DNA & Genomics Flashcards
What is the difference between ribose and deoxyribose?
A hydrogen atom is attached to the 2’ carbon atom of deoxyribose whereas a -OH group is attached to the 2’ carbon atom of ribose
Why is the RNA molecule more reactive than DNA?
The ribose sugar in RNA contains a -OH group which is polar and reactive.
Describe the formation of a nucleotide.
A nitrogenous base is attached to the 1’ carbon atom of the pentose sugar via a condensation reaction forming a nucleoside and a glycosidic bond.
A phosphate group is attached to the 5’ carbon atom of the pentose sugar via a condensation reaction, forming a nucleotide and a phosphoester bond.
What contributes to the stability of the DNA molecule?
Hydrogen bonds between the nitrogenous bases of opposite DNA strands by complementary base pairing and hydrophobic interactions between stacked nitrogenous bases
Why does DNA polymerase only add new nucleotides to the 3’ end of the growing strand?
The active site of DNA polymerase is only specific to the 3’ end of a deoxyribonucleotide
In what direction is the parental DNA strand being read in DNA replication?
3’ to 5’
In what direction is the daughter DNA strand being synthesised in during DNA replication?
5’ to 3’
What is the role of DNA polymerase I?
DNA polymerase I hydrolyses the RNA primer and replaces it with deoxyribonucleotides complementary to the DNA template strand
Why are the daughter strand synthesised continuously as a leading strand and the other synthesised discontinuously as a lagging strand?
The parental stands of DNA have an antiparallel arrangement
What is the role of DNA ligase?
DNA ligase catalyses the formation of phosphodiester bond between two okazaki fragments so that the gap between the two fragments are sealed.
It requires the free 3’ OH group of one DNA strand and a 5’ phosphate group of another DNA strand.
What is the difference in DNA replication in prokaryotic cells and eukaryotic cells?
Prokaryotic genome are small in size and therefore would only require one origin of replication.
Eukaryotic genome is very large so many origins of replication is present, forming many short daughter stands at each replication bubble which eventually link together to form a continuous daughter DNA strand
Explain the end-replication problem.
At the 5’ end of the newly synthesised daughter strand, RNA primer is hydrolysed which leaves a gap that should be filled.
The DNA polymerase I is unable to add deoxyribonucleotides to fill the gap at the 5’ end of the lagging strand.
This is due to the absence of an existing 3’ OH group.
Repeated rounds of replication leads to the shortening of DNA molecules.
What is a gene?
A gene is a specific nucleotide sequence within DNA which codes for a specific sequence of amino acids in a polypeptide molecules or a functional RNA molecule
What are the 5 features of the genetic code?
It is a triplet code (3 bases in 1 codon which codes for one amino acid)
It is degenerate as one amino acid can be coded by many codons
It is punctuated (presence of 4 codons - 1 start and 3 stop, signal the start and ends of translation)
It is non-overlapping, nucleotides of the genetic code are always read in triplets and read only once
It is universal as the codons which codes for a specific amino acid is used across all organisms
What is transcription?
Transcription is a process in which a sequence of DNA bases of a gene are read to form a complementary sequence of bases in a RNA molecule
Describe the transcription process in eukaryotic cells
Proteins called transcription factors bind to the promoter to facilitate the binding of DNA polymerase to the promoter, forming the transcription initiation complex.
The binding of DNA polymerase causes the DNA double helix to unwind and unzip.
One of the DNA strand is used as a template strand and the other will be a non-template strand.
The RNA polymerase reads the template strand in the 3’ to 5’ direction while the RNA strand is synthesised in the 5’ to 3’ direction.
RNA polymerase catalyses the formation of phosphodiester bonds between adjacent ribonucleotides added to the growing RNA strand.
RNA polymerase transcribes a terminator sequence
RNA and RNA polymerase dissociate from DNA molecule which reforms the double helix
What is the difference in the effect of adding a 5’ modified guanosine cap and the polyadenylation of the 3’ end?
The 5’ modified guanosine cap is not recognised by exonuclease so the 5’ end is not degraded by exonucleases, but exonucleases can still recognise the poly-A tail but it will take a longer time for exonuclease to degrade the poly-A tail due to its greater length
Why is post-transcriptional modification important?
The 5’ cap and 3’ polyA tail assist in the export of mRNA out of the nucleus
The 5’ cap and the 3’ polyA tail protect mRNA from degradation
The 5’ cap and 3’ polyA tail are involved in the initiation of translation.
What is the role of aminoacyl-tRNA synthetase?
Aminoacyl-tRNA synthetase has a specific active site which binds one specific type of amino acid to its tRNA molecule
