer, created
by primase enzyme, to initiate synthesis - DNA polymerase attaches to the 3’ end of the primer
and synthesises new DNA strand in the 5’->3’
direction, using the parental strand as a template - DNA polymerase forms phosphodiester bonds
between nucleotides, extending the new DNA strands - The process requires deoxynucleotide triphosphates
(dNTPs) to drive the reaction
Describe elongation
Replication of leading strand is continuous
▪ Same direction as the growing replication forkReplication of the lagging strand is discontinuous
▪ It requires many RNA primers
▪ Synthesis in short DNA fragments (Okazaki
fragments), each initiated by a new RNA primer
Describe termination
A nuclease degrades RNA primers
➢ The gaps are filled by DNA polymerase
➢ The enzyme DNA ligase joins adjacent Okazaki
fragments → DNA Replication is complete
What is a benefit of DNA Polymerase?
DNA polymerase has proofreading activity to check &
correct base-pairing errors
▪ When a mismatch occurs, DNA polymerase removes the
incorrect nucleotide and replaces it with the correct one.
What does DNA helicase do?
Unwinds parental double helix
What does DNA ligase do?
Joins Okazaki fragments and seals other breaks in sugar-phosphate backbone
What does primease do?
Synthesise RNA primers for DNA polymerase to start the polymerisation process
What does DNA topoisomerase do?
Unravel twists in DNA that occur as a result of DNA replication
